Zen for guitar power amp

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I think I may have seen a thread previously about this, so I apologize in advance if I'm not covering any new territory here.
I was thinking about using the zen for a guitar power amp, for these reasons:
1. low damping factor- should give nice tube-like low end when driven hard.
2. mainly low order harmonics- do I need to explain why this is good?
3. low power- would make an excellent amp to record with- just enough power to get a low-watt guitar speaker breaking up good, and not enough that you have to rattle the whole house to get some good power amp distortion.

Here are the problems that I am having with this theory:

1. Need to get the power supply to sag, to produce just the right "attack envelope". How do you do this with a single-ended solid state amp, without messing up the filtering? I was thinking of a pi filter with A LOT of inductance, and as little capacitance as I could get away with.

2. would it really sound good being slightly to heavily overdriven? I guess I can only find out by building it.

Any comments, suggestions?
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I don't think the ZEN would be the best amp for your application. Your asking this simple amp to do the exact opposite of what is was designed to do. Plus you'll be warming up your space with all the wasted watts.

You might do better w/ a low power tube amp or small class a/b amp. If you go out of the forum, you'll find pages to all sorts of amp projects.

Most likely, you'll want to distort/over-drive a preamp stage, not the amp output. The ZEN has only one stage.
Others might have a different opinion on this...

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Joined 2002
Thanks for the suggestions, but you're not exactly correct, or you're missing the point of why I wanted to use the Zen.
The zen was designed to be a simple amp, with one gain stage- Although it sounds good, it is not exactly "hi-fi", considering the amount of distortion that it produces at the top of it's power rating (although Nelson has provided ways to get it into the respectable range). The distortion characteristics are similar to tubes, at least in harmonic order, and from what I can tell, magnitude. I just thought it would be something different to use. Of course I could build (and have built) tube amps.
As far as distortion goes, the hard clipping is the job of the preamp tubes, as well as the tone shaping. After this has happened, it leaves a very compressed signal. The power amp has the job of producing the dynamic characteristic of the amp. This is accomplished throught the right amount of power supply sag- obviously easier with a vacuum tube rectifier. Power amp distortion is also a BIG part of the sound, along with a low damping factor (gives bass a certain sound) and speaker cone breakup. Of course, the power amp is not normally driven into hard clipping, but the right kind of harmonic distortion along with the low damping factor suggest to me that the zen could be a good candidate, and may have an original tone- a different flavor, you could say. I want the amp to drive about 5-10 watts into a junkie 15 watt paper cone guitar speaker, at 5-10 percent harmonic distortion, preferably mostly 2nd harmonic. This should probably be enough to get the speaker breaking up, and have the right volume to throw a microphone on for recording without ear-splitting levels. Of course the zen will have to be "tweaked" a little to get what I want- maybe lower the bias to get more distortion. Obviously, I want to get more lower order harmonics without bringing on the nasty upper harmonics. Any suggestions?
Don't take this the wrong way. Many people who don't play don't have an understanding of just how important the power amp is in a guitar amp. I was in the dark too, until I did a little experimenting. Guitar and hi-fi don't really have a lot in common, but I suspect that guitar amps may benefit from carefully-used hi-fi technology in the right places.
My original question was more or less how to get power supply sag without a lot of hum. I'm thinking big power resistor in series, after the first filter capacitor. Any suggestions?
SteveG said:
Don't take this the wrong way. Many people who don't play don't have an understanding of just how important the power amp is in a guitar amp. I was in the dark too, until I did a little experimenting. Guitar and hi-fi don't really have a lot in common, but I suspect that guitar amps may benefit from carefully-used hi-fi technology in the right places.

