Flanger questions

[cathexis]

Hi,

First off, some advice: building flangers is good, wholesome fun, but you should really stick to building one at a time I have several builds going on at the same time now, and my head is spinning. It's also a good idea to try the circuit out on a breadboard first, it makes tweaking a lot easier, and tweaking is often necessary to get the clock sweep right.

Anyhow, I've built the DOD FX75B flanger from this schematic:

http://files.muziq.be/schematics/dod_fx75b.gif

I have some debugging to do yet, but I was wondering if there's something missing on the schem. Shouldn't there be some kind of DC bias on the input of the BBD? I thought it didn't work otherwise? I compared this schem to the one for the FX75 here:

http://www.freeinfosociety.com/electronics/schematics/audio/dodfx75.pdf

and there's an extra trimpot on pin 3 of the MN3007 that seems to do just that...

LARS

[StephenGiles]

There may be fixed bias components - don't believe every circuit on the internet!

[cathexis]

I get about 1/2 V+ before the 47nf cap leading to pin 3 of the MN3007, but 0V on pin 3. The cap blocks DC, I suppose. As I've understood it, the input to any BBD in this application needs a DC bias voltage to ride on. On the FX75 schem there's a voltage divider with adjustable bias right on pin 3, after the cap. I have tried to tack one on there, but unfortunately I have some other probs in the signal path, and I was away from the scope, so I couldn't check the output of the BBD. I'll keep at it.

LARS

[Mark Hammer]

I can't verify everything, but the bias scheme shown in the second schematic (redrawn one) is appropriate.

You are correct that the .047uf cap before the BBD will block DC such that a bias has to be re-introduced, and the method shown in the 2nd schem does so.