How do I measure volts from a balanced output using a multimeter?
Fhl
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#1
15th June 2011
Old 15th June 2011
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How do I measure volts from a balanced output using a multimeter?

Hi,

I'm trying to figure out how many volts my motu 828 output is giving at a -18dBFS 1KHz test tone.

I also have a cable tester which can provide a +4dBu 1KHz tone.

When I use my multimeter on the latter and connect the black lead to pin 1 and 2 and the red lead to pin 3, I get a reading of 1.23v. Shouldn't I only get half that? Anyways.

When I try the same from the soundcard I get 0.30volts. What am I doing wrong? I'm using AC voltmeter.

Thank you
Fredrik
#2
15th June 2011
Old 15th June 2011
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I use a cable with either an XLR or 1/4 connector on one end and two raw ends on the other. Simply clip the DMM test leads on the raw ends to measure VAC.

The voltage reading should be taken into a load. +4dbm test tone and 1.23Vrms would suggest a 600r load. Are you strapping a resistor across the outputs when measuring?
#3
15th June 2011
Old 15th June 2011
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Quote:
Originally Posted by Fhl View Post
When I use my multimeter on the latter and connect the black lead to pin 1 and 2 and the red lead to pin 3, I get a reading of 1.23v. Shouldn't I only get half that?
Fredrik, the good news is that your AC multimeter seems capable of measuring 1 kHz sine waves OK. As you probably know, +4 dBu is about 1.23V.

For balanced audio on XLR connectors:
  • pin 1 is the chassis ground (cable shield),
  • pin 2 is the positive polarity terminal (hot), and
  • pin 3 is the return terminal (cold).
IMO, you ought to be measuring the output voltage between pins 2 and 3, without shorting either to pin 1. Some line driver circuits will only give you half the voltage between pins 2 and 3 when you short either of these to pin 1, but other circuits still give the full voltage.

It seems that your cable tester has the type of line driver circuit that provides full voltage when one of the signals is grounded. Your soundcard may or may not act the same way, and you don't need any more variables in your experiment. So, try measuring the soundcard between pins 2 and 3, without shorting either to pin 1.
Fhl
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15th June 2011
Old 15th June 2011
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Quote:
Originally Posted by lassoharp View Post
I use a cable with either an XLR or 1/4 connector on one end and two raw ends on the other. Simply clip the DMM test leads on the raw ends to measure VAC.

The voltage reading should be taken into a load. +4dbm test tone and 1.23Vrms would suggest a 600r load. Are you strapping a resistor across the outputs when measuring?
I'm afraid I don't have any resistors. Are you suggestion terminating pin 1 and 3 with resistors?

Quote:
Originally Posted by henryf
Fredrik, the good news is that your AC multimeter seems capable of measuring 1 kHz sine waves OK. As you probably know, +4 dBu is about 1.23V.

For balanced audio on XLR connectors:
pin 1 is the chassis ground (cable shield),
pin 2 is the positive polarity terminal (hot), and
pin 3 is the return terminal (cold).
IMO, you ought to be measuring the output voltage between pins 2 and 3, without shorting either to pin 1. Some line driver circuits will only give you half the voltage between pins 2 and 3 when you short either of these to pin 1, but other circuits still give the full voltage.

It seems that your cable tester has the type of line driver circuit that provides full voltage when one of the signals is grounded. Your soundcard may or may not act the same way, and you don't need any more variables in your experiment. So, try measuring the soundcard between pins 2 and 3, without shorting either to pin 1.
Thank you, Henry. I'll try once I get back, but I believe I tried that as well.
If I still can't get a correct reading. Is there anything else I could be doing wrongly?
Fhl
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15th June 2011
Old 15th June 2011
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Thread Starter
Quote:
Originally Posted by henryf View Post
So, try measuring the soundcard between pins 2 and 3, without shorting either to pin 1.
That worked. Thank you!

On the cable tester when I send -10dBV I hate to terminate pin 1 and 2 in order to get the correct reading. Why is that?

