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Old 5th February 2012, 10:52 PM   #1
dazed is offline dazed  Norway
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Default Balanced tube preamp schematic or kit?

I'm sorry if this has been discussed (a lot) before, but please bear with me.

I'd like to try a tube preamplifier with my two mono bridged Abrahamsen V4.0UP power amplifiers. In bridged mode they have a 50KOhm input impedance balanced. (100KOhm with SE adapters) They also each have a 1000VA toroidial transformer and a 100.000uF capacitor bank. Bridged they should give about 240W each @8Ohms, I guess. I don't know if this matters, but anyway...

My current pre has 6dB gain (Balanced. Unity gain when used single ended) , and I wouldn't think I'll need much more than that, but a little more won't hurt either, I guess.

I'd need at least one or two balanced inputs and a balanced output, as well as two or three single ended inputs. Controls could be limited to just power, input selection and volume control.

Are there any kits or schematics available for a DIY preamplifier that fits this description? If it used tubes that are easily available, it'd be even better. It should sound "warm and tubey" of course.

...Oh. And the power transformer will have to have primaries fo the European 230V power grid

Any and all help and tips will be greatly appreciated!

Thanks a lot in advance!

Last edited by dazed; 5th February 2012 at 11:11 PM.
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Old 5th February 2012, 11:01 PM   #2
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Do you want it to be fully balanced end to end? If not, then you can just use some high quality audio transformers with any single ended tube preamp design or kit.
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Old 5th February 2012, 11:07 PM   #3
dazed is offline dazed  Norway
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Quote:
Originally Posted by dirkwright View Post
Do you want it to be fully balanced end to end? If not, then you can just use some high quality audio transformers with any single ended tube preamp design or kit.
Preferably, yes. ...But I'd need 2-3 single ended inputs as well, and I guess I don't really know what the consequences of that are.

I guess i could just build single ended and use the XLR to RCA-adaptors that came with my power amps, but when my DAC has balanced outputs and my power amps have balanced inputs... Well, a balanced pre seems to make sense. (To me, at least. However, I'm a noob, so what do I know?)
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Old 5th February 2012, 11:11 PM   #4
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Quote:
Originally Posted by dazed View Post
Preferably, yes. ...But I'd need 2-3 single ended inputs as well, and I guess I don't really know what the consequences of that are.

I guess i could just build single ended and use the XLR to RCA-adaptors that came with my power amps, but when my DAC has balanced outputs and my power amps have balanced inputs... Well, a balanced pre seems to make sense. (To me, at least. However, I'm a noob, so what do I know?)
My personal preference would be to build something like this:
Power Supply Delay Finish PCB for DIY Tube Amplifier items in 8Audio store on eBay!
(no experience with his kits though!)
and use transformers for the balanced in and out. I like Jensen.

I agree that your situation warrants a preamp with balanced input and output. The single ended inputs can be connected directly to the single ended board, obviously.
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Old 5th February 2012, 11:15 PM   #5
dazed is offline dazed  Norway
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Thanks! Appreciate it! I'll have a look at that. I'll probably need some help on the "use transformers for the balanced in and out" part as well. I suppose that means it's easy to make an additional single ended output as well?
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Old 5th February 2012, 11:17 PM   #6
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Quote:
Originally Posted by dazed View Post
Thanks! Appreciate it! I'll have a look at that. I'll probably need some help on the "use transformers for the balanced in and out" part as well. I suppose that means it's easy to make an additional single ended output as well?
It's easy to use transformers if you are willing. Jensen has schematics and white papers under their "applications" section:
WELCOME TO JENSEN TRANSFORMERS, INC.

Sure, if the board is single ended, no problem.
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Old 5th February 2012, 11:30 PM   #7
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This one shows how to hook up a balanced input to a tube preamp input:
http://www.jensen-transformers.com/as/as087.pdf
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Old 6th February 2012, 06:57 AM   #8
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How about a simple differential amplifier without any transformers like this.


Click the image to open in full size.

You can connect this as a SE or balanced input and output as you wish.
The low frequency response can be lowered to 20 Hz by replacing C2 and C3 with 220 nF if necessary.

Last edited by artosalo; 6th February 2012 at 06:59 AM.
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Old 6th February 2012, 07:07 AM   #9
dazed is offline dazed  Norway
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Thanks again!

In your link above, it seems to point to the softstart module pcb. Do I need that for a preamp?

The easiest way fro me to do this would probably be to get a PSU board:

Tube Rectified Preamp Common Power Supply finish PCB | eBay

...And either one of the populated ARC SP10, CJ PV12, Cary SLP90 or Mac SP22 clone pcb's, right?
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Old 6th February 2012, 07:34 AM   #10
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These power supplies seem too complicated. Also no specifications are given, so what do you finally get is a question.

If you need, I can show some simple example of how I would do the power supply.

Quote:
...And either one of the populated ARC SP10, CJ PV12, Cary SLP90 or Mac SP22 clone pcb's, right?
What does this mean ?
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