Computer Processing in Guitar Amplification


There has always been a debate over which type of amplifier produces superior sound. With computer processors being produced that emulate vintage amplifiers, the similarities between solid state and vacuum tube amplifiers are becoming even closer.

I. Introduction

Over the years, many types of amplifiers and speakers were produced. This created a mixture of sounds that musicians wanted to imitate. When guitarists go into the studio to record, they can spend hours tweaking their settings to produce the best tone. It is best to mix the guitar tracks using a variety of different tones to help create a thick mixture when they are played together. This creates a full guitar sound and it is accomplished by overdubbing the same guitar track played through multiple amplifiers. If one amplifier produces a bright tone, it will compliment an amplifier that has a punchy bass sound when both tracks are mixed together.

As computer technology grew over time, so did recording capability. Recording studios that were set up using analog tapes were slowly replaced with digital recorders. With analog tapes, any time a guitar part was to be overdubbed, tapes had to be cut and aligned so that they could be mixed together. This was a time consuming and monotonous process. As tapes were cut and rerecorded on, they also lost some of their sound quality. Digital studios do not have the same limitations as analog ones as a recording can be aligned using software, and tracks can be cut and deleted with just a few clicks of a mouse.

Some musicians argue that the sound quality of vacuum tube amplifiers and analog recordings sound better than their digital counter parts. Voiced complaints against the sound of digital amplifiers say that the sound the amplifiers produce is too “tinny” or machine like. The sound from the digital amplifiers is said to not be as full or have the same characteristics of their vacuum tube counterparts. As solid state amplifiers became more popular, vacuum tube amplifiers became harder to find as there were not as many models produced. It is cheaper to produce a solid state amplifier and they are easier to maintain than tube amplifiers.

The debate over whether tube or solid state amplifiers is not new. In 1972, Sears Audio researched whether there was a difference in output between the two types of amplifiers. They found that when compared together the harmonic distortion output of solid state amplifiers is less than that of tube amplifiers [1]. This creates a cleaner tone, but means that when distortion is desired by a guitarist it cannot be produced as efficiently.

As guitarists started to show interest in vacuum tube amplifiers again recently, another concept was developed. A virtual or modeling amplifier was created. The purpose of the virtual amplifier was to emulate the sounds produced by amplifiers that are not manufactured anymore at the fraction of a cost. This allows a guitarist to switch sounds on the fly without having to carry around a ton of gear. Some of which, is impossible to find as it is not produced anymore, and reproductions are made using cheaper components so the sound produced is not of the quality the musician requires.

A virtual amplifier can also take advantage of vacuum tubes by mixing a solid state circuit with a tube gain channel. Since solid state circuits can amplify the guitar signal cleanly at high volumes, a tube gain circuit compliments that well in the gain department. The distortion sound of solid state amplifiers is usually the area where they receive the most criticism.

The future of virtual amplification has started to take shape in the form of software plugins. Computer sound cards are being produced that include an embedded digital signal processor to change analog signals from an instrument into a digital form that can be interpreted by a computer. This sound card works in conjunction with digital audio workstations to produce virtualized recreations of antique amplifiers and effects. The virtualized amplifiers and effects are stored in a virtual studio technology file. These are often referred to as plugins.

Plugins come from a variety of different sources and can perform one or many functions. A single plugin may serve only a single purpose, such as adding reverb to a signal, or it may emulate the entire circuit board of a discontinued effects pre-amplifier model. Some of the higher quality plugins are sold by workstation manufacturers while others are offered as free downloads on the internet. To install the plugins there needs to be a digital audio workstation installed on the computer. Depending on the workstation that is being used, the installation may differ, but most involve extracting the virtual studio technology file into the plugins folder for the workstation.

II. Details of Architecture

Universal Audio has produced a PCI accelerator card that takes an analog signal and uses a variety of plugins to recreate classic amplifier sounds. The idea behind the card is that a musician can choose from a variety of different set-ups to create whatever sound they want without having to spend more money buying other pieces of gear. The plugins run within the workstation itself so that once the audio signal is recorded, the musician has the freedom to change plugins and thereby change the sound of the recording on the fly.

