In article <C5qsBF.IEK@ms.uky.edu>, billq@ms.uky.edu (Billy Quinn) writes: |> I built a little project using the radio shack 5vdc relays to switch |> audio.  I got pretty bad 'clicks' when the thing switched.  I was doing |> most of the common things one is supposed to do when using relays and |> nothing seemed to get rid of the clicks. |>  |> My question is: |>  |> 	Is there a good relay/relay circuit that I can use for switching |> audio, so that there will be *NO* noise of any kind on the audio lines. |>  |> I will appreciate any advice or references to advice.  Also, exact part |> numbers/company names etc. for the relays will help!  The first step is to make sure that there is no DC component, intentional or not, on the audio lines to be switched. If neither the output or input presents an unintentional DC level (let's say they're capacitively coupled), then putting a 22 K resistor from each input to ground, and from the output to ground is a good precaution against leakage in any coupling caps presenting a DC level that is GUARANTEED to cause a click.  If the device presents an unintentional DC level that is from more than just coupling capacitor leakage, then you need to remove the DC before switching with coupling capacitors of your own.  If you have more than one relay (each relay switching one input through to a common output), you can use make-before-break switching where for a brief instant both sources are connected during a switch change. This may mask the switch click.  An audio switch that is clickless is made by taking a light bulb and making your own ersatz opto-isolator with a light-dependent resistor (LDR) sealed inside a piece of black heatshrink. Many broadcast consoles use this older, but time-honored technique. Claro actually makes a complete optisolator like this, but I don't have a part number handy. Radio Shack has a LDR grab-bag (276-1657) you might want to try.  For a simple, solid state clickless switch that is cheap, you might try  looking at the CD4066 analog switch that is commonly available (or its older predecessor, the 4016). Again, the DC component must be removed prior to switching. Other alternatives include JFET switches (both discrete and IC) and diode switching (which works relatively well, surprisingly). All the above, with their pitfalls and application tips could easily make a subject for a multi-page article.  Solid state switches may induce a click of their own when turned on due to charge being injected into the audio line through device capacitance when the control signal changes state. Techniques to minimize this include lowering the impedance of the signal paths through the switch, differential switching, and slowing the risetime of the control pulse. With the 4016, cutting the click can be as simple as putting 10K - 47K resistors in series with the control pins.  Hope this helps at least get you started.  --  --------------------------------------------------------------------        Dave Medin			Phone:	(205) 730-3169 (w)     SSD--Networking				(205) 837-1174 (h)     Intergraph Corp.        M/S GD3004 		Internet: dtmedin@catbyte.b30.ingr.com   Huntsville, AL 35894		UUCP:  ...uunet!ingr!b30!catbyte!dtmedin     ******* Everywhere You Look (at least around my office) *******