voip handset and Behringer aux send

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nobby6
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voip handset and Behringer aux send

Post by nobby6 »

Hey all,

Trying to use a cisco spa942 voip handset with Behringer mixer, the phone has 2.5mm headset in, I got the pinout sorted with T mic-in, R audio-out, and of course S ground.

Audio out from Cisco straight into the Behringer works great, audio sounds really good, but, trying to use Aux Send to the cisco is not working out so well, the audio level is so very low, but whats there is distorted.

I'm aware "line out" is usually 1-2v, and mic ins are usually millivolts, I tried making a 40db L-pad attenuator using a 10K inline resistor and a 100 ohm across T to S, didn't seem to make any difference. (I was surprised too, I thought I'd need the reduction on the phone out, but Behringer is happy - maybe self compensates on its inputs, who knows) but something is definitely awry on the aux send out.

I've verified with a cheap headset that the audio into phone via headset jack is all good.

There is obviously a mismatch somewhere (I thought that would involve distortion - but for the levels when maxed out to be so quiet confuses me).

Anyone had success in this sort of thing? are my resistors too high value perhaps, or maybe I only need an inline and not T to S resistor

Yes, as you can envisage, this is poor mans tel system

Thanks for any pointers
grich
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Re: voip handset and Behringer aux send

Post by grich »

Most mic inputs like this have a DC voltage on them for the condenser mics typical in small phone headsets. It may be messing with the mixer output electronics. Try a 1 uf non-polarized cap in series with the mic input before it gets to the pad.
nobby6
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Re: voip handset and Behringer aux send

Post by nobby6 »

Hi grich,

So if I get you, you're saying even those phone to mixer is all good, it might be causing the inference? even though aux send is off for the input which uses the TS input not XLR and phantom power switch was off whilst I was testing, was trying to move in baby steps so if it craps out I know at what point to roll back at

Or are you suggesting 1uf on mic-in to phone? Since thats the easiest to test I might try that one first anyway :)

I guess I'm about to find out if that 1uf is critical value? I do recall having a couple of 1.5uf tantalums but not 1uf
grich
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Re: voip handset and Behringer aux send

Post by grich »

Mic input to phone is where you want the cap. Value is not super-critical, except for the non-polarized part.
nobby6
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Re: voip handset and Behringer aux send

Post by nobby6 »

Just a quick update so you don't think I'm rude, the caps I had were polarized and different values, no non polarized available in my usual locals, so waiting for courier from down south, frustratingly due to flooding they are delayed until early or possibly mid next week, I will reply again with the updated results.
nobby6
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Re: voip handset and Behringer aux send

Post by nobby6 »

@grich

That 1uf made a 95% improvement, its now loud and most importantly crystal clear, I added a 47k resistor back inline to tame down the level, but you're right, that 1uf made all the difference, thanks again for your help, much appreciated.
Cheers
grich
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Re: voip handset and Behringer aux send

Post by grich »

You're welcome! :)
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