This gives many advantages, as before if the guitarist decided to change tone in the middle of a recording, they would have to connect a different amplifier and rerecord their part. If the current recording clashes with another instrument, say the recording is too bass heavy and it is interfering with the bass guitar sound, the guitarist can switch plugins to emulate a different amplifier, say one with a punchy mid tone instead. In turn, this saves time as the guitarist only has to worry about nailing the part once and then can worry about the tone later.

The accelerated card also supports up to eight analog devices [2] at one time, giving other musicians the freedom to record at the same time as the guitarist. It would also be possible to set up microphones on each piece of a drum set and record a complete drum session at the same time. Each track would record to its own channel so that effects can be added later. Compared to analog tape recording techniques of the past, digital recording has made the mixing and tracking of recorded parts easier and less time consuming.

The UAD-2 uses four SHARC ADSP-21369 processor chips. The ADSP-21369 is a 32/40 bit floating-point processor that was created for use with high performance audio processing. It features 2M bits of on-chip SRAM and 6M bits of on-chip mask programmable ROM. The code is compatible with all other members of the SHARC processor family. It also features onboard audio decoders in ROM and pulse-width modulation [2].

The ADSP-21369 allows two data transfers from the core and one from the I/O processor in a single cycle. Data from the processor is formatted as a 64-bit frame and divided into 32-bit words. Each of the eight inputs has its own clock, frame sync, and data inputs as well [2]. This allows for multiple recordings at the same time with no loss of quality and no synchronization errors.

In comparison, a lot of MIDI control devices from the eighties have used a Zilog Z80 microprocessor. I had an ADA-MP1 that had an internal Zilog Z80 microprocessor itself. It is an 8-bit processor that has been widely used over the years. It features a two-bit control mode that can be written into the output register by the CPU or read back to the CPU from the input register at any time [3]. Using the Z80 with a MIDI controller allowed users to access and store preset data from their musical instruments.

Data from the Z80 CPU goes into the BUS I/O. It flows to the internal control logic and continues until it is interrupted or the counter logic stops the process. The Z80 has 8-bit data paths and 16-bit addresses. It can search whole data words and directly address any byte in the memory. The base register value is D0 and it works down to the D7 register. Some of the register values are reserved for special functions. For example, C3 is reserved for Reset values. For the Z80 to function properly a single 5V supply and single-phase clock is required [4].

III. Features

The main feature of the ADSP-21369 is that it can handle eight inputs at the same time. Used in a musical recording environment, a producer would be able to record sessions from up to eight different instruments at the same time. In a typical rock band, this would allow input from a guitar, bass guitar, a microphone for a singer, and five microphones to be placed around a drum kit. Since all of them would be recording at the same time, it would allow the producer to catch the excitement of a song by using a live take. Then using a digital workstation, if there are any errors in a take, the producer would be able to isolate them and overdub fixes later on.

The ADSP-21369 also features an asynchronous memory controller that provides a configurable interface for up to four separate banks of memory. Each memory bank can be independently programmed with different timing parameters which enable connections from a wide variety of memory devices. This makes it a very versatile processor as it can handle SRAM, ROM, flash and EPROM as easily as it can handle I/O devices that interface with standard memory [2].

The features of the Z80 are quite different as there is quite a gap in the power of the two processors. With the Z80 only handling 8-bit operations, to control devices in a live setting there must not be any delay from the hardware. The Z80 has a very specific memory language to minimize the number of different opcodes that correspond to the basic machine set. This provides consistent operation from the processor [5].

The Z80 uses a buffered I/O technique for handling the assembly language source files. The assembler automatically determines the available workspace and allocates the correct buffer sizes [5]. Using buffers, the Z80 can handle input and output without any lag or delay in the processing signal. Also the Z80 can use the same instruction set as other processors, which is a bonus, considering when some manufacturers make chipset revisions they break some of their original functionality [6, pp. 203–208].

