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  #11  
Old 10-21-2019, 09:54 AM
jbreher jbreher is offline
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Default Re: Avid Pre calibration question

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Originally Posted by arche3 View Post
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.

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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?

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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.

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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.
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  #12  
Old 10-21-2019, 09:59 AM
jbreher jbreher is offline
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Default Re: Avid Pre calibration question

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Originally Posted by the.engineer View Post
Sorry if I’ve missed this...have you measured the output with a volt meter to double triple check it’s outputting 1.228V?
Yes. Although the voltage is somewhat shy of +4dBu. Initial measurement was 1.116V, later 1.184V.
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  #13  
Old 10-21-2019, 10:03 AM
jbreher jbreher is offline
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Default Re: Avid Pre calibration question

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Originally Posted by albee1952 View Post
Good suggestions above. Here is how I would try it(not knowing any better). I would create a session with an AUX track sending(use the Signal Generator plugin) a 1KHz tone out any output NOT in use for monitoring. Patch that(with a physical cable) to each input of the Pre and feed each channel to a separate track in Pro Tools. Put those tracks in Input mode and watch the track meters. Compare against each other(adjust trimmers on the Pre for identical readings). And, compare against the output level of the signal gen track. This might not be technically "perfect", but certainly can result in matching levels
Seems like that would work. Though it totally ignores Avid's User Guide mention that the inbuilt oscillator can be used to calibrate the Pre. I assume they had some reason to add the additional (non-free) circuitry to the unit.
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  #14  
Old 10-21-2019, 08:25 PM
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arche3 arche3 is offline
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Default Re: Avid Pre calibration question

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Originally Posted by jbreher View Post
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.



Sent from my SM-G975U using Tapatalk
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  #15  
Old 10-24-2019, 04:06 PM
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TVPostSound TVPostSound is offline
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Default Re: Avid Pre calibration question

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Originally Posted by jbreher View Post
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?


Test the cables
Stop overthinking this!!
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  #16  
Old 10-25-2019, 12:49 AM
jbreher jbreher is offline
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Default Re: Avid Pre calibration question

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Originally Posted by TVPostSound View Post
Test the cables
Stop overthinking this!!
Thanks so much for your attempt at a meaningful response. The cables are fine, thank you. Now that we have established that the cables are good, how do you propose that I move forward with calibration?
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