Tech

Technical blog posts

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Intro to Effectsblog

March 16, 2014
  • Early History of the Amplifier

    Last week, we talked about the history of electric guitars and how the first ones came to be. However, we may be forgetting about one very important item used for playing electric guitars; amplifiers. Amplifiers have some interesting history behind them, too, and they weren’t always as advanced and reliable as they are now. Let’s take a look at the history of amps, and how tube amps came to be.

    Around the time when the first electric guitars were created, the first amplifiers were already around and were being used for acoustic guitars. These amplifier systems, however, were not ideal. They were systems not meant for guitar players; they were meant for radio and PA systems, and they required big, bulky batteries to run. They were hard to come by, inconvenient, and unreliable, while also not providing enough volume amplification to really do the job. These systems were also often huge and expensive, which made them not feasible for most musicians to own and use.

    However, in the early 1930’s, PA systems began to move away from battery systems due to the use of electrolytic capacitors and rectifier tubes. Thanks to these inventions, PA systems could be plugged into a wall socket, which enabled them to be much smaller and overall better than the previous battery-based systems. Beauchamp and Rickenbacker, the innovators mentioned in our blog about electric guitars, began to experiment with modifying these; they altered the system to work with a pickup instead of a microphone, much like modern amps. This design would prove to be crucial to the development of amps.

    Beauchamp and Rickenbacke would go on to form the Electro String company, and would create the first production-model dedicated electric guitar amplifier in 1932. After creating this model, the company hired an engineer, Ralph Robertson, to improve their amplifier design. Robertson would go on to develop new circuitry that would put Electro String amps far above all other competition, to the point where their amps would inspire the designs of others. This amp design made guitar amplifiers much more popular, and they would steadily rise in popularity over the years. These amps, however, would have one big flaw; they had 10 watt speakers. That was just simply not enough power to get the volume that electric guitar players began to desire.

     

    Another person who took inspiration from the Electro String company saw this issue and worked to fix it. In 1949, Leo Fender would release a new 50-watt amp with much bigger speakers on it. Electric guitars and the amplifiers that worked with them would then begin to rise heavily in popularity.

    In the 1960’s, musicians began to discover that they could push their amplifiers past their limits and get an interesting distortion effect. This particular sound would become very popular in music of the era, and it would only further drive electric guitars into extreme popularity. The popularity led to more demand for a bigger, more powerful amp that was louder and more capable of handling such sounds.

     

    Jim Marshall would produce the biggest and baddest amplifier yet to meet these demands. The Marshall amp was 100-watts, which was a big jump in power. It would also use four speakers. The power of this amplifier would finally give guitarists what they wanted since the beginning; big, powerful sound.


    Sources:
    https://www.vintageguitar.com/1941/electrorickenbacher-amps/
    https://proaudioland.com/news/history-of-the-amp/
    https://en.wikipedia.org/wiki/Guitar_amplifier

  • Nuclear Secrets of the Tiki Room

    So, summer NAMM was cancelled and it’s unclear if the winter show in 2021 will proceed or in what form. This could put a stop to a show tradition for me visiting Trader Sam’s Enchanted Tiki Bar for some Tiki Tiki Tiki Tiki Tiki (five Tikis) Rum cocktails. You may ask what this has to do with this column, but if we follow a few connections, we can uncover a story of audio visual technology that has some interesting twists.

    Trader Sam’s is a tiki bar near the pool at the Disneyland Hotel. It used to be a bit of a hidden gem, but has become much more popular in recent years as word got around. The bar has various features that reference attractions in the Disneyland Park including some animatronics and acting performances from the bar staff. The aforementioned tiki rum is a variant of the Virgin Islands Painkiller cocktail, and the five tikis in the name are a nod to one of the songs in the Enchanted Tiki Room, an attraction in the Adventureland area of the Disneyland park.

    “The birds all sing, and the flowers bloom
    in the tiki, tiki, tiki, tiki, tiki room”

    This attraction is sponsored by the Dole Food Company and themed around Hawaii, the Polynesian islands, and pineapple which is a popular Dole product. The Trader Sam’s rum cocktail features pineapple as an ingredient.

    Anyway, the Enchanted Tiki Room is worth a visit. It’s an animatronics presentation that features some very impressive sound effects. There are several hundred separate animatronics characters, many of them with their own sound tracks and a large number of highly directional speakers. It’s a small theater and the effects are very convincing, especially the storm and drum effects. From the beginning, a convincing life like appearance for the characters was a major design goal for this attraction.

    First opening in 1963, a significant amount of equipment was required to operate the animated tiki room characters. Housed underneath the building are the networks of electrical, compressed air and water systems needed to operate the show. Driving these in synchronization with the audio to provide the desired realistic impression of the characters speaking and singing was a considerable engineering problem at the time. The original system utilized around one hundred separate speakers and 14 separate audio channels with hundreds of individual animatronic models.

    The technology used to resolve the challenge required some interesting sourcing. Walt Disney already had a relationship with Wernher von Braun, the German aerospace engineer who came to the US after the war and famously worked on the US Army Jupiter missile system before becoming instrumental in what developed into the NASA space program.

    Following on from Jupiter was the US Navy’s UGM-27 Polaris project, the first submarine launched nuclear ballistic missile system. One of the challenges this program faced was the constantly changing conditions that might impact a successful launch such as ocean currents, temperatures, hull movement, and vibrations. To resolve this, an audio based launch control system was developed that used pre-recorded audio cues to precisely time a missile launch based on the current conditions. This audio synchronization technology was a perfect fit for Disney’s attraction.

    Polaris operations began in 1960 and by 1963 much of this technology developed for use on the submarines was installed in a large basement under the floor of the Enchanted Tiki Room.

    Key to the system was the data storage on tape using PS200 tape drives manufactured by Precision Instrument in San Carlos, California. With 10 ½” reels of 1” tape providing up to 14 tracks in conjunction with racks of tube based computer switching and large banks of mechanical relays, the system precisely synchronized the audio with the mechanical movement of the characters creating a lifelike effect that was something to behold in its day. The installation generated so much heat, that the Enchanted Tiki Room became the first fully air conditioned attraction in Disneyland.

