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need help matching inputs to PA amp

Posted: Thu Sep 04, 2003 3:58 pm
by squonk
I want to use the output from a Plantronics telephone headset unit and feed it into a Radio Shack 100 watt PA amplifier. What do I have to do to match the impedance to either "microphone" or "CD/aux?" Assuming I can get that done, how can I get rid of any 60Hz hum that results? Can anyone help me? Thanks in advance.<p>Bill

Re: need help matching inputs to PA amp

Posted: Thu Sep 04, 2003 5:40 pm
by russlk
You don't need to match impedances, you just need enuf voltage out of the headset unit to drive the PA. If there is not enuf voltage, an intermediate amp or transformer will be required. There should not be a 60 Hz problem if shielded cables are used and all units operate from the same 60 Hz source (or batteries).

Re: need help matching inputs to PA amp

Posted: Fri Sep 05, 2003 12:19 pm
by Externet
Hi.
There should be no need to match impedances as Russ says.
But you should use a coupling transformer between the Plantronics headset audio source and the amplifier input to isolate the telephone line from any ground.
Miguel.

Re: need help matching inputs to PA amp

Posted: Fri Sep 05, 2003 12:23 pm
by chessman
To expand upon what Russ is saying, I'll add a couple things:<p>1) You may or may not have to match the input. If your amplifier has a 1/4" jack (looks like a headphone jack) for microphone input, you'll be fine. That's a "High-Z", or high impedence, input. If it has an XLR (three pins) jack, that's a low-z input (600 ohms). From www.musiciansfriend.com you can buy an adapter to go from high-z to low-z.<p>2) 60hz hum. you'll probably hear it even if you have a shielded cable, since you don't have a balanced microphone. balanced systems are different than un-balanced, shielded systems. all XLR cables/jacks are balanced, and they pretty much completely eliminate hum.

Re: need help matching inputs to PA amp

Posted: Fri Sep 05, 2003 4:08 pm
by squonk
Thanks, Guys for the great advice.<p>I have one of those small radio shack battery (9V)powered self amplified speakers which i can plug into that seems to work for the intermediate stage although it's noisy. Is there a better way to do this? It's interesting that if I use the earpiece and input it to the PA microphone jack, I get just what i want in terms of sound quality and noise free amplification. Not so for the microphone end of the headset. Why is this?