Virtual Amps for Rookies (Special Guest Mike Scuffham from Scuffham Amps)
Mike Scuffham was at one time the product designer for guitar legends Marshall Amps. This expert tells us what a virtual amp is and how it is created in software.
Mike Scuffham was at one time the product designer for guitar legends Marshall Amps. He was the man behind their JMP-1. In 2011 Mike stepped bravely into the crowded market of virtual guitar amps, and after few years Scuffham Amps has become a name that you could count on.
I like Amplitube, being that it offers millions of different guitar colors, I love Vandal’s metal darkness, I’m a big fan of Guitar Rig’s effects, GTR 3 is not bad at all, and there are few others worthy of attention. But lately I’m recording most of my guitars exclusively with S-Gear from Scuffham Amps. It reacts like a real amplifier, having the right dynamic and sound, and with a new amp added, The Wayferer, it brings back memories. I haven’t played a Fender for years, but now I have that sound and response back under my finger. The software market is very dynamic one, but at the moment, S-Gear is my sweet spot. I heartily recommend it to all.
So, take a deep breath, dear reader, because Michael (or Mike) Scuffham rolled up his sleeves and dove deep into this subject: what virtual amp is and how it is created. Do we get them by sowing small amps into the soil and waiting the harvest season? Or do we get them by implementing a old good Archimedes code (with the tiny touch of a slamming hammer)?
Let’s see what Mike will tell us.
Virtual amps for Rookies
Hi folks. Alex Arsov asked me to write a short article about virtual guitar amps, how they work and how they are made. Well, this is a very big subject and there is an enormous amount of accumulated knowledge that goes into producing a virtual guitar amp, or any other virtual model of a complex electronic device. So where to start?
What is a virtual amp?
If you have never played through a virtual amp, then you might find it difficult to conceptualise what these amps are and how they compare with a physical amp. The biggest difference is firstly that you need to plug your guitar into the computer, and secondly, that all the amp controls are presented on your computer screen. These differences may seem trivial to a younger generation of computer users, but older folk may need to see it and hear it to believe it! How can this digital computer compare with the beauty and purity of my all-valve guitar combo? Well it can, and you need to try it! Step 1: Invest in an audio interface for your computer, this will give you an instrument input and monitor or headphone outputs. Step 2: Download some virtual amp software and begin your journey.
The making of a virtual amp …
A virtual amp might be closer to the real thing than you first imagine. The amps found in the Scuffham S-Gear product are original designs created from scratch using traditional electronic representation, i.e. resistors, capacitors and thermionic valves. These components are arranged as amplification and filter stages to create an electronic amp design, which is then transformed into an equivalent software implementation. I cannot say that all virtual amps are made this way, but I can tell you that this is how we do it!
An initial concept:
The first stages of virtual amp design are really no different to designing a physical amp. The amp designer has an idea about the kind of amp he wants to create. Perhaps it is a simple vintage design like a ‘tweed’ Fender with two preamp tube stages and a power amp that can be driven easily into saturation. Alternatively, he might want a heavily saturated pre-amp with four or more gain stages driving into a tighter power amp stage. Another design consideration is the position of the tone stack (the traditional bass, mid and treble controls found on most amps). A tone stack positioned before the saturating gain stages will serve to shape the distortion but have less effect on the overall frequency spectrum of the sound produced. A tone stack positioned after the saturation stages will allow more sculpting of the overall equalisation.
Electronic circuit simulation:
So we have an idea about the structure of our amp design and maybe we have sketched a simple schematic diagram on paper. The next stage is to create a simulation of our design using an electronic circuit simulation tool or SPICE (Simulation Program with Integrated Circuit Emphasis) tool. This is quick and easy and will allow us to do some initial evaluation of the design, measuring voltage levels and plotting frequency responses at various points in the design. Later on, the SPICE model can be used to check that our software model is performing as expected.
Fig 1. SPICE schematic diagram and frequency response plot
Transforming the design to software:
The next stage is to transform our analogue design into a digital software equivalent. This is where the complex mathematics is required. Typically, each element of the design is expressed in a mathematical form that can be converted to an efficient digital software implementation for real time processing.
Amplifier elements like the tubes introduce ‘non-linear’ (distortion) characteristics, and require a bespoke mathematical model in order to emulate very accurately the characteristics of a real tube.