  • The Basics of Programming

    At their heart, computer processors run machine code. This is the series of 1s and 0s called bits(Binary digITS) that the device translates into voltage highs and lows used to control the display, keyboard, motor, switches, lights and so forth, in whatever system it’s being used. This is true whether the device is the CPU in your laptop, the microcontroller for your car brakes, or the DSP in your digital effects pedal.

    A binary bit stream looks like this:

    1001101110100111100111110011110100111011111101110

    In the early days of computing, engineers divided up the bits into groups of eight called octets, which came to be known as bytes. Eight 1s and 0’s provides 256 possible permutations, which can be used to represent human readable characters such as letters of the English alphabet. There’s a standard code for this that most systems use called ASCII. Let’s divide the bit stream into bytes:

    1001101 1101001 1110011 1110011 1101001 1101111 1101110

    That’s still a lot of digits. Hexadecimal, or base 16, is a convenient way to remap these values to something easier to handle, so let’s convert these to hex. For example 01100111 is 67 in hex. That reduces 8 digits down to two. Here’s our full binary stream in hex:

    4D 69 73 73 69 6F 6E

    Now if we If we convert those to ASCII, our long bit stream at the beginning becomes:

    Mission

    So, we may see a piece of machine code that looks something like this:

    0x 60 00 00 80
    0x A4 00 00 00
    0x 60 01 00 84
    0x A4 01 01 00
    0x 60 02 00 00
    0x 60 03 00 04
    0x 60 04 00 00
    0x 60 05 00 01
    0x 08 00 00 02
    0x 20 00 00 03
    0x 20 04 04 05
    0x 11 20 04 01

    Even using hex, it’s very slow and error prone writing complex programs like this, so there are some further translations we can do. Assembly language is specific to the actual processor, but uses some more human readable and writable language. After writing the assembly code, we run it through a specialized program called an assembler. This translates the human written text files into object files of machine code that the processor can run. Here’s an example of some assembly code:

    ADR r4,a ;     get address for a
    LDR r0,[r4] ;     get value of a
      ADR r4,b ;     get address for b, reusing r4
    LDR r1,[r4] ;     get value of b
    ADD r3,r0,r1 ;     compute a+b
    ADR r4,c ;     get address for c
    LDR r2,[r4] ;     get value of c

    Assembly language may be preferable in some cases in signal processing where certain timing issues may be easier to deal with. However, in many cases a higher-level programming language such as C may be appropriate. Here’s the equivalent to the above in C:

    x = (a + b) – c; // Calculate x

    C is a compiled language. You can write your code in any text editor, and then run it through a special program called a compiler that creates the output files to run on your particular device. Since C is so popular, there are usually compiler options for most devices.

    C code can also be moved between devices relatively easily. This is called porting. You may have to change processors in your products at one time or another. Maybe a model you use becomes obsolete, or you need something more powerful to add some more features. Assembly code is specific to the device, and would have to be rewritten. C is not device specific, and often the same code can be recompiled and run on different devices with minimal changes.

    Many digital effects have sophisticated programmable features such as upgradeable firmware that require the use of an editor. These apps run on another device such as a PC, Mac or smartphone. If you need to provide an app with your digital effect, you can also use C to write that. There are several object-oriented variants of C such as C++ and C#. These are C like in nature, but are optimized for reusable objects. Objects are pieces of code that can easily be moved around and reused. This is helpful when writing applications where things are often repeated such as drawing windows, creating menus etc. Object Oriented languages can be more complicated to learn, but a good investment if you need to write editor-like programs for your products.

    A great way to get started programming is with Arduino. You can buy a small Arduino board for just a few dollars, and connect it to a Mac or PC via USB. Arduino provides a software development environment called an IDE that you can download for free. This includes all the parts you need to get started right away:

    Editor: The window for reading and writing your source code. You can use any text editor to write code, but a code specific editor provides useful features such as color coding, and quick links to example code and help pages.

    Compiler: Compiles your code into object files that run on the processor. The IDE compiler options are pre-configured for the AVR processors used on the Arduino boards, so that you can compile your code for the Arduino without any additional setup.

    Programmer: Uploads the compiled object files to the Arduino board so you can run it.

    The IDE puts some wrappers around C to make some of the common Arduino functions a little easier, but it’s still basically C code. It’s a great environment for learning to program in C. Once you have learned some of the basics, Google ‘Arduino effects pedal’ for some project ideas.

  • Power Supplies: Which is More Quiet?

    Some people believe that modern switching power supplies and DC-DC converters are noisier, and that using a traditional larger AC power supply is going to be quieter. Others think that using a battery and isolating the pedalboard from wall power is going to avoid AC noise, ground loops etc, and so this will be quieter. So, which is correct?

    To find out, I did some tests connecting the same pedalboard to a high-quality AC reference power supply, a low cost generic switching power supply I got on the internet, and a Mission 529 with both battery and wall power, and measured the noise to compare them. Let’s find out what happened.

    Just before the turn of the 19th Century, engineers and industrialists were trying to figure out how to get a practical electrical supply into homes and businesses. Electricity was anticipated to be a cleaner, safer, and more reliable source of energy to replace candles and gas lighting in residences, and steam powered machinery in industry. In the US, a battle of technology and business took place between Thomas Edison, proponent of Direct Current, and Nikola Tesla, and George Westinghouse, pioneers of Alternating Current.

    The principle challenge was that low voltage DC, such as from a battery is ideal for small devices and local power, but a significant amount of energy is lost to heat when transferring over distance in cables. The voltage needed to be raised to much higher levels to be efficiently sent over a long distance, but this is hard to do with Direct Current. Tesla and Westinghouse developed Alternating Current which is much easier to convert between different voltages using simple transformers, and this is the key reason this won out over DC.

