The 81C55 (IC25) provides 8 data lines to a resistor network. This is the basic 8 bit D to A converter.
The signal is buffered by op amp TL062 (IC36) and is then fed to the 1 of 8 analog switch (IC8 - CMOS 4051).
The eight outputs of 4051 are fed into simple capacitor/resistor sample and hold stages. Each one is fed to a different buffer op amp.
I am not sure what the "odd looking arrangement" is on the VCF log line is about.
The signal is buffered by op amp TL062 (IC36) and is then fed to the 1 of 8 analog switch (IC8 - CMOS 4051).
The eight outputs of 4051 are fed into simple capacitor/resistor sample and hold stages. Each one is fed to a different buffer op amp.
I am not sure what the "odd looking arrangement" is on the VCF log line is about.
But given that the VCF cutoff parameter is linear and the control voltage on VCF log would be expected to be log.
Then I am speculating that the odd arrangement is two things.
On the X4 output side the oddness is a compensator circuit.
On the X5 output side the oddness is an integrator for smoothing the VCF MG waveform.
Mike.
From: "korgpolyex@yahoogroups.com" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Tuesday, February 3, 2009 2:52:03 PM
Subject: [korgpolyex] Digest Number 1252
Messages In This Digest (2 Messages)
- 1a.
- VCF controls From: Gordon JC Pearce MM3YEQ
- 1b.
- Re: VCF controls From: Michael Hawkins
Messages
- 1a.
-
VCF controls
Posted by: "Gordon JC Pearce MM3YEQ" gordon@... gordonjcp
Tue Feb 3, 2009 3:22 am (PST)
How does the VCF cutoff control voltage get generated?
Looking at the circuit, it seems that the control voltage is fed from
two outputs of IC8, through to half of IC7 forming a couple of
sample-and-holds, then into an odd-looking buffer arrangement.
Is it designed so that one output of IC8 provides fine tune and one
provides coarse tune? I would have thought that 256 steps wouldn't be
enough for a smooth VCF envelope.
Gordon
- 1b.
-
Re: VCF controls
Posted by: "Michael Hawkins" korgpolyex800@... korgpolyex800
Tue Feb 3, 2009 5:48 am (PST)
There are two sample and hold circuits used for controlling VCF.
One is the VCF cut off set point and keyboard tracking. That is output X4 on IC8.
The other is VCF modulation. That is output X5 on IC8.
Yes, all D to A in the Poly is 8 bits (0-255). Amazing it works so well!
Mike
____________ _________ _________ __
From: Gordon JC Pearce MM3YEQ <gordon@gjcp. net>
To: korgpolyex@yahoogro ups.com
Sent: Tuesday, February 3, 2009 6:21:14 AM
Subject: [korgpolyex] VCF controls
How does the VCF cutoff control voltage get generated?
Looking at the circuit, it seems that the control voltage is fed from
two outputs of IC8, through to half of IC7 forming a couple of
sample-and-holds, then into an odd-looking buffer arrangement.
Is it designed so that one output of IC8 provides fine tune and one
provides coarse tune? I would have thought that 256 steps wouldn't be
enough for a smooth VCF envelope.
Gordon
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