Avid Pre calibration question

[jbreher]

The readying gear for work saga continues...

I have a digidesign Pre. From my understanding, the Avid Pre is merely a cosmetic update of this unit.

AAR, the User Guide mentions the ability to calibrate each channel. It is however rather light on specifics.

There is an onboard oscillator. The manual states that this oscillator will -- after temperature stabilization -- output a 1.228Vrms sine wave out a balanced dedicated 1/4" TRS on the back panel.

There is also a means of feeding this oscillator to each channel (enable insert on associated channel when OSC is enabled).

Lastly, each channel has a Trim pot.

It is left as an exercise to the reader of the User Guide how to employ the above to actually perform a calibration.

My assumption: For standard cal procedure when all is working as it should (i.e., the OSC outputting the proper level), one should enable the oscillator, and assign it to each channel in turn, while monitoring that channel's output signal level, and twist the Trim pot until the channel outputs 1.228 Vrms (or whatever level you'd like to use for nominal, though I'm good with 1.228Vrms). Apply to each channel in turn.

^^ Question one - does that sound correct?

Next issue - my unit does not output 1.228 Vrms out the TRS when oscillator is enabled. It outputs 1.116 Vrms.

My assumption: While routing the 1.116 Vrms oscillator signal to each channel, I want to twist the Trim pot to yield a 1.116 Vrms signal on the channel output, in order to calibrate to 1.228 Vrms (in light of my out-of-spec oscillator output).

My reasoning: The Trim pot is probably a simple voltage divider after the gain stages of the channel. As such, output should be a linear function of signal level before the Trim pot. The oscillator signal is likely being injected after the gain stages, and before the assumed voltage divider Trim pot. The cal procedure would set the output signal to a fixed ratio voltage divider - this ratio would be identical between the oscillator output at the OSC TRS, and the output at the channel out, regardless of the actual oscillator output. Therefore, if my oscillator was outputting 1.228 Vrms as the manual states it should, then the output of the channel would be 1.228 Vrms.

^^ Question 2: Do these assumptions seem valid?

[jbreher]

OK, I'm floundering. Any help would be appreciated.

Seems my assumptions above on how one is to calibrate the Pre are incorrect.

First, while enabling the OSC and enabling INS on a channel does route the OSC signal to the output. However, using this routing, the Trim pot for that channel has exactly zero effect on the output as monitored on the DB25. Accordingly, from my naive viewpoint, the ability to route the OSC signal to each channel's outputs is useless for calibrating the Pre itself.

I can't imagine digi would have implemented a feature that has no use, so I must be confused about something. Perhaps its utility is to calibrate something (e.g. an ADC) downstream? But even so, such utility is hampered by the fact that the OSC level (on my admittedly possibly compromised unit) is somewhat below the manual spec. What am I missing?

Next, I envisioned/postulated the following cal mechanism. I used a TRS to route the OSC signal to each channel's line input in turn. I set the measured channel to line, and 0dB gain. I was gratified to learn the the Trim pot in this case did indeed affect the channel signal output at the DB25.

Figuring that 0dB gain indicated on the GAIN/PARAM display corresponds to an actual 0 dB gain from Line input to DB25 output, I tweaked the Trim pot to make the measured signal level at the DB25 equal to the measured signal level (when unloaded - maybe a small error introduced?) at the OSC TRS output. Or very close - approaching this level, channels stop changing level with additional pot rotations.

At the point that the pots stop affecting the output level, I measured the following unloaded Vrms levels:
OSC TRS: 1.184
Channel outs - 1.171 to 1.183, with the low reading being quite the outlier.

I'll consider this good enough for the work I'll be doing, unless I've made some error in procedure.

Anyone want to comment on how I went about this?

[LDS]

Originally Posted by jbreher

OK, I'm floundering. Any help would be appreciated.

Seems my assumptions above on how one is to calibrate the Pre are incorrect.

First, while enabling the OSC and enabling INS on a channel does route the OSC signal to the output. However, using this routing, the Trim pot for that channel has exactly zero effect on the output as monitored on the DB25. Accordingly, from my naive viewpoint, the ability to route the OSC signal to each channel's outputs is useless for calibrating the Pre itself.

I can't imagine digi would have implemented a feature that has no use, so I must be confused about something. Perhaps its utility is to calibrate something (e.g. an ADC) downstream? But even so, such utility is hampered by the fact that the OSC level (on my admittedly possibly compromised unit) is somewhat below the manual spec. What am I missing?

