From: wagner@mala.bc.ca (TOM WAGNER, Wizzard of old Audio/Visual Equipment........Nanaimo Campus)
Subject: correction of last followup re relays
Organization: Malaspina College
Lines: 73

In article <1993Apr20.102756.1709@mala.bc.ca>, wagner@mala.bc.ca (TOM WAGNER, Wizzard of old Audio/Visual Equipment........Nanaimo Campus) writes:
> In article <C5r60r.4ID@megatest.com>, alung@megatest.com (Aaron Lung) writes:
>> 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!
>> 
>> Are you switching high level signals or low level signals like pre-amp
>> out level signals?  Also, are the clicks you mentioning the big
>> clack that happens when it switches or are you refering to contact
>> bounce?  How are you driving the relays?  TTL gate output?  Switching
>> transistor?  How are the relays connected to what you are driving?
>> 
>> Need more specifics to answer your question!! :-)
> 
> As a general rule, no relay will cleanly switch audio if you try to tranfer
> the circuit with the contacts.  The noise you hear is due to the momentary
> opening and closing of the path.
> 
> The noiseless way of transfering audio is to ground the circuit.  In high
> impedance audio circuits a resistive "T" is constructed close to characteristic
> impedance of the circuit.  Grounding the imputs (connected to the T) transfers
> the audio.
> 
> In low impedance circuits transformers are usually used, and the inputs are
> shorted out or grounded.  Secondaries are paralleled at the characteristic
> impedance.
> 
> Sometimes if it is necessary to actually switch audio, a second contact is used
> to momentarily short the circuit output for the duration of the switching time.
> 
> Telephone relays are handy, because contacts can be adjusted to "Make before
> break and Vica Versa" but I haven't seen any of these for years.
> 
> Nowadys switching is done electronically with OP amps, etc.
> 
> A novel circuit I used to build was a primitive "optical isolator".. It consists
> of a resistive photocell and a lamp, all packaged in a tube.  When the lamp is
> off the cell is high resistance.  Turn the lamp on and the resistance lowers
> passing the audio.  Once again this device in a "T" switches the audio.  Varying
> the lamp resistance give a remote volume control.  Use 2 variable resisters and
> you have a mixer!
> 
> Lots of luck!
> -- 
> 73, Tom
> ================================================================================
> Tom Wagner, Audio Visual Technician.  Malaspina College Nanaimo British Columbia
> (604)753-3245, Loc 2230  Fax:755-8742  Callsign:VE7GDA Weapon:.45 Kentucky Rifle
> Snail mail to:  Site Q4, C2.   RR#4, Nanaimo, British Columbia, Canada, V9R 5X9  
> 
> I do not recyle.....   I keep everything!       (All standard disclaimers apply)
> ================================================================================
-- 
73, Tom
================================================================================
Tom Wagner, Audio Visual Technician.  Malaspina College Nanaimo British Columbia
(604)753-3245, Loc 2230  Fax:755-8742  Callsign:VE7GDA Weapon:.45 Kentucky Rifle
Snail mail to:  Site Q4, C2.   RR#4, Nanaimo, British Columbia, Canada, V9R 5X9  

I do not recyle.....   I keep everything!       (All standard disclaimers apply)
================================================================================