    This is the system we still use today. AC is generated in large power stations in industrial areas, stepped up to high voltages; sometimes hundreds of thousands of volts for transmission in power lines around the country. Then it’s stepped down again a few times, eventually to the hundred or two volts at the wall outlet. Then we often convert it to DC for use in our small devices such as guitar effects pedals.
    For the tests I used Pedaltrain Nano with a mixture of small analog and digital pedals. I added to the board an iRig Pro to provide the USB audio interface between the pedalboard and a PC to do the noise analysis. My thanks to the folks at IK Multimedia for providing the iRig Pro to test.

    For the AC power supply, I used the MXR MC403 power system. We use these in the Mission lab as our reference power supplies because of their good performance. This is a linear AC DC power supply. The wall power plugs directly into the side of the unit. For the low-cost power supply, I used the AGPtek CP-05. I purchased this on Amazon for around $30. This one uses a wall wart to convert the AC to 18V DC to power the unit. To test DC, I used a Mission 529 which uses a lithium ion rechargeable battery. The 529 can use any USB power source, so I tested this with a wall wart too, to see if there is any difference.

    The MXR uses internal transformers to drop the voltage, and rectifiers to convert to DC. With AC supplies we are looking for issues with 60 cycle hum. Unlike DC where current flows continuously in one direction, AC oscillates back and forth. This is what allows AC to be easily transformed between voltages, thanks to the properties of electro-magnetism. In the US, wall power oscillates at 60Hz. In some other countries, it’s 50Hz. Unfortunately, those same properties that allow transformers to work, can also cause electro-magnetic interference. We sometimes hear it as a hum in audio systems.

    Direct current does not oscillate, but we have another problem: Converting DC voltages. Battery voltage is determined by its chemistry; for example, 1.5V from an alkaline cell, 1.2 for Nickel, 3.7 for Lithium-Ion, etc. Transformers don’t work for DC, so to provide other voltages we use modern integrated circuit based DC-DC converters. A key mechanism behind how these work is the switching of current flow on and off quite quickly using transistors. By controlling the on and off times (called the duty cycle) the switching converters can easily convert between voltages.
    The trade -off is that now we are no longer just providing a continuous current flow in one direction, but are switching current on and off through inductors. This can create a similar issue with noise from electro-magnetic interference as we had with our AC supply. The main difference is in the frequency. Switching noise is generally a higher frequency and sounds more like a whine or whistle compared to the lower frequency hum or buzz from AC. A good power supply design will filter these out as much as possible, so let’s go measure these, and see how they do.


    AC vs Battery

    Here’s the first result. The pink trace is the AC linear supply with large toroidal transformer, and the blue trace is the 529 switching supply with a USB battery. The response is almost identical except the AC supply has a small amount of extra noise at 60Hz, which is our expected 60 cycle hum. The switching supply has nothing at this frequency because no AC is present. So, right there, the suggestion that DC switching effects pedal power supplies are inherently noisier is totally busted. Both of these are very quiet. Even the 60 cycle peak on the AC supply is at -98dBu which is slightly less than the -95 dBu at the very top of the noise floor.

    OK, batteries seem to work, but what about using the 529 with a wall power supply? Surely a wall wart is going to cause lots of switching noise?

    AC vs 529 with good adapter

    We’ll that’s busted too. Here the green trace is the 529 with a decent quality USB wall mount power supply, and although we see a little 60 Hz noise starting to creep in now, it’s still as good or better than the linear supply.

    So, what’s going on? Don’t some people get noise in their rigs when using switching power supplies? A conspiracy by makers of expensive transformers?

    AC vs Cheap switching

    Well it’s really just a matter of getting the best thing for the job. Here the orange line is the low cost power supply, and we can definitely see some increase in noise. It’s not terrible, but it’s there. Some can be much worse but switching converters that are properly designed for audio use filter this out or move into frequency ranges outside the audible spectrum.

    Battery vs AC effects on

    This last scan compares the 529 with the MXR, but this time with all the effects pedals switched on. Here we can see that any small difference in noise performance of the power supplies is wiped out by the increase in the noise floor once we enable a few effects. In guitar rigs, there is often so much noise from amps, effects, pickups etc that power supply noise is going to be the least of your problems.

  • Multimeter: The Invaluable Tool

    A digital multimeter (DMM) is an invaluable piece of equipment for any guitar player. One suitable for most guitar related duties will be under $20, and $90 will get a good one that should last a lifetime.

    Even if you are not particularly technically inclined, there are a number of simple multimeter tests that can save countless hours in set-up, tuning, and fault finding in guitar rigs. Here are eight of them.

    First, a warning. The tests described here are all low voltage, and are safe even if you get things wrong. Devices connected to wall power contain high voltages and currents that can cause serious injury or death. Guitar tube amplifiers are especially dangerous, with voltages often exceeding 300V and currents of several amps. Due to the large capacitors used in tube amplifiers, lethal conditions can remain present even when the amplifier is turned off and the power cable removed. Testing of amplifiers and other high voltage equipment should be done by a qualified service person.

    Cables

    Occasional instrument cable failures are pretty much guaranteed, especially with large rigs, and finding the bad one can be frustrating. The continuity feature of a DMM is designed to help with this. Connect the probes to the continuity/resistance inputs on the DMM. Markings can be different, so check the user manual if it’s not clear which these are.

    Select the continuity setting, and touch the metal tips of the probes together. The meter should beep, indicating a good connection. Touch one probe to the tip of one end of the cable, and the second probe to the tip at the other end. The DMM should beep indicating a connection. Hold the probes there for a few seconds and the beep should continue. Now do the same thing with the sleeve at each end, and check for the beep. If there is no beep, then you may have a broken connection in the cable.

    Next, touch the probes to the tip and sleeve at one end of the cable. There should be no beep. If there is continuity between the tip and sleeve, then you may have a short in the cable.

    Fuses and Lamps

    If your amp won’t power on, you might have a blown fuse. The DMM continuity setting can be used to check this. Switch the amp off and unplug it from the wall. Unscrew the fuse holder and remove the fuse. On the continuity setting, touch the probes to each end of the fuse and the DMM should beep. A blown fuse will have no beep.