Woaw, I really don't know much about this. But I'm learning a lot
#6
15th June 2011
Old 15th June 2011
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Quote:
I'm afraid I don't have any resistors. Are you suggestion terminating pin 1 and 3 with resistors?

AFAIK, For modern gear:

PIN 1 = shield

PIN 2 & 3 = signal conductors


You shouldn't be measuring anything with or on the shield (PIN 1).

I was referring to placing a resistor across Pins 2 & 3. Something like 5K or 10K is typical for most console input impedances
#7
15th June 2011
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Quote:
Originally Posted by Fhl View Post
On the cable tester when I send -10dBV I hate to terminate pin 1 and 2 in order to get the correct reading. Why is that?
Maybe the cable tester, by design, expects to be testing an unbalanced cable when set to -10dBV?

Last edited by henryf; 15th June 2011 at 03:29 PM.. Reason: a better thought
Fhl
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15th June 2011
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Quote:
Originally Posted by henryf View Post
Maybe the cable tester, by design, expects to be testing an unbalanced cable when set to -10dBV?
Ah I see. Yes, that's very possible. The multimeter I have states that is supports 20-20KHz. I've read that a lot of multimeters actually only measure around 50-400hz. But I think it's accurate. I found out the interface's outputs were -14dBFS = +4dBu
but I still couldn't get the correct reading. I got 0.56 or so. I'm confused.
#9
16th June 2011
Old 16th June 2011
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Quote:
Originally Posted by Fhl View Post
The multimeter I have states that is supports 20-20KHz. I've read that a lot of multimeters actually only measure around 50-400hz. But I think it's accurate. I found out the interface's outputs were -14dBFS = +4dBu
but I still couldn't get the correct reading. I got 0.56 or so. I'm confused.
If you are still using a 1kHz sine wave, I'd believe the multimeter readings. Maybe there are some volume controls affecting the output level?
Fhl
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16th June 2011
Old 16th June 2011
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Quote:
Originally Posted by henryf View Post
If you are still using a 1kHz sine wave, I'd believe the multimeter readings. Maybe there are some volume controls affecting the output level?
Hm, no only a neutrik patch panel, but could it be changing the voltage? All balanced.
#11
16th June 2011
Old 16th June 2011
  #11
Use an RMS DVM, they are accurate at 400 hz. Change frequencies and you may get misreadings.

Align the meters first.
#12
17th June 2011
Old 17th June 2011
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Quote:
Originally Posted by Fhl View Post
Hm, no only a neutrik patch panel, but could it be changing the voltage? All balanced.
I was thinking of something more like the main volume control on the motu 828mk3. According to its user manual, on page 12:

"The 828mk3 front panel provides two independent
headphone jacks with independent volume knobs,
one of which also controls the XLR main outs on
the rear panel."
Fhl
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17th June 2011
Old 17th June 2011
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Quote:
Originally Posted by henryf View Post
I was thinking of something more like the main volume control on the motu 828mk3. According to its user manual, on page 12:

"The 828mk3 front panel provides two independent
headphone jacks with independent volume knobs,
one of which also controls the XLR main outs on
the rear panel."
Ah, no I use the sends (analog outputs 1-8) which have no volume control. But thanks for the suggestion.
#14
17th June 2011
Old 17th June 2011
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Quote:
Originally Posted by Fhl View Post
Ah, no I use the sends (analog outputs 1-8) which have no volume control. But thanks for the suggestion.
OK, you are probably right, but you may just want to try it to be sure. The next sentence in the manual reads:

"Alternately, this MASTER VOL
knob can be programmed to control any
combination of outputs (analog and/or digital)."
Fhl
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#15
17th June 2011
Old 17th June 2011
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Quote:
Originally Posted by henryf View Post
OK, you are probably right, but you may just want to try it to be sure. The next sentence in the manual reads:

"Alternately, this MASTER VOL
knob can be programmed to control any
combination of outputs (analog and/or digital)."
Oh, that I have to look in to! I know you can set the level for each input, but I didn't know you can affect the output level. Thank you for the heads up.
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