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Old 28th May 2007, 11:58 PM   #51
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Conrad: " ... The real problem was that the caps are pretty tall, and the PCBs are quite a ways away ... [I established] a new star point right between the PCBs, near the amp inputs. Now the local bypassing works properly and the thing is stable as a stone. ..."

Effectively balancing distance and ground path impedence between the PCBs ... You seem to be well in advance of my previous rant(s): " ... Distance and timing (speed of electron flow and signal), conductor capacitance and resistance variations ... all resulting in an added contribution to resistance, inductance and capacitance variations ...", your clear thinking, bench work and intuition making up for and being better than "book learnin'".

" ... a 1K resistor in the [cap] base lead ... the same thing with op-amps, where a cap to ground on the input will give rise to intractable oscillation. ..." I believe that I have also seen this in some of the older op-amp cookbooks ... the theory being that there was "reflection" or "a bounce back" from certain types / sizes of caps, re-enforcing feedback ... or some such (I might be wrong here).

(Speaking of books, you might take a peek at "The Black Swan: The Impact of the Highly Improbable" by Nassim Nicholas Taleb, new at Amazon, et al ... definately good reading for DIY types.)
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Old 29th May 2007, 03:05 AM   #52
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Resistor & Cap on input of amp to keep from oscilating: http://www.national.com/images/pf/LM3886/01183301.pdf ... no?
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Old 29th May 2007, 06:10 AM   #53
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Yup, seen it before, but the big question for me is, "why?" Why is what makes the world go 'round. It must reduce some kind of positive feedback that causes the problem, but I don't really understand the path.
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Old 29th May 2007, 02:36 PM   #54
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Well, near as I can tell, there is an "RC time constant" relationship between resistor and cap at the (+) input ... the resistor / cap acting as a band pass or low pass filter to the positive feedback loop = allowing only the frequency band from ~0 Htz up to the intended upper limit of the amp or op-amp before out of control oscillations can set in ... and the resistor / cap does take care of that pesky "bounce" or reflection off of the cap, "squaring up" the leading edge of the signal ... If you have a 'scope you might be able to see this comparing a square wave signal trace with & without the resistor ...
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Old 29th May 2007, 05:18 PM   #55
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Quote:
Originally posted by FastEddy
Resistor & Cap on input of amp to keep from oscilating: http://www.national.com/images/pf/LM3886/01183301.pdf ... no?

???

That CR is a classic method of keeping the DC loop gain at unity, while maintaining an AC gain of 20 (Rf/Ri) above the Fc of the cap.

Nothing to do with oscillations or anything wierd.

Or are you looking at a different schematic?
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Old 29th May 2007, 06:05 PM   #56
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cliff: " ... That CR is a classic method of keeping the DC loop gain at unity ... Nothing to do with oscillations ..."

mmm ... Yes, approximately correct. (I'm wrong again ) My hydraulic model suggests that the resistor (1K Ri) "pinches off" the flow of electrons into and out of the cap (22 uF Ci) = acting as a high pass filter at the (-) negative feedback input (rather than a low pass type at the (+) input as stated). That would, I believe, do approximately the same (required) thing = allow the higher frequencies into the (-) negative input (pin 9) and thus, via the (-) negative feedback, keep the amp from oscillating ...

I guess I should breadboard these thing up myself before shooting from the hip ... or at least pay closer attention ... other than that, I had a great weekend ... You?
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Old 29th May 2007, 06:47 PM   #57
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"My hydraulic model suggests that the resistor (1K Ri) "pinches off" the flow of electrons into and out of the cap (22 uF Ci) = acting as a high pass filter at the (-) negative feedback input (rather than a low pass type at the (+) input as stated). That would, I believe, do approximately the same (required) thing = allow the higher frequencies into the (-) negative input (pin 9) and thus, via the (-) negative feedback, keep the amp from oscillating ..."

You are on a hiding to nowhere trying to analyse an audio amp feedback circuit using a "hydraulic" model of current flow.

There is no direct current in Ri because of the series cap.

Only using the frequency domain model makes sense:

1) Dc gain is 1
2) At high frequencies XCi is very low so gain is 20
3) As XCi gets to 1K at low frequencies the gain will be 3dB down on 20. This will occur at 7Hz

Whether the amp oscillates or not depends only on its internal frequency response and phase margin. It is impossible to tell from this block diagram what those values are. However, since this is a commercial audio amp IC, that will be detailed elsewhere in the datasheet.
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Old 29th May 2007, 11:38 PM   #58
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I'm not sure we're talking about the same resistor here. My interest is in Rb, the series resistor on the input. It prevents grounding the input when the pot is turned all the way down. If there were a RFI cap to ground (the situation in my amp), the resistor would limit the impedance to ground at high frequencies. No doubt it's something to do with the small but real input current, but I don't see how the signal gets reinforced to cause oscillation if the input is grounded directly.
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