2. amps-preamps
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Who is using passive preamps and why?

Seldom has there been any discussions on passive preamps in the forums and although my experience with them has been limited I have found them so far to be very enjoyable and refreshingly different. They seem to fall into their own category, somewhere between solid state and tube. Finding a preamp that is satisfing has been difficult. Some active solid state preamps can be very good but they seem to inject grain to some degree in the upper registers and some tube preamps are not too far behind. So far I think they should at least be matched up with an amp that has sufficient gain which is often overlooked. Which passives are you using and with what amp? Why do you like them?
phd
almarg's avatar
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almarg
9,670 posts
 
Almarg: In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.

The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).
To make sure it's clear, I should add that if you have a transformer ahead of the volume control which steps UP the incoming voltage from the source component, the source component will see a load impedance that is REDUCED from the 9.1K or 10K number, in proportion to the square of the turns ratio.

For example, if the transformer provides a 4x (12db) voltage step-up, the source component would see a load impedance of only 568 ohms to 625 ohms, depending on the volume control setting. Obviously that gets into worrisome territory.

Best regards,
-- Al
ngjockey
1,042 posts
 
Just to confuse things...

http://diyaudio.co.kr/wwwboard1/data/board1/compare.pdf

Is it safe to assume that the Goldpoint is shunt-type?
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almarg
9,670 posts
 
Is it safe to assume that the Goldpoint is shunt-type?
No, they are series types. From the Goldpoint website:
Goldpoint Level Controls used to offer Ladder and Shunt type stepped attenuators too. The fact is that with the very low noise, 0.1%, thin film resistors we are now using, we believe that there is no real advantage to Ladders and Shunts - and so have ceased offering them.
And a schematic of their Mini-V series attenuator:

http://www.goldpt.com/schm_ser.html

Best regards,
-- Al
pubul57
3,712 posts
 
I know it is a good thread when I understand half of it. On the issue of impedance matches and best pot ratings, I see the theoretical, and perhaps measureable difference, but are they audible (to most people)? I suspect that in my setup, whether the pot is 10, 25, or 50 I would ne hear the difference, though my test equipment might.
clio09
3,343 posts
 
Here is a schematic of what I built:

http://www.electra-print.com/pva.php

Here is some more information:

http://www.electra-print.com/techblog.php?blogid=1
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