The following circuit introduces a click noise into the audio signal (IN1, OUT1), whenever SW1 or SW2 is pressed. How can I eliminate/isolate that noise?

 

Put a capacitor around the switch, similar to these debounce schemes.

Just a suggestion, but scale the drawing to 800 pixels wide max, or keep it a thumbnail. It really detracts from the readability of the thread.

 

Tried the capacitor. Doesn't help at all.... I will see if I can change the drawing to a smaller image... Thanks!

 

I just reread your post, I misunderstood the question. You want to eliminate the sound. This is likely going through the power supply, not the button itself. Filter it a bit tighter.

It is hard to say for sure, since I can't see the schematic the way it's set up (if it were a thumbnail the browser would autosize it), but I don't think I saw a filter cap on the +5V line? You might also isolate the digital and analog with a choke, but the capacitors come first.

 

What would you suggest Bill? I have tried adding caps near the logic ICs and that didn't work. Also put caps at the LED display.

 

Much better! How big were the caps?

This kind of noise is hard to get rid of.

Studing the schematic I have a little trouble seeing what your doing. I think I see a variable impedance on the output, is this used to vary gain or what? The multiplexer is connecting different resistors to the output, from what I'm seeing.

I'm wondering if the click is the sound of U4 and U6 switching?

I've used variation of those mux/demux chips to route audio around, but that doesn't seem to be what you're doing. I'm going with volume at the moment. I'm not thinking power supply any more.

 

I just tried a 100uf cap on the +5... still get the noise. I think I used 1uf by each ic... still didn't fix it. How do I implement a "choke"?

 

You got it. I'm trying to vary the gain on the output.

 

It might be, but if the counter is at 7, the address lines on U4/U6 stay at logic 1. So it shouldn't switch. However, I still get the noise. The noise is more of a thump than a click. It is mid to low frequency.

 

Hi
By changing the switches you are changing the clock inputs to the 193 and that switches the 4051 which is connected to the audio line. The noise is probably due to the digital change in the circuit and is inherent in the design since that is what the switches were meant to do. The only way to see if it is a fault is to get a scope on the input and output of the 4051. It might show a switching spike that could be absorbed on the digital inputs of the 4051. Usually digital outputs are inhibited while they are being changed if used with audio circuits otherwise all the switching noise is sent directly to line.
Hope this helps.

 

How easy is it to try experiments on your board? My thought is to use a second +5 power supplies, completely isolated from the first, to power U4/U6. This should prove whether it is the power supply or not, and offer a clue how to fix the problem.

If it is the power supply, and I am loath to commit at this point, you need to isolate the two sections.

Here is a concept drawing to illustrate what I'm saying...

 

Looking at it, I think Ruptor may be close. Can you switch the inhibit line to see if it kills the click?

 

Looking at it, I think Ruptor may be close. Can you switch the inhibit line to see if it kills the click?

OK, I jumpered pin 6 of the 4051 to +5. Still getting the click noise.

 

YAHOO! I think you might be on to something Bill! I put a resistor on the 9V supplying the 7805 and the click went away! Now the only problem is that my LED has dimmed since I've lowered the current feeding the 7805.

 

I tried several resistors and found the 220 ohm works like a charm! Thank you so much! I have been working on this for months. You guys are great!

 

But instead of attaching your schematic to your post here, it is at another website that does not work.
So I don't know what you are talking about.

 

Thanks Audioguru. I have resolved the issue. Is there a way to remove a post?