Re: [korgpolyex] Digest Number 1252
2009-02-03 by Michael Hawkins
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. 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. ________________________________
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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 Korg Poly800/EX800 Users Korg Poly800/EX800 Users Messages In This Digest (2 Messages) 1a. VCF controls From: Gordon JC Pearce MM3YEQ 1b. Re: VCF controls From: Michael Hawkins View All Topics | Create New Topic 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 Back to top Reply to sender | Reply to group | Reply via web post Messages in this topic (2) 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 Back to top Reply to sender | Reply to group | Reply via web post Messages in this topic (2) Recent Activity * 4 New MembersVisit Your Group Yahoo! News Fashion News What's the word on fashion and style? New business? Get new customers. List your web site in Yahoo! Search. Biz Resources Y! Small Business Articles, tools, forms, and more. Need to Reply? Click one of the "Reply" links to respond to a specific message in the Daily Digest. Create New Topic | Visit Your Group on the Web Messages | Files | Photos | Links | Database | Polls | Members | Calendar MARKETPLACE ________________________________ From kitchen basics to easy recipes - join the Group from Kraft Foods Change settings via the Web (Yahoo! ID required) Change settings via email: Switch delivery to Individual | Switch format to Traditional Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe