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Re: A hum while on charge

Subject: Re: A hum while on charge
From: Walter Knapp <>
Date: Sun, 09 Mar 2003 11:22:47 -0500
Klas Strandberg wrote:
> Someone wrote:
> 
> ">It's not the cable between the Telinga and Portadisc. I've replaced that 
> 
>>cable and am using a 5 pin XLR connector and Canare star quad cable. So, 
>>it's all balanced cabling as far as the Telinga handle." 
> 
> 
> --------
> 
> Hm... Just to make sure:
> 
> There are two inputs in a balanced mic amplifier. Pin 2 is one input and pin
> 3 the other. When recording, both inputs must "see" the same impedance. 
> 
> The output of a PRO5W handle is 200 Ohms. If the signal lead is connected to
> pin 2 (hot / signal) the other lead (cold / no signal) must be connected to
> ground - at the PC board, inside the handle - with a 10 mfd condensor and
> 200 Ohms resistor.
> 
> Then, a induced hum in the cable going to pin 2 will be cancelled by the
> same induced hum in the cable going to pin 3, as the two amplifiers are
> working out of phase with one another. 

Just to make sure, are you saying I needed to add a 10 mfd condensor and 
200 ohm resistor to the circuit board for the cold lead, or are they 
already there? And, since this is in common on two channels, should 
there be two sets if they are needed. Not a lot of room in there for new 
components.

I did not go to the trouble to diagram the board while I had it out, so 
don't know for sure what it had. It was originally the model with the 
stereo miniplug and internal rechargable battery. I basically have just 
replaced that plug with the 5 pin. I replaced the cable at the same time 
figuring it would not hurt to put in new cable as my Telinga has seen 
lots of use. Electrically it's identical to the original in how 
everything connects to the Portadisc.

And the signal I'm getting off the Telinga has not changed. Gain 
settings are the same.

In the 5 pin system as I use it, pin 1 is still shield, pin 2 is right 
channel hot, pin 3 is right channel cold, pin 4 is left channel hot, pin 
5 is left channel cold. This duplicates the arrangement in the Sony 
ECM-MS957 mic I have. I've found that there are other arrangements used, 
it does not appear to be quite as standardized as the 3 pin system.

I've gone to using mostly 5 pin cabling to keep the cable snarl down. 
I'm virtually always recording stereo now.

Note that hum that's from somewhere outside of the cabling won't be 
cancelled in the same way, but will pass through even a balanced cable 
system. I think that's what we probably have here. I don't think it's 
induced in the cable.

Walt




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