figuring input impedance of balanced input

[dai h.]

(sorry if this is a stupid question!)

I'm trying to figure out the input Z of my Tascam 488mkII (XLR input, given as "2.8k" input impedance in the manual). (My understanding is that the balanced signal circuits through wires on pins 2 and 3.) Looking at the circuit, there is 13k (four 13ks with two in parallel on each side where phantom power is fed in), 3.3k(across pins 2 and 3) , then two 4.7k Rs from two transistor bases to ground. So, I'm thinking (13k, 3.3k, and 9.4k in parallel equals the input Z) :

1/((1/13000)+(1/3300)+(1/9400 - Google Search))

which is 2 056.19009, so roughly 2.1k. Is this correct? Thanks for looking.

[Enzo]

So why do you not believe the 2.8k IMPEDANCE they specify? Your calculations are DC resitances. I am looking at a a similar circuit in a different model, also called out as 2.8k. I see a couple caps to block the phantom from the transistors, you got those? Did you allow for them in your figures, if so? Not sure what they might contribute, if anything. I just am never comfortable adding up resistors and calling it impedance.

[dai h.]

thanks for your response Enzo.

re: the caps, yes they are present (10uF), and I'm not sure how they would figure in either. There is also a 10 ohm R and 2n2 cap to ground on each side right before the transistors(which *I think* is to filter out RF), but 20 ohms wouldn't seem to make much of a difference.

re: the figure, 1) just curiousity, and 2) to experiment with transformers so I have an accurate idea of what the secondary side load is (since a. I'm going to use a resistor to adjust(to whatever the actual input Z is), and b. a Zobel might be better but I don't have a signal generator and scope which are apparently needed to figure out the right values to damp the resonance that can happen with a non-ideal load).

(Maybe the transistors are the 700 ohms-ish(350 ea. side)?) Also, (I wasn't really sure if I was supposed to do this, but) I hooked up my LCR meter to the inputs and (although the reading was not completely stable) it did seem to show around 2k (1kHz, power off). (I also tried this with my joemeek VC3Q and the reading was around 5k, which didn't seem unusual for more modern transformerless inputs.)

[Steve Conner]

Negative feedback modifies the impedance of these circuits. They also have two different impedances, one for common mode and another for differential. The calculations for that are shown in one of Douglas Self's books.

The above could well account for the difference between your 2.1 and their 2.8.

[dai h.]

Thanks Steve. There is (what appears to be) a feedback adjustment (4.7k/820pF in a feedback loop on the opamp following the input transistor pair in series with a 10k rev. audio pot, then a series 1000uF cap, then another 4.7k/820pF to ground. I'm not quite sure what is meant by "common mode" and "differential" (input impedances--two x 4.7k to DC ground section at the transistor inputs being different?).

Also, another factor I found confusing was that there is an unbalanced input (that uses just one side of the input). The manual states 5.8k for the input Z (which appears to make sense--(3.3k and 4.7k in parallel + 3.9k series R (which "comes in" when using that 1/4" input) = 5.8k--which agrees with the manual).