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Old 5th April 2004, 04:44 PM   #1
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Default help cleaning up REALLY messy passive preamp

Hi, i recently built this passive preamp, but don't know very much about how to wire something like this nicely. The preamp has 2 10k resistors on the positive lead of the input coming in (attached to some more wire), and a 100k alps pot for the volume control. This thing looks like a rat's nest! Also, if anybody has any suggestions about what could be done to improve the sound, that would be great too. For some reason there is a little bit of grain in the sound on certain cd's.. Thanks!

Dave
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Old 5th April 2004, 09:31 PM   #2
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Where's the rest of it? I guess for starters you could put it on some kind of prototyping board. Using DB-9 connectors (serial port plug) to make the board removable from the chassis components is a favorite method of mine.
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Old 5th April 2004, 10:00 PM   #3
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Default rest of the preamp

This is it! There is very little to this preamp. The signal path is rather long, and i would like to be able to shorten it a bit, which hopefully would improve the sound.. The resistors are metal oxide 10k, and the wiring is 20 guage solid wire.

Dave
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Old 5th April 2004, 10:07 PM   #4
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You could mount the resistors directly to the pot and skip the connectors by hardwiring the in and out cables to the pot as well. That way you would preserve the shielding throughout.

That looks like an inexpensive pot. Are you sure you wouldn't rather buy one of my nice 50K Nobels?
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Old 5th April 2004, 10:16 PM   #5
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Cool Re: help cleaning up REALLY messy passive preamp

Quote:
Originally posted by bonsai171
Hi, i recently built this passive preamp, but don't know very much about how to wire something like this nicely. The preamp has 2 10k resistors on the positive lead of the input coming in (attached to some more wire), and a 100k alps pot for the volume control. This thing looks like a rat's nest! Also, if anybody has any suggestions about what could be done to improve the sound, that would be great too. For some reason there is a little bit of grain in the sound on certain cd's.. Thanks!

Dave
Hi Dave, the pot is totally hopeless. Build something with a Shallco 47 position switch as a shunt attenuator. Fred Dieckmann had a passive preamp like that back in the days he was called Harry Haller. Michael Percy has the Shallco forUS$ 95.
http://www.percyaudio.com/
Forget about the Elma or DACT. Fedde has also a bad experience with the Elma.
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Old 6th April 2004, 12:20 AM   #6
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Passive preamp=volume control???
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Old 6th April 2004, 12:59 AM   #7
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Default pots

$95 is a bit steep for a pot at the moment, are there any decent pots for a reasonable price? In fact, the entire preamp cost $20 to build, so that would be almost five times the price of the preamp! Since I don't have the opportunity to listen to it also, it would be hard to justify that kind of purchase. The pot i'm using at the moment is an alps that ran somewhere around $5..

Elso- to answer your question, this preamp was based on the Harry Haller preamp. Except his was a dual mono design, with 2 resistors in parrallel to make an 11k resistor instead of the 10k in this preamp. If i were to switch to a different pot, what kind of changes to the sound/benefits could be expected? I have heard that the Alps blue velvet is a fairly popular and decent pot, and with a pretty good price too.

Does anybody know if a buffer circuit has to be run by a power supply? (i.e. are there passive buffers??) Also, how would a different value pot change the sound ? Thanks

Dave
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Old 6th April 2004, 01:46 AM   #8
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Move the pot to the back of the case closer to the rca jacks. Then use a shaft extender to the front panel knob.

Cody
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Old 6th April 2004, 03:15 AM   #9
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what is this thing? just a volume control???
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Old 6th April 2004, 03:24 AM   #10
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A passive preamp is a device that is used instead of an active preamp, in order to control the volume. They are useful #1 as a starter, so that you can listen to music from your amp if you don't have the $$ to get an active one with gain stages. One benefit of using a passive is the transparency/lack of coloration. The sound is very natural and easy on the ears. However, if you are looking for extreme amounts of volume, or want to run a powered subwoofer, you should look to an active preamp. There is one notable problem with passives-the variance in the output impedance. Variances in the output impedance can cause certain frequencies to become more pronounced than others-hence the need for a buffer. A buffer circuit will keep the output impedance fairly constant, therefore delivering a more accurate reproduction of sound..

Dave
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