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.
When I replaced the cable, I got the circuit board out and duplicated
the connection I found. For the 5 pin connector that means the two hot
signals are on separate pins, and so are the two cold. But the
connection point on the circuit board for cold was one point, so the
cold is in common between the two channels. I did not connect the
shielding of the cable to the ground.
As I noted the cable shielding connects through the XLR to the
Portadisc. And since the unmodified Telinga also picks up the hum, I'm
sure it's not the cable.
I'm not at all worried about this hum, because I never use the mic that
way. Just a interesting interaction between Portadisc and Telinga.
Walt
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