discrete SSM2040-style filter

Joachim Verghese jocke at netcontrol.fi
Tue Jan 7 13:59:00 CET 1997


Hi Juergen and others,

Great description of your 2040 "clone"! Seems to be very similar to the
one I built last year. Incidentally, I recently did some more experiments 
with this filter too.

> The darlington works into a 500uA current
> source which is made from a 7th transistor (again BC550C).

Great idea. I used only a 10k resistor into -15V, which works ok,
but introduces some non-linearity because of the varying collector
current in the darlington (I used BC517s). Having an active load
also improves slew rate performance, one would think.

> Four of these stages are connected in series. The currents into
> the emitters comes from an exponential converter, built
> around a  CA3086 array and a dual opamp. One transistor
> for temperature compensation, 4 transistors for the 4 otas.
> Emitters all tied together. I chose a reference voltage of -1V
> for the expo converter, to allow some voltage drop over the
> transistor array. This results in a -1V instead of 0V summing
> node for control voltages.

I also used a 3086 array, but only as a 4-output current mirror.
I had a separate expo-converter, because I wanted the summing node
to be at gnd.

> One thing I had worried about were the offset voltages.
> But I only have approx. 100mV offset at the output, and the best
> thing is that this offset only changes by a few millivolts over the
> entire control voltage range. So it turns out that selected transistors
> are better than the monolithic transistors of the original in some
> respect. Temperature drift is worse, of course. If I touch one
> transistor of a pair with my finger, the offset increases to one volt.

Yeah, this must be the greatest problem with having several discrete
gain cells in cascade. I tried using EP2015/EN2016 arrays to get rid
of temperature sensitivity, but the problem is that these devices
don't boast particularly high gain, so offset and CV rejection becomes
a problem. I ended up using matched ZTX109C/ZTX212C pairs, and the
results were somewhat similar to those you got, but I'm still not
satisfied with the offset performance. Requires some tweaking.

> I also added a voltage controlled feedback loop (which is not
> part of the SSM chip). I tried a 3080 which works fine, but I finally
> used another discrete differential pair. Call me purist.

Me too!! In fact, I designed a complete 2VCO-VCF-VCA circuit using
only discrete devices in the signal chain. Well, the final output
stage uses a (high performance) op-amp, but everything else is discrete
and DC-coupled.

All control voltage inputs are 0...+5V, since I'm going to generate
CVs with a microcontroller.

Instead of having a level control for each VCO, I have a single
VCO A - VCO B mix control, and a separate "Filter Drive" control to
vary the amount of distrotion. Increasing Filter Drive decreases
final VCA gain, so that the overall amplitude remains nearly
constant. In a similar vein, the VCO waveform amplitudes are "averaged"
so that the VCO level remains constant through all saw-pulse-tri
combinations.

The design is just on veroboard at the moment, so there's s lot of
tweaking to be done. But the sound is FAT, that's for sure!

-joachim



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