Yeah, you're right. Preamp distortion is nice and has it's uses, but most people don't know that the "good", sweet distortion is generated when the power amp clips.
As for the Zen-based guitar amp, it might be a good idea... i once had plans for a ss guitar amp , but my approach was among the line of Tech21 stuff; you build an "amp" using jfet gain stages instead of tubes (complete, with preamp and power amp sections), then you feed that to a common power amp. I still have to experiment with this.
power supply sag


First, to get the power supply to sag on the loud stuff you are going to have to bias the Zen into the A/B range. (I think you know that but want to be sure.) Then, you will need some kind of resistance in the power supply. You are on the right track when you talk about putting resistors after the first filter cap. They do not have to be there, however, they could be in series with the primary of the transformer. (More care is required here due to the higher voltage.) I suggest using Nelson's idea of using light bulbs for resistors. You could wire several light sockets in paralell either after the caps or in the primary circuit. Then by changing the wattage of the light bulbs (or adding light bulbs) you can vary the resistance in the circuit. You will also be able to use the relative brightness of the bulbs to guestimate the amount of sag you are getting. To minimize hum, I suggest at least a few millihenries of inductance after the first filter cap followed with another cap to ground. A little inductance in the DC line will go a long way to eliminate hum. It also gets rid of the higher order harmonics of the 120 cycle ripple. See the disscussion on SOZ choke input? thread for more comments on inductors.
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Joined 2002
great idea!

I really like the light bulb idea! You get a built in "sagmeter".
Yeah, I know I'll have to go A/B to get the sag- the constant current draw of the class A will make that necessary, unfortunately. I hope it dosen't eliminate the sweet sound of the amp.
more saggin' ideas


I am glad you liked the light bulb "sag-o-meter" idea. I sure put my left foot on the keyboard when I said to bias the Zen class A/B.

Anyway, here are a couple of ideas to get the power supply to sag while you are getting loud on your guitar and the amp is behaving like Geoff pointed out it would.

First idea: Build a second amp to run A/B or B. Connect the second amp across the same DC supply and signal source as your Zen. The output of the second amp should go to a plain old power resistor. (A light bulb is not recommended here due to its changing resistance.) Now when you get loud the second amp will "suck electrons" out of your psu which will cause voltage sag due to the light bulb somewhere in the psu circuit. By varying the gain (and/or load) of the second amp you may be able to 'fine tune' the amount of sag.

Second idea: Build a second amp with the same power and input arrangement as before. This time capacitively (ot transformer) couple its output to a bridge rectifier. (A half wave rectifier might be OK.) Put a small cap and resistor across the output of the rectifier and connect the - side to ground. Connect the positive side to the gate of an FET. This FET will have its source tied to ground and drain to the + of your psu. (maybe thru a resistor to keep some heat out of the FET) By changing the R and C values on the output of the bridge you could control the decay of the sag. By putting a resistor in series with this RC network (or between the amp and the diodes) you could control the attack of the sag. By making the resistor across the output of the bridge a potentiometer and connecting the gate of the FET to the wiper you could control the loudnes at which the sag starts. (I hope that makes sense).

You MIGHT not need a second amp for the second idea. You might be able to run the rectifier(s) off of your Zen amp, but the diode(s) will conduct on peaks wich may introduce some distortion you don't want.

I hope one of those works for you or gives you (or someone else) an idea that does.
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Thanks for the ideas... sounds like a couple of good possibilities. I don't know what I was thinking when I thought about power supply sag with a class A amp. Must have looked at the distortion and damping factor and neglected to consider the realities involved. :eek:

Maybe a lower power Zen type stage driving a p/p output? Zen for 2nd harmonic, p/p for compression and dynamics control? That sounds like a possibility. Could be tubes on the output for the low damping factor of the output tranny. Interesting.
Anyone know how to magnify second harmonic in a zen type stage?
Your idea of using another amp to "sag" the supply of a zen stage is interesting... Maybe could be done with a series pass transistor of some sort inline with the power supply, monitoring the input signal with a peak/hold circuit to adjust sag decay- I believe that was similar to what you were refering to, correct?
Anyway- it's a lot of food for thought. You've given me some new direction, and I appreciate it. I think it's time to start building something, and see what happens.
2nd harmonic


You can get more second harmonic distortion by lowering the amount of current in your Zen stage.
(Keep your drain voltage the same.) But if you are not carefull you will get into clipping which is all manner of harmonics.

Unless you run A/B or B in some stage, class A will NOT vary the current to give you voltage sag.

Yup, some kind of in line variable pass transistor some how driven by your signal will cause voltage sag. That is a series "deregulator" my idea was a shunt "deregulator". Quite similar.

Time for me to go build something, too. Good luck!
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