impedance "balancing"

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rafafredd

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Joined
Jun 3, 2004
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2,409
Location
Rio, Brazil
I´m working on a 8 channel API 312-like (using diferent opamps), and I would like to know more about impedance balancing because I´ll have to save on output transformers this time. You know, eight channels and I have to save some money... The schematics I always see for impedance "balancing" is this:

impedance_bal.gif.gif


As I´m going to use SOLEN high quality caps for the 47uFs, I would like to know if I can omit C2. Will it affect the impedance "balance" too much if I omit it? Or will it work fine with just the resistor and no cap on the cold side?
 
I know absolutely nothing about this, but it seems to me that the bottom of J1 should go to ground, and the top should go to C2.

And I would assume that getting rid of C2 would change the impedance of the bottom leg, defeating the purpose of the circuit...

Peace,
Al.
 
here it is:

impedance_balanced.gif


I hope I can get rid of C2 with no big problems, or it won´t be cheaper then a Profile 4804 transformer at all...
 
I would think that the big difference would be in very low frequencies and DC. For higher frequencies, maybe you could look very carefully at the capacitor data and see what series resistance is. Then bump up the resistor by that amount, if non-trivial.

Regarding DC... what would you want to have happen if phantom power was accidentally applied? It would be blocked into the op amp output, while the balancing side would just go to ground anyway. Maybe an issue with how it puts some current through the resistor, though.
 
It´s a LINE OUTPUT device, and I don´t really consider the possibility of phantom power being applied.

Thanks for the cap resistance idea. I´ll have a look at this.

I´m thinking again about this impedance balancing thing, I don´t think it´s worth doing like I´ve planed. The solen 47uF caps costs 12 USD each, and the 4804 transformers costs just 15 USD from profile. So, maybe the saving is not enought. With most opamps, the 4804 can stand the DC offset with no caps, and it´s true balancing!

It would be worth if I use nichicon or ELNA high quality electrolytics thought, instead of the solen. Then I would not have to save on C2 also because those electros are way cheaper than big poly film caps.

So, I´ll go for
 
Why not just use an electrolytic for C2...something like a low ESR panasonic? Sure, it wont be exactly the same impedance as the Solen, but I think as far as performing the impedance balancing function it should work just fine. Remember, there is no signal in this line, only noise that we're trying to cancel at the receiving end.

Cheers,

Kris
 
There is no excuse for using 0.1% resistors here. Remember the other end is 10K, and rarely better than 1%. So 10K +/-1% is 9,900 to 10,100. A 100Ω variation in source resistor (the 47Ω) would be Bad. (Also impossible!) A 10Ω variation would be negligible compared to ~100Ω errors in the 10K input resistors. So you could get away with 37Ω-57Ω, or 47Ω 20%. Since I can't buy new 20% or even 10% resistors today, I guess I'd use 5%. Even to be fussy, 2% or 1% ought to be more than good enough.
 
I just wrote 0.1% because I select my resistors by handmatching, usually to +-0.1% ohm..., but no, I don´t buy 0.1% resistors. I go for the 5% and handmatch.

But PRR, what do you think about omiting the C2 cap? Is it really needed?

After reading your last post, I think it also wouldn´t make much diference...
 
No need to recalculate R2. It is ok to use R1=R2 because the output impedance is almost zero. Also you don't need C2.
Put C1 to the output of your opamp and two 50 ohm resistors from C1 to out and ground to out.
It works. All my preamps have two outputs, one with transformer and one transformerless. I use for transformerless this two resistor configuration.
For C1 if the value is low I use polypropilene caps, if it is a large one Panasonic FC in paralel with WIMA 100n polyprop.
Look at Fred's page, he has a schem with this impedance balancing.

chrissugar
 
> R2 should be recalculated as R1+output impedance of the opamp stage.

Exactly correct, but insignificant. The opamp output Z should be awful close to zero. And the tolerance is 10K +/-1% or 100Ω leeway.

I suppose you could have an opamp rise above 10Ω at the top of the audio band, and you might have an input that was better than 1%, and you might actually need "high" CMRR, so you might want to make the correction.

> omiting the C2 cap?

If you care at all, you probably need C2. Note that 47uFd is ~200Ω at the bottom of the audio band, ~60Ω at power line frequency, MUCH more significant down there than any 47Ω resistor and on the edge of my 10K +/-1% (100Ω) criteria. Either use C2 or omit C1 (or make C1 MUCH bigger, 470+uFd).

It works for Chris because his inputs are good enough for his local electrical noise. In fact all this "impedance balanced" stuff is overkill in many-many situations. The box-makers (should) know this, but "can't" give you an unbalanced output (people say "yuck"), so they came up with this 20-cent quasi-Balanced design. (Most boxes don't run to Wima output caps.) And the common 1-opamp "balanced" input really is sensitive to few-ohm source unbalance (and balanced-Z drive is only a partial fix).

> what do you think

What I think is: unbalanced outputs, proper differential inputs, transformers when you cross power grounds (including some PC interfaces: the PC "ground" is nothing like the wall-plug ground). And if that's not good enough, move to a quieter part of the world. Alternatives include improving your inputs (most so-called "balanced" inputs suck).

Proper balanced-DRIVE output have a place in large systems with many different signals in the same conduit. If the voltages on each line are equal-but-opposite, cross-talk is greatly reduced. This is rarely an issue in music production studios: everything in the room will end up in the same groove anyway.
 
PRR is absolutely right, I forgot to say that for C1 I use large values to be sure that the impedance at low freq in low enough. For example in the API output circuit where they use 470micro I use 2200micro Panasonic FC paralel with 100n WIMA polyprop.

Also I forgot to mention that I use in all my projects two different outputs (transformer balanced and impedance balanced) to have two sound options (transformer, transformerless) or in a chain like mic pre, comp, eq to limit the number of transformer in the audio path if needed.

chrissugar
 
Also I forgot to mention that I use in all my projects two different outputs (transformer balanced and impedance balanced) to have two sound options (transformer, transformerless) or in a chain like mic pre, comp, eq to limit the number of transformer in the audio path if needed.

Just wondering, do you leave the TX connected when using the TXless output ?
 
The right thing to do is to use a jack connector with switch that disconects the transformer. Some Moonlee gear do this.

chrissugar
 
Thanks, doing that feels good. :grin:
And I figure that with a more luxurious switching-output-connector the cap could be skipped when going TX.

regards,

Peter
 

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