According to Shima [7], the Z80 was created with the intent of trying to get higher performance than Intel’s 8080 and Motorola’s 6800 processors. It took two months to create the architecture design of the Z80. When the design was finished, the Z80 was able to update both registers that are in use concurrently. So even though the Z80 was created for a variety of different uses, it is still useful in the recording world. While the ADSP-21369 was created with the distinct purpose of being used for multi-track recording sessions, so it is better suited for that task.

IV. Application

Older amplifier models needed to be modified to make changes to their tone. I purchased an ADA MP-1 guitar pre-amplifier and completed the modifications myself to see if I could notice a change in the sound. The MP-1 is a guitar pre-amplifier that uses both solid state and vacuum tube components. The modifications I performed were used to swap out the tubes that are installed originally in the MP-1 with tubes and produce higher sounding gain. The modification is called 3.666.

The MP-1 was originally produced in the 1980’s and has remained popular with hobbyists as it is relatively quite simple to perform modifications. There are instructions on the internet for a variety of different modifications using readily available parts, but I chose the 3.666 mod to increase the gain while keeping the low end sound for playing rhythm guitar. The 3.666 mod makes the MP-1 sound like higher priced Mesa Boogie amplifiers. Another reason I chose to mod the MP-1 was because of the price and flexibility for the amplification itself.

To start the modification, the tubes need to be removed from the circuit so there is no chance that they can be damaged while soldering the new components to the circuit board. It is important when changing any hardware parts on any circuit board to take your time and make sure all of the parts are aligned correctly so there are no mistakes. It is also important to clean any excess solder off of the board so that all new parts are connected cleanly.

Replacing resistors and capacitors on the board is the next step for the modification. Once again, make sure to check to ensure that all connections are made cleanly. The circuit must be simple and fast to create a transparent or clean signal [8]. The new resistors and capacitors have different impedance values to help maximize the distortion of the tubes. This accomplishes the task effectively, but it comes at a price. Once the modification is complete, there will be an excess amount of unwanted noise that will be produced by the pre-amplifier. I handled this excess noise by running my MP-1 pre-amplifier through a noise gate before reaching the actual amplifier.

The next step of the modification is to change the trim pots of the equalization section of the amplifier. There are more resistor and capacitor changes that need to be made in this section as well. The changes in the equalization section create a larger range of values for the bass and mid tones. This allows the mid to be scooped out almost completely so that the sound output will consist of only bass and high tones. This creates a thumping guitar sound that compliments the kick bass drum and would be best utilized in heavy metal music. If the signal comes in too hot, there is a chance of high amounts of feedback that can cause clipping within the transistors [9].

The downside to the trim pot modification is that it will need to be set manually on the circuit board. Once the values are set, the only way to change them beyond the scope of the built in preset values is to open back up the case of the MP-1 and physically rotate the trim pot knob itself. This doesn’t really matter much once the MP-1 is set to match the speaker cabinet that will be used. It is best to adjust the trim pot while playing the guitar using a heavy palm muting technique to bring out a full bass tone from the guitar. This will help with the adjustment as it needs to match the speaker. Too much bass will create a muddy tone and not enough will leave the guitar with a weak sound. When the trim pots are set successfully set, the guitar tone will sound tight and punchy.

The benefits for using a pre-amplifier compared to a complete guitar head amplifier is the same a comparing a guitar head amplifier to a speaker combo amplifier. A pre-amplifier is separate from the amplifier itself so the musician has freedom when choosing the amplification section. Plugging a pre-amplifier into a full solid state amplifier allows the guitarist to achieve the full tone of the guitar and pre-amplifier without extra distortion added from a tube amplifier. If a guitarist wants to add even more distortion, they can set up the pre-amplifier to play straight into a tube amplifier. When adding too much distortion, excess noise can be added to the signal as unwanted white noise. For this reason, it is important to match the speaker impedance to that of the amplifier [10].