    If you have a tube amp with an incandescent indicator lamp, you can check the lamp the same way. Hold one probe on the metal tip in the center of the base of the lamp, and the other probe on the metal ring around the side. The DMM should beep if the lamp is good.

    Batteries

    Connect the probes to the voltage inputs and set the DMM to read DC Volts. For 9V batteries, touch the probes to one each of the metal connectors on the top. A new, fully charged alkaline 9V battery should read around 9.2V. For 1.5V batteries, touch the probes to the two metal ends. A new alkaline 1.5V battery should read about 1.6V.

    The order of the probes is not critical when measuring battery voltage out of circuit. If you reverse the probes connecting the negative probe to the positive terminal and vice versa, the DMM should just indicate it on the display, normally with a – sign. The voltage reading will be the same.

    Measuring batteries with no load is not very accurate and the reading will only be a guideline. If you want to measure the battery voltage more accurately, you can do so in circuit. Connect the battery to your effects pedal, and connect a guitar cable to the input so that you can turn the pedal on. With the DMM on DC volts setting, touch the probes to the battery terminals where they connect to the battery snap. The DMM should now give a more accurate reading with the battery under load.

    Speakers

    Speakers are rated with an AC impedance which varies with frequency and is difficult to measure, but you can easily measure the DC resistance with A DMM. Since guitar speakers are commonly 4, 8 or 16 Ohm, measuring the DC resistance is normally sufficient to determine which is which.

    If the speaker is in a combo amplifier, switch it off and remove the power cable. Disconnect the cables from the speaker tabs, taking note which is which so you can reconnect them afterwards. Connect the probes and set the DMM to measure resistance (Ohms). Touch the probes one to each of the speaker tabs. If the speaker is in a passive extension cab, just connect a speaker cable, and touch the probes to the tip and sleeve of the other end of the cable.

    The DC resistance will normally measure just below the nominal AC impedance rating of the speaker. For example, if the speaker measures, 7.1 Ohms DC resistance, it’s probably an 8 Ohm speaker. If it measures 3.8 ohms, then it is a 4 Ohm speaker, and so on.

    Pickups

    Pickups can be measured in a similar way to speakers. The pickups will need to be removed from the guitar, or at least the wires will need to be disconnected from the rest of the circuit first. Single coil pickups are straightforward as they normally have just two wires; ground and hot. Set the DMM to resistance, and touch the probes to the end of each wire. Compare the measured resistance to the manufacturers specification. If the resistance measurement is not stable and the values keep jumping around, the pickup may be damaged.

    Humbucking pickups will have more wires, so you will need to check which are the hot and ground wires from the manufacturers specifications.

    Tubes

    A specialty tube tester is required to fully test amplifier tubes, but there are a couple of basic tests you can do with just a meter. Turn off the amplifier and remove the power cable. If the amplifier has recently been switched on, give the tubes a good few minutes to cool down before handling. Carefully remove the tube to be tested, being careful not to bend the pins or damage the tube. Look up the data sheet for the tube type and determine which pins are the heater. Set the DMM to the continuity setting. There should be continuity between the two heater pins, but not on any others. Test for continuity between all the pins and make sure the meter beeps, only when connected across the heater pins. Make sure to replace the tube in the correct socket.

    Potentiometers

    To check the resistance value of a potentiometer, set the meter to the resistance setting, and touch the probes to the two outside pins of the potentiometer. Potentiometers will have a tolerance rating. A 500K Ohm potentiometer with a 10% tolerance could measure anywhere between 450K ohm and 550K Ohm for example, and still be in spec.

    To test the operation of the potentiometer, move one of the probes to the center pin and rotate the pot. The resistance should read the value of the pot when fully turned one direction, and something close to zero Ohms when fully turned the other.

    Power Draw

    Sometimes you may want to know what the power consumption of an effect pedal is; for example, if you are specifying a pedal board power supply, or calculating expected battery life. We can do this by measuring the current flow in Amps. This is a little more involved, as to measure current flow, the meter probes must be in series rather than the parallel measurements in the previous examples.

    Fortunately, there is an easy way to do this that works with many battery powered effects pedals that doesn’t involve cutting any wires or soldering any connections. Turn the battery snap so that just one of the connections is snapped to the battery. Connect an instrument cable to the pedal input so that the pedal can be turned on. Set the meter to measure current in milliamps. Note that on most DMM’s you will need to move the red probe to the mA current input. Make sure to do this correctly otherwise you may blow a fuse in the DMM, and be careful not to short anything out.

    Touch one probe to the unconnected battery terminal, and the other to the unconnected pin on the battery snap. Turn on the pedal and current should flow through the meter to the pedal. The meter will read the current in mA. This is the pedals power draw.

    If the pedal has different modes, try them all to see if there is any difference in the current flow. It’s a good rule of thumb to specify a power supply capable of providing 2x the total current draw of your devices.

  • The Power of Speakers

    Here’s a quick challenge: You want to make the most dramatic change possible in both the look and sound of a guitar rig, but can only change one item. What do you choose? Hands up all those who said speaker cabinet. It may not have been the first thing that came to mind, but think about it. How much of the difference between a small combo and that giant wall of Marshall stacks is really in speaker cabinets rather than the amps?

    For manufacturers cost optimizing their products, cabinets and drivers are often the first targets for reduction. Compromises here will normally yield much greater savings than the electronics, enabling them to hit the required price point. Budget units can often conceal some really nice amps hidden behind crummy, low cost cabs and drivers.

    With a modest amount of effort, you can choose the right speaker drivers, get your own custom cabinet built for you, and assemble it yourself in an afternoon. Ready? Let’s go.

    If you are using a tube head, then this should be quite straightforward. The purpose of separate heads, after all, is to allow the use of different speaker cabinets. If you are using a combo, then you may have an extension speaker out on the back that permits connection of an additional cabinet. Some cabinet builders can also build new combo enclosures for your amp. If you are using a software-based modeler, then you have a choice of using cabinet simulation software with a full range monitor or turning these off and using a regular cabinet.

    The first cabinet decisions are going to be the size of the cabinet along with the number and size of speakers. In general, a larger cabinet and more drivers is going to provide an increased Sound Pressure Level (SPL); in other words ‘louder’, and increased low frequency response.

