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Balanced PGA2311 volume control
Balanced PGA2311 volume control
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Old 21st March 2017, 07:47 PM   #21
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by sesebe View Post
I think the problem is wrong approached.
Check how the volume adjustment is done in PGA and you will understand.
The volume it is adjusted in PGA by referring to GND so differential work is for nothing.
Better convert it to unbalanced and use the PGA like in datasheet and if it is needed converted back to balanced.
No. See post # 15.
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Old 21st March 2017, 08:21 PM   #22
Gusser is offline Gusser  United States
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Quote:
Originally Posted by sesebe View Post
I think the problem is wrong approached.
Check how the volume adjustment is done in PGA and you will understand.
The volume it is adjusted in PGA by referring to GND so differential work is for nothing.
Better convert it to unbalanced and use the PGA like in datasheet and if it is needed converted back to balanced.
I'm with you on this. My box featured a few posts back works by running the PGA single ended with balanced receivers and line drivers around it. And as a professional engineer in the broadcast business for 35 years, I can tell you that's how it's done in commercial products.

Now I have not really analyzed the balanced PGA approach but it just seems like more of this audiophile nirvana where at the end of the day there is no audible difference.
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Old 21st March 2017, 08:25 PM   #23
jan.didden is offline jan.didden  Europe
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Originally Posted by Gusser View Post
Now I have not really analyzed the balanced PGA approach
Are you planning to?

Jan
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Old 21st March 2017, 08:29 PM   #24
Gusser is offline Gusser  United States
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Quote:
Originally Posted by jan.didden View Post
Are you planning to?

Jan
Well considering I already built my VCA box and then expanded it to 24 channels by adding another 8 channel board, i'm good.

My system is an Atmos HT system with 22 channels as the LCR's are triamped. That's why I need so many channels. This box is after the DACS.

So as it's running at +4, what exactly do I gain sonically by converting to a balanced PGA configuration? The SSM2141 and 2142 are well reviewed.
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Old 22nd March 2017, 12:01 AM   #25
Bonsai is offline Bonsai  Europe
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No one claimed 'nirvana' on anything wrt to running the PGA in balanced mode.

I wanted balanced in and balanced out so the approach seemed like a sensible way to do it.

It worked very well and I'd be happy to go back to using it again.

I'm quite familiar with integrated balanced line drivers and receivers as well.
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Old 22nd March 2017, 08:20 AM   #26
Bonsai is offline Bonsai  Europe
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Originally Posted by sgrossklass View Post
I was referring to a circuit like Bonsai's (or proposed by the OP), with single-ended buffers feeding two channels of the PGA directly. Common mode to differential mode conversion depends on channel tracking there. I haven't done the math in a while, but asking for 60 dB of CMRR (which would be a decent value for a line-level input though not SOTA) would require matching on the order of 0.1% or 0.01 dB. The part is specified for +/- 0.1 dB, no idea how good it is in real life.

Get why I'm seeing a potential problem there?
. . . you cant get an earth loop through an opamp input. So buffering the inputs before feeding the PGA input attenuator is as good as isolating them. Ditto the output circuit as well. The only issue you have is gain balance - but the PGA is pretty good in that respect.
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Old 22nd March 2017, 04:51 PM   #27
Gusser is offline Gusser  United States
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Quote:
Originally Posted by Bonsai View Post
No one claimed 'nirvana' on anything wrt to running the PGA in balanced mode.

I wanted balanced in and balanced out so the approach seemed like a sensible way to do it.

It worked very well and I'd be happy to go back to using it again.

I'm quite familiar with integrated balanced line drivers and receivers as well.
I enjoyed looking through your website!
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Old 22nd March 2017, 06:34 PM   #28
Bonsai is offline Bonsai  Europe
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Old 22nd March 2017, 10:47 PM   #29
sgrossklass is offline sgrossklass  Germany
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(Which is not to say that yours truly wouldn't have! )

Here's a lil' sim for you guys. I placed a very slightly mismatched "gain stage" between a buffer and an ideal bal/SE converter and fed a pure common mode signal into it. If not mismatched, attenuation is 180 dB, which is to say infinite within computational accuracy.

Increase one 10k resistor by 10 ohms (+0.1%), and you get 0.864 dB of gain mismatch or a CMRR of -60 dB, both inverting and noninverting. This mismatch converts part of the CM signal to differential. (The bal/SE converter on its own has ~infinite CMRR.)

I also tried the trick of referring the gain stage to a common-mode signal, and it does work quite well, though clearly some work on part of the opamp is involved so its effectiveness diminishes towards the higher freqs (still very good though). For a noninverting type of gain stage this reference would have to be buffered. Not sure about the noise penalty.
Connecting the two noninverting amps together INA style works great though, CMRR is up to maximum possible again. Not sure whether that'll fly with a PGA, but worth a try I suppose.
Attached Images
File Type: png bal-cmrr-model.png (35.6 KB, 118 views)
File Type: png bal-cmrr-model-cmref.png (37.3 KB, 118 views)
File Type: png bal-cmrr-model-ina.png (44.1 KB, 118 views)
Attached Files
File Type: asc bal-cmrr-model-3.asc (4.6 KB, 0 views)
File Type: asc bal-cmrr-model-4-cmref.asc (5.0 KB, 0 views)
File Type: asc bal-cmrr-model-5-noninv.asc (5.0 KB, 0 views)

Last edited by sgrossklass; 22nd March 2017 at 10:51 PM.
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Old 23rd March 2017, 07:50 AM   #30
AndrewT is offline AndrewT  Scotland
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Thank you Sgr.
I have seen a couple of articles that look at common mode to dif mode conversion of interference, but never fully understood the mechanism at play.
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