protecting inputs

[dnny]

how to protect inputs on our DIY gems?

i have just burned some IC´s by connecting the headphone output of MC-303 to robot-voice.
so now i learned that headphone signal is not suitable for this thing.
but, can i make some kind of simple circuit that would allow me to use different outputs and be safe?

limiting resistors, zener diodes, transformers, optocouplers, how do they work?
is there something else?


thanks

daniel

[Pehr]

Burned ICs in the 303 or the robot voice?

[dnny]

[Hank Reynolds 3rd]

I've used this method on guitar pedals I've made with no problems so far.

on a negative ground circuit, put a diode ( I always use 1N4001, but I think there's similar available) with the
cathode(-) towards the 9v supply ( silver or black banded end ) and the
anode(+) towards ground.
For a "positive ground" circuit, just reverse that position so the banded
end connects with the now "positive" ground.

[Hank Reynolds 3rd]

ack,my bad....you mean't audio I/P's ...

[jksuperstar]

It's difficult to say 1 good circuit for all uses, since one headphone jack might be expecting an 8ohm load, while another could drive a 600ohm load and be very happy about it. If you use some circuit to isolate things, then you could end up changing the freq. response inadvertently....but then again, we're hackers, and might not care!

Having some tools at your disposal such as a digital multi-meter, that can read both DC and AC voltages, is extremely helpful in try to understand what the two circuits are doing. Before connecting them, it might be helpful to get them onto the same ground, and pump the incoming signal through a capacitor, so you only get your audio signal (won't work for control voltages though, since they'll most likely need a DC offset).

optocouplers might be good for somethings, but they are typically only single-rail supplied, since the LED will only light in 1 direction! So an AC audio signal will get 1/2 rectified through this method. If you find some differential optocouplers, they might be your most flexible bet. Just put a current limiting resistor on the input(s), and the output is driven by your destinations power supply. You can build something like this with two opto-couplers, that have complimentary outputs (1 with npn driver, the other with pnp). This would diffintely be the best isolation you'd get between two circuits.

[zipzap]

I don´t really know much about thouse things, but i think it only gets dangerous when the voltage at the input is higher or lower than the voltage your circuit is runing on (that might be completly wrong!)
So one diode towards pos and one towards neg supply should do the trick, no?
About outputs i know even less, but i think since most of the circuits we do have high input impedance there should be no problem with overloading.

[bugbrand]

This snippet from the Soundlab Mods page may be of use:::

http://musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/SoundLabMods.html#PROTECTCVINPUTS

Zener diode method.

[mosc]

If you have inputs with op amps hooked up in the inverting mode then the summing node (the - input on the op amp) will always be at virtual ground. You can protect this input with two diodes connected back to back and connected to ground. These diodes should be silicon diodes. Since the forward turn on voltage of a silicon diode is 0.7 volts, this little circuit shorts out anything that exceeds plus or minus 0.7 volts.

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[bugbrand]

So could you replace those silicon diodes with zeners for, lets say, a 9v limit?
((I don't know much about zeners..)) - 0.7v is fine for guitar levels, but isn't much when you're looking at synth circuitry

That end to end silicon arrangement is the basis of many distortion stompboxes isn't it? --- of course, you could have several diodes in series for a larger voltage range.

[Blue Hell]

The diodes are at a point in the circuit that normally should have zero volts, no matter if you're driving it with 10 volts or whatever. Except when the opamp can't cope anymore, which means you're using the circuit out of specs, that's when the diodes kick in and keep the opamp healthy.

[zipzap]

let me refresh my basic understanding of electronics here: At that point there should be 0v because ther´s 0v at the non inverting input and the amp is feeding back any difference between its two inputs... Is that right?
But Anyway, Protection against what? what are the things that can destroy a circuit? (electronically, i don´t mean people, cats or hurricanes)

[Blue Hell]

Yes. It's the opamp's inverting input. When you drive that one high the output will go negative pulling the input back to zero. Now this will only work when the opamp's output can gow low indeed, and it won't go lower than it's negative power rail. So when you try to raise the input too much the opamp can't compensate and the voltage at it's input will go out of control. Now it depends a bit on the specific opamp you use (it's a figure in the data sheet), but eventually when you go on pushing it it will not survive. And of course the reverse holds when you make the input low. Exactly when it goes wrong depends on the actual values of the resistors, but with all resistors equal you get into this non compensating behaviour when the circuit's input voltage goes outside the supply voltage range, the opamp will survive a little bit longer though. But when the feedback resistor is twice the value of the other resistors (giving a circuit amplifiction of -2) the problems start on having about half the supply voltage on the circuit input.

See : http://en.wikipedia.org/wiki/Opamp and http://en.wikipedia.org/wiki/Operational_amplifier_applications for a good introduction and some basic circuits with calculation examples.

I can't really think of another way to damage an opamp ... well I once killed a CA 3130 I think with HF oscillation ... that was a badly designed circuit with a somwhat hard to handle (fast, for it's time) opamp ... maybe short circuiting the output, but I think most opamps handle that situation (and you could easily add a series resistor there of lets say a few hundred ohms).

[mosc]

I would only bother with this for inputs connected to external jacks. Sometimes there may be some large charge or a large DC offset voltage on a plug. This will provide some protection to the op amp and possibly some circuits down stream.

[zipzap]

[dnny]

thanks for all that info - i just remembered that i have seen a protection schematic like this:


its more circuit bending stuff and its used to protect the CPU from static discharges when using touch sensors / body contacts.

original schematic from CYBERYOGI =CO=Windler's keyboard site "WarrantyVoid"

[mosc]

comparitor ckt