GoodSoundClub - Romy the Cat's Audio Site

Audio Discussions
Topic: Out of the Signal Path?

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Posted by Romy the Cat on 01-11-2010

The Twogoodears posted on his blog an fascinating message about LDR volume controls.

http://twogoodears.blogspot.com/2010/01/optoelectronic-contact-free-volume.html

I never used them and wonder if it might be interesting.

This volume control uses non-contact optical SS devices to change resistance. There are many devises that vary resistance under different conditions – light, temperature, magnetic field, radioactivity, pressure… etc. I am not convinced that they all only by the fact that they have no physical contact are better for sound. I think in the case of the P&S preamp the quality that Twogoodears observe would derive not from the potentiometers itself but from the quality of the buffers that drive the load after the potentiometers.

Still, it might be interesting to hear the thing…

The Cat

Posted by Paul S on 01-11-2010

Well, there's noise, attenuation and impedance to reckon with, at a minimum.

The TAP TVC I use claims that the step circuitry itself is "out of the signal path". OK, fine; but so what? Doesn't the voltage have to "cross" something, somewhere, at some point, in order to be divided/attenuated, and doesn't the unwanted voltage/current have to go somewhere from the shuttle point? There's always plenty of noise to thwart, route and re-route, irrespective of the means of division. To be "good", any attenuator/pre-amp topology will yet have to deal effectively with the basics.

Does anyone know how the voltage gets turned into light, or if it then gets "re-converted"? Can any of this happen without some kind of diode effect?

Does the light conversion system itself require power from any source other than the signal? I would hope any active circuitry was limited to a well-isolated buffer, if only to avoid parasitic noise.

I admit I am leery of "active attenuation" for hi-fi.

Paul S

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