Modifying in-built distortion: transistors in an op-amp feedback loop? - diyAudio
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Old 10th September 2011, 12:50 PM   #1
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Default Modifying in-built distortion: transistors in an op-amp feedback loop?

Hi guys,

I rarely use the distortion built into my Hartke amplifier.

Now, I like the amp's clean sound, but the distortion isn't great, being a couple of LEDs in the feedback loop of an op-amp. Goes loud, though, so I'd like to make the rest of it more usable. Currently, I rely on a Boss multifx, which takes longer to set up and play.

Free Schematic Diagrams Hartke GT60 Schematic Diagram - you can download the schematic here.

I can't remember who it was, but I know someone on here has used (with success) transistors in the feedback loop of an op-amp. I'd like to try this, but I'm not entirely sure how such a thing would be wired.
Of course, I'd be willing to try switching other diodes in (I've heard germanium sound different to silicon), but I'm mostly curious about the thing with the transistors.

So, could whoever it was please post up a schematic of some form - it'd be very much appreciated.

Thanks in advance,
Chris
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Old 18th September 2011, 04:20 PM   #2
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Well I wasn't the guy who posted up but I can tell you some Fulltone pedals use transistors like diodes in a circuit to clip the signal and they sound great. Depending on what transistors you want to use you'll have either a NPN or a PNP. You'll probably want to use two of the same type and wire them in place of the LEDs from emmiter to base only and don't connect the collector.

My experience is that germanium sounds more tube like and much better for Bass guitar.
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Old 18th September 2011, 08:18 PM   #3
Minion is offline Minion  Canada
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You can take the LED"s out of the feedback loop and instead put them on the output of the opamp to get a different sound ...... never liked the sound of diodes in the feedback loop , I allways liked them better at the output ......
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Old 18th September 2011, 11:21 PM   #4
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Cook Your Own Distortion

not sure if its too late, but "shaka bradda" clipping seems to be what is mentioned on this website near the bottom of the page
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Old 19th September 2011, 08:40 AM   #5
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I added some wires and a connection block to the board so I can switch in different diodes and things. Tried a green LED one way, red LED the other.
Still not the sound I'm after.

So, I put a scope on the output of the Boss multifx I like so much. Turns out it's making a sort of rounded triangle wave, but all the points are moved to the right.
Picture a triangle wave, combined with a sawtooth wave where the leading edge is sloped, the following edge is vertical. Round off the results a little and that's the wave I'm after. I'll post a picture up this evening for clarity.
Methinks that'll be beyond a simple op-amp with a couple of diodes, so will probably revert it back to original.

Chris
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Old 19th September 2011, 08:21 PM   #6
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is it a bit like the "soft clipped wave" on the website diagram? if so i think that is beyond just clipping, theres also a bit of EQ that comes into getting that shape. mainly boosted treble and cut bass before the distortion, and plenty of mids... i hate it when i see an amp with scooped mids... the midrange is where distortion sounds best.
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Old 11th October 2011, 06:53 PM   #7
shanx is offline shanx  Canada
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I have found a schematic of a ''soft clip'' opamp circuit and is using both NPN and PNP pair in the feedback loop..one could be switched out if you want to only clip one half of the wave, apparently that would be more like tube preamp distortion. With both in the feedback loop you get a kind of tube output stage distortion where top and bottom of wave both soft clip. It is a bit messy on the schematic, but I will try to clean up and post it. Not tried yet so ''buyer beware''
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Old 12th October 2011, 01:01 PM   #8
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Hi guys, apologies for the delay, I've had other things on with college etc.

Anyway, here's a similar wave I found on t'interweb

http://i2.photobucket.com/albums/y41..._Mode1_Max.jpg

Except all the waves were of the width on mine. I don't think this can be done in the analogue domain so easily (some kind of expander, perhaps?), so I think I'll just stick to using my pedal for everything.

Shanx, I'd be interested to see that - cheers.

Chris
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Old 12th October 2011, 02:20 PM   #9
shanx is offline shanx  Canada
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Here is the circuit (not mine, so again just to illustrate). Showing a 2N2804 and 2N2222 in the FB loop and paralleled with a 500 K pot for sensitivity. This in theory would give a logarithmic curved output. Would be an interesting circuit to play around with, as the transfer curve should give a smoother clip.
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Old 12th October 2011, 02:22 PM   #10
shanx is offline shanx  Canada
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I don't think the attachment diagram uploaded properly..will try and do that again
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