For those interested in the maths …
The electrical filter circuit can be expressed in the s-plane using Laplace transforms, where the s-plane is used to describe linear continuous analogue systems in the frequency domain. An expression for the filter circuit is then converted to the z-plane which is the digital equivalent
for discrete-time (sampled) systems. This transformation to discrete maths is done using the Bilinear transform. It is amazing how much of our audio technology is based on the works of 18th century mathematicians. Look up Laplace, Fourier, Euler and Lagrange to discover more!
Testing the amplifier:
In order to test our software amp, we will use an oscilloscope to probe the virtual amp at different points in the circuit. Just the same as testing a real valve amp, we apply a pure sine wave and check the resulting signal at various stages in the design. We will also check that the controls are working as expected.
Another useful measurement technique is to apply a swept sine wave to the input of our circuit and measure the output to get a frequency response plot. Again we will take these sweeps at various points in the design, comparing the result with the frequency response plots from our original SPICE model to ensure that the software implementation is correct.
Listening and refining:
Listening tests and refinement is by far the most time consuming stage of the design process. If we are lucky, we might be happy with the design after some small tweaks, but on the other hand, we might end up going back to the SPICE model and making some radical changes to the structure of the amplifier.
Occasionally I have been asked whether we build real physical versions of the S-Gear amplifiers. The answer is that we do not, but we could do if we wanted to. The designs will translate back to physical analogue hardware.
Do we measure real hardware amps as part of our design process? Sometimes, yes. It can be useful to measure a real circuit to check that the software models are performing in the same way as their physical counterparts, particularly with respect to the non-linear circuit behaviour (i.e. the distorting stages) which is not so easy to simulate using SPICE tools. Taking measurements is particularly useful when dealing with connections between amp stages and ensuring that we have correctly understood how one stage interacts with the next.
Learning the fundamentals of audio engineering …
Whether you are aspiring to design your own audio software or whether you are recording and mixing your own music at home, some basic understanding of audio engineering can be very valuable and will help you gain a deeper understanding of your audio tools.
Learn how to express relative audio signal levels using decibels:
Learn about simple electronic circuits for low pass and high pass filters, how to express the filter cut-off frequency in terms of -3dB point, and how to express the filter slope in terms of dB per octave. A first order filter roll-off will be 6 dB/octave. Most of the passive filters in a guitar amp are first order, or cascaded first order filters.
Research the heritage …
If guitar amps is your thing, then you should read about thermionic valves and master the analysis of a simple triode amplifier. There are several good books available on thermionics. If you are interested, google for the famous Radiotron Designer’s Handbook by Frederick Terman.
Study the classic Fender and Marshall tube amp designs and try to find an opportunity to play some of these amps. If possible borrow or hire some amps at a rehearsal studio. If you are not able to get access to these amps, then search YouTube for the best recorded examples – listen as carefully as you can and keep a library of your favourite reference sound clips.
Master the tools …
If you have a collection of outboard gear around your studio and you are interested to make some accurate frequency response measurements, check out the software tool TrueRTA available from www.trueaudio.com. This software gives you a decent oscilloscope, real time spectrum analyser and swept sine analyser. You will need a good audio interface to take full advantage. You can also find spectrum analyser and oscilloscope plug-ins as part of some Digital Audio Workstation software.
To get going with electronic circuit design, go to the National Semiconductor website and download the free LTSpice www.linear.com/ltspice. Experiment by analysing some of the simple RC filter circuits. To get to grips with digital filters, download SciLab from www.scilab.org and experiment with some simple digital filter implementations.
Finally, a tip for all guitar tone designers …
Meditate on your tone, fill your mind with the tone that you wish to create. Listen to recordings and immerse yourself in the sounds. Play your guitar without an amp, and imagine how your virtual amp should respond under your fingers. The best guitar players can produce great sounds no matter what equipment they play through, it’s about getting your fingers to make the music. A good amp design should complement the player, it shouldn’t be too easy or too forgiving – he needs to fight a little and work his fingers hard to drive emotion into his music. You may not be a highly accomplished guitar player, but in order to get the best from your listening tests, concentrate on some simple phrases and single notes, try to discover what makes a great relationship between the fingers and the amp. Keep listening to your reference recordings for inspiration.
by Mike Scuffham – January 1st 2014
A clip recorded with a S-Gear virtual amp ( arrangement and instruments: Franci Zabukovec – Zabukowski )