Author Topic: MFOS Keyboard Controller Sample&Hold  (Read 543 times)

0 Members and 1 Guest are viewing this topic.

Offline nedhaywardTopic starter

  • Newbie
  • Posts: 1
  • Country: gb
MFOS Keyboard Controller Sample&Hold
« on: August 08, 2020, 11:56:47 am »
Hi
I've spent a bit of time recently trying to breadboard a 1v/oct keyboard controller circuit from Music From Outer Space, and have run into a bit of trouble! Part of the circuit is a sample and hold, the idea being that when a key is pressed, a short trigger pulse momentarily turns on a jfet which charges a capacitor up to what ever voltage is present at the source, and this is then buffered by an op amp, thus holding the source voltage. the problem I've been having is that instead of holding a constant voltage, the S&H part of the circuit causes the voltage to ramp up to V+. I've tried to trace this back to the root of the problem and I've discovered that if I connect the 10nF cap from the non-inverting of the buffer to ground, without connecting it to anything else, the output still ramps up like it was before, rather than staying at a constant voltage like i would expect. I just don't understand where it's being charged from! - Hopefully I'm explaining myself well enough, if not hopefully the attachment will help!
I've tried replacing the JFETs to no avail, so I'm thinking that, because the ramping still happens with the sampling capacitor/buffer disconnected from the rest of the circuit, it must be a problem either with the cap or the opamp? I've tried with TL074 and TL084 op amps and had the same problem both times, I know the circuit recommends LF444, however I haven't be able to source any yet! If anyone can offer some insight as to why I'm having this problem I'd be very grateful! Been tearing my hair out for a few days now...

The circuit I'm referencing, and explanation, can be found here
http://musicfromouterspace.com/index.php?CATPARTNO=PCBMFSNGBUSSKBRD&PROJARG=SINGLEBUSSKEYBOARD2007%2FSINGLEBUSSKEYBOARD2007.php&MAINTAB=SYNTHDIY&SONGID=NONE&VPW=1252&VPH=500

Thanks!
Ned
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf