[sdiy] Polymoog Resonator virtual ground

[rsdio at audiobanshee.com]

The only real drawback is the loss of ultra-low frequencies. Shifting signals to a virtual ground is almost always implemented via capacitive coupling at the inputs and outputs. Capacitive coupling means that you have a high pass, and can't reach all the way to subsonic frequencies.

I don't see any capacitors on the Input or Output of the Polymoog Resonator, so I assume the Polymoog runs everything with a DC offset, and then take care of it on the final outputs.

Then again, nearly all audio equipment is capacitively coupled, even when there is no virtual ground, and so this drawback is not much of a drawback at all - even if it is implemented that way.

Brian Willoughby
Sound Consulting


On Aug 14, 2014, at 10:45 PM, Dan Snazelle wrote:
> that sounds like a very simple way to do such a thing...any real drawbacks??
> 
> Sent from my iPhone
> 
>> On Aug 15, 2014, at 1:40 AM, "Donald Tillman" <don at till.com> wrote:
>> 
>> 
>>> On Aug 14, 2014, at 10:13 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>>> 
>>> I was just looking at some Polymoog Resonator schematics, and noticed that
>>> the virtual ground is not 0V, but is about 5V.  Why?  Does anyone here know?
>> 
>> I don't have the inside scoop or anything...
>> 
>> But my guess would be that they chose to use CD4000-series CMOS switches, which have a maximum voltage of 18 V, and chose to string them between +15 V and ground.  They probably thought it would be easier to bop the audio up to 5 V than to run the CMOS at +/- 7 V and level-shift all the logic signals.
>> 
>> -- Don