How do I measure volts from a balanced output using a multimeter?
Old 15th June 2011
  #1
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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
Old 15th June 2011
  #2
Gear maniac
 

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?
Old 15th June 2011
  #3
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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.
Old 15th June 2011
  #4
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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?
Old 15th June 2011
  #5
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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
Old 15th June 2011
  #6
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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
Old 15th June 2011
  #7
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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 04:29 PM.. Reason: a better thought
Old 16th June 2011
  #8
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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.
Old 16th June 2011
  #9
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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?
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.
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.
Old 17th June 2011
  #12
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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."
Old 17th June 2011
  #13
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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.
Old 17th June 2011
  #14
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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)."
Old 17th June 2011
  #15
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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.
Old 4 Weeks Ago
  #16
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I came across this thread after testing the outputs for my interface. If I test a signal coming out of my hardware mixer with 0db reading on the meters, it tests right at 1.23 volts. That's normal. However my audio interface tests at .275 with a -18dbFS tone and it needs me to get it up to -5dbFS to read 1.23.
Old 4 Weeks Ago
  #17
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Hi
If the interface is supposed to be a balanced output then ALWAYS measure between pin 2 and pin3 (or tip and ring).
Try this as 300 or 400 Hz, your meter may not be capable of accurate reading above 500 Hz.
If you attempt to measure between ground (pin 1) and either 2 or 3 you will get incorrect results.
Matt S
Old 4 Weeks Ago
  #18
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They're balanced for sure and I'm testing between the tip and ring. I tried from 50hz to 440hz and there's only a very slight difference, barely any at all. I put in a tech support ticket with the company (Focusrite, it's a 2i2). Hopefully they'll have an answer. I wouldn't think their converters are set up to be where there's only 5db of headroom.
Old 4 Weeks Ago
  #19
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This might be a stupid idea, but is there a chance the 2i2 maybe is set to -10 instead of +4? I don't know that interface, but I know with some interfaces (like in my echo audiofire), you can switch the outputs to either +4 or -10 in the control panel. Your measured 5dB plus the difference from -10 to +4 would bring it to -19dbfs, a lot closer to what one would expect.
Old 4 Weeks Ago
  #20
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I've considered that but the manual lists the outputs at +4dbu. I'm gonna see what tech support says and do some more investigation. I'm gonna slap myself if I discover it's something like I had fader down in Logic's mixer or something
Old 4 Weeks Ago
  #21
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You can do a quick frequency response check of your DMM.
All you need is a CD player and a test tone CD.
Old 3 Weeks Ago
  #22
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Would any of you meter-geeks give me a sanity check - look at the specs for the Fluke 179. (Fluke 179 True RMS Digital Multimeter) I interpret them to mean that the unit will measure ac up to 100 kHz, at .01 mV resolution. Seems like that would be perfect for testing audio interfaces.

Or... am I missing something.
Old 3 Weeks Ago
  #23
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Quote:
Originally Posted by TMetzinger View Post
Would any of you meter-geeks give me a sanity check - look at the specs for the Fluke 179. (Fluke 179 True RMS Digital Multimeter) I interpret them to mean that the unit will measure ac up to 100 kHz, at .01 mV resolution. Seems like that would be perfect for testing audio interfaces.

Or... am I missing something.
You're missing something. It's hard to find, but it's here:
http://media.fluke.com/documents/2155a.pdf
The Fluke 179 AC voltage accuracy is spec'd at 1% from 45Hz to 500Hz only (pdf page 3, detailed specs.)

That 100kHz figure is for frequency counter only, and it seems iffy since the pdf only lists 50kHz.

