<html><body><div style="font-size: 12pt; font-family: "times new roman", "new york", times, serif; color: rgb(0, 0, 0);"><div>The series Iabc resistor also makes the OTA resistance to any failures in the expo converter, as well as a slipped meter probe. If you keep the resistor</div>
<div>physically close to the OTA pin it limits the chances of a failure or accidental short circuit to the Iabc pin and one side of the series resistor.</div>
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<div>With no possible current limit, and the Iabc referenced to negative supply that is OTA murder waiting to happen...</div>
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<div>(but I never knew there was a better reason to include it.... :^)</div>
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<div>Harry</div>
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<blockquote style="margin: 0 0 0 .8em; border-left: 1px #ccc solid; padding-left: 1em;"><hr id="MESSAGE_DATA_MARKER"><strong>From: </strong>Ian <synth-diy@synth-diy.org><br><strong>To: </strong>Neil <metadata@gmx.com>; Phil <phil.macphail@liivatera.com><br><strong>Cc: </strong>synth-diy <synth-diy@synth-diy.org><br><strong>Date: </strong>Tuesday, 19 July 2022 12:54 PM EDT<br><strong>Subject: </strong>Re: [sdiy] VCO that goes SHARP in highest octaves??<br><br>Neil --<br><br>Good work troubleshooting. You've given us some important results.<br><br>I'd say leave the series Iabc resistor in -- just good practice. If you <br>can't get it to track well enough, then add the smaller cap and <br>reference current.<br><br>Ian<br><br><br><br>On 7/18/2022 4:17 PM, Neil Harper wrote:<br>> <br>> <br>> On 7/14/22 19:02, Ian Fritz wrote:<br>>> It would be interesting to compare the two separately, if you are so <br>>> inclined.<br>>><br>>> Ian<br>> <br>> I had a chance today to do exactly this!<br>> <br>> (cents) C4 C5 C6 C7 C8<br>> BEFORE 0 0 0 +3 +11<br>> IAN 0 0 -1 -3 -5<br>> PHIL 0 -1 -1 0 +2<br>> <br>> each one required a slight adjustment to the scaling calibration.<br>> <br>> Ian's solution (1/2 integrator cap, 1/2 reference voltage) returned the<br>> VCO to the expected operation matching the TH version. I'm used to using<br>> the HF-TRIM to null out this amount of error.<br>> <br>> Phil's solution (4.7k at 137000's Iabc input) flattened the dip as well.<br>> The results here might not be perfect, because I lost the original<br>> integrator cap after modifying for Ian's solution and could only replace<br>> it with a generic/noname 1000pF cap with no claims of being NPO/COG. So<br>> maybe that could account for the +2 cent rise at C8?<br>> <br>> Anyways, sorry for the bad science. (0805 components and a speckled<br>> epoxy floor do not mix)<br>> <br>> I gave Phill's solution another try on another oscillator:<br>> <br>> (cents) C4 C5 C6 C7 C8<br>> BEFORE 0 0 0 +2 +8<br>> PHIL 0 0 -1 -1 -2<br>> <br>> <br>> This time, with the original NPO/COG cap in place, there was no rise at<br>> C8 - although the initial rise was less.<br>> <br>> Anyways, I'm really happy with both solutions - thank you Ian Fritz and<br>> Phil Macphail for identifying the problem and coming up with solutions.<br>> <br>> <br>> -- <br>> /// Neil Harper<br>> /// Every Wave is New Until it Breaks<br><br>-- <br>ijfritz.byethost4.com<br>https://www.youtube.com/user/frijitz001<br>https://soundcloud.com/frijitz001<br>_______________________________________________<br>Synth-diy mailing list<br>Synth-diy@synth-diy.org<br>http://synth-diy.org/mailman/listinfo/synth-diy<br>Selling or trading? Use marketplace@synth-diy.org</blockquote>
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