Help with optoccoupler switching on Jet City amp?!?!

VaporDemon

VaporDemon

New member
I have a Jet City 100HDM that uses an Optocoupler switching circuit to switch channels. The circuit is very similar to the SLO circuit which uses two optocouplers to switch the gain controls and two for the channel volumes. There was a known channel bleed issue which I obtained the parts and schematic for the fix of the bleed issue and it did solve the problem however I noticed a drastic reduction in the overall available gain from the preamp section so it lost some of its growl. Just wondering optocouplers can vary in terms of signal loss from one batch to another or is that not typical for those types of components? I am considering coming up with my own circuit to switch the signal path using relays but I thought I’d put this up to see if someone with more experience might have some insight on optocouplers. I did replace the two original parts as well as adding an extra which is required for the updated design and all three are new replacements. Spec wise the parts are the same but I wasn’t sure how much those components affect signal or tone other than to allow or prevent continuity of the signal. Thanks in advance!
 
Optocouplers go high impedance but are not a true break in the circuit. They are noisy and their switching speed is awful - much slower than even JFETs used. For true isolation, relays are king.

My modded Marshall design is all around relays. SLO MKii was updated to relays along with the buffered loop. Peavey 5150s are relays.

Go relays and avoid optocouplers unless you intend to use them to mod an already existent circuit. They have their place, but if you can help it, go relay.
 
You shouldn't have lost gain. It sounds like you may have done something wrong. I would take some measurements with the amp in each switched state. I also might put signal into the amp and measure the A/C signal thru the preamp to see if that reveals where signal is getting dumped where it should not be. You could also put jumpers across the optos to check if they are truly closed.

Like glpg80 mentioned, these things go from 'really low' resistance to 'really high' resistance and aren't ever truly 'open' or 'closed'. And in addition to reacting relatively slowly, they turn 'on' and 'off' at different rates. So you might get some really quick, weird effects loop 'swells' when you switch channels depending on how the amp is set.
 
Thanks for the replies, I will have to go over my work one more time but if I can’t figure it out I should be able to change it to relays pretty easily. Appreciate the help!
 
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