AW: [sdiy] Re: OTA nonlinearities

[Czech Martin]

I know that some people don't want to hear it:

I used SSM2018 with linear and expo controll. They are harder to get now.
And then there is this On-Chip stuff PAsomething. Paul Schreiber is your man
in
this case.

Oh, and not to forget the differential pair with bias current control.
Basically the schemo of a four quadrant multiplier, crippled to
two quadrants. With emitter degeneration local feedback
this can be quite linear, I think you buy this with not so good 
control rejection and dc offset at the output. Matched duals,
or duals, or if anything fails discrete matched BJT out of the box.
I think Rene Schmitz has such a VCA on his site.

http://www.geocities.com/SoHo/Museum/4459/diy/ringmod.html

is a scetch how a discrete 4 quadrant multiplier could look like.
Easy to cripple.

m.c.

m.c.  

> -----Ursprüngliche Nachricht-----
> Von:	Nils Pipenbrinck [SMTP:np at inverse-entertainment.de]
> Gesendet am:	Mittwoch, 9. Mai 2001 13:19
> An:	synth-diy
> Betreff:	[sdiy] Re: OTA nonlinearities
> 
> Thanks for your replies (especially thank you, Jim.. your OTA analysis was
> very interesting to read!)
> 
> 0.5% THD for a OTA based VCA is a bad performer (in my eyes).. It might be
> ok to have such a distortion on mono synth-waves. I think the distortion
> will be hearable as soon as I route a complete audio signal (read: music)
> trough the OTA.
> 
> I wonder what options I have to build a better VCA... I looked up the
> datasheet of the SSM 2024, and it's not that much better.. the THAT-Chips
> look like a perfect choice, but they are a lot more expensive.
> 
> There are also a lot of 4-quadrant analog multiplier chips.. I never
> dealed
> with them, but it shouldn't be that hard to turn them into VCA's..
> 
> I even considered dropping VCA's all together and go for digital
> potentiometer chips.. Anyone has experiences with them? They are no VCA's,
> but suitable for my application (some kind of mixer-matrix for hifi audio.
> I
> want to control more than 2 channels with one pot, so stereo-pots won't do
> the job). Of cause I would need some digital stuff around my module if I
> choose the digi-pot way, but I would get midi-support and presets for
> free.
> Stuff that's always nice to have, but not really required for me.
> 
> Does anyone has another idea how to control lots of audio channels with
> just
> one pot?
> 
> 
> Nils Pipenbrinck
> 
> BTW: I'll try out the asm-1 VCA, just to get a feeling how bad 0.5% THD
> really is.. I mean.. I'm listening to my music with a self-build tranny
> headphone amp, and I doubt, that it's THD is below 0.5% or so. I don't
> care
> about that.. my amp somehow "flavours" the sound, and I like it that way..
> 
>