OTA's in audio circuits (operational transconductance amps)

[mikep]

I am in the process of tracing the circuit of a VERY awesome piece of kit, an equaliser made by C*e*l*l*o in the late 80's. Im going to post some pictures of it soon, the build quality is probably the best Ive *ever* seen. My internet computer is so F'ed up right now I can't post pictures, but I should be able to fix it soon.

anyway, there are three types of discrete OA's in here. Ive traced two of them. I simulated the simpler one and it doesn't seem to work. then I realized, I think these are OTA's. that would explain the diodes between the inputs and why they have "OTA-1" and "OTA-2" for the model numbers. doh. the simpler OTA-1 is used in the filters, one per band along with what seems to be a buffer.

Im kind of jumping the gun here, cos I can't post the schematic yet, but in general how common is OTA based EQ filters? I don't think ive ever come across anything even remotely resembling this topology. there must be other audio gear out there with OTA's, right? I gotta go do some more reading to try to get a stronger handle on how these amps work. As I understand it, it behaves like an ideal transistor...

mike

 

[gyraf]

In my understanding, OTA's are really not suitable for high-end audio - mostly because the inherent limitation on signal levels. I see them more as building blocks for effect-type things.

But it would be nice to see schematics..

Jakob E.

 

[clintrubber]

No idea about the available controls on that box, but obviously it sounds like some kind of variable equalizer that doesn't uses pots in the signal path but those OTA's for a variable element (pots now controlling the gm of the OTA).

Something like figures 11 - 14 in this file for instance:
http://www.national.com/ds.cgi/LM/LM13700.pdf

Like that they're often used in FX-boxes etc, like Jakob said.
Do your drawn schematics resemble the those figures a bit ?

Bye,

Peter

 

[Samuel Groner]

www.groupdiy.com/index.php?topic=16914

Samuel

 

[chrissugar]

[quote author="mikep"]I am in the process of tracing the circuit of a VERY awesome piece of kit, an equaliser made by C*e*l*l*o in the late 80's. Im going to post some pictures of it soon, the build quality is probably the best Ive *ever* seen.
[/quote]

hi Mike

I supose you talk about the P*A*L*E*T*T*E. I'm very curious about it's schem because I know about it since the late eighties, and had some internal pics. Also very curious about the Audi0 S*u*i*t*e.
Some very expensive audiofool hardware even used by some in mastering.
Although I'm a bit disapointed after listening recently to some audiophile record made by M.L. (the guy who made these products). sounded nothing like records by Chesky, XRCD, MFSL. Not even as good as some records made by myself.
Please, if you are ready with the reversing give me (us) a sign because at least me, I'm very curious.

chrissugar