    If you are looking for a gut punching, pants leg flapping output like no other, then there really is no substitute for a big 4×12. Everyone should play through one of these at least once in their lives. It truly is an addictive experience. Alas, as with most addictions, the trade-offs are numerous. A typical 412 is around 2.5 feet square, and a challenge for the trunk of most cars. At around 50lbs for just the empty cabinet, they are not light, either. With four vintage 30’s and some hardware such as casters, you are knocking on the door of 100lbs! If you can live with a slightly reduced flap of the pants, then a 2×12 vertical or diagonal is becoming a very popular choice. These provide a good chunk of the look and feel of a 412 but in a smaller, lighter package. With the right choice of drivers, you can get something that still covers a wide range of amps but comes in sub 50lbs fully loaded. A 212 slanted diagonal is my personal favorite all round cabinet.

    A straight 412 is also very directional. The high frequency tends to drop off quite significantly when you get off the axis, so you’ll get a very different sound depending on where you are standing relative to the cab. A slanted diagonal cab does a much better job of high frequency dispersion, giving a more consistent frequency response over a wider area.

    Side by side cabinets utilize two drivers in a low-profile form factor that’s not too much bigger than a 1×12. This is a great configuration to give you the extra output of two drivers while keeping the size manageable. Some 2×12 cabinets can be used in either horizontal or vertical configuration increasing the flexibility still further.

    After choosing configuration, the next choice is going to be cabinet material. There are several suitable woods with the most common being Baltic birch ply and pine. The Baltic birch tree grows around the Baltic area of Northern Europe, hence its name. The ply has been used by European cabinet makers for many years and is known for its strength and stability. Typical plywood is made from higher quality veneers on the outside and softer woods on the inside whereas Baltic birch ply uses the higher quality wood for every layer. The softer interior layers of standard ply also have areas of no wood at all called voids. Baltic birch ply is void free, so it’s 100% solid. It holds on to screws and inserts much better so it’s ideal for bolting oscillating speaker drivers to. Many good quality speaker cabinets are made entirely from Baltic birch including most of the classic British cabinets.

    The traditional American Fender style cabinets are often made from solid pine. Pine is a little lighter and, being a little softer, can contribute more to the overall sound. Since the wood is less uniform than the Baltic birch, there can be differences from cab to cab, and you have to be more careful matching the driver to avoid unwanted resonances. A good pine cab with appropriate speakers can give you that classic bright and lively American sound. Cabinets with pine shells and Baltic birch baffles can be a great choice. The Baffle is the front piece of wood that the speaker is bolted to. This way you get the strength and stability where it is needed most, and still some of that classic pine tone.

    Next up is to select the speaker driver. Most of the major guitar speaker driver manufacturers provide extensive details on their websites of the characteristics of their various drivers, and suitability for certain types of cabinets and the tones they are aimed at. Take the time to read through these. Mounting holes are sort of a standard. In most cases a 12” driver from vendor A and a 12” driver from vendor B can be exchanged and the mounting holes in the cab match up, but it’s not absolute, so check the specs that will be published by the vendor.

    Drivers can be front or rear loaded, which means the driver is put in through the baffle and bolted either through the front of the cabinet or the rear respectively. You should check both the cabinet and the speaker driver specs to see which orientation they require. Some cabinets and speakers will support both in which case you can choose which you like best. Otherwise, you should make sure they are compatible. For example, if your cabinet is rear load only and your speaker is front load only, then they won’t work together without modification.

    Drivers will have impedance and power handling ratings. Guitar speakers are most commonly 4, 8, or 16 ohm impedance. These figures are per driver and will change when using multiple drivers depending on how they are wired. Speaker manufacturer Eminence publishes some great technical resources on the different ways to wire together multiple drivers, and how this impacts total impedance. Check them out at www.eminence.com. Check to make sure that the total impedance of all drivers wired together the way you have chosen is supported by your amplifier. Mismatched impedances can cause improper operation and in some cases serious damage to either drivers, amp or both.

    Power handling for guitar speakers is usually from around 20W at the low end to around 200W at the high end. These figures are per driver and will change accordingly when using multiple drivers. Matching speaker power handling to amp output depends to a large extent on what your personal requirements are. In general, if the effect of speaker break-up is part of your required tone, then you should aim for the low side of the power handling. It’s OK in most cases (and sometimes desirable) to use lower rated drivers with higher rated amps, as long as appropriate precautions are taken. For example, if you are looking for some speaker break-up at lower volumes, using a 30W rated speaker driver with a 50W rated amp is appropriate because the driver is going to begin to break up long before you hit maximum volume. If you pound away with a bunch of high gain pedals at full volume with the amp cranked to the max at a stadium gig then you do risk damaging the speaker driver, but if you are running the amp at lower volumes then you’ll likely remain well within the driver specs and get towards your desired tone at the same time. Vice versa if you want to make sure that your speaker tone remains clean and does not break up at all, then you can over-rate the driver. Using a 100W or 150W speaker with a 50W amp could be suitable in this case.

    There’s only so much you can determine from specs, and eventually you are going to have to start putting speakers in cabs and trying them out to see how they sound to you. Don’t be too surprised if you must try it two or three times to find something you really like. Start by following the speaker drivers’ recommended cabinet configurations and going with something that’s a known quantity such as 30W British style drivers in a birch 412, or American style drivers in a 2×12 pine, and you’ll be relatively safe. Later on, you can start experimenting by trying unconventional combinations like American style drivers in British style cabs. You can even mix different drivers within the same cab. For example, putting low frequency optimized drivers at the bottom of a cab and higher frequency optimized ones at the top can help give a more hi-fi like tone.

    For wiring up, your speaker cabinet builder may offer pre-assembled wiring harnesses so you can just plug in and play without any soldering or crimping required. If you are au-fait with these techniques and have the right tools, you can do your own wiring. Remember that for both internal connections and when connecting the finished cabinet to the amp, the wires carry power amp level outputs so you must use suitable gauge speaker wire. Don’t use a guitar cable to connect your 200W tube head to your 412! If you are not comfortable with basic electrical wiring, then you should have a qualified person do it for you.

