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Old 26th March 2006, 09:46 PM   #1
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Default Decoupling cap placement - virtual ground dual op amp?

Evening, all.

I've trawled the net (and DIYa) for some further advice on decoupling. I'm aware of the "keep it close to the pins" basics, but I'm wondering what the best way to decouple a dual op amp is - is it any different from a single op amp? I'm trying to improve the sound of some Rohm 15218 dual op amps (which I presume is their take on the generic, bog-standard dual op amp theme). Having had a quick look at the circuit board in my Denon DRS-710 cassette deck, I'm wondering if the two op amps which seem to form the playback front end and buffering are connected to a virtual ground. Both op amps run from +8V and -8V rails

An application note for the AN-202 shows a number of approaches to decoupling. In the case of a grounded load, it talks about minimising the current loop by placing the decoupling caps between the load ground and the relevant power pins of the op amp which drives the load. When it talks about 'load', does it mean the resistor connected between the input of the following op amp and ground?

In the case of my tape deck, there are two 15218s. The load resistor (if I'm guessing correctly above) doesn't seem to be connected to ground. Does this imply a virtual ground (although I can't see another op amp performing this duty, clueless as I am in these matters)

I'd like to improve the decoupling as the existing caps which decouple these two op amps are quite a way from either op amp (one cap is connected from the positive rail to ground, the other from negative rail to ground).

Is it possible just to connect a single cap between the respective power pins?

Maybe I'm just getting my knickers in a twist, so any guidance would be much appreciated.

Cheers, Jon.
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Old 27th March 2006, 06:30 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
I think an extra decoupling cap (bypass) between the power pins will do no harm. I also read it tends to be more effective with faster opamps.

If the existing decoupling is more than a few cm from the opamp power pins then you should go ahead and put in some additional decoupling near the power pins (of each) to ground.
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