My ancient Fluke 8060A was designed for audio work and is spec'd RMS 2% to 50kHz. You have to spend to get one of the Fluke advanced meters to get wideband AC response, like the 289.
Old 3 Weeks Ago
  #24
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Hi
Not only the restrictions as just noted but it does not even apply to all ranges. Some meters have a graph that shows the accuracy and resolution in each 'range' which can show large tolerances at low levels.
I am not suggesting they are 'bad' by any means (I have several Flukes and prefer them to others) but you do have to understand the limitations.
The 179 is a reasonable choice for 'casual' audio work. I know mine reads a little 'low' at 1KHz so I wrote down it's actual measurement on the back for when I need to be 'accurate'.
Matt S
Old 3 Weeks Ago
  #25
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Quote:
Originally Posted by GLouie View Post
You're missing something. It's hard to find, but it's here:
http://media.fluke.com/documents/2155a.pdf
The Fluke 179 AC voltage accuracy is spec'd at 1% from 45Hz to 500Hz only (pdf page 3, detailed specs.)

That 100kHz figure is for frequency counter only, and it seems iffy since the pdf only lists 50kHz.

My ancient Fluke 8060A was designed for audio work and is spec'd RMS 2% to 50kHz. You have to spend to get one of the Fluke advanced meters to get wideband AC response, like the 289.
Thanks! I'm gonna buy a used O-Scope instead. It'll give me more value, I think as I can use it for car stuff too (I've already got a decent multimeter, it's just not good for audio frequency ranges). I'll play with the BK 2120 if I win it.

Stand by for a new thread on this whole "how do I compare my DAW meters to voltage at my audio interface" topic.
Old 3 Weeks Ago
  #26
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I have two Fluke 179s (one from work); great little meters. With a 1 kHz sine at 1.228 Vrms from an HP 33120A (1% accuracy), both measure 1.216 V, which is a bit less than 1% error. At 500 Hz they both read 1.226, and at 100 Hz 1.228 V.

But if you already have a decent meter, getting a scope is an excellent idea.
Old 1 Week Ago
  #27
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Follow-up, just as an FYI... I have a Focusrite 2i4, and a Fluke 115 True RMS volt meter. I used an ASIO signal generator to generate a 100 Hz sinewave at 0 dBFS (48 KHz sample rate). Measured 2.56 Vrms with the meter connected to tip and ring unloaded, which works out to 10.38 dBu, which is close enough for me to the Focusrite spec'ed max output at +10 dBu for the balanced interface. I then set the signal generator to -6 dBFS, and got 1.28 Vrms - as expected, a halving of voltage being a 6 db drop. So the device measures up to its specifications. Did a similar test using Reaper as the source and got the same numbers. Also put on the o-scope and saw a nice clean sine wave start to square when I tried to send signals above 0dBFS out to the unit.

This has been a good exercise for me, I finally got my head around the how the reference between the digital dBFS and the analog interfaces in dBu or dBV will vary depending on where the audio interface is set. Essentially, I track and mix with 20 db headroom between 0 VU and 0dBFS. That means that my nominal 0 VU output will be at -10 dBu on the Focusrite, but might be higher or lower with a different interface. I suspect that a self-powered interface (rather than USB powered) would probably use different D/A stages and result in a higher signal for 0 dBFS.
Old 1 Week Ago
  #28
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Quote:
Originally Posted by T.V. Eye View Post
This might be a stupid idea, but is there a chance the 2i2 maybe is set to -10 instead of +4? I don't know that interface, but I know with some interfaces (like in my echo audiofire), you can switch the outputs to either +4 or -10 in the control panel. Your measured 5dB plus the difference from -10 to +4 would bring it to -19dbfs, a lot closer to what one would expect.
It sounds like the 2i2 is set for -10 dBv nominal output.

One small correction:

The difference between the common "nominal" audio line levels of -10dBV and +4 dBu is actually 12.2 dB (not 14 dB)

If the Saffire 2i2 is set for -10dBV "nominal" then the DAW level that produces 0.316 volts (-10 dBV) across the output will indicate the true headroom.

If 1.23 Volts (+4dBu) is reached at -5 dB(fs) then the headroom for a nominal -10 dBV will be -5 -12.2 or -17.2 dB(fs).

Last edited by Lotus 7; 1 Week Ago at 07:00 PM..
Old 6 Days Ago
  #29
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Just to be a pedant.

Not 12.2 but 11.79dB

+4 = 1.228v

-10 = 0.316v

20log(1.228/0.316) = 11.79dB

Gareth
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