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Old 28th November 2012, 10:27 PM   #1
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Default IV output stage volume control

Hello,

I have a PCM1704 DAC with IV output stage shown bellow. I would like to add a volume control system before the XLR output connectors since those XLR outputs are connected to two separated studio monitors and it not handy to change volume on both of them

I am ready to change the whole IV stage if you have better sounding proposals

Thank you for your help
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Old 29th November 2012, 01:30 AM   #2
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You might insert balanced resistive attenuators between the op-amps and the XLR jacks, but that will either significantly increase the output impedance or load the op-amps, depending on the attenuator design. A better way might be to insert autoformer (magnetic) based attenuators, such as sold by intact audio and others.

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Last edited by Ken Newton; 29th November 2012 at 01:32 AM.
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Old 29th November 2012, 01:47 AM   #3
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If you're not too bothered about the higher noise I'm getting great sonic results from using passive I/V and AD603 post amplifiers. They have in-built DC driven volume controls.
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Old 29th November 2012, 03:02 AM   #4
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This can be done VERY simply. Since there is a 30ohm series resistor on each output line, all you need to do is connect a 10k-100k log or linear pot(one section of a dual pot per channel), and connect the counter-clockwise terminal to XLR pin 2, and wiper terminal to pin 3, leaving the CW terminal open or tied together with wiper terminal. This creates a fairly typical balanced output attenuator setup, with the 300ohm series resistors preventing the two opamps from driving excessive current. Just for the sake of good balance tracking, I'd suggest using an Alps RK27 series "blue velvet" pot. Since the pot is strictly a shunt accross the output lines, this control arrangement will have zero negative impact on the sound, regardless of how high or low it is set.

Have to say, though, that the i/v circuit shown, and it's implementation, is NOT the best way, or even close, to get the best sound from PCM1704's.
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Old 29th November 2012, 03:09 AM   #5
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you might want to buffer after the attenuator
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Old 29th November 2012, 03:24 AM   #6
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I can't endorse stevensank's volume control suggestion myself even though he's normally a fount of knowledge. Consider for a moment what happens:

With a 300R series resistor, the -6dB point here is going to be where the pot is presenting 600R. For a 10k linear pot that's 6% of its travel. For a 100k, that's 0.6%. -6dB setting is very high volume and mostly you're going to want to listen at levels below this - but you have at most 6% of your pot's range available. Now a log pot will improve on this but not by a lot. Also the opamp's distortion will be unnecessarily high over the working range of the pot as no opamps like loads below 600ohms much.

Agree though that OPA627 as I/V is probably a waste of a very decent DAC chip.
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Old 29th November 2012, 10:31 AM   #7
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Hi all,

Thank you for your answers,
I just read Nelson Pass Zen I/V article which highlight that OPA I/V conversions are not the best sounding ones,
So since I need to add volume control at the output of the DAC it seams that it is a good opportunity to review all the output stage and go for a nicer IV conversion.
Do you have any suggestion of a good IV stage that works good with PCM1704 output?
Thank you for your advices
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Old 29th November 2012, 02:32 PM   #8
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There are some suggestions here:

best balanced volume control solution ?

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Old 30th November 2012, 01:59 AM   #9
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The 10K value was off the top of my head, thinking mainly about not wanting to drop the maximum output level with versus without the pot. A 1k would would give more useful control range, but would drop the max output level substantially. A 5K might be the best compromise. As for opamp loading, any opamp much better than a 5532 can easily handle a 300ohm load, but it would only see that load at full down volume setting.
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Old 30th November 2012, 08:29 AM   #10
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Thank you all,
First I will go for a the easy pot output solution using a 5k pot as suggested by stephensank,
But regarding your answer I must change this OPA IV conversion stage for a better sounding one.
Please suggest me a good sounding IV converter for PCM1704.
I saw the Zen Cen, Sen thread about IV conversion, does they sound great on 1704 output?
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