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October 19, 2014, 10:24:45 PM
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DIYstompboxes.com  |  DIY Stompboxes  |  Building your own stompbox  |  Diodes to Ground/VREF from Feedback Loop 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Diodes to Ground/VREF from Feedback Loop  (Read 2015 times)
WGTP
Posts: 2458


Diodes to Ground/VREF from Feedback Loop
« on: November 18, 2004, 10:39:15 PM »

Has anyone else tried running the diodes to ground from the feedback loop instead of parallel to the resistor or to ground after the loop?  I don't recall seeing this anywhere.  

It seems to work, but I haven't had time for a really good listen yet.

I'm wondering if the clipping is harder than the Tube Screamer, but softer than the Dist + :?:  :twisted:  Cool
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Nasse
Posts: 2325

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Diodes to Ground/VREF from Feedback Loop
« Reply #1 on: November 19, 2004, 12:46:50 AM »

I have once done it by mistake. The results were somewhat "interesting" but I never bothered to solder the parts in place, just tried different diodes in a feedback loop.
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petemoore
Posts: 18794


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..
« Reply #2 on: November 19, 2004, 12:56:48 AM »

Would not the diode to ground just take over the clipping on that side?
 It would be interesting to see what two diodes clipping the same signal at different thresholds on either side of the wave would be like....Excuse If I'm excluding an existing circuit.
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Nasse
Posts: 2325

Eagle eats a piece by piece


Diodes to Ground/VREF from Feedback Loop
« Reply #3 on: November 19, 2004, 01:34:20 AM »

Sorry I am not awake yet good morning anyway my memory is going bad. I believe what I did was some resistor-cap combination to ground  from feedback diodes and inverting input junction, no diodes to ground.
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Steben
Posts: 1491


Steven Burvenich


Re: Diodes to Ground from Feedback Loop
« Reply #4 on: November 19, 2004, 02:48:34 AM »

Quote from: WGTP
Has anyone else tried running the diodes to ground from the feedback loop instead of parallel to the resistor or to ground after the loop?  I don't recall seeing this anywhere.  

It seems to work, but I haven't had time for a really good listen yet.

I'm wondering if the clipping is harder than the Tube Screamer, but softer than the Dist + :?:  :twisted:  Cool


I bet the sound of a TS9 - besides the pregain lowcut - is very much determined by the feedback loop placing, like the Dist+ sound is by the the diode grounding. With feedback loop (non-inverting) the clipping results in a gain compression (unity gain beyond a voltage), while in the dist+ the clipping is really a hard cut to ground (zero gain beyond a voltage).

This means you will rarely find an in-between-sound. Maybe the very filtered TS will more sound like a RAT with diodes to ground than a Dist+.
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Paul Perry (Frostwave)
Posts: 7470

Paul P.


Diodes to Ground/VREF from Feedback Loop
« Reply #5 on: November 19, 2004, 03:29:00 AM »

What would possible be interesting might be to split the feedback resistor into two in series, and run a diode from the junction of the two to earth. The effect would be less feeback on one half of the cycle (at peaks). So a slightly different harmonic mix, maybe.
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Nasse
Posts: 2325

Eagle eats a piece by piece


Diodes to Ground/VREF from Feedback Loop
« Reply #6 on: November 19, 2004, 06:03:47 AM »

Quote
The effect would be less feeback on one half of the cycle (at peaks)


Yep. Something like this happened to me, and trough a cap so it was freq dependent, filtery. Unfortunately I was not able to study or document that thing further, my life was in a suitcase then. But the feedback was sucked away when the diodes started to conduct more and more, so I believe.
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petemoore
Posts: 18794


As Yet Unrated


...
« Reply #7 on: November 19, 2004, 08:26:33 AM »

Another speculative approach...two seriesed diodes for clipping the + or - cycle somewhere in a circuit, one having a small rolloff cap across it.
  These two would be part of a clipping arrangement...
  --->l--->l----
   ---<l---<l---
     --ll--
  The left bottom diode and the cap would be seriesed.
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WGTP
Posts: 2458


Diodes to Ground/VREF from Feedback Loop
« Reply #8 on: November 19, 2004, 09:35:28 AM »

I realized staring at my cereal bowl that it's not going to ground, but to VREF.  Anyway, my thinking was that by taking the diodes to VREF from the feedback loop (the output side) would have the harder clipping of the Dist+/Rat, but it would be "recirculating" thru the negative feedback loop like the Tube Screamer/SD-1.

