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Old 26th August 2004, 03:42 AM   #1
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Default Unity Gain Unbalanced to Balanced Phase Splitter

Here's a problem that I've been puzzling over for a while. I sure would appreciate some other thoughts, because I'm not able to solve it.

There are a number of ways to go from unbalanced to balanced signals. One of them is a simple split load (concertina) phase splitter. If both outputs of this topology are loaded properly it can give a very nice balanced output when the signal is taken between the outputs. Here's a picture just for reference, even though I know you guys don't need it.

But, what I'm trying to figure out is this. Even though the gain of each leg is less than one (for most triodes that we use, let's say .9), the "effective" gain when you take the balanced output is twice this number (say 1.8). That is, with 1V unbalanced coming in there will be 1.8V balanced coming out measured across the outputs.

What I'm trying to figure out is this: is there a way to have unity gain from unbalanced to balanced. 1V unbalanced in gives 1V balanced out. Is there a way to do this other than the simple technique of using a triode with low enough mu so that each leg has only a gain of .5? In other words, can this topology be made to look like a transformer with a 1:1 ratio?

Thanks for the help.
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Old 26th August 2004, 08:25 AM   #2
EC8010 is offline EC8010  United Kingdom
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Default Have the gain, and like it!

No. Consider the lower output to be a cathode follower. You'd need mu = 1 to achieve A = 0.5. Even a 6080 can't go that low. Worse, that would mean that the resulting stage wouldn't have any distortion-reducing feedback...
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Old 26th August 2004, 08:40 AM   #3
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Sure no problem... split the two resistors , the anaode and the cathode each in 2 equal resistors and take the output from the middle of them.. as a side effect both output now have equal output inpedance (or atleast more similar) of about halve the original anode output impedance ...
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Old 26th August 2004, 09:10 AM   #4
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Quote:
as a side effect both output now have equal output inpedance (or atleast more similar) of about halve the original anode output impedance
You dont need this trick to achieve equal output impedances in a split load phase inverter, as long as both outputs are equally loaded also the output impedances are equal, this has been debated and described many times in this forum.

However you are correct in that splitting anode and cathode resistors is one way of achieving lower gain, but I want to know why Runeight want to achieve this, if needed i would probably put a resistive splitter on the input.

Regards Hans
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Old 26th August 2004, 06:43 PM   #5
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A fair question Hans. Perhaps I should describe the problem first.

I have a pair of monoblocks that are quite sensitive. They only need 1v peak to reach full power. But, their inputs are balanced.

So, I don't need a preamp, just an attenuator for most reasonable sources. I know that there are CD players with balanced outputs and I can go there, but they are generally more expensive than average CD player. I don't know about phono sources yet because I have not looked into their availability.

Assumming then that the sources are single-ended, I need to translate from unbalanced to balanced without adding any gain.

I've looked at transformers and can easily go this way. But, I'm not in a terrible hurry so I'm just looking at other ways to get from unbalanced sources to balanced amps.

of course, the resistive splitter on the input is just the volume control. but, for many sources, i'll barely be able to crack the volume if the preamp itself has a gain of 1.8 or so. i could put another divider at the input and probably wouldn't suffer too much from doing it, perhaps a little less s/n. but at these levels not necessarily an issue.

And since the "preamp" doesn't need any gain, my question.

tschrama, i can indeed use voltage dividers in the loads. there are some problems that occur in doing this, but this is one possible option.
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Old 26th August 2004, 07:12 PM   #6
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You can have a pair of Jensen 11P1s in your hot little hands in 48 hours. They work great as unity gain input unbal to bal converters.
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Old 26th August 2004, 07:56 PM   #7
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Default Sowter TVC

Quote:
Originally posted by runeight
A fair question Hans. Perhaps I should describe the problem first.

I have a pair of monoblocks that are quite sensitive. They only need 1v peak to reach full power. But, their inputs are balanced.

So, I don't need a preamp, just an attenuator for most reasonable sources. I know that there are CD players with balanced outputs and I can go there, but they are generally more expensive than average CD player. I don't know about phono sources yet because I have not looked into their availability.

Assumming then that the sources are single-ended, I need to translate from unbalanced to balanced without adding any gain.

I've looked at transformers and can easily go this way. But, I'm not in a terrible hurry so I'm just looking at other ways to get from unbalanced sources to balanced amps.

of course, the resistive splitter on the input is just the volume control. but, for many sources, i'll barely be able to crack the volume if the preamp itself has a gain of 1.8 or so. i could put another divider at the input and probably wouldn't suffer too much from doing it, perhaps a little less s/n. but at these levels not necessarily an issue.

And since the "preamp" doesn't need any gain, my question.

tschrama, i can indeed use voltage dividers in the loads. there are some problems that occur in doing this, but this is one possible option.
Hello ,
I'm currently using a Sowter TVC . These can be configured to provide an unbalanced to balanced conversion and can be driven directly by a CD player . It sounds very good in comparison to resistive attenuators too

cheers

316a
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Old 27th August 2004, 05:54 AM   #8
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Default Am I missing something?

If I understand you correctly, feeding each of your monobloc amps from a balanced input of 1v RMS each side will cause them to give full output. Feeding them from a shared single-ended 1v RMS input via a concertina splitter should give ~0.9v RMS per side. I don't see why this is a problem.
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Old 27th August 2004, 02:53 PM   #9
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my fault for the confusion ray. the amps run full output with 1vp across the inputs (.5V into each side).
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Old 28th August 2004, 01:30 PM   #10
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Hi tubetvr .. are you sure... I dedn't know that, but if it's true, than my text book is wrong, but that wouldn't be the first time...


gr,
Thijs
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