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Vintage threads from the first ten years

Re: Soft clipping, CMOS and the Muff

4/8/1998 8:22 PM
Jack Orman	Re: Soft clipping, CMOS and the Muff Speaking of waveshapers for generating distortion... I breadboarded a version of the FET sine wave shaper (the one with two diodes across the gate) and tested it for distortion characteristics. I found a variety of mods that make it more useful than the typical circuit that is always shown, but even so, it isn't that great. The biggest problem is that you have to hit it with a very strong signal to get it to begin rounding the signal over. It is much simpler and more easily controlled to simply make an FET gain stage and drive it into the non-linear region. I'm sure that you're aware that this is somewhat similar to the technique that I used in the FET-MUFF design. Fet common source amps don't have a lot of gain, and by putting a diode in the feedback loop and limiting the input signal with a couple of resistors, you get a soft knee clip. i don't really care for MOSFETs for distortion because they are enhancement mode devices (the most common ones anyway) while JFETs work in depletion mode similar to the way vacuum tubes operate. I have some depletion mode MOSFETs that I'm working with. regards, Jack

4/8/1998 8:22 PM

Jack Orman

Re: Soft clipping, CMOS and the Muff

Speaking of waveshapers for generating distortion... I breadboarded a version of the FET sine wave shaper (the one with two diodes across the gate) and tested it for distortion characteristics. I found a variety of mods that make it more useful than the typical circuit that is always shown, but even so, it isn't that great. The biggest problem is that you have to hit it with a very strong signal to get it to begin rounding the signal over. It is much simpler and more easily controlled to simply make an FET gain stage and drive it into the non-linear region.

I'm sure that you're aware that this is somewhat similar to the technique that I used in the FET-MUFF design. Fet common source amps don't have a lot of gain, and by putting a diode in the feedback loop and limiting the input signal with a couple of resistors, you get a soft knee clip.

i don't really care for MOSFETs for distortion because they are enhancement mode devices (the most common ones anyway) while JFETs work in depletion mode similar to the way vacuum tubes operate. I have some depletion mode MOSFETs that I'm working with.

regards, Jack

4/8/1998 9:34 PM
Hi	I, for, one am most emphatically NOT an electronics wiz like a lot of ya'll, so I'm happy to see things explained. And I am also very much in favor of redisigning this particular wheel. I'm not happy with modern distortion devices, but I was never that happy with the old ones. The devices Hendrix used were obviously fiddled with by Roger Mayers, 'cause even the ones he designed were obviously very different than the commercially available models. And of course David Gilmour uses a Big Muff, but "tweaked" by Peter Cornish to get rid of the noise. I'm all for anybody who wants to build distortion boxes that don't have to be reworked after you've already paid (too much for what you get, usually) for the thing. Of course old ones had "better" tone in general, but even then I was always dissatisfied by the buzzy sound and the signal breaking up on long sustains, eyc. Reinvent that wheel! And more power to you. Hi

4/8/1998 9:34 PM

I, for, one am most emphatically NOT an electronics wiz like a lot of ya'll, so I'm happy to see things explained. And I am also very much in favor of redisigning this particular wheel. I'm not happy with modern distortion devices, but I was never that happy with the old ones. The devices Hendrix used were obviously fiddled with by Roger Mayers, 'cause even the ones he designed were obviously very different than the commercially available models. And of course David Gilmour uses a Big Muff, but "tweaked" by Peter Cornish to get rid of the noise. I'm all for anybody who wants to build distortion boxes that don't have to be reworked after you've already paid (too much for what you get, usually) for the thing. Of course old ones had "better" tone in general, but even then I was always dissatisfied by the buzzy sound and the signal breaking up on long sustains, eyc. Reinvent that wheel! And more power to you.
Hi

