Some questions on that varicap circuit posted a while back..

[lightningfingers]

This is the one, posted by puretube. My application isn't strictly stompbox related...but anyway, as the circuit is shown if it is used to replace a "real" variable capacitor, will DC used to power the varicap get into the existing circuit? And also the resistance of the 10K pot..... could this possibly cause problems?

many Thanks In Advance

[R.G.]

Look at the circuit.

There is a cap from input to output. No DC offset there.

Then the input voltage is buffered by a complementary emitter follower (including about a volt of crossover distortion) which feeds a pot. The pot taps off some of the input voltage as buffered, then is itself buffered to the output of the cap.

The second buffer sets the output DC level to be 0V - it has to, it's input is at 0V and it will make the output be the same or die (literally) trying.

If the input is sitting at other than 0V DC, the output will follow the input.

So what we have is a circuit that makes for a capacitive signal passage, with no DC blocking whatsoever.

[toneman]

RG
what's this thing supposed 2 do?
can U give an example where it would B used?
Also, where's "in" & "out"
inquiring minds & all that.......

[puretube]

the simulated cap sits between points 1 & 2 : so it`s a cap to ground;
(not floating = i.e.: shunt-cap; not: series-cap!).
e.g. for lowpass filtering purposes, gyrating-, or timing-circuits...

http://diystompboxes.com/sboxforum/viewtopic.php?t=24307&highlight=varicap

http://diystompboxes.com/sboxforum/viewtopic.php?t=25504&highlight=varicap

[lightningfingers]

thanks ppl,

could it possibly be a floating cap if it were given its own power supply?

[Paul Perry (Frostwave)]

thanks ppl,

could it possibly be a floating cap if it were given its own power supply?

I don't think so, but a floating cap IS possible.
For a hint, see the National Semiconductor LM13700 data sheet, with the single ended and floating voltage controlled resistor circuits.
(I could be wrong..)