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Old 16th March 2006, 05:41 PM   #1
Glowbug is offline Glowbug  United States
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Default Passive line stage with tubes

Hey...

Sort of juggling around some equipment here, and most likely going DIY for my next preamp due to budget constraints. I've read a lot about passive "preamps" as being highly transparent and introducing nothing to the signal chain, but I've read quite a few negative comments about them at the same time. Articles such as this: http://www.enjoythemusic.com/magazin.../coolaudio.htm tend to make me confident that it can be done with tubes, but I figured I'd ask you all before I went out and bought anything.

Here's my situation. As the signature says, I'm going to be keeping my SV811 monos - they use a 6EA7 input stage. I'm not sure of the input impedance, but I've heard that anything from 25k to 100k is the value to use for a vacuum tube passive system. I found this on Ebay: http://cgi.ebay.com/DACT-CT2-Stereo-...QQcmdZViewItem ...seems to fit the bill and my budget quite nicely.

My sources are going to be mainly a phono preamp (not decided exactly what model yet) and a CD player or tuner.

Thank you for any help in advance.
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Old 16th March 2006, 06:45 PM   #2
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The English language takes another beating. They've been doing this in the Chip Amp forum a while, but I don't take them seriously. Please don't call a <1 gain active preamp a "passive preamp". It drives me crazy.
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Old 16th March 2006, 07:40 PM   #3
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Hi Glowbug

The value of the pot is determined mostly by the sources. Many phono preamps are simply not capable of driving well a 10k load. All are perfectly fine with 50-100k. Of course cable effects seem more audible at higher driving impedances.
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Old 16th March 2006, 08:48 PM   #4
Glowbug is offline Glowbug  United States
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Thank you both for your input.

One more question...I've found the DACT for the main volume control, but I'm planning on mounting a balance control on the rear of the unit, as a sort of "set and forget" control. More intuitive to my preferences than dual mono controls, and is cheaper Any suggestions on, well, good pot?

I'm sure Radio Shack probably has something that will work, but I doubt the quality will be up to par...I'm not looking to spend $100 on a balance control, but middle-of-the-road quality wouldn't be a bad thing.

Thanks.
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Old 16th March 2006, 11:32 PM   #5
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I'm still not completely sure what you are building. And I'm not sure what is the point of using a switched attenuator but then still putting a pot in the signal path for the balance. BUT, since you asked, I would probably look for either an ALPS or one of those PEC mil spec conductive plastic pots that were the rage a while back. A quick search should find you a good price.
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Old 17th March 2006, 06:20 AM   #6
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Yup. Bad idea with the balance pot. It will completely negate any advantage of a nice stepped attenuator. What does one use a balance control for anyway?
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Old 17th March 2006, 03:14 PM   #7
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Quote:
Originally posted by analog_sa
What does one use a balance control for anyway?
On a commercial amp, I only use them for 2 things: (1) verify that I wired the speakers correctly, and (2) an A/B switch for crude crossover development.
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