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Old 5th May 2012, 01:40 PM   #1
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Default Volume Control with symmetrical out and in

Hello, I was searching for a very good volume Attentor and didn't find something usable and well priced on the market.
My Goal is a "simple" active Volume controll with very high part quality and a negable difference between right and left channel.
A few Day's ago, i found those Volume Attentuator:

High Quality Audio & Industrial Attenuators

Now i started to think about the schematics. The Opamps in the front are not choosen out till now.
My idea was a operational amplifier for a high input inpedance and with some trimmers to get a CMRR>60db. The DC servo is to handle a DC offset of +-100mV (but i need to calculate it again). Do you think it is possible to get a high CMRR with DC Servo in one stage?

The Output stage uses a 1632 to get a symmetrical Signal again. Then Id like to use LME 49990 with the BUF 634 to drive long cables. Mabe it is possible to drive the two BUF directly?
My experience with the buf teached me that the BUF's are not easy to control ner unitiy gain, so i'd like to be on the save.

What do you think about the schematics?
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Last edited by Stefan0815; 5th May 2012 at 01:43 PM.
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Old 6th May 2012, 12:53 AM   #2
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I worry about schematics that connect XLR connectors pin # 1 to analog ground.
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Old 6th May 2012, 08:39 AM   #3
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Oh, you are totaly right!
I'll correct this. What do you think about the schematics in general?
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Old 7th May 2012, 03:52 PM   #4
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There is a "mistake" in my shematics.
The 1632 is builded for an AD converter. It is possible to get a "near symetrical signal" out of a Single ended input.
Depending on my measurements today, it was 20% off
My idea is now to buy a finished Balanced SE konverter and then a SE Balanced Converter from Funk Audio Berlin.

Last edited by Stefan0815; 7th May 2012 at 03:56 PM.
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Old 7th May 2012, 07:44 PM   #5
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Yes xlr pin #1 to chas. Please have a look at THAT 1642 as bal. outputamp.
//Hans
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Old 7th May 2012, 07:58 PM   #6
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You could put the pot meter directly across the balanced signal insted of one sided to ground.
That way you can lose the bal to se and the se to bal conversion. Saves you most of the opamps

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