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Modifying a S&H to only sample with a gate signal ?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Modifying a S&H to only sample with a gate signal ?
johnstilton
Hello,

I am have finished soldering an MFOS Ultimate PCB but am wanting to make a modification to the S&H circuit.

http://musicfromouterspace.com/analogsynth_new/SOUNDLAB_ULTIMATE/pdf/s chematic_pg8_schem.pdf

Effectively, I would like an option so that the circuit only samples upon receiving an external gate signal instead of the internal pulse generator.

Any suggestions on how that could be done?

Full description of the circuit can be found here : http://musicfromouterspace.com/index.php?CATPARTNO=&PROJARG=SOUNDLAB_U LTIMATE%2Fpage2.html&MAINTAB=SYNTHDIY&SONGID=NONE&PLAYONLOAD=FALSE&VPW =&VPH=&CATMOBISTATUS=&SYNTHDIYSTATUS=CONTENT&SEARCHTERM=

THX
guest
it is probably best to send your signal into the oscillator opamp, and use it to convert your gate to the same +/- output signal. its the falling edge of the opamp output that activates the sample phase, and the DC blocking capacitor C62 allows the gate voltage to float back up for the hold phase. so you need the rising edge of the gate to cause a falling edge at the opamp, so the signal needs to go into the inverting pin of the opamp. but, you need to disconnect the other circuitry from that pin so it doesnt oscillate as well as gate. to do this, you can use a normalled jack. cut the trace going to pin2 of the opamp, and send the passive components to the normalling switch on the jack, and then connect a 10k resistor between the jack tip and pin2 of the opamp. this resistor is just here to protect the opamp, and wont effect the circuit elsewise. the final thing you need to do is set the threshold. if you want a 0-5V gate signal, the thresholds should be 1V and 4V, for example. to do this, put a pullup resistor from pin3 to +12V. something around 680k should be ok, but the values might need some testing. this will change the frequency of the oscillator, so C64 might need to be changed out to get it back to where you want it.
guest
just ran the numbers, the pullup should be 470k.
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