Passive preamp - best solution?

sobazz · 17th July 2003, 03:17 PM

I stumbled across a one year old post about a passive preamp. It used a somewhat different approach to volume attenuation than those normally used. Instead of relying on the potentiometer a resistor was used in series and the pot's only function was to shunt a part of the signal to ground.

The thread is here

For the preamp I plan to use a spare ALPS RK40 10K. But would I benefit anything from using the above solution? If yes, what value should the resistor have (it's only a 10k pot)?

What are the different connections on the pot? There are four pairs of connections, the first 3 are numbered from 1 to 3.

jbateman · 17th July 2003, 03:37 PM

I'll leave it to the platinum eared to argue which sounds better, but part of his reasoning for doing it that way was due to the fact that he had a 100k pot. In a passive preamp you have to maintain a high enough impedance to not overload your source, but low enough to retain frequency response to the amp. 100k is too high. Your 10k should be just about right, if used in the traditional manner.

Peter Daniel · 17th July 2003, 03:43 PM

Quote:

Originally posted by sobazz
what value should the resistor have (it's only a 10k pot)?

The value of a series resistor will greatly depend on your setup, gain of the amp, input resistance, speakers efficiency, even the type of Alps pot. I would suggest using a trimpot (variable resistor) initally and trying to find the best resitance range in this way. A 10k or 5k timmer should be a good value for a start.

sobazz · 17th July 2003, 05:12 PM

Thank you both. I think I'll wire the Rk40 in the traditional way, then. I don't feel like experimenting so much at the moment.

What are the fourth set of pins on the pot for? I read somewhere that the pot was a fake ALPS if the stícker on the back was not glossy but more like a photo copy. This is hopefully true. To me it sounds far out. What do you guys say? How can you recognize a fake ALPS RK40? I bought mine from LCAudio a couple of month ago.

sobazz · 17th July 2003, 08:39 PM

Quote:

This is hopefully true.

This should of course have been hopefully not true. It seems to me, that I cannot edit my post. I don't know why.

[edit]: Time limit, obviously.

17th July 2003, 03:17 PM	#1
sobazz diyAudio Member Join Date: Apr 2003 Location: Jutland	Passive preamp - best solution? I stumbled across a one year old post about a passive preamp. It used a somewhat different approach to volume attenuation than those normally used. Instead of relying on the potentiometer a resistor was used in series and the pot's only function was to shunt a part of the signal to ground. The thread is here For the preamp I plan to use a spare ALPS RK40 10K. But would I benefit anything from using the above solution? If yes, what value should the resistor have (it's only a 10k pot)? What are the different connections on the pot? There are four pairs of connections, the first 3 are numbered from 1 to 3.

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17th July 2003, 03:37 PM	#2
jbateman diyAudio Member Join Date: Oct 2000 Location: The People's Republic of Maryland	I'll leave it to the platinum eared to argue which sounds better, but part of his reasoning for doing it that way was due to the fact that he had a 100k pot. In a passive preamp you have to maintain a high enough impedance to not overload your source, but low enough to retain frequency response to the amp. 100k is too high. Your 10k should be just about right, if used in the traditional manner.

17th July 2003, 05:12 PM	#4
sobazz diyAudio Member Join Date: Apr 2003 Location: Jutland	Thank you both. I think I'll wire the Rk40 in the traditional way, then. I don't feel like experimenting so much at the moment. What are the fourth set of pins on the pot for? I read somewhere that the pot was a fake ALPS if the stícker on the back was not glossy but more like a photo copy. This is hopefully true. To me it sounds far out. What do you guys say? How can you recognize a fake ALPS RK40? I bought mine from LCAudio a couple of month ago.

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