It sounds similiar to the standard options and I'm using LED's so I can see how much clipping is going on (I also like the pretty lights)

That didn't last long until I started mixing this "alternative" option with the 2 standard options.  

I may not have seen it anywhere before because it sounds just like one of the standard options.

I have tied the cap in series with one diode deal, but not much  I will have to try the diodes between 2 series resistors.

Actually the Blues Breaker Mutant I'm working with sounds pretty good with NO diodes.  I recommend that be tried as a BB mod.   :twisted:  Cool
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R.G.
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Diodes to Ground/VREF from Feedback Loop
« Reply #9 on: November 19, 2004, 10:53:40 AM »

Let's think for a minute.

It helps to think of diodes as voltage variable resistors. Try drawing the circuit with the diodes replaced with 1-ohm resistors with a switch in series. When the voltage across the diode is greater than the diode drop, the switch closes and it looks like a 1 ohm resistor. Then evaluate what the opamp does each way.

I can't quite tell from your description whether you're using a back to back clipper to ground from the output end of the feedback loop or the inverting input end of the feedback loop.

Diodes to ground from the output end can be connected up either directly to ground or through a capacitor. In a single supply setup, directly to ground clamps the output of the opamp to one diode above ground, which may be hard on the opamp if it's being fed a reference voltage of 4.5V. It sits in current limit until the signal pulls it below the diode drop.

If you're using bipolar supplies, or tie the diodes to Vref or through a capacitor, then the diodes act as a hard current limit on the output. The voltage can't go above the diode's voltage drop at whatever current limit the opamp has. So the opamp is running dead linear until you trigger the diode switch on, and then the diode can eat as much current as the opamp can pour out.

This is only varied a bit by the fact that a resistor/resistor/capacitor Vref or a capacitor to ground is not a perfect short. These setups will let the capacitors charge up and unclamp the thing at some point. Diodes to Vref may cause low frequency motorboating because they give you mixed positive and negative feedback, and in some cases could cause oscillation. Not often, but could.

Diodes from the inverting input to Vref will have different effect depending on whether you drive signal into the noninverting input or the inverting input. If you use the inverting input for signal in, the diodes have no effect whatsoever. The inverting input in this case sits at virtual ground, moves no more than millivolts, and never turns the diodes on.

If you drive it from the noninverting input, the diodes do their one-ohm dance because signal on the non-inverting input moves the virtual ground around, and will eventually cause a diode to switch over to low resistance.
When that happens, feedback is made inoperative and the opamp blams against the power supply in whichever direction the signal is pushing it, as it cannot supply enough current through the feedback resistor to balance.
So signals under a diode drop are normal, and peaks over a diode drop at the + input will cause a sudden wham to the power supply.

Anyway, use the switched resistor model to think about what happens.

And *do* think. Good practice.
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R.G.

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WGTP
Posts: 2458


Diodes to Ground/VREF from Feedback Loop
« Reply #10 on: November 19, 2004, 12:05:28 PM »

Thanks for the explaination, as always.    Cheesy
If I wanted to think, I wouldn't have been a guitar player : :lol:
I'll have to work on this awhile.   Cool

Whew, I think my forhead is sweating.

I was experimenting with diodes to ground/VREF from both input and output side of the opamp and using an inverting (Blues Breaker) stage driven by a non-inverting stage.

I can't tell from your explaination which methods are creating "soft" diodes in the loop vs. "hard" diodes to ground type distortion.  I think there are some hints, my understanding of the process is sort of like my understanding of women (no offense).

I have speculated that the diodes in the loop produce "soft" clipping as a result of the clipping occuring around or in parallel with the resistor and being recirculated with fresh unclipped signal.  "Hard" clipping occurs to the signal as it gets "wacked" on the way to the output.  Is that anywhere close??? :?:
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