Asymmetric balanced outputs on Trident 65

leigh · Aug 11, 2011

I'm doing some tech work on a Trident Series 65, and at the end of a long day got stumped with this issue: the balanced output levels I was looking at on this board (namely, Aux and Mix outs) are not the same on pin2 and pin3.

For example, running a test tone through the board, and looking at one of the Aux outs with a voltmeter, I was seeing about 900mV between pin3 and pin1, and only about 100mV between pin2 and pin1.

Some of the Auxes are more symmetric than others - there is not a consistent asymmetry. So, between that and a rebuilt power supply providing nearly equal positive and negative rails, I'm ruling out an "across the board" power supply issue.

That said, I'm stumped about where to start with fixing this - and my brain is fried - so I'm throwing it on here hoping someone recognizes these symptoms...

Thank you,
Leigh

abbey road d enfer · Aug 11, 2011

You can't really expect measuring the same absolute voltage on the hot and cold points of a balanced output.
With a transformer output, you should measure virtually nothing on each leg when the other leg is left floating.
On floating electronically balanced outputs, it can be anything. Most of the time, the +output will be higher than the -.
Only hard balanced non floating outputs deliver perfectly symmetrical outputs.
Don't waste your time trying to fix it; it's a non-issue. What matters is the levels are correct.

leigh · Aug 11, 2011

Thank you for your thoughts. I'll say right up front that I'd still like to hear others' takes on this, but here's some further info in response to the points you raise.

First, since this is a specific issue on an actual board (a Trident Series 65), we don't have to guess which type of output we're talking about. It's a pair of opamps in a standard push-pull circuit (see the attached clipping from its schematic - input is on the left, output pair is on the top right). I wouldn't call it "floating" per se, since the non-signal input side of each opamp is referenced to ground.

Second, this asymmetry is not a non-issue when it comes to hooking up unbalanced gear to the Aux outputs. Case in point: Aux 3 and Aux 4 feed the L and R inputs of a spring reverb. I noticed this whole "asymmetric" thing first because the L input of the reverb was getting WAY more signal than the R input. I checked all the cables and connections, and traced the issue back to the board. Here were my voltage readings on a test tone, where Aux 3 and 4 both measured about 1.4V across pins 2 and 3:

	Aux 3--	Aux 4--
pin 2	800	30
pin 3	660	1400

(readings in mV, referenced to ground)

So you see, whereas Aux 3 is roughly symmetric, Aux 4 is almost completely one-sided. Which means, when connected to unbalanced equipment (where the signal it sees is pin2 referenced to ground), I've got almost nothing there.

And before anyone suggests just switching stuff around so the unbalanced input sees pin3 referenced to ground, consider the imbalance between levels in that case...

Thank you,
Leigh

abbey road d enfer · Aug 11, 2011

Provided the resistors are of adequate value, this is the standard cross-coupled EBOS, which is indeed floating (not much, and not in a very symmetrical way). this type of circuit NEEDS to have both legs connected, which means you must ground the -leg when you drive an unbalanced load.
This circuit is quite sensitive to component tolerances.
Even 1% difference can lead to 6dB variations in the unloaded voltage balance.

leigh · Aug 11, 2011

Thank you, that makes sense. And grounding pin 3 did indeed bump the level up on that Aux 4.

I've had this same issue happen before with transformer ins & outs, but did not expect to find it on this non-transformer output. I guess I wasn't aware of the needs of an "Electronically Balanced Output Stage".

Leigh