Once I had completed the modifications, I was unable to get a clean sound from my guitar when using a tube amplifier running from the pre-amplifier. The tube amplifier added its own distortion on top of the distortion that was created by the modification of the MP-1. This worked great for solos but really muddied up the tone of the guitar while trying to play rhythm pieces. A major factor that contributes to the extra distortion is the extra waveforms that are created from the overloaded modulating signal. These extra waveforms conflict with each other making the individual notes harder to distinguish creating a muddy sound [11].

To clean up the sound for rhythm guitar I had to use an A/B switch after the pre-amplifier to change the signal from a tube amplifier to a solid state amplifier. The solid state amplifier allowed for a full recreation of guitar and pre-amplifier sound without adding any distortion. To test to see how much extra distortion was created from the tube amplifier compared to the solid state one, I plugged my guitar straight into the A/B switch and tested each separately. I cranked both of them up and switch backed and forth between the two amplifiers. With the gain knob around six, the tube amplifier began to distort. At full volume the distortion was that of which you would hear on an AC/DC album.

The modification to the pre-amplifier took away a lot of the mid tones creating a very punchy bass sound and a bright high end that really cuts through any recording. Running clean gain settings on the MP-1 still distorted the signal even at the lowest volume. To prevent any distortion I had to use another A/B switch and bypass the pre-amplifier altogether. When I did that I sent my signal straight into a separate effect pre-amplifier to add a chorus effect to my signal and then it continued into the solid state amplifier.

For my rhythm guitar sound I ran my guitar through the MP-1 and the solid state amplifier. To get the extra distortion that I needed for solos I ran the MP-1 through the tube amplifier. I also ran some other varieties of my signal through the amplifiers and the effect pre-amplifier to achieve other effects such as delay and reverb which I wanted to be able to use in some songs.

To accomplish the complex switching that was needed, I had to employ the use of a MIDI foot switch. I placed all of my pre-amplifiers and amplifiers together in a rack mount and wired them altogether. I also placed a sound normalizer and equalization unit at the end of the chain to help keep my signal consistent. I found through my earlier A/B test that the tube amplifier signal came out louder than the solid state signal even though both units were rated at 100 watt output. Through tests of other amplifier units I found that tube amplifiers generally sound louder than solid state units rated at the same output.

To Z80 handled channel input from the MP-1 and the control was used to store and recall data from the built in equalizer. It is possible to have channels that matched the same amount of gain but had different equalization levels to scoop out the mid tones or brighten the high tones. This is useful from the perspective of a musician as the MP-1 can store up to 128 channel settings on internal memory and another 128 on external memory using the bypass setting. This enables the user to create another 128 settings using another processor like I did with the effects pre-amplifier.

The settings created can also be backed up to a local computer. To accomplish this task using modern equipment I had to purchase a MIDI to USB cable to connect the MP-1 to my computer. Then I was able to store the MIDI file settings on my local computer. It was also easier to change the vales in the files themselves and re-sync them with the MP-1 as I was able to change all of the values in the file using the keyboard instead of navigating the up and down arrows on the front panel to find my settings. The only downside to changing the values on my computer was that I wasn’t able to hear my changes as they were occurring.

The values of the MP-1 are stored as hexadecimal addresses. The hexadecimal address for each value is stored as one byte each. Each hexadecimal address denotes a separate value. For example, the value of the first overdrive channel is stored at the hexadecimal address 00. The actual channel switch itself, between the solid state and tube circuitry, is stored at the hexadecimal address 0A.

The settings for the other parameters handled by the MP-1 are controlled by MIDI values that are stored at their hexadecimal address as well. Depending on the setting, the MIDI value changes the actual value for the setting by using different amounts. For example, the overdrive values increase by two tenths of a percentage while the bass and treble value settings increase by three percent with each MIDI value increase.

