Balanced input impedance

[fotios]

Question:
In the specification brochure of an amplifier which includes except the usual single (unbalanced) inputs also balanced inputs, is refered except the single input Zin also the Zin of the balanced input.
For the single input, the Zin which refered is quite comprehensible, because is measured between the signal input terminal and the ground.
For the balanced input i can not understand where is measured: Between the Hot pin and the Gnd? Between the Cold pin and the Gnd? Between the Hot and Cold pins?
I have to refer an example. In the specs of a well known power amplifier, is refered:
Zin unbalanced = 50KÙ. By measuring it with a multimeter - in rough of course - it is true. The multimeter shows 47,5ÊÙ.
Zin balanced = 15ÊÙ. By measuring again with the multimeter, the impedance between the Hot pin and the Gnd it is almost 15ÊÙ. Instead, between the Cold pin and the Gnd it is 30ÊÙ.
When the Zin of a balanced input is measured with a normal impedance meter, between which pins is reffered? It is obvious that the Cold signal input has absolutelly different Zin (respect to Gnd) from the Hot signal input.
Can someone to explain me this?

Thanks
Fotios

[Jan Dupont]

I think it would help if you post a schem of the actual circuit

[janneman]

Fotios,

Nrmally there should be two impedances: the differential mode impedance which is measured between hot and cold, and which is what would be important for the balanced out driving stage.
Then the common mode impedance is the impedance measured from both the hot and cold to gnd. Normally this is not important for the driving stage.

jd

[fotios]

Thanks guys for the replies.
@ Jan Dupont: To help you, i talk about a Bryston 4BST-Pro that i have in my stock. You can find very easy the whole schematics from the site of Bryston (i suppose you have those already).
@ Janneman: I did a rough differential measurement between hot and cold pins, and the multimeter shows 41,5ÊÙ. This is a big diference from the refered 15ÊÙ from Bryston.
I have report the rest measurements in my first post.
To my eyes, Bryston refers as balanced input impedance this between HOT pin and GND

Fotios

[janneman]

Originally posted by fotios
Thanks guys for the replies.
@ Jan Dupont: To help you, i talk about a Bryston 4BST-Pro that i have in my stock. You can find very easy the whole schematics from the site of Bryston (i suppose you have those already).
@ Janneman: I did a rough differential measurement between hot and cold pins, and the multimeter shows 41,5ÊÙ. This is a big diference from the refered 15ÊÙ from Bryston.
I have report the rest measurements in my first post.
To my eyes, Bryston refers as balanced input impedance this between HOT pin and GND

Fotios

Yes, could be, if they don't specify whether the number is for differential or common mode you don't really know. Also, input impedance measured with the multimeter may not be the same as the input impedance for the input (ac) signal.

jd

[cbdb]

Heres the schema. The input impedance when switched to ballanced should be across hot and cold (pins 2 and 3). The Zin3 goes thru the 10K right into the OP Amp so a meter probably wont read this properly.

[AndrewT]

the asterisk * on the 10k next to the -IN pin is the virtual ground of the (discrete) opamp.
XLR3 sees this 10k as the input impedance.
XLR2 sees the 6k65+3k32 as the input impedance.
These should be matched to <0.1% for the balanced connection to work well at attenuating interference.
The capacitance from these two XLR pins also needs to be matched to maintain rejection to higher frequencies.

[AndrewT]

Hi,
in the same way that you can measure output impedance, you can measure your Hot to Cold impedance.

Check your source output impedance.
Add a resistor into the hot line and measure the voltages at the resistor and across the XLR.
As you vary the in-line resistor you will be able to calculate the input impedance.

[cbdb]

The * means the components are matched ( says right on the schemat ). The -ve in pin is not a virtual gnd. It is at the same voltage as the +ve in pin, about a 1/3 of the voltage at XLR2.