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Topic: inhibit pin high without power?! (Read 5181 times) previous topic - next topic


I'm building a simple audio switch using a HC4052 (mux/demuix). Pin 6 is an inhibit pin that needs to be low for normal operation.
I did plan on adding a toggle such that it's high for a mute function which is easy enough, but when there's no power to the circuit there's connectivity across (at least one of the) input/outputs.

It's not desirable that there be any connectivity when the circuit is off, but I'm a bit stumped being a bit of a noob... Is this the job of a suitable transistor such that the two output +s for L and R are blocked until there is a +5v rail available?
Will this be an expensive way to do it bearing in mind that I'm trying to keep the audio distortion to a minimum (I'm not an audiophile, perhaps minimum isn't the right word here...)


I'm a little bit confused.

It sounds like your question is:  "how I do I keep the Pin HIGH while there is no power in the circuit?"  If there is no power, why does it matter?
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but when there's no power to the circuit there's connectivity across (at least one of the) input/outputs.

When there is no power to a circuit there must be no signal into it. That is the way electronics is designed. Putting signals into a chip with no power applied is a good way of destroying a chip or at least latching it up.


Ok, ok... I was trying to give as much information and background as I could - perhaps I could have been more succinct and saved some confusion!

My question is not really how do I have the inhibit pin high when there's no power; I understand that this wouldn't work. But when there's absolutely no power to my circuit, the 4052 allows current to flow between inputs/outputs which means I'll get sound coming out my speakers from and and all input sources. I've proven that this happens in a test circuit, and is not good.

I was really asking for alternative solutions, hense my direct question about using transistors.
The real requirements is that the outputs (two of them - L and R) from the 4052 should be blocked when there is no power, but allowed when there is a supply. This part of the circuit needs to not interfere with audio signals. So finally, my question is what is the easiest / cheapest way to achieve this?


When no power is applied to a chip it looks like a collection of diodes. Therefore to block audio going through you will have to reduce it to below 0.7v and then amplify it up. Probably using op-amps.

Anyone else got any other ideas?


Interesting. I didnt know the signal(s?) got through those analog multiplexers with no power.

Maybe use opto-couplers on the inputs. Digital signals for the opto input (on/off), analog inputs with some circuit for the "other side".. Which may be as simple as just a resistor to GND. But then you might not need the HC4052 :)

Or, keep it powered but inhibited.

It gotta be possible somehow! (Without resorting to relays).
(FYI, I'm not an audiophile either).


Maybe use opto-couplers on the inputs.

Problem is that opto couplers are not linear devices so you need an opto with one LED and two photo transistors and an op-amp to get a liner response.
But basically passing signals through unpowered equipment is a rather odd thing to want to do.

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