Voltage reading for MOTU 828 MKII outputs

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druedger

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Joined
Jul 30, 2004
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This is probably an esoteric post, but I'm in the process of building some gear and discovered something quirky with the output level voltage from the unit. I hooked up my multimeter to the output so I could measure the voltage, and I ran a 1K sine wave at -3dB out of the interface to check the voltage level. Based on a 0dbu/dbm reference level of 0.775 volts RMS, I was expecting to see a reading of 0.548. Instead my multimeter read 1.34 volts! I'm pretty sure the multimeter is measuring RMS and not peak to peak, but even if it were, the p-p reading should have been 1.55, not 1.34.

I tried contacting MOTU to see what their output reference is based on, but I got this gem of a response "As of now, the information of voltage read out on the 828 mk2's analog outputs is not published." Does any one else out there have any insight into this issue and can possibly explain or point me in a direction to understand why this readin is off so much? The way I discovered the problem initially was by hooking up a VU meter, and it read close to 0db with a -3db signal out. That's when I broke out the multimeter to take the voltage reading and saw the voltage was way off.
 
Interesting. Let me start by saying i can be of no help to you and i barely understand what you are talking about. Interesting. I have one of those units and would be interested to know, if you'll tell me, how this relates to it's sound. |: )
 
I'm not sure it has any bearing on the sound per se, but it does affect the matching of signal levels between gear. I am designing an external summing buss, and I'm tring to match the level of the 828 to the VU meters so I can accurately set each input channel's gain to ensure the input voltage to the mix buss is the same as the output voltage from the 828. Since the 828's voltage level is so hot, it actually decreases the overall headroom of the mix buss as 16 channels of summed voltage at these output levels can potentially exceed the power supply voltage of the op amp providing the summing circuit. That would be very bad and would lead to clipping and distortion in the output signal. And I suppose it could lead to damage of the circuit as well if run that way for too long.

Hopefully someone out there knows what the reference voltage for these units should be or can point me in the right direction. MOTU certainly can't, and I can't say that I'm all too surprised by that.
 
I really don't see much wrong with your unit..
Most (maybe some) units have 0VU level set at -18dBfs. I personally calibrate my interfaces to 16 or 14, whenever possible.
Try setting a signal generator from inside your DAW at -18dBfs and see what that reads on the VU. (or voltmeter). I think it should read 1.23v.

take care
Gil
 
Note that MANY multimeters have crummy frequency response. Stay at "power line" frequencies, 50Hz-400Hz, until you verify the meter's quirks.

> Based on a 0dbu/dbm reference level of 0.775 volts RMS, I was expecting to see a reading of 0.548. Instead my multimeter read 1.34 volts!

The currently popular line-up reference level is +4dBu, 1.22829 volts. Your reading is 0.756 dB off, which is no big deal.

However you say you were running "-3dB", so it does not make sense. We do have to ask: "-3dB compared to what?" And MOTU's reply is "we don't say". Gee, thanks.

I thought they maxed-out at +16dBu and "zero VU" was 14dB below that.

> -3dB out of the interface ...my multimeter read 1.34 volts!

Try zero dB. If their meters don't suck, that should be 1.34*1.414= 1.9V. This is +7.765dBu. +8dBm was once a very common reference level in Broadcast work. It was also the maximum average level allowed on the phone system. It is only 10dB below +18dBm, which is the maximum clean output of padded tube line-amps or IC line amps. 10dB headroom was OK for radio, especially AM radio, but not for fine music, especially if you listen to it over and over. Most "+8dBm" gear in recording rooms got recalibrated to +4dBm to get that last peak-clip out. 14dB headroom is a good compromise for disk/tape systems; digital can be more generous.

Turn it up until the meters peg and the output is clearly clipped. What is the clipping level? It should be 12dB to 20dB above the nominal VU level; 12dB is awful thin, 20dB is awful generous, 14dB to 18dB are popular numbers, and 16dB is very common. Whatever the clipping level is, you want to keep VU meters hovering at least 16dB less. With only peak meters, the notion of "nominal level" has no real meaning: just don't clip peaks.

> 16 channels of summed voltage at these output levels can potentially exceed the power supply voltage of the op amp providing the summing circuit.

If you set the mixer unity-gain for one channel, and then mix two uncorrelated channels, they will sum to potentially +6dB (very rarely) or +3dB (very often). If you mix 16 equal channels, you will average +12dB (with very-very-rare +24dB peaks; you may not live long enough to find one). For the more common case of 4 nearly equal "lead" tracks and 12 backup tracks, figure +9dB most of the time.

And yet the same song may have a solo with just one track in front, which wants to be near peak levels.

Mixers MUST have faders. If you don't have knobs on the mixer, you need to fade in the digi-box. Then you fade until the mixer output is appropriate. Despite what you see on the MOTU, +4dBu is the popular line-up level, with +20dBu a typical clipping level. This may mean that you have to call the "-4dB" mark on the MOTU meters "line-up level" (+4dBu).

If they won't talk about voltage, what do they say about impedance? If the box output is 350 ohms, then with a true 600 ohm load the level will drop 4dB lower than you see with your Mega Ohm meter. That would be a very untraditional way to do things, but MOTU does not have to follow audio tradition.
 

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