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Balanced outputs on Apogee Quartet Audio Interfaces
Old 11th August 2018
  #1
Balanced outputs on Apogee Quartet

Hi all,
I was using my Apogee to calibrate another piece of gear and I realized that the balanced line outs are...not very well balanced!
I was playing from my DAW a 1KHz [email protected] RMS-0dBu (measured with an oscilloscope and with a DMM) but I found this value only on pin 2 of the ch1's XLR connector. On pin 3 I found 0.622V, more than 100mV is too much in my opinion (>12%). Every measure used pin 1 as reference (connected to GND).
On line2 out I measured 0.786V and 0.744V (RMS), a difference of about 5% which is still high.
I measured with and without load on outputs and the results are the same.
I suppose they are using something like SSM2142 inside the Quartet: is there something that I can check or do I measured something wrong? Any suggestion?
Thanks
Stefano
Old 11th August 2018
  #2
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ruffrecords's Avatar
This is probably because a balanced output does not mean the signal between ground and hot is the same as between ground and cold. What is supposed to be balanced is the source impedances of the hot and cold. It is these, in combination with the equal input impedances of the receiver, that rejects common mode noise. When the combined impedances are the same, the balance is good and a high common mode rejection ratio (CMRR) is achieved.

This is why so called impedance balanced outputs still work where the hot is an unbalanced signal and the cold is just an impedance to ground.

Also remember, in a balanced signal, ground is not involved. The signal is that between the hot and the cold. In theory there is no need for a connection between the grounds of the sender and receiver.

Cheers

Ian
Old 11th August 2018
  #3
Thanks Ian, it's right, but what about the levels? If I have 0.5 and 0.4 at the output pins 2 and 3 the receiver will see 0.45V. If have to deal with precise levels this is not the same as having 0.5 or 0.4v. I still have a differential signal with all its benefits but with a wrong level. If the difference is quite big this could imply potential unwanted distortion or unwanted behaviour at the receiver as the level is not what I'm supposing it would be: is it right? And, if the two channels are not identical, this would damage the stereo image and everything related to channel balance (L/R).
Thanks
Stefano

Last edited by tubeing; 11th August 2018 at 12:29 PM.. Reason: Adding infos
Old 11th August 2018
  #4
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Richard Crowley's Avatar
 

Measuring either side of a balanced signal TO GROUND is undefined. A proper balanced input is NOT ground-referenced. It should ONLY care about the DIFFERENTIAL voltage between the two. Traditional audio gear (which was transformer-coupled) had ZERO voltage to ground.

While Apogee has a fine reputation for making good-quality audio gear, it is not laboratory-grade test gear. But then you probably don't need laboratory-grade for your calibration activities.

More detailed discussion is impossible without knowing exactly which Apogee gadget you are using, what the destination gear is, and exactly what you are trying to do.
Old 11th August 2018
  #5
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ruffrecords's Avatar
Quote:
Originally Posted by ruffrecords View Post
This is probably because a balanced output does not mean the signal between ground and hot is the same as between ground and cold. What is supposed to be balanced is the source impedances of the hot and cold. It is these, in combination with the equal input impedances of the receiver, that rejects common mode noise. When the combined impedances are the same, the balance is good and a high common mode rejection ratio (CMRR) is achieved.

This is why so called impedance balanced outputs still work where the hot is an unbalanced signal and the cold is just an impedance to ground.

Also remember, in a balanced signal, ground is not involved. The signal is that between the hot and the cold. In theory there is no need for a connection between the grounds of the sender and receiver.

Cheers

Ian
As has been mentioned, in a balanced system, the signal exists only between the hot and the cold. The voltage from hot to ground and from cold to ground is not defined and does not matter. Think of a floating transformer balanced output such as we used at Neve when I was there back in the 70s. If you measure from hot to ground you will get nothing, nada, zilch. Ditto from cold to ground. But measure between hot and cold and there is your signal. The balancing action does not involve the ground. You don't even need a ground connection between two pieces of balanced equipment.

This only became confused when people invented so called electronically balanced outputs and inputs where it just so happened that each output was referenced to the local power supply ground and ditto for the inputs. This does not alter the fact that the balancing action is independent of the ground.

Bottom line is measuring from hot to ground and from cold to ground tells you nothing about the equipment. The fact that the measurements are different does not matter at all.

Cheers

Ian
Old 11th August 2018
  #6
Ok, thanks guys. I'll try to measure only in differential to see what's going on. I'm sending the Apogee Quartet's outs to a Drawmer 1961 that needs to be calibrated. The Eq's outs are completely different from cold to hot and this confused me more. I understand that it's only the difference that matters but I was thinking that, with electronically balanced signals, the ground reference should have been the "middle point"
Old 11th August 2018
  #7
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Richard Crowley's Avatar
 

It might be interesting to try the experiment to connect the Quartet's output to the Drawmer's input in the normal fashion, and then in "inverted phase" (exchange Hot and Cold). That would identify whether the Drawmer is sensitive to any "unbalance" of the hot and cold signals relative to ground. Ideally, it should show no difference
Old 12th August 2018
  #8
Quote:
Originally Posted by Richard Crowley View Post
.... That would identify whether the Drawmer is sensitive to any "unbalance" of the hot and cold signals relative to ground. Ideally, it should show no difference
That's a good idea, I'll try as soon as possible.
Thanks
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