Old 01-24-2018, 10:15 PM   #1
puneettaneja
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Default Audio Interface output too low

My 4.5 years old Tascam US 1800 is starting to show some issues. I have connected my Line 6 Helix -> Tascam (via XLR). I have to put the preamp gain to 10 to even be able to hear anything. In the past, with preamp at 5 and monitors output at 5, I was able to get a loud enough sound. Nothing has changed on that front except that now the output is way too low.

I already checked direct Helix -> Monitors and that sounds fine. Couple of things I need to check are (I will do that tomorrow):

1) Different XLR cable
2) XLR output on Helix

The reason why I feel like the issues are with Tascam is because the other day, I recorded some vocals and the recording was too low even with preamp set to high (although this was a different preamp input on the Tascam).

Any thoughts or ideas? Could the Tascam preamps have gone bad?
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Old 01-25-2018, 11:55 AM   #2
Philbo King
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You should do some basic testing.

- Put a tone generator on a track in a new reaper project. Turn off speaker feed. Set all faders and audio interface controls to zero dB. Put in the JS Signal generator plug, at 0 dB, with a 400 Hz sine wave.

(You can skip this next step if you don't have a voltmeter)
If you have a voltmeter, measure the AC voltage out of you interface.
For a -10 dBu interface it should read 0.244 V RMS.
For a +4 dBu interface its 1.228 V RMS.

Edit: I just checked. The US-1800 is a 4 dBu unit.

This calculator comes in handy.
http://www.analog.com/designtools/en/toolbox/dbconvert

Now loop the output back into an input, arm a new track, and monitor the level coming in. If it says 0 dB all is well.

If you want to go a step further, put a spectrum analyzer plugin on the input track. Ideally there should be a single spike at 400 Hz and nothing else. Harmonic distortion (a possible symptom of a sick interface) will show up as smaller spikes at multiples of 400 Hz (800, 1200, 1600, 2000 Hz).

Since you are at 0 dB, these are on the verge of these showing up (You can see them by setting the sig gen plugin above 0 dB). As long as they are no more than -60 dB with all levels set at 0 dB things are good.

Last edited by Philbo King; 01-25-2018 at 12:16 PM.
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Old 01-25-2018, 12:45 PM   #3
ashcat_lt
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Ummm... My interface is something like 18V at RMS with a 0dbFS sine wave...

Edit - in order to actually measure +4dbu at the output, you'd need to look up how much headroom the interface has over that nominal level, and set the tone generator that far down. You'd also probably need to measure between between the T and R of the balanced output.
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Old 01-25-2018, 01:48 PM   #4
Philbo King
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Quote:
Originally Posted by ashcat_lt View Post
Ummm... My interface is something like 18V at RMS with a 0dbFS sine wave...
Wow - Thats +27 dBu !

Plenty of headroom there...

Quote:
Edit - in order to actually measure +4dbu at the output, you'd need to look up how much headroom the interface has over that nominal level, and set the tone generator that far down. You'd also probably need to measure between between the T and R of the balanced output.
Yes, Tip to Ring if outputs are balanced. It'll drop by about 6 dB if you measure to the ground ring.
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Old 01-25-2018, 02:30 PM   #5
ashcat_lt
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Quote:
Originally Posted by Philbo King View Post
Wow - Thats +27 dBu !
Yeah, it might be a bit of an overestimate, too, I think it's nominal +4dbu, with 22db headroom. Even if we assume the "standard" 18db headroom (the source of all that confusion about "proper levels"), it'd be a lot more than 1.228V RMS, and the real point is that you need to know the specifics of YOUR device in order to know what to look for on any meter either in or out of the box.
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Old 01-25-2018, 08:08 PM   #6
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Quote:
Originally Posted by Philbo King View Post
You should do some basic testing.

- Put a tone generator on a track in a new reaper project. Turn off speaker feed. Set all faders and audio interface controls to zero dB. Put in the JS Signal generator plug, at 0 dB, with a 400 Hz sine wave.

(You can skip this next step if you don't have a voltmeter)
If you have a voltmeter, measure the AC voltage out of you interface.
For a -10 dBu interface it should read 0.244 V RMS.
For a +4 dBu interface its 1.228 V RMS.

Edit: I just checked. The US-1800 is a 4 dBu unit.

This calculator comes in handy.
http://www.analog.com/designtools/en/toolbox/dbconvert

Now loop the output back into an input, arm a new track, and monitor the level coming in. If it says 0 dB all is well.

If you want to go a step further, put a spectrum analyzer plugin on the input track. Ideally there should be a single spike at 400 Hz and nothing else. Harmonic distortion (a possible symptom of a sick interface) will show up as smaller spikes at multiples of 400 Hz (800, 1200, 1600, 2000 Hz).

Since you are at 0 dB, these are on the verge of these showing up (You can see them by setting the sig gen plugin above 0 dB). As long as they are no more than -60 dB with all levels set at 0 dB things are good.


Nothing to add here.

I just wanted to say: Philbo, that's one helluvan answer !
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Old 01-25-2018, 10:51 PM   #7
Philbo King
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Quote:
Originally Posted by Cableaddict View Post
Nothing to add here.

I just wanted to say: Philbo, that's one helluvan answer !

I used to chase electrons in circles for a living. I was a lab rat in an electronics company.
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