EL84, Balanced Input, Push-Pull Amplifier

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Mike H
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#16

Post by Mike H »

Nick wrote:Well, I have built a few balanced things now. I think part of what you are hearing Steve is the lack of the treble damage your typical valve phase splitter produces.
Interesting!
 
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#17

Post by Cressy Snr »

Nick wrote:Well, I have built a few balanced things now. I think part of what you are hearing Steve is the lack of the treble damage your typical valve phase splitter produces.
I'll have to agree there Nick. In my very limited experience, valve phase splitters seem to add a subtle fizz to the top end, that sounds impressive and detailed but is actually acting as a veil, hiding the finer details and harmonics. The Sowter phase-splitting input transformers also added a slight fluffy quality that I didn't realise was there until it was removed.

I honestly think that if you are going to use push pull then it needs to be in its purest form, ie. balanced operation sans phase splitter. No-one argues against single ended amps needing to have the simplest, purest signal path to sound at their best, so why not apply the same logic to push-pull operation.

Here's something to think about: your modern DAC invariably uses differential operation, so it is a no-brainer for the manufacturer to make a feature of it and offer balanced outputs, but your average transistor or valve push pull amp cannot take advantage, because all its inputs are usually single ended.
The option therefore rarely gets used.

With conventional push-pull, your data stream goes into the DAC, gets converted to twin phase analogue via the differential convertor, gets converted to back to single ended * and exits via the L/R phono sockets, enters your amp, gets amplified by the single ended input stage, gets converted again to twin phase, by the phase splitter/driver, enters the output stage, gets converted back to single ended by the OPT and then goes on to the speakers.

*Unless the DAC just throws one half of the signal away for use at the phono outputs

So your analogue signal has been through maybe 3 stages of analogue splitting and recombining before it gets to your speakers. Each conversion has the potential to add distortion, lose information, or both; and that's before you even start with feedback.

Single ended OTOH only has to go through one recombining process as it leaves the DAC chip and enters the L/R phono outputs. After that it stays single ended all the way through.

Balanced push-pull also only has the one recombining process but this time at the OPT end, so you have, by using balanced operation, at a stroke removed 2 layers of signal manipulation and leveled the playing field.

With vinyl, SE has the advantage, as with a turntable you are SE right the way through to the speaker. But with a turntable you can of course with an MC cartridge, wire it for balanced operation and send it through a balanced phono stage/preamp so you can get pp all the way to the speaker if you are prepared to fork out the dosh.

Maybe to get the best out of pp, you have to do what the single ended crowd do as a matter of routine; keep the signal in the same format from the start of the reproducing chain to the finish and use a 2 stage amp with direct coupling.

Guess what is next on my agenda. :wink:
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Nick
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#18

Post by Nick »

Yep, I agree with all of that, and its part of my plan. We will have to try my DAC which is transformer coupled and balanced from the dac onwards. If you want, I will fit a AES input so you can have balanced in as well :-)
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#19

Post by Cressy Snr »

Nick wrote:Yep, I agree with all of that, and its part of my plan. We will have to try my DAC which is transformer coupled and balanced from the dac onwards. If you want, I will fit a AES input so you can have balanced in as well :-)
Sounds like a plan to me Nick :D
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#20

Post by Neal »

Interesting info and feedback Steve, cant wait to hear it at Owston. You can get very good phase splitter balance in a non balanced amp with careful use and design of a CCS with a LTP but I suspect it's still not as good as what you have achieved here.
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#21

Post by Nick »

Neal wrote:Interesting info and feedback Steve, cant wait to hear it at Owston. You can get very good phase splitter balance in a non balanced amp with careful use and design of a CCS with a LTP but I suspect it's still not as good as what you have achieved here.
Yep, but with a LTP, the cathode coupling will mean the two phases will still drift out of phase at higher frequency. Mind you I havent tested that, so high frequency in this case may be way beyond audio anyway.
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#22

Post by Mike H »

This is echoes of the discussion we had at Eggborough last time, why does SE sound better than PP, because SE does not have a valve phase-splitter.