4/9/1998 2:05 AM
MHelin	This approach of varying the input resistor can lead to some interesting sounds. Doesn't increasing the input resistor add noise, too? Are there other factors that could be modified, how about biasing the diodes in FB loop. If you put the diode(s) between two caps you could change the DC voltages at both ends. Maybe you could take the bias voltage from envelope follower so that increasing level would keep the diode just below conducting, or you could use there a differential amplifier to keep the voltage across the diode below Vf (don't know). Though it would be simpler to get a DSP kit with A/D and D/A and start thinking numbers. I would sugggest (c) setting a DC bias in the input voltage so that you can chose the operating point on the feedback diode V-I curve. I have simulated some DC bias & diode clip circuits, and they seem to produce nice distortion when used together. The DC bias can be generated by the envelope of the signal, it can be inverted, and it's attack/decay times changed by changing cap/resistor values. I'm going to build a prototype which combines DC bias shift with soft, symmetric JFET clipping. I think even Tube Screamer circuit could be given a new life with the addition of dynamic DC bias shift. I just tend to go a little basic (excuse me, obvious) in my replies in such things as feedback amps, gyrators, boards layout, wave shaping, etc. cause I note that a lot of AMPAGErs don't have formal education in electronics. That's good, as I'm one who doesn't have any formal education in electronic... -Mikko

4/9/1998 2:05 AM

MHelin

This approach of varying the input resistor can lead to some interesting sounds.

Doesn't increasing the input resistor add noise, too? Are there other factors that could be modified, how about biasing the diodes in FB loop. If you put the diode(s) between two caps you could change the DC voltages at both ends. Maybe you could take the bias voltage from envelope follower so that increasing level would keep the diode just below conducting, or you could use there a differential amplifier to keep the voltage across the diode below Vf (don't know).
Though it would be simpler to get a DSP kit with A/D and D/A and start thinking numbers.

I would sugggest
(c) setting a DC bias in the input voltage so that you can chose the operating point on the feedback diode V-I curve.

I have simulated some DC bias & diode clip circuits, and they seem to produce nice distortion when used together. The DC bias can be generated by the envelope of the signal, it can be inverted, and it's attack/decay times changed by changing cap/resistor values. I'm going to build a prototype which combines DC bias shift with soft, symmetric JFET clipping. I think even Tube Screamer circuit could be given a new life with the addition of dynamic DC bias shift.

I just tend to go a little basic (excuse me, obvious) in my replies in such things as feedback amps, gyrators, boards layout, wave shaping, etc. cause I note that a lot of AMPAGErs don't have formal education in electronics.

That's good, as I'm one who doesn't have any formal education in electronic...

-Mikko

4/9/1998 7:42 AM
Jack Orman	I'm going to build a prototype which combines DC bias shift with soft, symmetric JFET clipping. I think even Tube Screamer circuit could be given a new life with the addition of dynamic DC bias shift. Mikko, I did this while working on a revision of the Fet-Muff. I used diodes to set a bias point and a switch to select the different points. It worked okay, but I was able to get sounds just as useful by varying the input resistor or drive voltage. I also did an AC bypass around the diodes to see how that changed the sound - the answer is, not much. Keep us filled in on your dc bias experiments. regards, Jack

4/9/1998 7:42 AM

Jack Orman

I'm going to build a prototype which combines
DC bias shift with soft, symmetric JFET
clipping. I think even Tube Screamer circuit
could be given a new life with the addition of
dynamic DC bias shift.

Mikko,

I did this while working on a revision of the Fet-Muff. I used diodes to set a bias point and a switch to select the different points. It worked okay, but I was able to get sounds just as useful by varying the input resistor or drive voltage. I also did an AC bypass around the diodes to see how that changed the sound - the answer is, not much.

Keep us filled in on your dc bias experiments.

regards, Jack

4/16/1998 3:51 AM

MHelin

Re: DC bias experiments

quote:
"Keep us filled in on your dc bias experiments."

