re suggestions on audio relays in article c qsbf iek ms uky edu billq ms uky edu billy quinn writes i built a little project using the radio shack vdc 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 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 you might want to try for a simple solid state clickless switch that is cheap you might try looking at the cd analog switch that is commonly available or its older predecessor the 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 cutting the click can be as simple as putting k k resistors in series with the control pins hope this helps at least get you started dave medin phone w ssd networking h intergraph corp m s gd internet dtmedin catbyte b ingr com huntsville al uucp uunet ingr b catbyte dtmedin everywhere you look at least around my office the opinions expressed here are mine or those of my machine
