Pretty cool distortion from a $30 amp

[ianmgull]

I traced out the preamp from a friends practice amp because it has a nice crunch. I wanted to put it in a pedal. The amp is a Drive CD100. I've never traced anything like this out before so I'm hoping someone can give me some advice. I traced out everything from the input until I hit the power transistor. I think I have everything, except power filtering. Does this look like it would work? Does anybody recognize this as a derivative of something else?

[ianmgull]

Ok, I just realized how hard that probably was to read. Here it is with the values on the schem:



A couple things:

1) I realized there is no output cap on the schem, this should be a 1uf presumably.
2) I took a measurement of the pin 8 on the 4558 in the amp and it measured a bit over 12v, I'm not sure if this will make much of a difference.

If anyone who knows anything about simulators can verify this I'll be happy to put together a PCB layout for it. Thanks!!

[anchovie]

Measure pin 4 of the op amp with respect to a ground point such as the input jack as well. If it's 0V then great, it's running on a single supply and using a 9V battery rather than 12V won't make much of a difference. If it's -12V then the amp supply is bipolar so a single 9V will give a lot less headroom, in which case I'd change the gain pot to 100K and the clipping diodes to 1N4148 or a similar silicon.

I don't recognise the tone stack - looks like an output cap would be best as on your schem there isn't a cap in every possible route to the volume pot.

[earthtonesaudio]

I don't recognise the tone stack - looks like an output cap would be best as on your schem there isn't a cap in every possible route to the volume pot.

There's one (220nF) right on the output of IC2.  Output is referenced to ground.

[anchovie]

Ah yes, my eyes must have only been 95% un-bleary at that point in the morning!

[Tonemonger]

From memory I think it looks like a Marshall Guv'nor.

[ianmgull]

Measure pin 4 of the op amp with respect to a ground point such as the input jack as well. If it's 0V then great, it's running on a single supply and using a 9V battery rather than 12V won't make much of a difference. If it's -12V then the amp supply is bipolar so a single 9V will give a lot less headroom, in which case I'd change the gain pot to 100K and the clipping diodes to 1N4148 or a similar silicon.

I don't recognise the tone stack - looks like an output cap would be best as on your schem there isn't a cap in every possible route to the volume pot.

This worries me. I do remember seeing a electro cap or two with their negitive lead pointed away from "ground". I attributed to my lack of sleep but seems suspicious nonetheless. I'll take the amp apart later and double check it.

[Joe Hart]

How did this work out?
-Joe Hart

[GibsonGM]

Cool schem, and a good idea....VERY similar to a Guv'nor. 

[JKowalski]

You'd want to decrease the gain I think, to to get unity. Being a preamp, the output of this circuit will be much much greater than your typical guitar signal.

No output cap is needed, there is a cap on the output of the second op amp stage and therefore the whole tone stack is capacitively coupled.

[ianmgull]

Sorry about the drop out guys, I've been swamped. I suppose the only remaining question is the issue of bi-polar power supply. Hopefully I'll get some time later this week to check on that. Also... There really isn't any need for the funky input section is there? I'm calculating the corner frequency of those lowpass filters at 10266645Hz 

[ianmgull]

I checked pin 4 of the opamp and sure enough -12v. So this thing is running on a bipolar supply. Damn.

[JKowalski]

Its okay, you dont need a bipolar power supply for this thing.

The only reason you would need a bipolar supply is for the headroom needed for the high voltage preamp signal. Since you are putting it in a pedal, you are gonna have to dial down the gain anyways. I would suggest a 12V power supply with a virtual ground.

I messed with it in LTspice and came up with this circuit - effectively the same results, with a 12V supply. I had to dial down the gain a bit, because the op amps hit their supply rails faster and clip before the +/- one does. On the scope it looks pretty similar in response to the original. I didn't add the tone stack cause that would have been a waste of time...

Also, on the treble pot 2nd lug, before the output volume control, you might want to put a resistor, say 47k or so, to limit the volume max... this thing will get loud, let me tell you.

It seems there is alot of excessive filtering on this thing... You can probably take out some of the caps with no effect. I took out the input filter (maybe your amp has digital effects, and a 10MHZ clock signal was possible interference?

EDIT: Also, you might want to add another stage to flip the signal 180 degrees.... Right now it is inverted compared to the input, and you typically don't want that in pedals (in case you want to mix signals later on, or whatever)