Page 10 of 10

Re: That1580 mini mixer

Posted: Mon Jun 07, 2021 7:43 am
by mediatechnology
It also occurred to me that you might consider combining the 124X common mode stage with the modulator driver.

Use a "Birt" topology similar to those shown in THAT's DN-140 (figure 11 basic Birt) and the THAT 5263 data sheet figure 12.

http://www.thatcorp.com/datashts/THAT_5 ... asheet.pdf

I did the development work on both of those circuits and the modulation driver works quite well.
It not only saves you a stage it allows the common mode rejection circuit to have lower resistor values than the 124X.

Re: That1580 mini mixer

Posted: Mon Jun 07, 2021 11:48 am
by weroflu
can't do the polarity digital because signal is still analog at that point.

the adc happens after the virtual earth summing:

5x2 analog mixer>> stereo analog buffer for adc >>stereo adc>>mcu>>microsd

i thought about what you're saying (sorta) with nixing the 1240's.

pro: save a stage, less noise probably, signal remains differential right through to the adc
con: time, money, redesign. summing will have to be differential.

i already have the boards done and in hand. one channel is populated but not tested.

i think it comes down to noise/distortion. if i won't hear any difference then i'm inclined to leave things as they are. but, convince me!

1240 was not in stock last time i checked mouser and it looks like ina134 is drop in replacement so i might go with those.

not sure how the birt configuration you posted compares to what cirrus recommends.
see figure 2

https://statics.cirrus.com/pubs/appNote/an241-1.pdf

Re: That1580 mini mixer

Posted: Mon Jun 07, 2021 5:09 pm
by mediatechnology
For some reason I was thinking each pair of channels fed an A/D.
Forgot you were analog summing.

For the single modulator driver the Birt might still be advantageous.
The Cirrus circuit requires anti-polarity inputs.
The Birt can be fed single-ended or differential but still provides differential output.
One of the advantages of the Birt output.mod driver is that the second inverting op amp stage's noise appears in common mode and is thus rejected by the A/D's CMRR.

If you think you're going to need INA134's I would order them now.
Deliveries on everything is getting crazy long.