Cure for nasal fuzz?

[Steve Conner]

The idea is that it gives you more options per money/time spent on wiring up switches. Each switch selects nothing, LED, MOSFET, silicon diode, germanium diode, etc. If you set the two switches the same, you get symmetric clipping, set them different and you get asymmetric.

[R.G.]

I don't see the distinction between placing the clipper on the output and placing it on the feedback network. To me they do just about the same thing.

There is a slight difference. Diodes on the feedback path are effectively driven by a constant current source. This is a bit different from them being driven by a fixed resistance as they are on an output.

@Ashleyslidewhip:
If you want to experiment with distortions, I suggest that you find a stereo graphic equalizer, the kind with 10-30 sliders. Put one channel in front of the distortion, one after, and spend an afternoon - or a weekend - diddling the sliders. The ones in front of the pedal affect what frequencies get clipped most, the ones afterwards affect what frequencies are emphasized or cut after clipping generates harmonics. Be prepared to spend a lot of time finding neat sounding settings.

[Ashleyslidewhip]

RG - thanks - that's partially why I thought there would be a difference between clipping on the output and clipping on the feedback - as the feedback goes "back into the input" and get's processed again, when it eventually gets out it's been messed up more than just clipping the output (although I have no idea if my process engineering analogy works at all for electronics!)

[JoeM]

Joe,
I tried several diode arrays, and I ended up using 3 x 1N4148s in an "asymmetric/symmetric" clipping configuration (the third diode can be shorted by means of a switch, I used a DPDT, the second section of the switch changes the color of the "FX on" LED, clearly indicating which kind of clipping is currently being applied to the output signal).

Germanium diodes sound too fuzzy in a TS, at least to me, and tend to take away some of the original TS "roundness", but they're excellent if you plan to build an MXR D+ based distortion pedal; thanks to their very low forward voltage, they square the signal very heavily, and give a very nasty, fuzzy distorted sound (after all, a square wave, as good ol' Mr. Fourier tells us, is made only of "nasty" odd harmonics).

Hope this helps

Best regards

Bob

Thanks, Bob. Appreciate the information.

[R.G.]

RG - thanks - that's partially why I thought there would be a difference between clipping on the output and clipping on the feedback - as the feedback goes "back into the input" and get's processed again, when it eventually gets out it's been messed up more than just clipping the output (although I have no idea if my process engineering analogy works at all for electronics!)

It doesn't. The output is not "processed again" in this setup. What the feedback connection does is cause a kind of balance between the output and what's going into the inputs, as opposed to running packets through the amp again.

Actually, running diodes in the feedback loop can cause less clipping and "messing up" than a resistor limited output, depending on a large number of other things, of course.

[Ashleyslidewhip]

Ah - so my mental model is wrong, but hey, I'm learning. Think I'll definitely try it then, as it seems the potential for feedback clipping and output clipping to be different is definitely there. I've spent hours looking at schematics wondering why they were designed the way they were, and a decision like where to put the clippers seems pretty fundamental to me

Thanks for all the suggestions and help - for a first post to the forum I've flushed out some things I'd never have thought of

Great stuff

[loudthud]

I spent some time driving my pedal with a sinewave generator and looking at the output with a scope. It does put out a non-symetrical wave, 60% high and 40% low, but there is a really long time constant on the bias shift. I'm planning to build a prototype to experiment on and leave the exsisting pedal as-is for comparison.

I've go a 50W Marshall (4 input 5F6A type circuit) that puts out a non-symetrical wave, 40% high and 60% low. If I drive it with my pedal will I get a 50/50 wave? What is the effect if you overdrive the first tube stage but attenuate at the volume control and send a small signal to the power amp that won't drive it to clipping? Seems like there will be some interaction there. My prototype is going to have a phase reverse switch to see if that makes a difference.

[loudthud]

It just occured to me that diodes in the feedback loop are going to act differently depending on whether the op-amp configuration is inverting or non-inverting. In the inverting config (Av=Rf/Ri) the diodes just clip the input. In the non-inverting config (Av=1+Rf/Ri)you get the clipped signal riding on top of the input signal. You can emulate the non-inverting wave by inserting a resistor in series with the back to back diodes.

[R.G.]

It just occured to me that diodes in the feedback loop are going to act differently depending on whether the op-amp configuration is inverting or non-inverting. In the inverting config (Av=Rf/Ri) the diodes just clip the input. In the non-inverting config (Av=1+Rf/Ri)you get the clipped signal riding on top of the input signal. You can emulate the non-inverting wave by inserting a resistor in series with the back to back diodes.

Oh, yeah!

There are other things you can do too. If you have an output-clipper, with diodes to ground, try inserting a 10K pot hooked up as a variable resistor between the diodes and their ground connection. Play with that for a while... 8-)

[jmaf]

Wow a rare major league discussion in here. Popcorn in hand!

PS. I felt proud now because the only thing I thought I could contribute to this while reading through was the suggestion R.G gave on his last message ^^^ - because I've used a 12AX7 for a diode clipper and you need to elevate it so it doesn't ground all the signal, so I thought I'd suggest the inverted germanium diode pair elevated by a pot so you can dial the exact amount of clipping you want(Vpk - 0.3V - Vpot for germanium). Since RG suggested that, I'm proud of my insight Also, the shunt capacitor with the clipping diodes should help round the corners, which is part of what RG teaches in his "Technology of ..." TS articles (IIRC)...

[Ashleyslidewhip]

I was thinking a "Technology of the Distortion+ / DS1 / etc etc" might be useful?

[Robert M. Martinelli]

The technology of the Distortion + is very simple, it's only a single op-amp (a 741) powered by a single 9V supply (and a voltage divider to supply Vcc/2 to the non-inverting input) and a voltage gain of 100 (40dB) plus a couple of Germanium diodes to heavily clip the output signal. IIRC the "distortion" pot has an RC network in series to change the (bass) frequency response of the circuit as the pot is turned up.

With a gain of 100, however, the signal gets squared even before reaching the diodes because its amplitude "hits" the power supply rails even with a low-output pickup.

IIRC the D+ is nothing more than an MXR micro-amp plus a clipping output section.

Hope this helps

Best regards

Bob

[Ashleyslidewhip]

That's great - thanks. Assume the Ross and DOD are the same from looking at the schematics, which are virtually identical. I have a BOSS DS2 with it's "turbo" function but have never used a DS1 which I assume is similar

Ash