There are twenty-nine preset values included with the MP-1 when it is first booted or reset. The settings are set to emulate well known amplifiers or styles that are highly sought after by guitarists. The first setting emulates a Marshall brand tube head amplifier. After modifications have taken place, the presets will not work as planned and will need to be adjusted based on the modifications that are performed. When emulating other amplifiers the waveforms produced are close in relation, but there are slight differences between the two. The variance created by the actual tube circuit may add to the richness of the distortion while a solid state or emulated amplifier would not have that variance [12].

V. Conclusion

Over the years, musical styles have changed drastically. From the instruments that musicians favor, to the overall approach in composition, music has become louder and more aggressive. One of the most drastic changes in a singular instrument has taken place in regards to the sound of the guitar. Originally, the guitar was produced as an instrument with strings that vibrated over a hollowed wooden body to amplify the sound. The concept and design of a guitar has changed throughout the years.

In competition with louder instruments, such as drums, when played live guitars looked for ways to increase the volume output of their instrument. Guitar designs were changed to accommodate the needs of musicians by changing the structure of the instrument and adding electronic components to help amplify the sounds that the guitar produced. Originally, the guitar body was changed from a hollow one to a semi hollow body with a magnetic pickup attached above the bridge to help amplify the sound. This helped keep the warm tone that is naturally produced by full hollowed body acoustic guitars.

As live music continued to evolve and become louder with the invention of amplifiers and speakers, so did the design of the guitar. Manufacturers stopped hollowing out any part of the body, instead using solid blocks of wood. This created a more aggressive sound. Guitars were also able to be produced at cheaper costs as manufacturers could now bolt the neck of the guitar to the body. This allowed them to craft the guitars out of smaller blocks of wood instead of one large piece which was needed at the time to create full acoustic guitars.

Along with the changes made physically to the guitar, there were vast changes in the way the signal from the electric guitar was amplified. A guitar pickup is basically a magnet that has been wrapped with wire. This creates an electromagnetic signal that travels through the cable to the output jack of a guitar. Some guitars have knobs and switches hardwired to change the tone of the signal or the volume from the guitar to the output jack. Some pickups are also considered “active”, and these pickups use battery power to help boost the electromagnetic signal. The pickups that do not use battery power are considered passive.

Once the signal reaches the output jack of the guitar, it travels through a wired instrument cable into an amplifier. Amplifiers come in a variety of different designs, but they all serve the same purpose — to boost the signal of the guitar. The components of an amplifier have changed along with the guitar and the technology of the times. The original intent of the amplifier was to boost the signal of the guitar so that it could compete in a live setting with instruments that are naturally louder such as drums. The amplifiers were created using vacuum tubes which then increased the volume of the guitar output.

Amplifiers have two main designs. There are combo amplifiers that combine the circuit that boost the signal and include one or more speakers wired together. Using tube combo amplifiers can be cumbersome to carry around as tubes themselves are quite heavy. This also limits the sound that that certain amplifier can make as the speaker and the amplifier circuit are wired together to produce the same tone.

The second amplifier design is to create a “head” which is just the amplifier components themselves. This design includes a separate output jack which gives the musician the freedom to choose which speakers they want the amplifier-produced sound to run through. This allows the musician to change the tone of their amplifier by changing the type of speaker that produces the sound. This also breaks the amp into separate parts allowing easier transportation depending on how large the speaker cabinet is.

Both design types have their pros and cons, but the most drastic advancement made in amplifier development has been in regards to the circuit components. When the transistor radio was produced, the idea to use it in amplifiers was also implemented. This would allow manufactures to replace the vacuum tube circuitry of an amplifier with solid state components. Some benefits of using a solid state amplifier instead of a vacuum tube amplifier would include lighter weights, cheaper costs, and an unlimited number of different tones produced by the amplifier.

By placing a computer processor in an amplifier, the amplifier can be used to create a variety of different sounds. The amplifier can be controlled remotely using programmable switches to help the musician change tone during performances. An amplifier can also take advantage of memory banks so the musician can preset tones and then recall them as needed.

