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Old 18th November 2008, 10:16 AM   #1
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Default converting BAL to UNBAL using tubes and without transformers

....

i want to convert 2Vrms, balanced signal to unbalanced audio signal using tubes with no transformer to do the conversion and with gain of arround 1 .... it would be nice to have not to much stages (one stage will be perfect)..... does anybody has a suggestion......

thanks for any help

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Old 18th November 2008, 10:28 AM   #2
SY is offline SY  United States
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Diff amp.
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Old 18th November 2008, 11:26 AM   #3
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Hi

The Broskie cathode follower could be an option as well

http://www.tubecad.com/october99/page9.html

Erik
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Old 18th November 2008, 11:26 AM   #4
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SY's anser is correct but kinda brief.

You need a differential stage sitting on a very good current source. That is naturally balanced in on the two grids, and you can have a perfect unbalanced signal on either of the two anodes, one in phase and one out of phase.

Bit hard to only have a gain of one, but you can certainly get it down by using larish unbypassed cathode Rs in the diff pair.

Regards, Allen (Vacuum State)
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Old 18th November 2008, 01:25 PM   #5
KaDe is offline KaDe  Germany
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hmm.... I was in this task on a balanced phono pre amp output a couple of years ago.
I found it to be completely unnecessary to convert to unbalanced with an extra diff-amp buffer.
I ended up with simply using one signal phase, leaving the other phase disconnected.
To me 1Vrms seems plenty sufficient.
Btw, your source is not a transformer output ?

Klaus
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Old 18th November 2008, 01:31 PM   #6
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Long necked pair, differential "4th circuit".
Shared CCSink above common plates.

This cuts gain down to 1 (or slightly less?).
Output impedance would be favorably low.

And PSRR should be excellent. As good as
your CCS vs output cathode impedance(s).
Referenced almost entirely to GND.

Make sure there is plenty of Mu headroom
for the plates to couple. Voltage swing at
the shared "neck" might be quite large.

Power supply may not need much filtering,
as that chore is redundant to the action of
the common plate current sink. Free bonus.

Unique to the relatively low plate impedance
of Triodes, the plate can be abused as either
input or output, or both simultaneously...

Same basic concept as long tail pair, but
completely upsidaisified, and without any
voltage gain (but lots of current gain)...
Do you need a picture?

----------------------------------------------------

Parts saved include two cathode resistors
(previously suggested to reduce gain to 1).
And almost all power supply filtering, but
perhaps for one cap.
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Old 18th November 2008, 01:39 PM   #7
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Hi Kenpeter

I am interested in a picture

Erik
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Old 18th November 2008, 01:56 PM   #8
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I gotta get out and go to work right now.
But when I get home, I'll draw you up an
LTSpice.

Its too damn simple, we speak of only five
components. One shared CCS, two triodes,
each with an independent cathode resistor.

OK OK, so you gotta add grid leak and some
coupling caps... Goes without saying.
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Old 18th November 2008, 02:19 PM   #9
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Hi Kenpeter

I think I got the picture...but I still have no idea how it works (but that is my lack of knowledge).

Erik
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Old 18th November 2008, 03:09 PM   #10
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KenPeter,
I am a 40 year tube audio professional, and I have NO idea of what you are talking about - and I'm not ashamed to admit it.

I can't belive a newbie can understand what you have written, a pix would be essential, IMO.

Regards, Allen (Vacuum State)
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