For solid state guitar amplifiers, the preamp is probably the single most important part. It shapes the tone and often adds distortion that can enhance the sound you want to create. It is the “user interface” for the amp, giving a wide range of control over how the amp will sound.

Before starting this project, I had cloned the preamp of the venerable Ampeg VH140c, and used it in this amplifier. It is a two channel preamp, with one clean channel and the other distorted, which is great for a larger amplifier. For something a little smaller, two channels is not really practical, and I thought I could come up with a more compact design. I combined features of both channels from the VH140 into one, mostly with respect to tone shaping, and added in a few changes of my own. The objective is to make a clean preamp, but with some edge when the gain is turned up.

For this project, I did a side by side comparison of the actual results to the simulated results, as the build progresses. I use simulation for much of what I do, since I lack a lot of the deeper understanding and knowledge required to design or modify circuits on my own. This is just a hobby for me, learning as I go.

To get started, the first thing I did was to verify the amount of gain I had from the FET/BJT input stage, as the simulation didn’t give me the expected results. Since I’m not overly experienced with jfets, I breadboarded this circuit

I discovered that the simulation model I have for the J201 is a bit off, giving a lower than expected gain for the stage. In fact, there isn’t a jfet model in the Multisim database that gives near to an accurate result for the gain through this stage.
To make the stage simulate the correct amount of gain, I had to change R24 to 880 ohms (gain for the stage is actually supposed to be ~R17/R24).

The gain for this first stage needs to stay relatively low, with just enough headroom to cover the typical output of a guitar being played fairly aggressively. This, according to my measurements, is around 1Vpp. The idea is that clipping will happen in the first gain stage and not in the following stages (they clip, but it’s controlled via diodes and not rail to rail on the opamps). That’s the primary purpose of this first stage, or at least how I envisioned it.

Despite the problems with accurately modeling the fet/bjt combo, I expect the opamp stages to sim within reason. In particular, the amount of gain and the quality of the distortion, with regard to wave shape.

After the input stage, there are two ‘shaping’ stages. The first has the “high” tone control included in its feedback loop. What I have tried to do is limit the clipping in this stage to ~50% of the gain control and up. So, with the gain set at 25%, there should be no clipping, as shown in the screen shot of the simulation:

This result in a very clean final output:

With the gain set to 100%, there will be clipping:

resulting in a nicely distorted final output:

This is a balancing act and it took some considerable fiddling with levels to get it here. Hopefully, the actual circuit mirrors the simulated performance. Based on the measurements I made on the distorted channel of my other guitar amp build, it shouldn’t be far off.

A prototype of the whole circuit needed to be built before I can test the accuracy of the data in the previous post. That will be a matter of doing a rough board layout, etching the board, populate the board with the correct components and test. If my previous experience counts for anything, that first board won’t be the final one.

In my original build, I followed the Ampeg schematic to the letter, except for one rather important detail: I didn’t notice that there were a few log pots (and a couple of reverse log pots) on the schematic. I ordered and used linear pots only. This was more of an issue for the clean channel, as it uses more log pots than the distorted channel. The problem this causes is an abrupt transition in adjustment of the tone controls on the clean channel.

With this redesign, I hope to minimize that, and still use linear pots. Early results show that I’ve done a fairly good job, if the frequency response of the real preamp matches the frequency response of the simulation.

Here is the simulated response with the “bright” switch off. The 5 traces represent the response of the preamp with the tone controls set to 0%, 25%, 50%, 75% and 100%, with 0% being the bottom (green) trace. Not bad, with more than 10db of boost over the pots travel:

This one shows the response with the “bright” switch on. High frequency shifted up 5-6db, pulling the mid up slightly as well. This looks good.

I did try to jack up the mid a bit more but it killed that deep null when the pot is at 0. I think that deep null might be more desirable than a hump, especially as the speakers have the highest output at that point. Could be interesting, again, if the real circuit matches the sim.:

To get started on the real circuit, I have the test board layout done:

Ready to etch and I can do some testing once it’s stuffed.