That was our conclusion anyway. :D
 
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#23

Post by simon »

Mike H wrote:This is echoes of the discussion we had at Eggborough last time, why does SE sound better than PP, because SE does not have a valve phase-splitter.

That was our conclusion anyway. :D
Ooof! Light blue touch paper moment! :lol:
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#24

Post by Cressy Snr »

Mike H wrote:This is echoes of the discussion we had at Eggborough last time, why does SE sound better than PP, because SE does not have a valve phase-splitter.

That was our conclusion anyway. :D
Probably right.
Here are my findings; for what they're worth:

Valve phase splitter - OK
Transformer phase splitter - much better
No phase splitter (balanced throughout) no apologies need to be made for your choice of amplification style. :D
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#25

Post by Neal »

Nick wrote:
Neal wrote:Interesting info and feedback Steve, cant wait to hear it at Owston. You can get very good phase splitter balance in a non balanced amp with careful use and design of a CCS with a LTP but I suspect it's still not as good as what you have achieved here.
Yep, but with a LTP, the cathode coupling will mean the two phases will still drift out of phase at higher frequency. Mind you I havent tested that, so high frequency in this case may be way beyond audio anyway.
They do Nick but only past 12~15Khz or so and then by a small but growing amount... clearly its not perfect and depends on the design.
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#26

Post by Neal »

Mike H wrote:This is echoes of the discussion we had at Eggborough last time, why does SE sound better than PP, because SE does not have a valve phase-splitter.

That was our conclusion anyway. :D
...but some of us have never thought that Mike :P :D
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#27

Post by Neal »

Neal wrote:
Nick wrote:
Neal wrote:Interesting info and feedback Steve, cant wait to hear it at Owston. You can get very good phase splitter balance in a non balanced amp with careful use and design of a CCS with a LTP but I suspect it's still not as good as what you have achieved here.
Yep, but with a LTP, the cathode coupling will mean the two phases will still drift out of phase at higher frequency. Mind you I havent tested that, so high frequency in this case may be way beyond audio anyway.
They do Nick but only past 12~15Khz or so and then by a small but growing amount... clearly its not perfect and depends on the design.
I take that back, quick test of the splitter in my K5881 shows approx .5% voltage difference and no phase difference at 20KHz @1W

Near full output it gets worse 5% with some phase difference at 20Khz but virtually no difference at 1Khz...anyhow I'm thread crapping so I'll stop.
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#28

Post by Mike H »

Yes and as Nick said at Eggborough that time during this discussion we had, it gets MUCH worse with a composite signal, i.e. music like what you would normally use it to listen to (as opposed to a simple sine wave :D )

IIRC the discussion was prompted by a question from Steve S, re SE versus PP, plus, we'd heard Nick's balanced pre-amp design, that uses p-p transformers (a bit like the Raven). In other words phase-splitting is done by transformer(s) and we thought this is probably the best way to do it in a p-p amplifier as well.

HTH :D
 
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#29

Post by Andrew »

I am also of the mind that the phase splitter is the achilles heal of the PP amp, make it a good one and it will be a good amp, however most are not good phase splitters. Why the old WAD PP is so highly by SE enthusiasts? Because it has a well executed phase splitter.
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#30

Post by Cressy Snr »

Harvey Rosenberg talked of re-inventing push-pull. In one of his articles he referred to the Sun Audio 300B amplifier, that was being manufactured in Japan at the start of the 21st Century.
This had the interstage transformer phase-splitter. Legend has it that once the Doctor had heard the amp along with Lynn Olsen, that their eyes were opened to the fact that the Japanese were getting right, what the American push pull hedgemony was getting so, so wrong.
The rest is Amity :wink:

The work Nick is doing himself and the work that he inspired me to do with push-pull amps, via the Little Martha preamp and the EL34 breadboard, is I suppose, taking things a little bit further.


I just know that there is still more to be had, as I am only at the beginning of the learning curve, and it is this that makes this little quest so exciting; for me at any rate.

One can only marvel at the possibilities a fully balanced, globe PX25, push-pull amp would present. Those valves are however so valuable these days, that no one in their right mind would even attempt it.

Onwards!
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