I have built guitars and amplifiers consisting of a variety of different parts trying to recreate the sounds of different musicians. The composition of a guitar itself can change the sound of a player, along with the technique used to play the guitar. But over the years, I have found that the greatest change in tone occurs in the amplifier itself. Speaker change has a slight effect on the sound as well, but again, I have found that the amplifier itself produces the greatest change.

An amplifier consists of a circuit board, capacitors, resistors, transistors, vacuum tubes or computer chips. An amplifier does not have to use all of the previously mentioned components, just some variety of them. I have seen amplifiers that only consist of transistors. The intention of amplifiers changed over time as musicians started to incorporate sounds of distortion into their music. Originally, an amplifier was only used to boost the signal from the guitar. Distortion was an unwanted byproduct of using vacuum tubes. When the gain of an amplifier is turned up past the maximum level that the vacuum tubes can handle, the sound begins to distort.

Guitarists started to take advantage of the distortion by using it to boost their signals while soloing. This was most prevalent in the blues, but was soon incorporated into rock and roll music. Guitarists started to look for ways to add distortion to their guitar sound and control the amount of distortion. The distortion can be controlled by boosting the signal and or by varying the attack that the player uses. Manufacturers started looking for ways to allow musicians to control the tone of their sound while playing.

They accomplished this by creating amplifiers with multiple channels that were able to be switched by using a foot pedal. The foot pedal sends a signal to the amplifier telling it to switch channels on the circuit. Some amplifier manufacturers used different types of tubes and capacitors for each channel to increase or decrease the distortion. The benefit of this approach was that guitarists could play rhythm guitar on the clean channel and switch to the distortion channel when soloing. This would allow the solo guitar to cut through the mix.

In the future, I believe that digital recording will overtake all forms of analog signal processing and recording. There will be a demand for tape recording gear along with tube amplifiers, but in the end the cost of that type of equipment will vastly outweigh the benefits of it when compared to digital equipment. The best sounding recordings that I have ever made have been created using a variety of different gear with different tonal qualities. With virtual signal emulation, different sounds can be implemented on the fly so that it is easy to create great sounding recordings by mixing emulated signals.

VI. References

  1. R.O. Hamm, “Tubes Versus Transistors- Is There an Audible Difference?,” Sears Sound Studios., vol. 21, no. 4, May 1973.
  2. SHARC Processor ADSP-21369 User Manual. Analog Devices, 2013.
  3. Zilog Z80 PIO User’s Manual, ZiLOG Worldwide Headquarters, 2004.
  4. Z80 Family: CPU Peripherals User Manual, ZiLOG Worldwide Headquarters, 2004.
  5. Z80 Assembly Language Programming Manual, Zilog, Inc., 1977.
  6. R.B. Thompson and B.F. Thompson, PC Hardware in a Nutshell: A Desktop Quick Reference, 3rd ed., Cambridge: O’Reilly, 2003.
  7. M. Slater, F. Faggin, M. Shima, and R. Ungermann, “Zilog Oral History Panel on the Founding of the Company and the Development of the Z80 Microprocessor,” Computer History Museum, 2007.
  8. E. Sanchez-Sinencio and J. Silva-Martinez, “CMOS Transconductance Amplifiers, Architectures and Active Filters: A Tutorial,” IEE Proc.-Circuits Devices System., vol. 147, no. 1, February 2000.
  9. P. Quilter, “Amplifier Anatomy — Part 1.” Sound & Video Contractor., February 20, 1993.
  10. J. Portilla and R. Jauregui, “Studies on Small- and Large-signal Noise in Solid-State Amplifiers,” UPV/EHU,. 2013.
  11. T.E. Rutt, “Vacuum Tube Triode Nonlinearity as Part of the Electric Guitar Sound,” Convention of Audio Engineering Society., October 8–11, 1984.
  12. M. Karjalainen and J. Pakarinen, “Wave Digital Simulation of a Vacuum-Tube Amplifier,” Laboratory of Acoustics and Audio Signal Processing., vol. X-06, pp. V-153-V156, 2006.

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Tags: music, guitar, amplifier, amplification, computer processing, computer processors