    Most of all, experiment. There are a few basic rules such as the mechanical and electrical ones described earlier, but other than that try different stuff out and see what you like. As long as it’s put together safely and sounds good, then it is good!

    Links:

  • Of Pots and Tapers

    You may not have thought about it, but when you turn a volume or tone knob on a guitar, or a setting on an effects pedal or amp, some complicated things happen. The little potentiometer that’s hiding behind the knob has some interesting properties. There are already some excellent articles on the web about how pots work, so there’s no point in rehashing that here. I’ll link a good one in at the end. However, one thing you’ll find in most of them is the discussion of taper. This describes a relationship between the mechanical and electrical properties; how much electrical change happens, relative to the mechanical change. For example, you may hear people describe a volume pedal with terms like ‘smooth linear sweep’. In fact, a linear taper is usually the last thing you want in a volume control, because human hearing is not linear. For a volume change to SOUND proportional, the ACTUAL change in terms of voltage, SPL etc should be logarithmic.

    Anyway, sometimes you may want to change a pot. Maybe the tone control on your guitar is too sensitive, or not sensitive enough. Maybe your volume pedal seems to make all its change in the last few degrees of travel. Was your amplifier gain fine at one time, but now you are older and deafer it doesn’t seem to work the same? If you read up on potentiometer tapers you’ll find that some may not be exactly what they seem. If we could measure exactly what taper our pots are, then we could compare them and decide what change could be made. So, let’s do that.

    You’ll need some things. A function generator, multimeter, and an oscilloscope would be awesome, but we can get by with much less. You will at least need a multimeter, but it doesn’t need to be fancy. A hardware store or low cost Ebay or Amazon one will work fine. Depending on the device, you may be able to take the measurements in circuit or you might need to remove the potentiometer that you want to test by de-soldering or cutting the wires. You’ll have to do it anyway if you are going to replace it. Measure the resistance across the outside two pins of the pot (sometimes marked 1 and 3 or CW and CCW). This is the maximum resistance of the pot. This one is 10K Ohm.

    You’ll also need something to generate a fixed voltage. A tone generator works well. If you have a cable tester with a tone generator on it, you can use that, or you can use a computer generated one if you have an audio interface. There are some low-cost tone generators available from the usual sources. It’s helpful to have a simple tone generator for many audio projects. If none of these are available you can use a DC source. A small battery will be fine. Make sure not to use a larger voltage battery than the pot is rated for.

    Decide how many samples you want to take and at what points. We are essentially going to be doing digital sampling by hand. For example, if we measured our potentiometer max resistance at 10K Ohm, we could take 11 samples starting from 0 and then at 1 KOhm intervals. Turn the pot until it’s all the way as close to 0 Ohm as it goes. Now move the + of the resistance meter to the center pin on the pot and connect the voltage source to the open pin. Measure the voltage and record it next to the resistance. Then measure the resistance again and turn the pot until it reads about 1 Ohm. Measure the voltage again and record it. Keep doing this until you have rotated the pot all the way to the other end and have all your samples. For example:

    0 Ohm – 0V
    1K Ohm – 0.1V
    2K Ohm – 0.3V
    …..
    10K Ohm – 1.5V

    The number of samples we have taken (11) is analogous to sampling rate. The accuracy with which we record the voltage is analogous to bit depth. The more samples and the more accurately we record the voltages, the more detailed our plot will be.

    Once you have all the values, you can graph them out. A spreadsheet program like Excel will work for this, or there are a number of free online graphing tools on the web. I like www.chartgo.com. Plot resistance (rotation) against voltage and if all went well you should get a plot of the taper of your potentiometer.

    Now you can tell if everything is how you expect, and if not, how you might change it. For example, if your control seems to change too fast and you measure a linear pot, then try changing it to a log one. If the control appears ‘backwards’ try swapping a log for anti-log or vice-versa. Some low-cost log pots are not true log, but just two different slopes. If you have one of these it will show as two straight lines at different angles on the graph rather than a continuous curve. If you have one of these, and it’s causing problems, then replacing it with a true log part may help.

    In the link, there are some instructions on how you can add tapering resistors to a pot to change the taper. If you do this, you can use the plotting technique to visualize the change. Measure the pot first without the resistors (the stock pot, ha ha), then add the tapering resistor and measure again. Plot these as two lines on the graph. With two plots on the one graph, you’ll easily be able to compare them.

    The Secret Life of Pots

  • For the Love of Tubes

    Tubes rule; we all know that. Even those of us firmly in the digital modeling camp still enjoy a good tube amp. There’s something about the warm glow, the smell, and the sense of history that appeals to us.

    Many pedals require some sort of amplification circuit, so putting a preamp tube or two into a pedal would seem like an obvious choice; and indeed, there have been some great tube- powered pedals over the years. The Mesa V-Twin and Bottle Rocket both utilized a pair of 12AX7’s for preamp and overdrive capabilities.

    As a manufacturer of tubes themselves, EHX has a range of five or six tube pedals including EQ and modulation pedals. Effectrode out of the UK is one of the few companies manufacturing exclusively tube-powered pedals including buffers, delays, and modulations, as well as gain pedals.

    Compared to solid state though, the choice of tube powered pedals is small. Our expectations for effects pedals require that they are small, low cost, low powered, and bullet proof. Tubes don’t fit well within these constraints. Tubes are large and fragile, and they require protection against damage yet need ventilation to keep from overheating. They deteriorate over time, sometimes needing replacement, and require high voltages and special power supplies to operate.

    It would be nice if we had a small, low power, low temperature, robust and long-lived miniature tube. Wouldn’t that be great for effects pedals? Enter the Korg Nutube 6P1 directly heated dual triode.

