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Old 2nd July 2009, 03:15 AM   #1
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Join Date: Mar 2009
Default help with single supply for two EQ/preamps


I'm building a single stand alone tube power supply to power two EQ/preamp units. The power supply I've built works well with the single preamp I've completed. My question for the tube veterans is what would be the best way to parallel the 2 B+ rails to feed the 2nd unit to keep crosstalk and extra voltage drop to a minimum?

Here is a link to the schematic for reference

Note the 2 B+ rails are taken from the ends of the 47K and 2.2K resistors. To phrase the question another way, from the ends of these resistors, should there be a small isolation circuit between each rail going to the 1st unit and each rail going to the 2nd unit, and what would give the best results?

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Old 3rd July 2009, 04:05 AM   #2
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Old 9th July 2009, 12:47 PM   #3
Gordy is offline Gordy  United Kingdom
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Join Date: Nov 2006
One possible method: From the top of the first 40 microFarad capacitor (i.e. the junction of the capacitor, the 470 Ohm and 1000 Ohm resistors) take a branch out to an additional 1000 Ohm + 40 microFarad combination, and from there to the second channel feed-in point (the junction of the 47K and 2K2, as you say).

Keep the ground returns separate from the first channel also should help.

Without meaning to sound patronising, remember to check that the rectifier tube and transformer can take the additional current draw of a second channel.

(I know that this is a PSU related question, but to get more response on tube related issues you might want to move this question to the Tubes part of the forum. If you send a Private Message to a moderator, such as SY, I am sure he will help you move it).
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Old 9th July 2009, 03:24 PM   #4
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Join Date: Mar 2009
Thanks Gordy. I've had good luck so far doing essentially what you suggested but using an 80uF cap for the second(post 1K) cap which helps keep the current from dropping.

Definitely a good point about the rectifier/ transformer. The particular transformer I'm using is from an old hi-fi power amp & FM receiver that originally powered about 6 preamp tubes and 4 EL-84s via a 5u4GB rect. Probably overkill for my application but it was sitting around in a junk drawer. So I actually use a heavy duty 15K pot in place of the 1K to dial in the correct B+ and this works very well.
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