<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 2, 2006, at 4:13 PM, Jeffrey Needham wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">Crap.</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">ARP Trigger IN spec:</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">+8V min.</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">10 usec min.</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">I guess I still need to find at least 8V somewhere since the Pro-1 GATE OUT</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">is 0-5V, so it needs to be level-shifted.</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">I guess I could level-shift the Pro-1 and then add the cap to the Avatar?</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">OK, that's pretty ugly.<SPAN class="Apple-converted-space"> </SPAN>It has some appeal as it means I don't have to</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">build a circuit (providing I can level shift the Pro-1 GATE)</FONT></P> </BLOCKQUOTE></DIV><BR><DIV>Hello Jeffrey,</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>If I was doing this, I would probably put the modification inside the ARP. This is a problem for pretty much all ARP synths, and here is your chance to correct their "mistake". Install the two-transistor level shifting circuit somewhere inside the ARP, and connect it internally to the ARP's Gate input (at the terminals of the Gate input jack). That way, the ARP will always gate/trigger correctly no matter what you connect it to. (BTW, you can use pretty much any small NPN transistor for that circuit. If you can't find the 2N2222, look for a 2N3904 -- or a BC548 if you're outside the US. Also, that 5K resistor can be a 4.7K, since you'll likely have a hard time finding an actual 5K resistor)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Regarding adding a cap in series to <A href="http://www.physics.ucla.edu/demoweb/demomanual/electricity_and_magnetism/ac_circuits/rc_integration_and_differentiation.html">differentiate</A> the Gate signal into a trigger pulse: It's a good idea to add a diode to ground after the cap before connecting it to the ARP's trigger input. This will insure that the trigger spike only goes positive, and will also guard against possible double-triggering. Otherwise, you'll get a bipolar double spike every time the cap receives a gate signal. </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Use any regular small-signal diode, such as a 1N914 or 1N4148. Connect your level-shifted Gate to one side of the cap, and connect other side of the cap to both the ARP's trigger input and to the <A href="http://en.wikipedia.org/wiki/Diode">cathode side of a diode</A> (the side with the little line mark). The other side of the diode (the anode) should connect to ground. This will eliminate the negative-going part of the spike that's created by the diode, leaving just a clean positive spike for your trigger.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Depending on the inout impedance of the ARP's trigger input, you may also have to add a resistor in series with the cap's output. Sorry, I don't know what value. The resistor will control the discharge rate of the cap, and thus control the duration of the trigger spike. You may have to experiment a bit to get it right.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Do look at <A href="http://www.cgs.synth.net/modules/cgs24_gatetotrigger.html">Ken Stone's circuit</A> -- it's more complex than what I've suggested, but it's really perfect. See how he has the diode connected to ground in each of the two sections -- that's how the aforementioned diode would be connected. You could also use one half of Ken's circuit to take care of the needed Gate level shifting stuff really nicely. Simply eliminate the 10nF series cap in the first of the sections (just jumper across it). You might have to juggle a few resistor values on that circuit half to get it right, but the end result would be a clean level-shifting comparator circuit for your low-voltage Gate. You would then connect that shifted gate to the input of the second circuit section (with the cap in place) to derive the trigger.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Good luck!</DIV></BODY></HTML>