Impedence Balanced help

[abechap024]

Hello,
I am impedance balancing the buss outputs on my console, but wondering what value resistor to use. Sorry it seems like a stupid question. I've read literature on the subject, but Can I just measure the  DC resistance to ground from the + pin and match it with the - pin? or could I just add a ~2.5K resistor to each pin to ground? The circuit is pretty simple. 

http://bolstad.net/quantum/Quantum%20QM-168%20Manual.pdf    (page 24)
But I put a high impedance vu buffer with  a 100k to ground.

op amp out with 2 transistor buffers to the unbalanced line out. The part the confuses me is there is no resistor to ground to match.

Also how important is the Capacitor on the - pin? It shouldn't cause too many problems if I omit it ?

Thanks!!

 

[abechap024]

wait doing some more reading...in the schematic there is a 10ohm resistor in series from the output transistors to the + do I just add a 10ohm resistor from the - to ground? that seems awful low! I'm still a little confused by this seemingly concept???

 

[VictorQ]

Hello Abe
Not a stupid question...
Unfortunately, the excact output impedance of this circuit depends on hard-to-know parameters like transistor beta (Hfe), op-amp output impedance and loop gain - even to a very small extent the impedance of the preceding stage, assuming bjt input op-amps (this small contribution would probably be swamped by the series 10k, though).

Anywho it looks to be a pretty small number, probably lower than 1 ohm at the feedback point, which would mean that the output impedance would be dominated by the 10 ohm series resistor (like you noticed).
As I understand impedance balancing, excact matching isn't crucial as long as the input impedance of the following stage is much larger (which it almost certainly is if it's a line input)..

You could measure it by by grounding the input, applying a sinusoidal voltage at the output and measuring the amplitude across the 10 ohm series resistor. If I'm right you should see most of the voltage dropped across the resistor, if not you could calculate a figure with the voltage divider formula.. that all seems a little elaborate, though.

Adding a 2k to ground would not change the impedance much as it would be in parallel with the (much lower impedance) output.

I would just stick a 10 ohm in there and be done with it, but please don't take my word for it - I'm only a second year EE student... Someone who actually tried this with a similar circuit should chime in...

Hope I didn't confuse you further  ??? Just thinking out loud (well.. you know)...
Victor

 

[abbey road d enfer]

Hello,
I am impedance balancing the buss outputs on my console, but wondering what value resistor to use. Sorry it seems like a stupid question. I've read literature on the subject, but Can I just measure the  DC resistance to ground from the + pin and match it with the - pin? or could I just add a ~2.5K resistor to each pin to ground?

The 2.5k impedance is for the Cue outputs, because the actual volume pot is not followed by a buffer. In fact 2.5k is the maximum value, but the actual value depends on the rotation of the pot - going from zero to 2.5k to 10R.

But I put a high impedance vu buffer with  a 100k to ground.

That is a good decision; the original use of a direct connection after the 10 ohm resistor is on the verge of malpractice.

Also how important is the Capacitor on the - pin? It shouldn't cause too many problems if I omit it ?

I would recomment not omitting it, since it governs the LF CMRR. Use 10 ohms and 100uF.

 

[abechap024]

excellent. Cheers to both of you for clearing away a little of this fog. I'll wire it up after I order some caps
I'm also in school for EE but unfourunatley we spent the first semester talking about general engineering stuffs. Can't wait till next semester to take the Analog Circuits Class!
Thank you 

 

[Conductor]

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