VCA bus mixer using all THAT all the time

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mediatechnology
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Re: VCA bus mixer using all THAT all the time

Post by mediatechnology »

The VCA distortion will surely be uncorrelated since the individual audio inputs are all uncorrelated.
Well, yeah. I guess we don't run correllated test tones in the real world. Forgot about that... :oops:

I got that argument muddled with the parallel/differential VCA argument such as the GSSL folks who run them differentially to cancel THD - they don't. In parallel configurations the signal rises by 6 dB/pair where THD and noise rise by 3 dB/pair. So 6 dB in cost buys 3 dB.

The THD mechanisim though is internal transistor Vbe match which is random. You might get some cancellation.
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JR.
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Re: VCA bus mixer using all THAT all the time

Post by JR. »

I guess I could look this up, how does the noise floor compare for newer small geometry VCAs versus previous stuff?

If I was trying to market a stand alone mixer, noise floor would be a metric of interest to buyers, even though I've been saying for decades the bus noise floor will be dominated by one mic preamp at nominal gain.

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mediatechnology
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Re: VCA bus mixer using all THAT all the time

Post by mediatechnology »

JR -

The 22 kHz BW noise performance is about the same comparing a 2162 to a 2181 around -98 at 0 dB gain.

The primary difference is THD performance with the larger-geometry parts being better.

IIRC correctly the output C was 3 pF vs. 15 pF.

Wayne
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JR.
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Re: VCA bus mixer using all THAT all the time

Post by JR. »

I feel like I can design around the capacitance... and in a large console you might need to accommodate a lot of inputs... I don't know what it's like these days but I did one console with some 120 inputs to L/R and I doubt it was the biggest around. So the difference between 3 pf and 15pf is 5x but still potentially an issue at 100x.

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