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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

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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