In my first experiment I used a diode from a cap to ground through a resistor (100k pot) to charge a capacitor to get the DC bias. The cap was between two opamp (TL072) stages, input (non-inverting) inpedance of the second stage is now 10k, the 470k I tried muted (locked) the circuit altogether (the cap had no way to discharge). I have to yet try change the cap bigger as well as that 10k input resistor to 47k-100k (or pot). The sound was different than with unbiased clipper, but I got to sample the sound yet to look at it on the PC (CoolEdit) to see how it works. It seems to work, as it produced big second harmonic when the sound started decaying (as the cap was charged). It would be better to try conventional envelope follower circuit for DC bias generation, as it's easier to control, and it could be used for other purposes like for an automatic wah or for any other voltage controlled waveshaper.

I used a 20k resistor from the second opamp stage (no cap as it should be a DC circuit) to back-to-back LEDs (red) to ground for clipping, this produced also little bit softer clipping than 3k3 I also tried (100k was too much).

-Mikko

4/13/1998 6:38 AM
GFR	Re: Soft clipping, CMOS and the Muff Doesn't increasing the input resistor add noise, too? Sure, the input resistor will have great influence on the noise level. One solution to this would be including a carefully designed impedance matching stage before the waveform altering (let's not say "clip") stage. Or you can just hear it and decide if the noise level is acceptable. Though it would be simpler to get a DSP kit with A/D and D/A and start thinking numbers. That's the "digital distortion" found at many rack effects. They are the living proof that it is not only the VxI curve that matters for a good sound, but also the dynamics, since they can emulate any transfer curve with perfection but are not regarded as the best fuzzes ever. In a real amp or vintage fuzz, things charge/discharge, heat, etc. so the actual behave of the device changes with the way you play. That's what lets the tone be on your fingers. Newer modeling technology is trying to take these factors in account. The DC bias can be generated by the envelope of the signal The "bias shift" effect happens to a litlle extent in the FF. As you play, the second transistor works like a rectifier that charges the 22uF capacitor, shifting a little the bias of the first transistor, so distortion characteristics are a little different on the attack and sustain parts of the notes. And that's a way to make things touch sensitive too. GFR

4/13/1998 6:38 AM

GFR

Re: Soft clipping, CMOS and the Muff

Doesn't increasing the input resistor add noise, too?
Sure, the input resistor will have great influence on the noise level. One solution to this would be including a carefully designed impedance matching stage before the waveform altering (let's not say "clip") stage. Or you can just hear it and decide if the noise level is acceptable.

Though it would be simpler to get a DSP kit with A/D and D/A and start thinking numbers.
That's the "digital distortion" found at many rack effects. They are the living proof that it is not only the VxI curve that matters for a good sound, but also the dynamics, since they can emulate any transfer curve with perfection but are not regarded as the best fuzzes ever. In a real amp or vintage fuzz, things charge/discharge, heat, etc. so the actual behave of the device changes with the way you play. That's what lets the tone be on your fingers. Newer modeling technology is trying to take these factors in account.

The DC bias can be generated by the envelope of the signal
The "bias shift" effect happens to a litlle extent in the FF. As you play, the second transistor works like a rectifier that charges the 22uF capacitor, shifting a little the bias of the first transistor, so distortion characteristics are a little different on the attack and sustain parts of the notes. And that's a way to make things touch sensitive too.

GFR

4/9/1998 2:10 PM
R.G.	First of all, I do apologize if I sounded a bit gruff - I still hadn't had my coffee. The use of the CMOS inverter does have the advantages you note ( I count low open loop gain as an advantage) while still not using any input current, but even bipolar transistors can profit from this, witness the Big Muff itself. Setting a DC bias hasn't proven necessary in the simulation at least. A simple back to back pair around the inverter works pretty well.

4/9/1998 2:10 PM

R.G.

First of all, I do apologize if I sounded a bit gruff - I still hadn't had my coffee.

The use of the CMOS inverter does have the advantages you note ( I count low open loop gain as an advantage) while still not using any input current, but even bipolar transistors can profit from this, witness the Big Muff itself.

Setting a DC bias hasn't proven necessary in the simulation at least. A simple back to back pair around the inverter works pretty well.

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