Passive pre-amp

Davy · 22nd August 2008, 11:00 PM

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.

Cheers

fredex · 22nd August 2008, 11:42 PM

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.

Davy · 22nd August 2008, 11:54 PM

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?

unclejed613 · 23rd August 2008, 12:00 AM

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.

AndrewT · 23rd August 2008, 11:41 AM

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.

richie00boy · 23rd August 2008, 11:51 AM

You really need to know the Naim input impedance before you can choose a pot.

Davy · 23rd August 2008, 01:42 PM

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?

Cheers

richie00boy · 23rd August 2008, 01:59 PM

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.

analog_sa · 23rd August 2008, 02:06 PM

A 100k pot can only drive a very short length of wire.

AndrewT · 23rd August 2008, 02:08 PM

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?

22nd August 2008, 11:00 PM	#1
Davy diyAudio Member Join Date: Sep 2006	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. Cheers

23rd August 2008, 12:00 AM	#4
unclejed613 diyAudio Member Join Date: Dec 2006	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. __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

23rd August 2008, 11:41 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

23rd August 2008, 11:51 AM	#6
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	You really need to know the Naim input impedance before you can choose a pot. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

23rd August 2008, 01:59 PM	#8
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

22nd August 2008, 11:42 PM	#2
fredex diyAudio Member Join Date: Jul 2005 Location: New Zealand	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.

22nd August 2008, 11:54 PM	#3
Davy diyAudio Member Join Date: Sep 2006	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?

23rd August 2008, 01:42 PM	#7
Davy diyAudio Member Join Date: Sep 2006	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? Cheers

23rd August 2008, 02:06 PM	#9
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	A 100k pot can only drive a very short length of wire.

23rd August 2008, 02:08 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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? __________________ regards Andrew T.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Passive Crossover for Passive Subwoofer?	Toast_Master	Subwoofers	23	30th April 2006 12:14 AM
passive pre amp	mschwilson	Solid State	10	27th July 2005 01:43 PM
dB loss by using passive crossovers? Active vs Passive and 1st vs 4th order	Hybrid fourdoor	Multi-Way	3	11th July 2004 09:16 PM
Help with passive pre	sejarzo	Everything Else	9	27th April 2004 06:11 PM
Passive EQ?	Johan_Ch	Everything Else	1	4th September 2003 04:11 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read