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Bypassing, Goldilocks, and the Sound of Nothing

Posted 31st May 2012 at 06:28 PM by rjm
Updated 1st June 2012 at 11:53 PM by rjm

Douglas Self writes,
The 5532 and 5534 type op-amps require adequate supply decoupling if they are to remain stable, otherwise they appear to be subject to some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope. The essential requirement is that the positive and negative rails should be decoupled with a 100 nF capacitor between them, at a distance of not more than a few millimeters from the op-amp; normally one such capacitor is fitted per package as close to it as possible.
He's someone who should know. Anyway, it doesn't take much digging on the internet to confirm beyond reasonable doubt that bypass caps should be as close to the op amp power pins as possible. So thinking about my previous experiments with bypassing the Sapphire, by adding bypass caps around the transistors I also effectively also added a bypass for the op amp, but a rather poor one as the power-pin-to-power-pin round trip loop distance is probably 10 cm or so.

It's hard to describe, it just sounds off somehow. Unpleasant, but very subtly so. A slightly hard edge to the midrange, a strange tang to stringed instruments, vocal sibilance, excessively strong bass.

Solution: a fifth 0.1 uF ceramic bypass cap under the Sapphire boards soldered directly between the socket pins 4 and 7. Getting crowded down there. This did the trick. Completely I think. All the weird stuff, heck, all the stuff, period, dropped out. Balance re-asserted itself, all is right with the world.

What's funny, looking back on this saga, is how unremarkable "just right" actually sounds. The Goldilocks principle I guess. The Sapphire does not call attention to itself, in the way that it did before.

I can still hear a difference between the NE5534A and OPA134. The NE5534A is rougher, less distinct, but warm and solid. The OPA134 is clean, almost crystalline, but with a faint shimmer (perhaps gauziness is a better word..) that tends to float the soundstage in a way that may not be to everyone's taste. JFET input stage making its presence felt? I'm not sure. As a guilty pleasure, I prefer the more immersive presentation of the OPA134 though.
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