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Old 24th April 2014, 12:45 AM   #1
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Default Headphone system - volume pot placement.

My current system consists of the following:
  1. PC with JRIVER Software
  2. DAC (AD1865 non-oversampling w/ USB input)
  3. Passive current-to-voltage with resistors, balanced output
  4. John Broskie "Unbalancer" differential input gain stage with Broskie cathoe follower unbalanced output (circuit attached)
  5. Headphones

Currently I control the volume via the 'slider' on JRIVER pc software (or Foobar), although I have read that digital volume control in fact causes loss in bitrage/dynamic range.

If this is indeed the case, then where is the best place to put an analogue volume control? Would it be between the DAC and the headphone amplifier, or between the headphone amplifier and the headphones?
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Old 24th April 2014, 12:19 PM   #2
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Hi,

if you don't mind using dual mono volume controls then I'd suggest putting them at the input of the first stage.

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Old 24th April 2014, 12:26 PM   #3
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Ok. Would a stereo balanced pot work? What impedance would be best? I did consider this, but wasn't sure how it would affect the load on the DAC.
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Old 24th April 2014, 12:52 PM   #4
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Hi,

You could use a dual ganged stereo pot per channel ( 50k or 100K) which allows for trimming out gain differences, or a single stereo pot per channel with well matched tracks (the latter assumes using well matched valve halves).

Naturally a single double stereo ganged pot would work as well but wouldn't allow for fine inter channel balance trimming.

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Old 6th May 2014, 01:04 PM   #5
TheGimp is offline TheGimp  United States
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There is a discussion in the tube section on placement of the pot. After the first triode has advantages for frequency response, however you have to be careful as the greater the resistance the more noise it introduces. With the rp of 25K for the 6072A, you would be better off with 100K from a distortion/loading standpoint, but slightly worse for resistance noise.

If you put the pot in front of the first stage, the lower value would be better from a noise standpoint (presuming the driver is a modern low impdeance driver spec'd for 10K load), but you will need a large coupling cap, and frequency response will shift more with pot setting relative to having the pot after the 6027a.
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Old 6th May 2014, 06:51 PM   #6
Fsatsil is offline Fsatsil  United States
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I would try the digital volume control in Jriver and see how it sounds first before you add a potentiometer. I haven't used Foobar or Jriver in years but I remember there was a way to increase the bit depth to 24bit so that the volume control wouldn't effect the dynamic range. I don't remember the details but it might be worth googling.
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