More VCA Hum but this time from a 2700
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Author:  Kyhotay [ Mon Oct 17, 2011 7:23 pm ]
Post subject:  More VCA Hum but this time from a 2700

Hi Scott,

Now that you fix mine and everybody's 9700 VCA issue, how about some guidance with a 2700 VCA? I have two of them and one is quite and the other has a hum. I'm cheating my running a 9741 and modulating it with the 4700 ADSR - which works terrific, BTW - but wanted to see if you had a capacitance/resistor substitution in mind. I can't recall how long I've had these but I always replace the old electrolytic caps of older PAiA gear.

Please advise,


Author:  PAiA-Scott [ Mon Oct 24, 2011 11:11 am ]
Post subject:  Re: More VCA Hum but this time from a 2700

The filtering of the dc supplies on the 2720-1 VCA is with resistors R20 (470ohms) and R21 (1K), and, capacitors C3 and C4 (each 100uF/25V).

Two nine volt batteries can be used as a temporary power source connected to the +9 (+) and +18 (++) supplies. Connect the two batteries in series with the plus pole of one to the minus pole of the other. The junction is the +9V node, the free minus pole is the Ground node, and the free plus pole is the 18V node. If there's no hum with them, try replacing C3 and C4. Otherwise, inspect and possibly make new solder connections on the grounds and wiring for the same (the audio connectors too).

Try running the audio to the input of the VCA through a capacitor link via one of the 2720-7 power supply patch section points (the connectors shown joined with "C" to block any possible DC component, or, substitute a different signal source (one known to have a coupling capacitor in series with the output (or with zero offset), or, the one used when listening to the VCA which didn't have noticeable hum).

Author:  Kyhotay [ Mon Oct 24, 2011 6:53 pm ]
Post subject:  Re: More VCA Hum but this time from a 2700

OK, I'll give it a shot and see what ends up working the best. I'll let you know.

Thanks for sharing your encyclopedic knowledge!

Brian Folkes

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