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Old 1st January 2011, 04:41 PM   #1
Loren42 is offline Loren42  United States
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Default Balanced to Unbalanced Line Conversion

Greetings & Happy New Year!

My next project is a preamp to match my tube power amp. The issue is that the run between the preamp and the power amp is about 20 feet.

This is problematic for a single conductor high impedance cable, correct?

What is the advantage to going to a balanced line?

What losses would I have using transformers as a means to do this? What am I giving up?

I know I could do this without transformers, but the power amp is done and I do not want to change the power amp circuit, so I am resigned to look at other alternatives.

Thanks in advance!
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Old 1st January 2011, 07:07 PM   #2
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Many would say that transformers are the best way to do this. Then they might list the problems with some of the other methods.
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Old 1st January 2011, 07:27 PM   #3
SY is offline SY  United States
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Unless you live in a high RF area, a 20' single ended line at line level (volts) with a low source impedance shouldn't be a big deal as long as you don't use some sort of goofy (and expensive) high capacitance or unshielded cable.
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Old 1st January 2011, 07:37 PM   #4
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Unless you live in a high RF area, a 20' single ended line at line level (volts) with a low source impedance shouldn't be a big deal as long as you don't use some sort of goofy (and expensive) high capacitance or unshielded cable.
True, but since you are talking about a passive solution anyway, just put the transformer at the destination instead of the source, and bathe in the comforting glow of the common-mode cancellation that balanced lines give you, even if you are extra goofy.
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Old 1st January 2011, 09:07 PM   #5
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Use 6 Meter shielded cables, with 2 signal conductors, between the preamp and the power amp. Connect the shields to ground only at the preamp end. The preamp has to have a LOW O/P impedance and good current handling capability. An IRFBC20 source follower set up for a 25 mA. drain current will do the line driver job very nicely, indeed. Yes, a miniature power amp is what the situation calls for. If some gain is needed in the line stage, a 12B4 would be excellent and DC coupling the FET to the triode is easy.
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Old 1st January 2011, 09:44 PM   #6
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What Eli said...with the following suggestion: run the source follower in cascode; use two and deal with the variable gate capacitance. Even if D-S voltage *SEEMS* to be well outside of the high-variability zone, the cascode is worth the effort. You're going to need something that would drive a near short circuit for best results.
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Old 1st January 2011, 10:35 PM   #7
Loren42 is offline Loren42  United States
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Originally Posted by Eli Duttman View Post
Use 6 Meter shielded cables, with 2 signal conductors, between the preamp and the power amp. Connect the shields to ground only at the preamp end. The preamp has to have a LOW O/P impedance and good current handling capability. An IRFBC20 source follower set up for a 25 mA. drain current will do the line driver job very nicely, indeed. Yes, a miniature power amp is what the situation calls for. If some gain is needed in the line stage, a 12B4 would be excellent and DC coupling the FET to the triode is easy.
Thank you!

I assume that one of the inner pairs is also connected to ground at both sides of the cable, but the braided shield is only connected at the preamp end?

Now, how do I do that with tubes?

My whole point was to get all the silicon out of the loop by building the power amp and pre amp using tubes. I would imagine a cathode driven output could do that?

Last edited by Loren42; 1st January 2011 at 10:38 PM.
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Old 1st January 2011, 11:00 PM   #8
SY is offline SY  United States
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OK, I'll have to resort to Blog-whoring.

SYclotron Audio The Heretical Preamp

Most of the stuff you need to know is in there, especially the equations for determining what you need for preamp output impedance for your situation.
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Old 1st January 2011, 11:01 PM   #9
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Originally Posted by Loren42 View Post
Thank you!

I assume that one of the inner pairs is also connected to ground at both sides of the cable, but the braided shield is only connected at the preamp end?

Now, how do I do that with tubes?

My whole point was to get all the silicon out of the loop by building the power amp and pre amp using tubes. I would imagine a cathode driven output could do that?

Yes, 1 wire of the interconnect cable is "hot" and the 2nd is "cold". The shield plays no role in signal transmission.

Doing the line driver hollow state is much tougher than using FETs. You will need a high gm power pentode cathode follower. You might have to choke load, which is expensive. DC coupling between voltage amplifier and line driver will be highly problematic. Prepare to spend on PTFE film interstage coupling capacitors. IMO, FETs (darned near heaterless pentodes) are the proper choice for this job.

As I sense a determination to be SS free, a candidate for the pentode cathode follower is the 12BY7 or a similar type whose gm is higher.
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Old 3rd January 2011, 08:15 AM   #10
tomchr is offline tomchr  United States
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Originally Posted by Loren42 View Post
My next project is a preamp to match my tube power amp. The issue is that the run between the preamp and the power amp is about 20 feet.

This is problematic for a single conductor high impedance cable, correct?

What is the advantage to going to a balanced line?

What losses would I have using transformers as a means to do this? What am I giving up?
I'd trust SY's judgment and experience on this. 20 feet is not that far. Single-ended will be fine. Use a good quality shielded MIC(rophone) cable with two conductors (preferably twisted pair) and a shield. Connect as described above by others.

If you already have differential out on the preamp, you might as well run differentially to the power amp.

Differential signaling will give you common-mode rejection if you implement the receiver correctly. So any noise/interference/hum injected on the cable will be rejected. Transformers are one way of converting from single-ended to differential (and back). The Jensen JT-11P is a good and reasonably priced ($70) one. The main drawback of transformers is the cost. They also have relatively high THD (around 0.01 % mid band for 0 dBu). But if you worried about 0.01 % THD you probably wouldn't be writing in the tube forum... The trafos tend to have very high Common-Mode Rejection Ratio (CMRR). The Jensen transformer has CMRR of 130 dB. That's hard to beat.
Note though, that the same job could be done by an LME49724 differential op-amp for $4 with THD below 0.00003 %. Yes. That's four zeros after the decimal point.

~Tom

Last edited by tomchr; 3rd January 2011 at 08:18 AM.
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