    Those of us old enough to have used calculators and Hi-Fi systems in the seventies and eighties will recall the now retro-cool blue glowing displays that were often used on these devices. Wikipedia describes these vacuum fluorescent displays as hot cathode, anodes, and grids encased in a glass envelope under a high vacuum condition. Doesn’t that sound familiar? Some folks at Korg thought it did. They have partnered with display specialists, and VFD pioneers Noritake Itron (who are still in the business fifty years on) to produce an audio tube module based on a VFD.

    The Nutube 6P1 has some impressive advantages compared to a 12AX7:

    • 30% of the size
    • 2% of the power
    • 30,000 hours continuous life expectancy

    At just 12mW per channel from a 5V input, the Nutube should be appropriate for a typical Hammond-style effects pedal enclosure. Small linear regulators can be used so the pedal could be run from a battery or conventional 9VDC pedal power supply. The design generates almost no heat compared to a conventional vacuum tube. The module also looks cool too, with its distinctive blue glow from the phosphor on the VFD.
    Let’s take a quick look at some of the figures compared to a GE 12AX7.

    12AX7

    • Filament Voltage: 6.3V
    • Filament Current: 300mA
    • Anode Voltage: 250V
    • Anode Current: 1.2mA

    Nutube

    • Filament Voltage: 0.8V
    • Filament Current: 17mA
    • Anode Voltage: 80V
    • Anode Current: 20uA

    The low voltages compared to a traditional tube make Nutube an easier proposition for boutique pedal builders that can create pedals that run on a 9V battery. They also do not have to worry about high voltages and the associated safety concerns and regulatory requirements.

    Korg doesn’t divulge exactly how they have optimized the VFD into an audio tube, but we can determine the basic function just by observing the part. The 6P1 is a twin triode like a 12AX7. The two white squares are the anodes (plates). The centers of the anodes have a phosphor coated layer that glows in operation in a similar way to the original displays. I’m not sure if this serves any functional purpose. It may just be there for visual appeal. The square grid overlays the anodes. You can see the grid pattern if you look closely. The filaments are the two thin wires running across the center of the glass. The circle at rear center is probably the getter that’s used to help maintain the vacuum. I’d recommend checking out the Applied Science Youtube channel, and search for the CRT disassembly video. This includes a great explanation of how a getter works in a vacuum tube.

    The Nutube is potentially suitable for any pedal that can benefit from a tube amplifier; including buffers, boosts, overdrives, distortions, modulations, and delays. Because it’s a directly heated device, it may be susceptible to generating microphonic noise, where vibrations are picked up by the filament and reproduced as noise in the signal. This could be an issue for pedals that are typically floor mounted. Acoustically isolating the component inside the enclosure in some form such as plastic assembly, rubber mounts or damping material in the chassis may be necessary.

    Nutube was announced a couple of years ago. Korg has produced a number of proof of concept devices, mainly small amplifiers. The output from the tubes is not sufficient for a power amp, but pairing with a Class D power stage is possible. The first full commercial Nutube product looks to have done just that. The Vox MV50 range of small form factor guitar amps (Vox is a subsidiary of Korg) utilizes a Nutube preamp stage with a solid state power amp. I’ve seen a few one-off effects pedals but nothing on a commercial scale yet. I imagine that’s about to change.

  • The Significance of Certification

    Have you ever looked at the underside of a pedal or the rear of a guitar amplifier and wondered about all those little symbols such as FCC, CE, CSA, TUV or UL? Why is it that we typically see those on products from the big players, but not on the boutique devices? What exactly does ‘This equipment has been tested and found to comply with the FCC Part 15 limits for a class B digital device’ mean exactly?

    The short explanation is that these are marks indicating that the products comply with various safety and performance standards around the world. The standards vary quite significantly between different nations, which is why we often see many different marks on one product. If the manufacturer is expecting to sell their product around the world, they will often indicate compliance with multiple standards with labels on the device. The European Community has a set of harmonized standards for different types of devices. The CE mark that you see on many products indicates the manufacturer is confirming that their product complies with these standards.

    An interesting thing about the United States is that the majority of the standards bodies are independent groups. There are often few laws requiring compliance to these standards to be able to sell a product. So, this largely answers our question about why we typically don’t see these marks on things such as boutique effects pedals: There is no law that says they must comply, and complying is a significant undertaking. The standards are often complex and difficult to follow. Testing requires hugely expensive specialty facilities with vast arrays of costly equipment and expert test engineers.

    If certification is not compulsory, it begs the question; why do the larger manufacturers even bother? This usually comes down to a couple of things. In some countries, certain levels of compliance are compulsory, so a manufacturer will at least need to test those if they expect to sell into those regions. Some standards bodies will recognize tests of other groups, so if you have to do compliance for one location, then some others may come almost for free. Some distributors or resellers may require compliance in order to resell a product, so that might have an influence. Lastly, it just makes sense for larger manufacturers to develop and test to standards. It can help with design decisions and quality control, minimize support and legal issues, and most importantly, give the company and their customers reassurance that the product they have made is safe and functions reliably.

    If you are manufacturing a digital device though, one set of rules in the US that you WILL have to comply with is the FCC Part 15. This recognizes that certain digital devices emit radio signals, even though this is not part of their intended operation. We refer to these devices as ‘unintentional radiators’. The radio frequencies that these devices emit have the potential to cause interference with intentional radiators, and its part of the FCC’s job makes sure that your whizz bang digital delay does not result in your neighbors cell phone being blocked or an aviation accident in your backyard.

    I was involved in testing some Mission products for FCC compliance so I thought I would share some of the photos from the day.

    We had a very informative and productive day at the lab. Thanks to everyone at EMT Labs for helping us out and completing our testing within a day. I’m happy to say we passed all our FCC emissions tests.

  • Cables: Which Are Best?

    In this blog post, we are going to take some different cables into the lab, and look at how they stack up in terms of some basic, measurable numbers.

    In our test, we have seven different cables varying in price from less than $6 to over $250. There are a variety of different core materials and jack plating’s, as well as the same cable with different lengths, so we can compare the impact just of the length of the cable.

