Hi guys!
I was encouraged to create this topic to answer a question thought of the end consumer or any uninformed person who plugs anything into any jack… >.<
I’m developing this Opto VCA with 3 switchable modes (HPG / Full Range / LPG), and the questions are:
1- Is the connection of the diodes after the Jacks protecting against negative voltages and excessive voltages?
2- Are the diodes after the switch isolating the 2 positions that use the same Op-amp?
3- This Op-Amp from the question above is also protected when it is in disuse?
3- Is something wrong? What would you improve on this schematic?
Note: Op-Amps will be switched to TL072 soon.
Please: Mention MY CREDITS If you use this schematic as a reference elsewhere
If you’re switching to TL072s, and I assume to ±12 V power, then I’d say there really isn’t a need for protection diodes — unless you expect to use it in a system that has, say, ±15 V potentially finding its way onto a cable. In that case yes, use diodes from the input and output lines to ±12 V. But I’d suggest using Schottky diodes, such as 1N5817. The forward voltage of 1N4148 is around 700 mV. That means it’d allow negative voltages down to about -12.7 V. The TL07x spec says the lower voltage limit is -12.3 V, so that’s out of spec. The 1N5817 forward voltage is 0.32 V, so it’d keep you, well, almost in spec.
D12 and D13 are all right, but D2, D8, D9, D10 need a series resistor between them and the jack to protect them. 1k should be enough.
But again, if you’re using this in a ±12 V system with ±12 V power on your op amps, the protection diodes (and resistors) really aren’t needed.
I’m not sure what your intention is with D3 and D4 so won’t comment on them.
And I don’t really understand your question about the op amp, but it looks fine.
Anything else wrong? Just nit picking, “KΩ” should be “kΩ” and there’s a “c” missing from “electronics” at the lower right…
The power would be 0v +12v but since we are on a Eurorack system, it would be better to use it at ±12v if not for the optocoupler, should I put a 1N5817 Schottky diode just at this point?
I’ve heard about Schottky, but I only have zener in my hands now…
D3 and D4 would be to use the same Op-amp as a buffer at positions LPG(C1) and HPG(R4) without the Audio In signal coming out from pin 4 going to the opposite position. I imagine it won’t work if it’s not a 1N5817 either…
You already answered the Op-amp question from the above question: ±12v = unnecessary diodes.
Thanks for the enlightening reply!
“KΩ” was just an oversight and I am extremely embarrassed to have exposed my mark without realizing the spelling is wrong! (>.<)
(Un) fortunately, English teaching is weak in my country, as you could see…
If it’s a bipolar AC signal around GND, I’m not sure the diodes on the audio input are going to do what you want. Maybe it depends on what the emitter voltage is - but in that case I think you’d want an AC coupling cap on the input so that it all works properly. I’d be very tempted to build at least that part on a breadboard and take a look at the waveforms to make sure you’re not accidentally truncating your audio in with the protection diodes.
Seems to me AC coupling is just what you would not want, at least not by itself. Now that I’ve looked closer I see this whole thing is set up to handle positive voltages. I don’t think it’ll work with an AC signal. And maybe it’s intended to be used with an input that produces something with a positive bias. But if it’s supposed to handle an AC input, then yes, I’d think AC coupling, but then add a known DC offset to put the signal in the middle of the op amp input range.
I note the passive version posted elsewhere had the audio in connected to the phototransistor collector and here it’s on the emitter. Does that matter? I’m not much familiar with phototransistor optocouplers. Do they work on AC signals? I would have thought not.
OH! Just remembered… if you post untested schematics (or stripboard layouts) it’s best to write UNVERIFIED on the image, just so if it gets picked up without context people know not to expect it to work.
I think the input must be at least prepared to receive AC and take only DC to the optocoupler led, the same it will not occur for Audio In and Out as the Op-Amps will be TL072 at ±12v.
Yes, I know you can do it with AC through a driver led, but that’s not the objective here. The idea is a simple and peculiar module compared to transistor circuits.
I’ll update the schematic with more time, but CV In will discard DC in GND so we don’t extend the circuit (I want it to fit in 2x8cm).
In tests there was no practical difference between emitter and collector, even inverted. I think it’s like a vactrol, only faster/cleaner, and less linear…
While I’m still not sure I understand what the circuit is trying to do, you may want to be careful with which way 'round you connect the transistor in the photocoupler - the datasheet says Vec max is 6V.
I really forgot to pay attention to the datasheet…
I see then that the phototransistor works at ±6v if reverse voltage refers to DC on the datasheet (or is it the inversion of pins 3 and 4?).
However 6v is the maximum voltage of the “Emitter Collector” and 35v that of the “Collector Emitter”. So if you use the pin in the right direction, will it withstand a voltage of up to 35v at the input, and when CV is triggered it will transmit a maximum of 6v? I’m confused about that too…
If so, only the output and the led must be protected with 1N5817. Right?
Cheers to you too!
If the audio input is AC centered on 0 V then you’re going to need to redesign to get rid of diodes D3, D4, D1, and D7. I still haven’t figured out what you’re doing there, so I don’t know how to do it, but those diodes won’t pass the negative part of the waveform.
Absolute maximum ratings are one thing and typical/min/max electrical characteristics are another. Absolute maximum reverse voltage is 6 V; you must design to prevent larger reverse voltages. As for what you’ll get out of it under smaller reverse voltage, the maximum reverse current is 10 µA at 4 V, as compared to 30 mA forward collector current at V_CE = 5 V. Whatever you may have seen on a breadboard, this is a device intended to work in one direction only, with positive signals applied to the collector.
Hi friends, I came to update you on the schematic:
As expected, most of it didn’t work with the 1n4148, the 3-position switch didn’t either (I tried in several ways).
While my order for TL072 and schottky diodes does not arrive this schematic is functional with VCC = 12v and GND = 0v and CV from 0v to 6v.
The Swich is now 2-position (LPG/HPG).
Capacitors were readjusted during testing.
There is a resonant saturation that reminds the MS20 Filter all the time, probably because of the Audio In overload or the very bad LM324.
Personally I liked that, so it might be a good idea to put another switch to select between the Optocoupler (distortion) and a vactrol (traditional gate)…
Tested with EMW Multi Wave, Cre8Audio Chips and EMW 4x ADSR.
If the audio input is AC around 0 V then you’ll get distortion because the LM324 cannot output negative voltages. You’d have to add a DC bias to the audio signal to get good results out of that op amp.
After a lot of brainstorming, I found that a channel in my Mixer module is damaged (probably from connecting and disconnecting jumpers with everything turned on). Testing on other channels everything works fine, (Wave ok) including the passive circuit if it wasn’t for the weak LFO of the Cre8Audio Chips…
So my decision will be to keep it as a low cost VCA, as the Optocoupler has a transistor and doesn’t have the LPG effect of an LDR.
As for the topic title, I have 2 options:
1 - Led Driver Circuit after the CV input buffer for the anode to work with DC
2 - Ground DC with resistor + diode
I imagine that the led driver will not work, because it will transform the DC part of the wave into AC, just deforming it…
