Descret OTA was "that Japanese site"

[jhaible]

>Hi Juergen:
>
>I took a look at your SSM2040 clone, and found it very interesting. The gm
>cell as used by ARP is the same circuit.  What ARP did different is run
the
>output of the gm cell into the summing node of an op-amp, instead of a
>darlington buffer.  The limited output swing of the gm cell no longer is
an
>issue, as the output of the op-amp is where the output swing occurs.  Now
>your voltage swing is limited by the op-amp.  If you replace the feedback
>resistor of the op-amp with a capacitor, creating an integrator, you then
>get a current controlled first order low pass function.  Again, the output
>voltage swing is limited only by the op-amp being used.
>
>I am puzzled why the SSM2040 was designed with this inherent limitation on
>voltage swing.  Instead of trying to fix it by keeping the input level
down
>with a 10:1 divider, all you need do is use the op-amp integrator approach
>as described above.  All voltage swings are nearly rail to rail.
>
>Unless, of course, the asymmetrical clipping was intentional.  
>
>Terry Michaels

Very good points. I don't know for certain why he didn't use the opamp
integrator
approach. I got one single email from Dave Rossum when I asked about
the 2040, but not a second one when I asked for further details.
He mentioned that they experimented with zener diodes to increase
the level shift of the buffers, but dropped this idea. He also mentioned
they used these minimal transistor count OTAs because of a limited
number of transistors available on chip. Maybe he had the choice between
internal darlington buffer or external opamp for the same reason.
Who knows.
But as a fact both the old data sheet and this email from a few years ago
only mention the limited voltage swing as a restriction, not a feature.
And the Prophet 5 was built that way, too: avoiding higher levels at the
filter.

So I still tend to believe that this "feature" was unintentional. No one
mentioned it, the first applications didn't make use of it ... I am
curious which synth really was the first to make use of it ! (Any ideas ?)

Speaking of ARP and opamp integrators: Do you have a feeling (or numbers
even) how much using opamps as buffers actually slow down the circuit ?
I vaguely remember an EN article about that. Interesting enough, ARP 
split the capacitors in two parts in one of their filter modules, one
in the feedback loop of the opamp, and one from the -input to GND.
I guess that was to increase the noise gain of the opamp, so they
could use a fast opamp without getting in trouble with stability.

JH.