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Old 9th July 2012, 03:24 PM   #1
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Default Adding volume control to DAC

Hi all, I bought this DAC some time ago

Finished CS4398 + CS8416 USB DAC With R-core Transformer + case | eBay

it performs quite well but sometimes volume control is problematic (some software allows for control, some don't)

So I would like to add a stero pot to the output (unloaded output is 6.5vvpp)

Will an ordinary stero log pot (1M) be adequate?

Thanks
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Old 9th July 2012, 06:27 PM   #2
DF96 is offline DF96  England
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Ordinary stereo log pot is fine, but 1M is a bit high. 50k might be better.
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Old 9th July 2012, 06:46 PM   #3
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Or even lower; I prefer 10k.
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Old 9th July 2012, 07:45 PM   #4
Pano is offline Pano  United States
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Yes, I have used 10K (A taper) with very good results. The output impedance is probably around 100 Ohms.
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Old 10th July 2012, 07:47 PM   #5
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Thankyou for your answers. I will go for a 10k log pot and vheck things out but i am confident it will work perfectly

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Old 10th July 2012, 11:03 PM   #6
qusp is offline qusp  Australia
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nobody has asked what hes driving with it exactly. 100 ohms on top of whatever was the outputZ before that driving a low impedance input = bad juju. 1M was more than 'a bit high'
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Old 11th July 2012, 06:37 AM   #7
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I should drive a tube amplifier with 68k grid resistor and 470k to gnd resistor.

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Old 11th July 2012, 01:10 PM   #8
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There are two effects of inserting a high value resistive attenuator (pot.) directly to the DAC box output. One is a permanent reduction in the maximum output signal level, although this does not alter the system frequency response. The other would be the formation of an RC low pass filter in conjunction with the shunt capacitace of the interconnect cable and that of the amplifier input stage, which will alter the system frequency response. Whether either effect is of audible consequence depends on the relative impedances involved. The higher the pot. resistance, the greater those two effects will become.

The most obvious solution to those two effects is to use a relatively low resistance pot. However, even a 10K pot. could load the DAC output stage enough to raise the distortion profile of your DAC, especially higher order distortion products. Providing a high impedance to the DAC's output stage, while providing a low impedance to the interface shunt capacitance, is the area where inductive volume controls really have an advantage in the application you describe.

Bye the way, the effect of interconnect cable shunt capacitance can be eliminated by locating a resistive volume control pot. at the amplifier end of the interconnect. This would require mounting the pot. inside your amplifier chassis (in essence, changing it in to an integrated amplifier) rather than inside your DAC chassis.
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Last edited by Ken Newton; 11th July 2012 at 01:34 PM.
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Old 12th July 2012, 09:35 AM   #9
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Thank you all for your replies. To achieve volume control and still be able to avoid the consequences Ken Newton described I added two switches on the back of the DAC which enable me to either use the 10k log stereo pot or bypass it completely. The two switches were used to avoid loops of any kind. When both are pointing "left" the output is directly connected to the board and shares no path to the POT, if turned right (pointing the knob) the opposite happens.

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Old 12th July 2012, 10:18 AM   #10
DF96 is offline DF96  England
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A correctly wired pot will not drop the maximum signal level by more than a trivial amount, unless the source has unusually high output impedance. Bypass switch not needed.

Ken's suggestion to put the pot at the receiving end of a long interconnect is worth following. Not necessary for a short interconnect.
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