Which is the best? Order: Amplitube 4 Guitar Rig 5 Bias Amp 2 Bias Fx (JamVox is obsolete). Guitar on the right: 1 impulse of Vox AC30 Blues cab (loudspeaker: Blues) through a microphone M8 in position Cap Edge position, at a distance of 0.5 inches (1.3 cm). Stereo panning: 40% right. 1 impulse of Vox AC30 Blues cab (loudspeaker: Blues) through a microphone Audix i5 in position Cap Edge Off Axis, at a distance of 2 inches (5 cm.

Elliott Sound Products

Project 47

Vox AC30 Guitar Amplifier Simulator

Stephan Möller (Edited by Rod Elliott)Introduction

This project is a little different from most. It was designed in Germany by Stephan Möller, and was quietly residing on his web site until I spotted it. Stephan was very happy to have it reproduced here, as he will get more exposure, and you will get something quite extraordinary. In his own words -

Dear guitarists who love the VOX AC-30 !

The secret is revealed !

This is my project 'AC30SIM', a Vox AC-30 Simulator, based on OP-Amps. I hope you enjoy to read or even test it. I am very happy to publish my page on Rod's page, that's really nice from him, and if you have any questions, I will answer them soon.

Have fun,

Stephan has done a lot of work on this, and I only have the essential elements here. Much of the support material is linked back to Stephan's site. These include sound files (MP3 files are local, but the wave files are remote), so you can hear what the simulator and the original Vox AC-30 actually sound like, and some plots from PSpice showing frequency response, circuit simulations, etc.

The material is essentially Stephan's original, reformatted to match the rest of my pages - I have made a couple of comments, and have changed some words - Stephan's English is excellent, but a few words were obviously hard to translate properly.

Comments I have made are indented, and in this type, so it is really easy to see what I added to the original. If this is not enough, I have also added my 'tag'. RodE

Vox AC-30 Simulator

Please Note: This is a general description of the circuit AC30SIM, the simulator of the VOX-AC30. Some resistor values are not shown to protect the circuit against commercial use. If you are privately interested to know the values, so that you can build the circuit for your own private use, print, sign scan and e-mail me back the non-disclosure agreement

My E-Mail: stephan.moeller@hamburg.de
I will then e-mail you back the values as soon as possible.

Figure 1 - Simplified Circuit of VOX-AC30 Preamp

The original : The brilliant Input Preamp of the VOX-AC30 Top Boost. The Volume pot is turned to full and therefore replaced with R4. I must say that my PSpice simulations with triodes do not show the behaviour of my VOX-AC30. I don't know why, maybe because of the old tubes in my old VOX (1961) or the PSpice models are not valid enough.

Figure 2 - Block diagram of AC30SIM Preamp

This is a block diagram of the AC30SIM Preamp. U1 simulates the first VOX Valve stage V1 and U2 simulates the second VOX Valve stage V2/V3. U1 and U2 are inverter amplifiers with non-linear transfer functions. Between them is a high pass C1 / Re2. Ra1 simulates the Anode resistor R2, and Rdx / Dx simulates the grid current of V2.

Figure 3 - Circuit of AC30SIM Preamp

My intention was to develop an opamp circuit which simulates the VOX AC30 Preamp. Eclipse download for java for mac. The main problem was that the non-linearities of the VOX are hard to determine. For example, it is not possible to measure the DC transfer function of the VOX, because it has capacitors in the signal path. A sine signal is the only signal on which distortions can be identified.

The thing is, that the different input levels of the sine wave will cause different output signals. Like when you play a single guitar tone into an amplifier. At the beginning the tone has full volume and the amplifier has a lot of distortion. Then the tone gets lower and lower, and the distortion gets smaller and smaller. The distortion changes with the input level. So I calculated a sine burst signal with an exponential volume decrease. You can see a comparison of measurements of the preamps (VOX-AC30 and AC30SIM)

If I use a circuit which has exactly the same amount of stages and the same filters between them, it is only necessary to find the non-linearities of each stage to simulate the behaviour of the VOX. It's enough to use an exponential sine stimulus of only one single frequency. Which frequency is not important. I took a 400Hz expontial sine stimulus. That signal I fed into the VOX and began in parallel to compute with PSpice some circuit simulations to find possible solutions to get near to the behaviour of the VOX.

Some readers might be baffled by Stephan's claim that the frequency is not important, however this is the case provided the chosen test frequency is within the normal bandwidth of the amp - preferably at an approximate centre frequency. For guitar and most music, 400Hz is actually a better test frequency than 1kHz. Having found the non-linearities and the frequency response, the two combine to provide the overall characteristics of the original. RodE

When I had a good solution, I took some measurements of my circuit and compared it to the VOX. When the measurements of my circuit where not near enough, I modified the circuit and took some more simulations with PSpice. And so on.

At low input levels it is hard to detect distortions on a sine wave. So the final measurement was to compare the FFTs at low input levels. I took a 400 Hz sine wave which has an exponential volume decrease and measured the FFTs of my circuit and the VOX.

To show the behaviour of the complex non-linearities of the AC30SIM Preamp circuit, I have computed some PSpice Simulations.

The inverter opamp circuit is a good basis to form non-linear transfer functions. Each Opamp inverter is driven by a Opamp buffer to give an independent high input impedance. The complex non-linear network in the feedback loop of U2b is necessary to simulate the unique behaviour of the 2nd stage of the VOX-AC30 preamp. The dual power supply of +/- 15 Volt seems high, but is needed to let the first buffer U1a work without clipping.

Some guitar pickups can produce relatively high output peaks which would easily overdrive the first buffer. The potentiometer at the positive input of U2b is needed for DC offset trimming. Because of the DC coupling between U2a/U2b and the high gain of U2b, a small DC offset of U2a can be significant at the output of U2b and therefore influence the non-linear transfer function very much. The potentiometer is trimmed for 0V DC at the output of U2b.

Instead of the NE5532 any good Fet Input Opamp could be used. The advantage is, that the bias circuit is not needed anymore, because of lower input bias current which causes much lower input offset voltage.

If you would like to see my Pspice simulations for comparison of non-linear Opamp circuit:

Figure 4 - Circuit of VOX AC30 End stage

This is the simplified circuit of the VOX AC30 Top Boost Power amp. The tone control filter has fixed resistors instead of pots. The treble potentiometer is RK1/RK2 and bass potentiometer is RK4, which stay for my favourite positions. Free download manager not working with firefox.

Figure 5 - Block diagram of AC30SIM End stage

Figure 5 is a block diagram of the AC30SIM End stage.

U3 simulates the VOX Phase splitter V1/V2.
U4 simulates the VOX End stage V3/V4/V5/V6.
U3 and U4 are inverter amplifiers with non-linear Transfer functions.

The front end is the tone control filter. Between them i

Free Vox Ac30 Vst