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Old 22nd August 2008, 11:00 PM   #1
Davy is offline Davy  Scotland
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Default Passive pre-amp

Hi im looking for some advice please, im planing to make a passive pre to control my pc's soundcard output.
Im hoping it will drive a Naim NAP140 to a decent volume. Im thinking alps blue for the volume pot but im unsure which value to get - im looking at probably either 50 or 100k value, can anyone help.

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Old 22nd August 2008, 11:42 PM   #2
fredex is offline fredex  New Zealand
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The higher the pot value the more the cable to the amp will affect the highs. The input circuitry of the amp will also affect the outcome. A lower value is better but if too low it will load the output of the sound card making it work harder = more distortion. So it is a balancing act. The general consensus with solid state gear is 10k.
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Old 22nd August 2008, 11:54 PM   #3
Davy is offline Davy  Scotland
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Thanks Fredex, im planning on using some of DNM's new interconnect wire for internal wiring because thats what im using for the rest of my inters so keeping capacitance low.
Stunning wire IMO, so's his new speaker cable.
BTW, any tips on where i can source a quality pot with that low a value?
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Old 23rd August 2008, 12:00 AM   #4
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it depends on which sound card out you use. the line out would work best with a 10k or 20k pot, while the headphone out would work best with a 1k, 2k, or 5k pot. if it's an actual speaker out, use a 100 or 500 ohm pot.
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Old 23rd August 2008, 11:41 AM   #5
AndrewT is offline AndrewT  Scotland
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I don't know how good or bad the PC's soundcard output is, so I suggest you cobble together a very cheap 10k pot with DC blocking cap on the input and a high value grounding resistor from the wiper.
Keep your cables as short as possible (less than 1m) for this first test set up.

If you get a sound that seems acceptable then try to source a better attenuator that suits the PC source and suits the Naim load.
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Old 23rd August 2008, 11:51 AM   #6
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You really need to know the Naim input impedance before you can choose a pot.
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Old 23rd August 2008, 01:42 PM   #7
Davy is offline Davy  Scotland
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Thanks all, it appears that most if not all Naim poweramps have an input impedance of 18k.
Not sure about the soundcard, i did'nt fit it but im told it's decent,
Creative Audio PCI (ES1371,ES1373) (WDM) - at least thats what my pc tells me is fitted, makes no sense to me at all may as well be in urdu or chinglish.
So what value pot suits an 18k impedance?

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Old 23rd August 2008, 01:59 PM   #8
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I would fit a 100k linear taper pot. The 18k input impedance will load the pot to give a good approximation to a logarithmic characteristic.
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Old 23rd August 2008, 02:06 PM   #9
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A 100k pot can only drive a very short length of wire.
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Old 23rd August 2008, 02:08 PM   #10
AndrewT is offline AndrewT  Scotland
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but the 25k source impedance of the 100k pot will demand a very low capacitance between pot and power amp.
40pF will create a filter with F-3dB = 16kHz
4pF will give a much more acceptable treble roll off characteristic.
What is the value of the RF filter capacitor inside the Naim?
What is the parasitic capacitance of the internal wiring and connectors in the Naim?
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