Archive of the former Yahoo!Groups mailing list: Discussion about the Korg PolySix synthesizer

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Subject: Re: [PolySix] RE: Polysix tuning difficulties.

From: Bob Grieb <bobgrieb@yahoo.com>
Date: 2014-03-02

Hi Malte,

    As you said, there are two "extra" time slots that are constantly being stepped through by the ABC mux select signals.   (I checked with a scope this morning)   These are slots 6 and 7. 

Time slot six connects the output of the converter to the opto-coupler feedback circuit.
The RC time constant of 330K/47 uF is 15 seconds, so this would be a slow correction,
not quick feedback.   This feedback affects the mv/oct scale at the input of the converter
by adjusting the R98/R85 voltage divider.  I am wondering if this feedback is more for the higher frequencies (lower output voltages) since the Tune High VR affects it. 

Time slot 7 also feeds back, through IC18-1,2,3   Again, the time constant at the output is about 5 seconds, so this is also a slow feedback path.   This one seems to affect the "center" setting, as it feeds an offset voltage, instead of changing the input attenuation.  When this feed back is selected, VR15 is also enabled, so I guess it makes sense that this would be a "center" feedback.

     Bob


From: Malte Rogacki <gacki@gacki.sax.de>
To: PolySix@yahoogroups.com
Sent: Sunday, March 2, 2014 1:40 PM
Subject: Re: [PolySix] RE: Polysix tuning difficulties.

 
This is just from the top of my head:

The optocoupler is in a feedback loop from the demultiplexer output to the
antilog amp.
If I remember correctly there are two unused "slots" in the DAC (and hence
two "unused" multiplexer outputs). But those two "slots" are always at the
same fixed value (I think). By using such a feedback loop it should be
possible to account for any changes to the circuitry (temperature,
voltage...) because this output would carry the changes and hence counter
them in the optocoupler.
I haven't checked what exactly the second "unused" multiplexer output does.
That's the one that leads back to vicinity of the 4051 in front of the
antilog amp (or the one on KLM-396 for old production boards).

At 6:15 Uhr -0800 02.03.2014, Bob Grieb wrote:
> One thing I don't understand is how the 4051's are being used to change the
> behavior of the circuit. And what is the fcn of the opto-coupler? Any
> thoughts?