What a difference a feedback resistor makes!? - diyAudio
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Old 28th October 2003, 07:11 PM   #1
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Default What a difference a feedback resistor makes!?

I recently replaced the cheaper resistors and caps in the signal path in my LM3875 chip amp with more expensive parts: Vishay S102K for the 10k resistor to the - input, Caddock 132 (332kohm) resistor for feedback, Axon input cap. The result, near disaster. Perhaps that is overstating it, but the amplifier definitely sounded better with the cheaper components.

So, I took the amp to the scope and started looking around for trouble. Gain vs. freq.: perfect. Max. ouput signal: Fine. I was looking at high-frequency roll-off and I cranked up the frequency of the sine wave to 100 kHz. Here's where the strangeness lies. The output wave was phase-shifted from the input wave. Okay, I think, maybe that is just the phase-shift due to the GBW product.

Then I looked at a square wave and with a small-signal input the response was nearly indistinguishable from an RC low-pass filter - extremely overdamped; I hadn't installed a cap across the feedback resistor. With a large signal input, the slew rate was fine at about 15 V/micro-second, so it wasn't slew-rate limited.

Has anyone else observed this behavior?

I went back to check the phase-shift more closely. It turns out that there was about a 1.1 micro-second delay between the input and the output signals at all frequencies I measured, down to about 15kHz, where the gain is still flat. It was difficult to measure the phase delay accurately below 15 kHz. I am not sure of the Caddock MK-132 construction, but if it is like the Vishay, it has a signal path in the resistor that is longer for higher resistance values. The Vishay data sheet says 1kohm resistors have a 1 nano-second delay. So, I paralleled lower value carbon film resistors with the feedback resistor and, voila, the delay decreases to about 0.3 micro-seconds with a 44kohm in parallel with the 332 kohm resistor.

I haven't yet taken it apart to replace the feedback resistor, since I don't have the right value on hand.

Has anyone else had similar trouble with high-value Caddock or Vishay resistors? It kind of defeats the purpose of a very short feedback path length if the resistor adds 80 meters of wire from lead to lead.

Jeremy
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Old 28th October 2003, 07:31 PM   #2
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This doesn't sound abnormal to me. It seems like a normal result of the internal frequency compensation of the chip amplifier.

Even with perfect resistors, if you use a conventional operational amplifier or a chip amp at a closed-loop gain considerably higher than the minimum for which its stability is guaranteed, it is normal that the circuit has approximately a first-order behaviour, with a small-signal step response with approximately a
(1-exp(-t/tau))
shape.

Theoretically, the time constant should be equal to
tau=(R1/R2+1)/(2*pi*GBP),
where R1 is the feedback resistance from the output to the negative input, R2 is the resistance from the negative input to ground or to the signal source, GBP is the gain-bandwidth product (in Hz) and pi is 3.14159265358979...

If the amplifier is well-designed, the step response will gradually change into a well-damped second order response when you reduce the closed-loop gain to the minimum recommended value.
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Old 28th October 2003, 07:33 PM   #3
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I forgot to mention: for frequencies well below 1/(2*pi*tau), one would also expect the output sine wave to be delayed by a delay time approximately equal to tau.
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Old 28th October 2003, 07:53 PM   #4
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Thanks for the details about the delay. That makes good sense. It surprised me because I hadn't seen such an obvious shift from input to output before. Of course, I have worked mostly with DC in the past.

Assuming the National Semiconductor literature to be accurate, the numbers work out to be very close to the measured 1.1 microsecond delay observed.

That leaves the question of the poor sound... I'll have to go back and change one thing at a time and see what the effects are.

Jeremy
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Old 2nd November 2003, 09:36 PM   #5
Pedja is offline Pedja  Serbia
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Hi Jeremy,

Something like 1us delay is exactly what I see running the LM1875 (similar GBW) model. This delay goes down together with the amps gain. However, I am not sure why there will be such a difference between small and large signal pulse response that you found. That is interesting.

Joe Rasmussen found strange behavior above 320kHz and that started to happen rapidly at 330kHz.
http://members.ozemail.com.au/~lisaras/design.htm
(scroll down to the bottom half of the page)
Note that he uses a higher gain (1M/(18k+4.7k+Zout) feedback). If this freq would be shifted with lower gain, Id rather expect to see it shifted up.

If you repeat that measurement with 100kHz (not bad to go even higher) using the cheaper/other resistors, please, keep us informed about it. It will be interesting to know if anything of this is related to the used (type or brand of the) resistor.

Pedja
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Old 4th November 2003, 06:48 AM   #6
ppl is offline ppl  United States
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One thing to keep in mind is that Bulk Metal Foil Resistors are highly inductive while carbon composition and Metal Oxide are relatively low inductance. Bulk Metal Foil resistors while great at noise specs are not the best choice for feedback networks. This can cause time delay in the larger values because the self inductance of the resistor is larger at higher values and can get to the point of reacting with the parasitic capacitance both on Board and on the Chip to create a L/c resonant circuit.
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Old 4th November 2003, 10:44 AM   #7
pooge is offline pooge  United States
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Bulk metal foil resistors are NOT highly inductive. They are particularly designed with a pattern to have very low inductance.
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Old 4th November 2003, 11:29 AM   #8
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I think, reading all the answers, that kropf really must do a comparison with a "normal" resitor. The thread is called "what a difference ....", but I cannot find what this difference is, if there is any. Without that, any cause attributed to the feedback resistor remains pure speculation, if not fantasy.

Jan Didden
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Old 4th November 2003, 03:31 PM   #9
ppl is offline ppl  United States
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While Bulk metal foil resistors have alot lower inductance than wirewound thay are not as low as carbon compisistion and Metal oxide.
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Old 4th November 2003, 04:29 PM   #10
ppl is offline ppl  United States
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it appears that the caddock Bulk metal foil resistors are rather low in inductence the 10 nH inductence is real Low and on par with the Wima Polopropylene Box capacitors often used for bypassing thay also have about 10 nH of inductence the caddock resistors can be found at
http://www.caddock.com/Online_catalo...000_Series.pdf
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