ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: R.G. >RG, sorry but I think you are sort... -- 4/8/1998 7:54 AM

Re: Soft clipping, CMOS and the Muff

4/8/1998 12:26 PM
GFR	Re: Soft clipping, CMOS and the Muff Yes, you sure has something new (or at least out of ordinary) going on, because the standart procedure in designing fuzz boxes is to vary the resistance in the FB loop to achieve different degrees of distortion by "linearizing" more or less the transfer function. This approach of varying the input resistor can lead to some interesting sounds. Also, besides having a natural soft compressor built-in, CMOS have less open loop gain than most opamps, which can also help to "soften" things. And since we will deal with low level signals (to avoid saturating the diodes) the absence of crossover distortion is also an advantage over some opamps. Besides "a) limiting the input voltage excursion and (b) setting the input resistor to pick the limits of excursion on the feedback diode V-I curve to get the amount and kind of clipping you want.", 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. Also you can put diodes, bipolar transistors, FETS, CMOS gates, OTAs, even tubes (like in the Butler tube DI) in the feedback loop to achieve different sounds. I once put a PWM modulator in the loop of an opamp to make a compressor. Since we're looking for flexible designs, there's also the "wave shaping" amps used in signal generators to convert triangle waves to sines. Basically you have various diode chains in parallel with resistors at the feedback loop of an opamp so that the actual gain is determined by which diode chains are conducting. This approximates a "S" shaped transfer function by small straight lines (linear by parts). With enough diodes and resistors, you can approximate any shape you want, even assymetric, and by using pots instead of fixed resistors... Well, it would be very flexible, but very hard to tune. Or you could have a rotary switch that would chose preset resistors and set emulations of different devices. GFR PS RG, I don't have any doubt about your familiarity with electronics, in fact I keep learning new things with every post of yours. 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 and think they can benefit a little from this, even if the reply is to someone with enough knowledge to find these things basic. So please excuse me if sometimes I'm to boring or sound like I'm teaching the priest how to pray.

4/8/1998 12:26 PM

GFR

Re: Soft clipping, CMOS and the Muff

Yes, you sure has something new (or at least out of ordinary) going on, because the standart procedure in designing fuzz boxes is to vary the resistance in the FB loop to achieve different degrees of distortion by "linearizing" more or less the transfer function. This approach of varying the input resistor can lead to some interesting sounds. Also, besides having a natural soft compressor built-in, CMOS have less open loop gain than most opamps, which can also help to "soften" things. And since we will deal with low level signals (to avoid saturating the diodes) the absence of crossover distortion is also an advantage over some opamps.

Besides
"a) limiting the input voltage excursion and (b) setting the input resistor to pick the limits of excursion on the feedback diode V-I curve to get the amount and kind of clipping you want.",
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.

Also you can put diodes, bipolar transistors, FETS, CMOS gates, OTAs, even tubes (like in the Butler tube DI) in the feedback loop to achieve different sounds. I once put a PWM modulator in the loop of an opamp to make a compressor.

Since we're looking for flexible designs, there's also the "wave shaping" amps used in signal generators to convert triangle waves to sines. Basically you have various diode chains in parallel with resistors at the feedback loop of an opamp so that the actual gain is determined by which diode chains are conducting. This approximates a "S" shaped transfer function by small straight lines (linear by parts). With enough diodes and resistors, you can approximate any shape you want, even assymetric, and by using pots instead of fixed resistors... Well, it would be very flexible, but very hard to tune. Or you could have a rotary switch that would chose preset resistors and set emulations of different devices.

GFR

PS RG, I don't have any doubt about your familiarity with electronics, in fact I keep learning new things with every post of yours. 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 and think they can benefit a little from this, even if the reply is to someone with enough knowledge to find these things basic. So please excuse me if sometimes I'm to boring or sound like I'm teaching the priest how to pray.

Replies:

Jack Orman Speaking of waveshapers for generat... -- 4/8/1998 8:22 PM
MHelin This approa... -- 4/9/1998 2:05 AM
R.G. First of all, I do apologize if I s... -- 4/9/1998 2:10 PM