The prototype finished, ready to try:

My method of prototyping is not the most cost effective or efficient, but it serves me well. I have had no luck with breadboarding and very rarely use one. The times that I have used one, I spent most of my time troubleshooting the breadboard connections, frustrating my attempts to debug the test circuit.

I reuse most of the components, except for the resistors. The resistors I use for prototype are a very low cost variety and I would not use them for a final circuit.

With the prototype finished, I can run some tests.

First, I had to make a small adjustment to the simulation to make it match the real circuit on the fet/bjt input stage. I had already established that the fet model is less than perfect.

So, with that done, I simmed the first opamp gain stage. Note the rms V at the output of U1A. With all of the pots set to the same level as in the sim, I measured the same (give or take a bit – setting a rotary pot with no scale and no knob to exactly 25% is pretty hard to do) rms voltage on the actual circuit:

Scope shot at that node: That was with the gain pot at 25%:

The following is with it at 100%:

Then a scope shot from the actual circuit:

Output voltage is once again within tolerance. The real circuit is clipping just as predicted by the simulator. This is good news. Aside from build a preamp, I wanted to verify (or debunk) the simulator. I want to find out first hand how reliable it’s results are. So far, so good but there is some more demanding content ahead.

Second gain stage. Here there is some deviation that I can’t chalk up to tolerance.
With the gain at 25%, the output on the actual circuit is ~60mVACrms while the sim shows this:

Not a huge difference but significant.

The actual scope shot (above, right) shows the waveform is correct and that is the important thing:

With the gain at 100%, the simulation:

and the actual:

Measured output with gain at 100% was ~260mVACrms. Quite a bit less than the simulation. I’ve tried a few different opamp models to see if that makes any difference – nothing so far.

After spending more than an hour double checking my circuit (the real one) for mistakes, and finding none, I believe the reason for the disparity in voltages comes from the way the simulation handles the passive components between opamp stages. Individually, the gain stages are accurate but NOT when linked together, via passive components (resistors especially). I think the simulator is not accurately calculating the voltage drop between stages, or at least that is my best guess.
For example: to bring the actual circuit results in line with the simulated results, I changed R15 to half its value (5K):

This brought the voltage up in the following stages, closer to the simulated results.

Moving on.
Next step is to check the frequency response of the tone controls and compare to the simulation. I tried to use RMAA but it will not let me record each new setting of the tone controls at the same volume setting. So I used Adobe Audition, recording a white noise sample through the preamp. I’ve used this program for this kind of analysis before and I believe it does a better job than RMAA.

Here is the combined response:

Gain at ~25%, volume set to ~50%, each of the tone controls advanced equally – 0%, 25%, 50%, 75%, and 100%, in the same way as I did in the simulated frequency response charts above.

The “bright” switch is ‘on’. One difference from the simulated response is that I’m using a 10K pot for the High tone control, when it is supposed to be 25K. I don’t have a 25K pot on hand when making the prototype.

Overall, not a bad match. Some good variability in the tone settings, with more than 6db for each.

I spent most of a day either measuring and listening to this preamp. I’m now satisfied that it is performing the way I intended.

I made a few small changes:

Added a 47pF cap from the gate to the source on Q1.

Increased the gain of the input stage to ~10. I did this because I reduced the gain of the second stage by adding a 100K resistor from the wiper of the ‘gain’ pot to its low side. This effectively cuts the value of this pot in half (from 100K to 50K) and gives an approximation of reverse audio taper. My listening tests and measurements said that this was a worthy change, giving unclipped signal up to ~50% of the pots travel.

With an input signal of 100mVrms, the output with all controls maxed is ~1.8Vrms. At first I thought this might be too low, considering that the maximum gain setting for ‘clean’ is 50%, but having driven a power amp/speaker combo, I can say that I was mistaken. Still lots of volume on the clean setting.

The tones really have broad effect and tailor the sound very well. I was on the money concerning the midrange null – very good effect with high tone at max and the “bright” switch on.

Final schematic (click picture to download a pdf of the schematic):

I used this preamp in a smaller practice amp. It has a 10″ Eminence “Copperhead” speaker driven by a 50 watt power amp. There is a sample of how that amp sounds here.