Phasers (AW: noise of arrays)

Haible_Juergen#Tel2743 HJ2743 at
Tue Jan 23 01:20:00 CET 1996

> No, I just happened to have a spare single channel compander board,
> I think it was one of those Maplin kits, so I didn't even try the
> original. It's a nice design, though, using a 4007 gate as a
> variable resistance.

I think I will try it. Especially I like the idea of having all the FETs on 
one chip
*and* driving them with the same control voltage.

> Yes, the 2N3958 dual FET looked too exotic for me, so I used a
> 3-JFET op-amp instrumentation amp instead. I guess you'd have to
> put a balance trimmer in there if you use two unmatched FETs.

I did a spice simulation meanwhile, and I found that the CMRR is very
dependant of the FETs' matching. So I'll better go Instrument Amp
here, too.

> I'm not familiar with Moog's 12-stage, does it use OTAs or discrete
> gm stages? I've been thinking about implementing this design too,
> and 2024s definitely would be better than 3080s or 13600s.

The Moog uses OTAs, 3094 if memory serves (3080 + buffer).
So a 13700 would be quite equivalent. The Moog has a very
sophisticated compander, however, with differential photo resistors
and a attack/hold/release envelope follower ...

The problem with the 2024 is its reduced output swing. So I must first
figure out a good all pass circuit with the OTA driving a virtual GND
summing node! Any ideas?

> How much does the 2024 cost these days? I used to buy them from
> Maplin for around GBP4.

And I pay about DM 10,- at RS; quite comparable.


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