Caps connected to virtual ground?

[ugly]

Why do so many effects have a cap with one side connected to the (typically +4.5V) virtual ground? As far as I can tell there should be no difference between connecting it there and actual ground. Several examples are the mid-boost/bass-cut cap on a TS from the inverting input or the 1n cap from the non-inverting input in the filter section of the Hollis' Omnidrive. I guess it kindof drives me nuts because a blocks DC so whats the point of feeding it DC? Am I missing something?

[bioroids]

Hi!

The point is exactly that, making sure there is no AC at the Vref point, so it can act as a virtual ground for AC. If it weren't for the capacitor the Vref wouldn't be virtual ground.

The capacitor makes the AC signals go straight to GND, skipping the parallel resistor. That way you get a good clean DC point at 4.5v (or whatever the voltage is).

Hope it is clear enough. I'm sure there is a lot of info about this in the archives. I reccomend you to take a look at them using the search function (third button on the menu above).

Welcome to the forum!

Miguel

[R.G.]

As far as I can tell there should be no difference between connecting it there and actual ground...  I guess it kindof drives me nuts because a blocks DC so whats the point of feeding it DC? Am I missing something?

You're missing the secondary effects.

If the virtual ground is clean (that is, low impedance to ground) at the signal frequencies, then the caps connected there do not have to charge up with the DC voltage from ground to the bias voltage before the circuit works correctly. Depending on the charging path available to them, this may take a noticeable time to happen. This is one of the things that some poorly designed effects have that causes the sound to slowly get better over the first few seconds of operations. This is a good reason to connect to virtual ground.

If the virtual ground is not a good, low impedance to ground, the connection to virtual ground can introduce signal bleed through, peakiness and oscillation. This is a reason that connecting to virtual ground is bad.

If the virtual ground is perfect, there is no difference to steady state sound at all. The details of the rest of the circuit matter.

[bioroids]

Oops I totally misunderstood the question. In fact it was a doubt I also had, until now 

Miguel

[ugly]

Thanks, I had forgotten all about what happens to a cap with DC applied. Sometimes I wish everthing used a bipolar PS because I like the asthetics.