    Contenders

    Neewer. This was the lowest cost 10’ guitar cable I could find on Amazon Prime at the time of purchase. I paid $6.95. The cable has a braided Tweed style cloth jacket and large jack plugs. The tip of the jack plug is gold colored, although I could not tell if it is actually Gold plated. The sleeve of the jack appears to Nickel.

    Rapco Horizon. The Horizon Standard is available at Guitar center for $7.50. I took this one from our collection in the lab. This cable has 116 reviews on Musicians Friend, so I imagine it is a very popular low cost, no frills instrument cable.

    Quantum Audio Designs. The first of the Oxygen Free Copper cables. This one was purchased from our local music store for about $15. It has heat-shrink over the jack and first few inches of the cable.

    Best-tronics Pro Audio. We tested a 10ft TRS cable. TRS cables are sometimes used with active pickups, dual magnetic/Piezo, and other types of guitar that require the additional conductor. The cable utilizes OFC conductors and Switchcraft ¼ Nickel jacks. Best-tronics will custom make cables to order to any length. Price is around $24 for TRS 10’.

    Kirlin Stage. The Kirlin Stage is described as ‘exclusively designed for live performance’ The 18 AWG OFC conductor is thicker than the others on the list. The large metal jack plug has gold plated contacts, and a black painted metal body. The Literature lists a couple of China Patent numbers: ZL 201230626606.8, and ZL201320086667.9, if you want to try looking them up. Price is $25 at Musicians Friend.

    Zaolla Silverline. The Silverlines are the only Silver cables that we tested here.
    They are described by Zaolla as using a mixture of Silver and Copper conductors: ‘all Zaolla Silverline instrument cables feature a solid Silver center conductor and an enamel coated, stranded copper ancillary conductor in a unique hybrid configuration’. The Jack plugs are listed as having both Rhodium and Silver plating layers. Similar to Best-tronics, Zaolla can build cables to custom lengths on request. We tested a 15’ cable and a 2’ cable from the same line. A 15ft cable is priced on zaolla.com at $254.95, and a 10’ at 199.95.

    Tests

    A cable connected to a magnetic guitar pickup creates an RLC 2nd order low pass filter. This creates a small boost to the signal at the resonant frequency, and then gradually attenuates the signal at higher frequencies past that. By adjusting the amount of capacitance and resistance in the circuit, you can control the center frequency and slope of the filter.

    Since the resistance and capacitance of your guitar pickups, tone and volume controls, as well as the number and length of cables all work together to impact this, every rig is going to be different. This is potentially even true in the same guitar. If it is a multi-pickup guitar where the pickups have different inductance and resistance, just switching between pickups will change the characteristics of the filter.

    With this in mind, I measured the resistance and capacitance of the cables and jack plugs themselves as best as I was able. If you characterize the rest of your circuit by looking up the numbers or measuring your guitar pickups, you can then plug these numbers into an online RLC filter calculator (or do the math yourself if you prefer) and predict the effects.

    Measurements

    All measurements were taken at 68 degrees F. The cables were all kept in the same room as the measuring equipment for at least five hours, and the measuring equipment was left on for at least one hour before for temperatures to stabilize. I measured the resistance of each cable from tip to tip, and sleeve to sleeve. I measured the capacitance of each cable from tip to sleeve at each end. The results are shown in the table.

    Results


    The biggest single factor impacting change was the length of the cable. Total resistance of the cable assembly (including jacks) is reduced by more than half on the 2ft Silverline compared to the 15ft. Capacitance was reduced almost six times, which is directly proportional to the change in length within the margin of error.

    The 15 ft Silverline had the lowest resistance on the signal conductor at 0.013 Ohms/ft. The 18 AWG Kirlin was close behind at 0.016 Ohms/ft. The 2ft Silverline measured a higher resistance per foot, but this was most likely due to the contact resistance at the measuring point on the jack making up a greater proportion of the number on the short cable. As cables get longer, the resistance of the cable becomes much more significant than the resistance of the jack.

    The Horizon had the greatest resistance on the tip. At 24AWG compared to 22AWG for the Best-tronics and 18AWG for the Kirlin, this is not unexpected. The thinner wire should have a greater resistance and our measurements bear this out. The Horizon also had an order of magnitude higher resistance on the sleeve. The cable was not new, and was taken from our demo room, it possibly had some damage, which just goes to show that it is worth testing your cables on a regular basis, even if they appear to be working.

    The resistance of the cable however, generally makes up only a small percentage of the overall resistance of the circuit. With the pickups, volume, and tone controls usually being in the hundreds of Kilo Ohms range. The capacitance of the cable usually has a more dramatic impact on the audible tone. Here again, the best way to reduce the capacitance of the cable is to shorten it. The short 2ft Silverline measured just 84pF, compared to 969pF for theNeewer which was the worst performing in our capacitance test. Calculating a theoretical pickup configuration, that moves the cutoff frequency of the filter from 12KHz to 3.6KHz, which would be a noticeable difference.

    The lowest measured capacitance per foot of all out cables was the Quantum Audio Designs at 25pF/ft, followed by the Silverline 15 at 33pF/ft. and the Silverline 2 and Best-Tronics both at 42pF/ft.

    Conclusions

    Shorter is better. Although it’s obviously not practical to use a 2ft guitar cable, minimizing the number of cables and keeping down the length helps. Use as short a cable as possible from your guitar, and use a buffer first in line if at all possible. If your environment requires a really long guitar cable, consider using active pickups, or a wireless system instead.

    The very low cost cables in our test did not perform great. The Neewer had a much higher capacitance than the others which would be audible in most guitar setups. The Horizon had a higher than normal resistance reading on the sleeve (which may have been due to some damage).

    The Zaolla Silverline was the most expensive cable in our test, and had the lowest tip resistance and the second lowest capacitance so there’s no doubt it scored well. The Silverline was also noticeably lighter than the other cables. The 18AWG core in the Kirlin helped it turn in a low resistance measurement very close to the Silverline, although capacitance of the Kirlin was not great.

    From $20 up, the differences between the expensive and mid-priced cables were small, especially in capacitance where it most matters. At around $20 ea, the mid-priced Quantum and Best-Tronics cables were close to or better than the highest price cable.

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