can someone please help me identify this board? SVF wah

[pinkjimiphoton]

someone sent me this like a long time ago i think, and the schematic is lost to the ages.
i've tried a bunch of google searches, looked under old accounts at various forums and stuff, and i just can't find it.

the board is marked SVF Wah, and uses two ca3080's and two tl072's, thats pretty much all thats marked on there. i am too much of a newb and too deeply idiotic to figure this out on my own i think... been trying for like a week or two now.
so any help identifying the circuit and hopefully finding a schematic would be greatly appreciated.
thanks fam

[vigilante397]

SVF = State Variable Filter

I know that isn't a complete answer to your question, but it's all I've got

Edit: Still doesn't look like a winner, but I found this:



And also this:

[ElectricDruid]

Nathan's covered the two likely scenarios there: Either it's a SVF based on OTAs with a cap to ground and a buffer, or it's an SVF based on an OTA followed by an integrator (cap wrapped round an op-amp). Those two options cover a lot of variations in the filter world. You can also do them with VCAs instead of OTAs, but that's an aside since we know we've got CA3080 here.

It's possible to build an SVF using OTAs without the highpass output by using the -ve and +ve input of the OTA, but since you've got 4 op-amps on your board and only two are required for the buffers/integrators, I think there's an input mixer as shown on Nathan's CA30380 schematic. The output from that op-amp is the highpass. The output from the first buffer or integrator is the bandpass, and the output from the second is the lowpass.

There are feedback paths from both of those stages to the input mixer, and there are a couple of variations on that too. Occasionally the resonance control is in the feedback path from the lowpass output (there's a Roland synth filter like that, for example), but mostly it's wired in reverse in the feedback path from the bandpass output. Why in reverse? Because the bandpass signal is out of phase with the input, so in fact it acts as a "damping" control to tame the resonance that would be there without it.

Incidentally, how was a person supposed to know where to put which resistor if the board isn't marked with either reference designators *or* values?! One or the other would have been nice...presumably there was a bit of paper once upon a time.

Tom

[pinkjimiphoton]

thanks brother nathan,
appreciate the help
i saw both of these too, but am not savvy enough to figure this one out. if i have time to beep out the connections i will try and draw a schematic. but this is WAY above my paygrade as a simple fuzz monger

Nathan's covered the two likely scenarios there: Either it's a SVF based on OTAs with a cap to ground and a buffer, or it's an SVF based on an OTA followed by an integrator (cap wrapped round an op-amp). Those two options cover a lot of variations in the filter world. You can also do them with VCAs instead of OTAs, but that's an aside since we know we've got CA3080 here.

It's possible to build an SVF using OTAs without the highpass output by using the -ve and +ve input of the OTA, but since you've got 4 op-amps on your board and only two are required for the buffers/integrators, I think there's an input mixer as shown on Nathan's CA30380 schematic. The output from that op-amp is the highpass. The output from the first buffer or integrator is the bandpass, and the output from the second is the lowpass.

There are feedback paths from both of those stages to the input mixer, and there are a couple of variations on that too. Occasionally the resonance control is in the feedback path from the lowpass output (there's a Roland synth filter like that, for example), but mostly it's wired in reverse in the feedback path from the bandpass output. Why in reverse? Because the bandpass signal is out of phase with the input, so in fact it acts as a "damping" control to tame the resonance that would be there without it.

Incidentally, how was a person supposed to know where to put which resistor if the board isn't marked with either reference designators *or* values?! One or the other would have been nice...presumably there was a bit of paper once upon a time.

Tom

i honestly have no idea where it came from, i seem to remember someone talking about making a SVF and i said it sounded like it would make a great wah... so they sent me a board, but no paperwork. but god... that was like, windows 95 days i think. i've held onto it for years but no idea what goes where, or what component is which. the opamps are marked, but that's it.

so, for a bandpass filter like a wah you'd just use the bandpass output maybe tom?
sorry, again, my bag is fuzz. i can make a fuzz out of a 9volt battery and a wet paper bag, but... this is above my paygrade a bit

[vigilante397]

i can make a fuzz out of a 9volt battery and a wet paper bag

Pretty sure I've seen your schematic for that floating around somewhere, it's on my "to-build" list

[pinkjimiphoton]

hahah i  call it the serious design flaw fuzz 

the bag has to be wet so it will conduct electricity. you use plastic bread ties for insulators. works great
ground the e
stick the 9volt to c
use a doobie as input and output caps
get your friend to hold the guitar cord against the wet paper bag. its brilliant.

and green.



sounds kinda like a farting frog in a blender of nuts, but, hey, some people like that sound

[ElectricDruid]

i honestly have no idea where it came from, i seem to remember someone talking about making a SVF and i said it sounded like it would make a great wah... so they sent me a board, but no paperwork. but god... that was like, windows 95 days i think. i've held onto it for years but no idea what goes where, or what component is which. the opamps are marked, but that's it.

Yeah, I've got a box of stripboard prototypes which is a bit like that - there's stuff in there that I'm not even sure what it was supposed to be any more.

so, for a bandpass filter like a wah you'd just use the bandpass output maybe tom?

Yeah, you could do. But quite a few SVF filters use the lowpass output, since with resonance you can get a nice midrange peak anyway and you don't lose so much fundamental. That's pretty common on envelope-follower controlled auto-waas.

[pinkjimiphoton]

thanks tom, i'll try and beep the board out this weekend and see if i can figure out something to do with it.
rock on bro!

[Kipper4]

The guys are right there's only so much you can do with 4 op amps and 2 OTA's.
And call it an SVF.

I can't see if there's any other identifiers on the pcb. My eyes ain't what they were.

The SVF was used in the mutron iii. Albeit an op amp only version.

Any idea what the control element is?

[ElectricDruid]

2 OTA's.

Any idea what the control element is?

[pinkjimiphoton]

The guys are right there's only so much you can do with 4 op amps and 2 OTA's.
And call it an SVF.

I can't see if there's any other identifiers on the pcb. My eyes ain't what they were.

The SVF was used in the mutron iii. Albeit an op amp only version.

Any idea what the control element is?

hi rich,
i think it was supposed to be a drop in for a crybaby or something.