Next, I envisioned/postulated the following cal mechanism. I used a TRS to route the OSC signal to each channel's line input in turn. I set the measured channel to line, and 0dB gain. I was gratified to learn the the Trim pot in this case did indeed affect the channel signal output at the DB25.

Figuring that 0dB gain indicated on the GAIN/PARAM display corresponds to an actual 0 dB gain from Line input to DB25 output, I tweaked the Trim pot to make the measured signal level at the DB25 equal to the measured signal level (when unloaded - maybe a small error introduced?) at the OSC TRS output. Or very close - approaching this level, channels stop changing level with additional pot rotations.

At the point that the pots stop affecting the output level, I measured the following unloaded Vrms levels:
OSC TRS: 1.184
Channel outs - 1.171 to 1.183, with the low reading being quite the outlier.

I'll consider this good enough for the work I'll be doing, unless I've made some error in procedure.

Anyone want to comment on how I went about this?

That sounds as good a game plan as I have heard. Have you tried taking a measurement with the gain set higher? Maybe +12? To see how well the Pre actually maintains those calibrations in real world use.

Originally Posted by arche3

Why do you want to calibrate the pre? I suppose we can start there.

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Ambisonic recording is a big reason why you would want the preamp calibrated. You need closely matched gains on all channels for the decoder to work properly. One of the upsides of the PRE is that Pro Tools will automatically link channels based on the track type you create. 1st order ambisonics = 4 linked channels.

[jbreher]

Originally Posted by LDS

Have you tried taking a measurement with the gain set higher? Maybe +12? To see how well the Pre actually maintains those calibrations in real world use.

Good idea. I'll do that.

[jbreher]

Wrong subforum? Stupid question? Nobody has a clue?

[arche3]

Why do you want to calibrate the pre? I suppose we can start there.

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[jbreher]

Originally Posted by arche3

Why do you want to calibrate the pre? I suppose we can start there.

So metering is consistent with actual signal levels throughout my system.

[arche3]

Originally Posted by jbreher

So metering is consistent with actual signal levels throughout my system.

How are you using the pre? Microphones? Summing mixers? If microphones I don't see the need to calibrate. If bringing level up from summing mixer the metering is not needed imo as all that matters is the adc level into protools. You can calibrate the input on the audio interface for pairs of mic pre outputs of the digi pre.

I'm not even sure what the trim pot does on the mic pre units.

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[jbreher]

Originally Posted by arche3

How are you using the pre? Microphones?

Mostly. Also guitar and bass to the INST inputs, and various line level gear to the LINE inputs and insert RTNs.

Summing mixers?

Not at this point. It would seem to me to make more sense to run a summing mixer direct to the ADC, skipping the additional gain stages of the Pre, no?

You can calibrate the input on the audio interface for pairs of mic pre outputs of the digi pre.

Not sure what you are saying here. In a previous thread, we covered the calibration of my HD I/O. But that cal is per channel, not per pair.

I'm not even sure what the trim pot does on the mic pre units.

I guess that makes two of us. Or more (given any lurkers). All I know so far is that the trim pot affects the level as measured at the DB25 of any signal injected at the LINE input.

It would be nice if Avid provided more info on exactly where in the signal flow the trim pot is placed. Somewhat astonished that they provided a user-accessible control with no solid information regarding its use.

[arche3]

Originally Posted by jbreher

Mostly. Also guitar and bass to the INST inputs, and various line level gear to the LINE inputs and insert RTNs.



Not at this point. It would seem to me to make more sense to run a summing mixer direct to the ADC, skipping the additional gain stages of the Pre, no?



Not sure what you are saying here. In a previous thread, we covered the calibration of my HD I/O. But that cal is per channel, not per pair.



I guess that makes two of us. Or more (given any lurkers). All I know so far is that the trim pot affects the level as measured at the DB25 of any signal injected at the LINE input.

It would be nice if Avid provided more info on exactly where in the signal flow the trim pot is placed. Somewhat astonished that they provided a user-accessible control with no solid information regarding its use.

Oh. I'm assuming people use passive summers like me. I have custom passive summing boxes built and I patch in stereo pairs of mic pres to taste. The avid pres are really clean and work well for this. The good part of the bespoke passive summing is you get to choose the pres for the makeup gain.

I calibrate the adc in pairs for the different summed sub groups.

Imo for the way you are using the avid pre you don't need to calibrate it at all...just use it to get the level you want into protools.



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