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Old Yesterday, 01:36 PM   #1
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Default Unity gain opamp circuit

Hello, I'm a newbie when it comes to "designing" my own circuits, so I would like some input on the attached circuit.

The goal is to have a capacitor coupled opamp that will serve as a buffer for a 25K volume pot. Source impedance will be less than 1.3k ohms, I would like the circuit to be able to play with amplifiers that have an input impedance as low as 30K. The opamp will be the LME47910 or LME47920/LM4562. Power supply pins will be decoupled with multiple low value ceramic capacitors.

What are your thoughts on the resistor and capacitor values?
Would you add or remove components to improve stability?

Any input would be greatly appreciated.

Thank you
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Old Yesterday, 02:25 PM   #2
Mooly is offline Mooly  United Kingdom
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Its basically fine. R2 and R3 are not needed. R4 is to large, it need only be perhaps 33 ohm, just enough to isolate the opamp from cable capacitance effects.

(Although this is AC coupled you do realise it still needs a spilt supply as drawn)
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Old Yesterday, 03:01 PM   #3
forr is offline forr  France
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Quote:
Originally Posted by Mooly View Post
Its basically fine. R2 and R3 are not needed.
When the pot cursor is at ground, it happens sometimes that the circuit oscillates.
I always keep R2.
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Old Yesterday, 06:32 PM   #4
Mooly is offline Mooly  United Kingdom
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Must be honest and say I have never encountered that. Did you find it IC specific ?
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Old Yesterday, 06:33 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by forr View Post
When the pot cursor is at ground, it happens sometimes that the circuit oscillates. I always keep R2.
Didi you have long wires from the pot to the op amp?
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Old Yesterday, 07:34 PM   #6
forr is offline forr  France
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I have encountered the problem a few times.
There is always some parasitic capacitance at the input of an amplifying device and some parasitic inductance in the wiring, so a small value resistor will tame the tendency of the LC circuit to resonate.
This may look like an excessive precaution but I think it is often used, it is innocuous . It is sure that Blehmbo has already seen it somewhere.
There was a paper by Erik Margan on the subject at the input in an old Electronics World issue.
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Old Yesterday, 07:52 PM   #7
Mooly is offline Mooly  United Kingdom
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Thanks forr. Its obviously a problem you have encountered and know the solution that works to prevent it.
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Old Yesterday, 08:30 PM   #8
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Very low-noise opamps are particularly susceptible to this, as their Rbb' inevitably is low and does little to isolate input capacitance (or even negative input impedance).
That's also why a direct connection from out to -in is recommended against with the AD797. Other, less "hot-rodded" parts can take that just fine (Yamaha must use NJM2068s like that by the bucketload, I think 5532s don't mind either, nor do NJM4556As - note how none of these are remotely close to 0.9 nV/sqrt(Hz) in terms of e_n).

The only common audio part that potentially needs a 470 ohm output isolation resistor is a TL072 though. Most any other part should be perfectly happy with 47-100 ohms. Something like an LM4562 should also drive loads down to less than 600 ohms with ease.

R3 can also be used to reduce impedance imbalance between -in and +i if the noise penalty isn't an issue but low distortion is. The source impedance presented by a 25k pot fed by a low-impedance source can be up to 6.25k after all, with a minimum of near-zero ohms. It's not a big issue for the LM4562 (LME49860 was found to exhibit about 0.04% of input impedance distortion at 100k imbalance, 10 kHz @ 7.75 Vrms, unity gain, so at 6k25 it would be like 0.0025%, proportionally less at lower levels - so at 1.5 Vrms it would be down to 0.0005%), but a lot of FET input parts are about one order of magnitude more critical.
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Old Today, 09:28 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
Thanks forr. Its obviously a problem you have encountered and know the solution that works to prevent it.
It's a solution I am aware of, but rarely implement.
So far it has not bitten me.

Many discrete amplifiers show a low value resistor just before the base of the LTP pair, like a base stopper.

I have recommended R3 to many of our Members.
Some datasheet demand R3.
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