9V discrete VC LP filter?

[Isaiah]

Ooops, thought I had replied to this topic.

I meant to say no worries if you can't find it Mark.
I appreciate you offering to look. But if you have found it now, that's even better! Thanks!

Thanks for the links Slacker!

Ahaha! I think you hit the jackpot, Widdly!
I'm almost certain that would run on 9V too.

I had an idea last night after looking at all-pass filter schematics (mostly the Univibe) -
http://www.diystompboxes.com/smfforum/index.php?topic=60293.0

Thanks again guys!

[Mark Hammer]

Okay, here you go, as promised.

[Paul Perry (Frostwave)]

mark, I think that filter needs + AND - rails, as opposed to a mere 9v all up!

[Mark Hammer]

Perhaps.  It IS untested, after all.  The schematic, however, only shows single-ended power from +9v with Vref represented by the inverted T.

[widdly]

The bias section is a bit strange there.  It doesn't look like Vref would give 1/2 the supply.  Any ideas on that one?

[Paul Perry (Frostwave)]

Sorry Mark, my mistake.. I was thrown by the way it looked like + and - in the top left corner - that thing I thought was a minus is apparently a dash!

I will say this though, those Tom Gamble circuits were often subject to patching up afterwards, he was a pretty brilliant & very fast designer - people would ask for something & he'd have it put up the next day - but often there were a few bits to tidy up!!
I guess the bias circuit could be quite OK, remembering that a bias line isn't always half of the main power rail.

[widdly]

If you compare it to the pixie one http://www.elby-designs.com/pixie/pixie-vcf3.pdf which is based on the tom gamble one, the biasing is a little different.  The bias to transistors is lower than the biasing to the output op-amp.  I think that change would give more headroom on the output.

[Isaiah]

Ah, thanks for posting that Mark, much appreciated.

It seems the 'Pixie' diode ladder filter that Widdly posted is based on the Tom Gamble circuit (looking at the notes on the schematic).
That probably means a lot of the problems have been ironed out, I guess.

Yes, I thought the biasing on the schematic that Mark posted was a little unusual.
I don't see why you wouldn't just use half the supply voltage.

Anyhow, I will probably breadboard the 'Pixie' filter when I get the chance.
I'm still looking into modifying a Univibe stage for a low-pass response at the minute -
http://www.diystompboxes.com/smfforum/index.php?topic=60293.0
Any help with that would be appreciated, please!

Thanks again, all of you!

[Mark Hammer]

I'm going way out on a limb here since I'm outside the perimeter of my expertise, but it seems to me that with a diode ladder arrangement, won't a +5v supply present issues with respect to getting all those diodes to conduct?  My understanding is that the diodes figure like  voltage-controlled resistors.  Presumably, even though one can monkey around with op-amp choices that permit use of smaller supply voltages, there is an absolute requirement with respect to the control voltages needed to make the number and type of diodes used conduct.  Am I incorrect about that?  If I'm in the ballpark as far as correctness goes, what sorts of minimum supply voltages might be needed to make this work at +5 or even +9v?  Alternatively, what sorts of control voltages are needed/recommended in a circuit like the one shown?

[Isaiah]

Oh, wait, does the Pixie use 5V?
Ah... well, when I get around to it, I plan to breadboard that, but run it on 9V.

[Paul Perry (Frostwave)]

I guess we won't know till it's built, but 9 v would surely be enough. The transistors are acting as high impedance current sinks, so provided there is enough voltage available to completely turn on the diodes (after allowing for say 4V to run the transistors) all should be well.

[aron]

I'm interested in how this goes. The wasp was a cool synthesizer!

[snufkin]

im interested in this to i hope you report back

[dirk]

The Roland TB-303 also works on a 9V adapter. But the circuit works on 12V.
Its a 4 pole diode filter, with transistors as diodes.
Cool thing about it is that the first pole is 1 octave higher than the other 3.
This gives a realy nice resonance behaviour, and can easily be done on other filters as well. Just half the first filter cap.

http://www.hylander.us/images/schematics/roland/tb303.gif