=?ISO-8859-1?Q?Re: [sdiy] Looking for OB-8/?=CEM3320 filter gurus

[jhaible at debitel.net]

>I finally started working on that dead OB-8 filter.
>Although the schematic looks a bit weird at first sight, it's starting
>to become clearer, once all the 2 pole circuitry is omitted, leaving the
>basic 4 pole part.

In 2pole mode, one gm cell is used as an opamp (reducing the
filter cap, but not to zero, because you need a phase compensation).
It's remarkable that this is stable, because this "opamp" gets the whole
range of bias currents for its input stage (just like the stages that are
used as integrators).

>However, I don't quite understand the way the gain cells are working.
>Is there a more complete data sheet than the 2 pages found at synthech.com?

Look for the Curtis patents. All the details are there. It's basically the
same OTA + cap + buffer topology as the SSM2040, but a very different
circuit design. Linearized, a current input that needs to be biased for
a slightly positive voltage.


>To me each gain cell/buffer combo looks like an inverting op amp (plus
>filtering of course), with a summing node, Rc for input, and Rf for
>feedback.

Well yes, a simple opamp is nothing else than an OTA, a compensation cap,
and a unity gain buffer. But here you have a rather high cap, and a variable 
transconductance in the input stage. (And splitting hairs, it's not a
transconductance, but a "current gain" in Curtis chips.)
If you want to understand it better, get the data sheet for the "missing link"
between OTA and opamp, LM346. (www.national.com)


>But in the OB-8 circuit there are additional resistors (RX44, 59, 58)
>that appear to add a negative DC offset current into the summing nodes.
>These resistors are not present in the data sheet sample applications, so
>they don't seem to be required by the 3320.
>What's the use of this? Is it to improve the chip's operation in some way,
>or is it to intentionally cause asymmetric clipping in each stage, in
>order to spice up the filter's sound?

This is to bias the input stage properly. What you want is an output voltage
between positive supply and GND (no negative swing, here!), and for that
you need a bias current at the input. Part of it comes from the feedback
resistor of the filter stage's own output: If output is at 7.5V and input is
at 0.6V (typical conditions), there is a bias current flowing.
Sometimes you have an additional bias current from a previous
filter stage (DC coupled, antother resistor from approx. 7.5V to 0.6V).
Then you must compensate this with a resistor to th enegative supply
voltage, in order to keep that filter stage in the intended output voltage
range. For various filter conigurations, and for various negative supply voltages,
these resistor values must be different.

Speaking of negative supply: Curtis filters are typically working between
positive supply and GND for audio. (Complicated to calculate, but
nice to interface with 4000 series CMOS switches!) A negative supply is
only needed for internal CV biasing. It's limited to a small value by an on-chip
zener diode (I think the process they used was only 24V, so no +/-15V supply
without extra means of voltage limiting!). Application note circuits mostly
use this internal zener, but the OB-8 uses an external -5V supply instead,
effectively bypassing the on-chip zener and giving more precision.

There was a *lot* of design work behind the OB-8 (and a lot of experience
from the OB-Xa), and the switchable 2pole / 4pole filter is simply amazing.
This is neither a 4pole filter that is tapped after the 2ns stage, like so many
others do, nor two SV filters as in the Jupiter 6 (also easy to design), but
a complete re-configuration from 4-pole polygon-filter to 2pole state variable
filter. Real pole stages become integrators, one stage tunrns into an opamp,
and all that with the DC bias current issues everywhere. 

Enough of OB-8 praise for now. (;->)

When you have fixed yourOB-8, how about doing some modifications?
http://home.debitel.net/user/jhaible/hjob8mod.html
and
http://home.debitel.net/user/jhaible/hj_pmob8.html

JH.