Author Topic: Can Feedback loop soft clipping be done with a discrete transistor stage?  (Read 16983 times)

midwayfair

I was wondering if it's possible to do soft clipping like a pair of diodes in the feedback loop of an op amp, but with a transistor stage. Putting them in the positive feedback loop (base-collector) is still just hard clipping.

I was positing that perhaps it could be done with a Fuzz Face style arrangement, where the 100K between the base of Q1 and emitter of Q2 is a pair of diodes. Some other biasing would need to be worked out to turn on Q1.

I know, I know, just use op amps. I can't help it, I like transistors. :P
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!

tca

Yes:



http://www.muzique.com/schem/projects.htm

Just put two diodes in the feedback loop.

Cheers.
« Last Edit: March 03, 2014, 02:14:00 PM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson

midwayfair

I'm pretty sure I've read that that's hard clipping, as I mentioned in my first paragraph.
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!

tca

^ It looks soft and sounds like soft :)



P.S.
New image.
« Last Edit: March 03, 2014, 11:52:35 AM by tca »
"The future is here, it's just not evenly distributed yet." -- William Gibson

midwayfair

huh. okay. Problem solved, then! :) Thanks!
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!

thelonious

I wonder if the series resistor trick would soften it even more. (I've never been quite clear on whether that only works in hard clipping situations, or if in a feedback loop it would still soften the transition itself even though it's limiting overall feedback.)
« Last Edit: March 03, 2014, 12:17:39 PM by thelonious »

tca

Edit picture above. The lower plot shows what happens when R6 varies from 1k to 100k.

Cheers.
"The future is here, it's just not evenly distributed yet." -- William Gibson

midwayfair

Edit picture above. The lower plot shows what happens when R6 varies from 1k to 100k.

Cheers.

That's interesting. Why does that happen (becomes more asymmetric)?
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!

tca

That is because the way the transistor is biased. Take a look. When you increase the value of the resistor, the diodes contribute less and less to the feedback.

Look at the next picture (no diodes).



Cheers.
"The future is here, it's just not evenly distributed yet." -- William Gibson

DougH

You can do it with the fuzz face feedback loop too.
"I can explain it to you, but I can't understand it for you."

Lurco

I was wondering if it's possible to do soft clipping like a pair of diodes in the feedback loop of an op amp, but with a transistor stage. Putting them in the positive feedback loop (base-collector) is still just hard clipping.

I was positing that perhaps it could be done with a Fuzz Face style arrangement, where the 100K between the base of Q1 and emitter of Q2 is a pair of diodes. Some other biasing would need to be worked out to turn on Q1.

I know, I know, just use op amps. I can't help it, I like transistors. :P

C-to-B is NEGATIVE feedback.

Gus

Have you seen this thread?
http://www.diystompboxes.com/smfforum/index.php?topic=104029.20
reply 29, look at the 2nd gain stage

midwayfair

Have you seen this thread?
http://www.diystompboxes.com/smfforum/index.php?topic=104029.20
reply 29, look at the 2nd gain stage

Thanks, I did see that one.

Smallbear's Ursa overdrive uses the same feedback loop arrangement; I even used it in my Mossy Sloth fuzz (with a MOSFET) -- I just didn't realize it was softclipping.
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!

R.G.

... I just didn't realize it was softclipping.
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R.G.

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pappasmurfsharem

Not if you called ‘em stench blossoms.

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Your name makes this post even funnier
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Gus

Have you seen this thread?
http://www.diystompboxes.com/smfforum/index.php?topic=104029.20
reply 29, look at the 2nd gain stage

Thanks, I did see that one.

Smallbear's Ursa overdrive uses the same feedback loop arrangement; I even used it in my Mossy Sloth fuzz (with a MOSFET) -- I just didn't realize it was softclipping.

Not really, look closer at the ASDF and tca posts note the added series resistor and how the waveforms change
Also the cap that is in series with the diodes value matters and the cap value from C to B(if used)

The BMP uses this clipping with out the added series resistor
The bluesbreaker has series resistance with the diode clipping
US patent 3,223,936  has diodes from C to B.

I believe I understand the point R.G. is making with the post, however the adding of a series resistor to the diodes makes a difference

merlinb

Apparently the ubiquitous LM317 will behave as a very soft diode if the OUT and ADJ pins are shorted. I wonder what it would sound like in a clipping stage?
http://www.edn.com/design/analog/4422122/Power-Zener-using-the-LM317

samhay

Apparently the ubiquitous LM317 will behave as a very soft diode if the OUT and ADJ pins are shorted. I wonder what it would sound like in a clipping stage?
http://www.edn.com/design/analog/4422122/Power-Zener-using-the-LM317

I was playing with an LM317 as an amplifier a couple of weeks ago. Not sure if I tried that though. Perhaps we should start a new thread.
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midwayfair

Apparently the ubiquitous LM317 will behave as a very soft diode if the OUT and ADJ pins are shorted. I wonder what it would sound like in a clipping stage?
http://www.edn.com/design/analog/4422122/Power-Zener-using-the-LM317

I was playing with an LM317 as an amplifier a couple of weeks ago. Not sure if I tried that though. Perhaps we should start a new thread.

Or revive the "pleasant distortion without clipping" thread, which had a bunch of soft clipping discussion. http://www.diystompboxes.com/smfforum/index.php?topic=103129.0

Fig. 3 here looks intriguing, but I don't know if it was exactly what I had in mind when I said "soft clipping using a transistor" ... http://diale.org/img/soft_clippling.png
My band, Midway Fair: www.midwayfair.org. Myself's music and things I make: www.jonpattonmusic.com. DIY pedal demos: www.youtube.com/jonspatton. PCBs of my Bearhug Compressor and Cardinal Harmonic Tremolo are available from http://www.1776Effects.com!