MFOS phase shifter input decoupling

modulus in moduli · Joined: May 14, 2010 Posts: 17 Location: USA

Hey,

I have build a couple of MFOS boards, and I am loving them.

Now I am building the eight stage phase shifter and I noticed that at the input there is
- a pot that we can ignore for the moment if set to max.
- a 1uf electrolytic cap
- a 10k resistor to the virtual ground of an opamp
Now there is a couple of things that I am wondering about
- the cap is polarized. The input is bipolar
- the cut off is 16Hz. -- not all that low.
So, if choosing electrolytic, why not choose 10uF? or should I put in a polyester cap? Or, can't I just short it? I don't see why a phase shifter would be hurt in case there was a little DC on the input?

slacker · Posted: Sat Dec 04, 2010 4:13 am Post subject:

I'm not sure why you'd want to make the cap bigger, 16Hz is below audio so I can't see that a bigger cap would have any effect. A film cap would work just as well.
You could short the cap and probably never have any problems, personally I'd leave it in to be on the safe side.

modulus in moduli · Joined: May 14, 2010 Posts: 17 Location: USA

Well, the thing works!

I am kind of missing the manual control, like there is one on the Doepfer A-125. The internal vco makes up for it though and one can always connect a DC voltage to the CV inputs. But if I keep missing it, I'll just retrofit this function.

The effect of the 4 phase and 8 phase feedback and the difference of the two outputs are rather subtle.

Then about the caps. There is another polarized elec. cap (c20, 10uF) that gets the audio input over it, without one pole being lower than the other. (ehh, well I think it actually is lower but only by a marginal amount, less than the singal amplitude). This means that if I would short the input cap, this cap could get DC (possibly in reverse polarity) over it.

I will replace the input cap with a plastic film cap and c20 with a NP elec. type. I just think it is a poor choice to put polarized caps on these spots. These caps will degenerate due to the reverse voltage, while it is easy to do it right. Well, I hope I am not missing something here but changing the caps seems safe to me anyway.