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Old 25th February 2009, 04:17 AM   #1
wicked1 is offline wicked1  United States
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Default PP to balanced input

I'm thinking about modding a mullard style push pull amp to balanced input.

Could I simply bypass the phase splitter, and maybe replace the input tubes which are now a single triode per channel, w/ dual triodes?


Then just out of curiosity, could you turn two stereo SET amps into a stereo balanced amp by simply changing the output transformers?

I'm building a DAC w/ balanced output capability and it has got me thinking about future projects!
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Old 25th February 2009, 05:10 AM   #2
twystd is offline twystd  United States
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Replace the phase splitter driver stage with input transformer>long tailed pair driver, with a CCS in the tail, and cap couple to the driver stage. Do this with left and right channels, and have a balanced amp with one stereo PP amp, no need for two. Easier said than done, but should sound nice.

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Old 25th February 2009, 03:22 PM   #3
wicked1 is offline wicked1  United States
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I'm just trying to understand this, so I am probably wrong, but couldn't I completely take out the LTP?
Since the balanced signal is already split, I don't need the LTP to split the phase,right? The DAC is outputting a perfectly phase split signal. Then do I really need a transformer for the input, since the output of my source will already be balanced, and at a normal line level?
I'm thinking that since the phase is split, I basically just need a single ended amp (input tube, power tube) for each 1/2 of the signal, then the output tx re-combines it. (which is why I added the question at the bottom, of could you turn two SET amps into a balanced PP amp by only replacing the output tx.. The phase is split.. Just need to amplify each 1/2)
And again... I don't know what I'm talking about, but w/ my very limited knowledge, that makes sense to me.

Are there any good schematics floating around for balanced amps? (I tried searching, but you could imagine, there are a lot of threads w/ the word balanced in them!)
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Old 25th February 2009, 05:56 PM   #4
DougL is offline DougL  United States
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Quote:
I'm just trying to understand this, so I am probably wrong, but couldn't I completely take out the LTP?
You could change it, but why would you?
Quote:
Then do I really need a transformer for the input, since the output of my source will already be balanced, and at a normal line level?
If you are only using balanced input, the input transformer is not required.
Quote:
I'm thinking that since the phase is split, I basically just need a single ended amp (input tube, power tube) for each 1/2 of the signal, then the output tx re-combines it. (which is why I added the question at the bottom, of could you turn two SET amps into a balanced PP amp by only replacing the output tx.. The phase is split.. Just need to amplify each 1/2)
That would work.

There are several levels of engineering. First level is "Will it work?". Next level is "What are my options?". Third level is "What are the tradeoffs?" and "What are my goals?"

HTH

Doug
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Old 25th February 2009, 06:05 PM   #5
wicked1 is offline wicked1  United States
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Why would I remove the LTP and go straight off the balanced output from the dac?
My thought behind it is that the phase splitter is usually the weak point of a PP amp. If I already have something that is perfectly splitting the phase, wouldn't that both be more simple and sound better?

Why take the balanced signal, turn it into an unbalanced signal, put it in the amp, then phase split it again before the output tubes?
Am I missing something here?
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Old 25th February 2009, 08:21 PM   #6
DougL is offline DougL  United States
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Why take the balanced signal, turn it into an unbalanced signal, put it in the amp, then phase split it again before the output tubes?
A long tail pair is a differential amplifier with one input at AC ground. If you use both inputs, its will act as a deferential amp. Since there may be long cables, the Common mode rejection of the stage might be an advantage.
The difference in distortion between a Diff amp and 2 grounded cathode amps is very small. The biggest difference is the LTP typically needs a negative supply to operate correctly.
As I see it, you gain the ability to have an amp that can be both single ended and balanced without any significant compromises.

If you were talking about one of the other phase splitters, I would be less baffled.

Doug
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Old 25th February 2009, 09:39 PM   #7
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Quote:
Originally posted by wicked1
I'm just trying to understand this, so I am probably wrong, but couldn't I completely take out the LTP?
Since the balanced signal is already split, I don't need the LTP to split the phase,right? The DAC is outputting a perfectly phase split signal. Then do I really need a transformer for the input, since the output of my source will already be balanced, and at a normal line level?
I'm thinking that since the phase is split, I basically just need a single ended amp (input tube, power tube) for each 1/2 of the signal, then the output tx re-combines it. (which is why I added the question at the bottom, of could you turn two SET amps into a balanced PP amp by only replacing the output tx.. The phase is split.. Just need to amplify each 1/2)
And again... I don't know what I'm talking about, but w/ my very limited knowledge, that makes sense to me.

Are there any good schematics floating around for balanced amps? (I tried searching, but you could imagine, there are a lot of threads w/ the word balanced in them!)

hey-Hey!!!,
If you fed a pair of SE amps a balanced signal, and then bridged their output, you'd have a PP amp. It is easy enough to try, and you could at some point substitute a PP opt and see how it works.
cheers,
Douglas
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Old 25th February 2009, 10:21 PM   #8
wicked1 is offline wicked1  United States
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Ok, thanks. It sounds like I have a few things to try now. I just wanted to make sure I wasn't completely off base w/ my ideas.
I've got a pair of Ampex 6v6 speaker amps just sitting around, which have the mullard PP topology. If this works out how I am thinking, I'd build it all into one chassis.. would basically be a USB input amp.. DAC use only. Lossless digital music is all I play these days, anyway. So for this, as far as the audio is concerned, the cables wont be longer than a couple of inches.

The problem is that I never really liked the sound of these amps. They dont have a negative supply as they are. All the suggestions I've had so far about improving the amp have been to add negative supply and CCS to the LTP, or remove the LTP and use a tx in its place, always something about the LTP, though, as most seem to think that is the area that needs the most improvement.

I read this over in a thread about DAC's which is what got me on this path;
"Running your gear fully balanced results in better dynamic range, which is why I think I will probably never have a single ended power amp again. "
I know the person who wrote that is into solid state amps, but I'd imagine the same idea would apply here. So far, however, I've been WAY happier w/ my single ended amp than my PP amps. I've got several PP amps now, because I keep reading about why technically, they should be superior. Each time I build or repair a PP amp I use better and better components, but still my SET wins all the listening tests.
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