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Old 30th Apr 2019, 4:52 pm   #1
The General
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Default So what's the deal with single ended hifi amps?

OK, serious question & apologies if this has been asked before.
I have an audio engineering background & have been repairing & restoring valve kit since I was 15, but there's one thing I still don't understand.
Now we know that although a single ended valve audio output stage is cheap, compact & good enough for most radio/TV/tape recorder applications, conventional wisdom says that for a quality valve amplifier push-pull is the way to go. More efficient, the standing DC currents in the output T/X cancel out, the nasty even harmonics (mostly) cancel out, lower distortion, etc.
So what is the deal with audiophile single ended power amplifiers?
They usually have a big power triode like a 300B, a huge output transformer - which I'm guessing needs an air gap in the laminations to avoid saturation from the standing current - & relatively low output power.
Surely you could get better performance & sound quality from a pair of EL84/EL34/6L6/6550 (insert favourite valve here)?
Not sure how to call this one. Is it audiophoolery? Or what? Am I missing something fundamental here? Need help with this one.
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Old 30th Apr 2019, 5:05 pm   #2
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Default Re: So what's the deal with single ended hifi amps?

Fashion? Audiophiles/phools like the sound they make, even harmonics and all (which at least sound nicer than odd ones).


True HiFi amps of whatever persuasion all tend to sound the same insofar as they simply make their input signals bigger without adding anything to them so there's nothing to get excited about if looking for differences.
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Old 30th Apr 2019, 5:45 pm   #3
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Default Re: So what's the deal with single ended hifi amps?

Yes, it's basically fashion as with much of the hifi world. At least you will get lots of that lovely 'valve sound' (aka distortion and poor linearity).

Going for class A does make entry level valve hifi more affordable, but the high end stuff is nuts.
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Old 30th Apr 2019, 5:46 pm   #4
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Default Re: So what's the deal with single ended hifi amps?

SET amps using 300Bs or PX4 / PX25s may sound very attractive due to the predominance of even order distortion and if that`s the sound you like, go for it.

It`s not High Fidelity though......
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Old 30th Apr 2019, 5:46 pm   #5
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Default Re: So what's the deal with single ended hifi amps?

It must be that beguiling combination of high distortion and low power output...
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Old 30th Apr 2019, 6:25 pm   #6
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Default Re: So what's the deal with single ended hifi amps?

They are terribly pretty, terribly expensive and terrible technically. But as said second harmonic sounds good to the ears, if I wanted more second harmonic I would build a box with a knob on it to adjust same. I might have a go using an ECC8x at low HT and a low load. Or DSP using a soundcard, I bet there is something downloadable to do the job.
 
Old 30th Apr 2019, 6:48 pm   #7
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Default Re: So what's the deal with single ended hifi amps?

You could use Audacity on your computer and an appropriate plugin to produce most distortion effects without going to the expense of a 300B or lumps of metal. Just a thought.
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Old 30th Apr 2019, 6:50 pm   #8
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Default Re: So what's the deal with single ended hifi amps?

Quote:
Originally Posted by The General View Post
...push-pull is the way to go. More efficient, the standing DC currents in the output T/X cancel out, the nasty even harmonics (mostly) cancel out, lower distortion, etc.
Actually, odd harmonics are even nastier! Even harmonics of very low order are less objectionable. But p-p cancels them completely, you're just left with the odd.

Why single-ended? Hopefully someone on here will provide a reason, because I can't think of one! Having designed a few output transformers, I definitely know which is less of a compromise! (Yes, you are right, a SE transformer needs an air gap - unless it's capacitively coupled to the output valve, in which case you need a thumping big choke, which has its own air gap).

For equivalent power, the only down side of P-P is the phase inverter. Everything else is a big win!
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Old 30th Apr 2019, 7:25 pm   #9
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Default Re: So what's the deal with single ended hifi amps?

I usually run a KT88 push-pull stereo amplifier (50+50 Watts) which I like very much. However, I recently built a power triode single ended amplifier that gives 8 Watts before flattening and I also find it very nice. It was meant to have no feedback as with triodes it should not be necessary, but I found the frequency response all over the place so put in around 12dB. That made a tremendous difference and what I notice more is the extra Bass I get compared to the Push-Pull. It certainly does not have the same amount of grunt (if I may call it that ), the Push-Pull seems to deliver loud energetic parts of the music with more ease. However I can say that if the KT88 amp blew up for some reason, I would be more than happy to switch back to the single ended amp. It runs with 50mA standing bias, the same as the P-P amp but of course that is 100mA draw from two valves. The magnetic field may be cancelled in the output transformer but the currents still add up . Visitors show more preference for the push-pull amp.
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Old 30th Apr 2019, 8:11 pm   #10
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Default Re: So what's the deal with single ended hifi amps?

Some of the owners might feel that it's the same as the deal with the E-type Jag. Newer designs have better brakes and steering, are less prone to rusting, more fuel efficient and can be faster (and the manufacturers somertimes don't even fib about how fast they actually will go).

But, well, just look at it ...

Cheers,

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Old 30th Apr 2019, 8:23 pm   #11
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Default Re: So what's the deal with single ended hifi amps?

Reasons for SE popularity:
1. fashion
2. simplicity - lots of people believe that a 'simpler' circuit must be a better circuit
3. preference for some nonlinear distortion and, in many cases, a limited frequency response
4. a belief that a circuit which changes what it does when you swap components must be better than one which just does what it does whatever brand of output valve you use
5. in some cases, SE combined with a poor PSU with high subsonic output impedance can create artificial volume pumping which is not recognised as such but thought to be 'improved dynamics'
6. a fear of feedback - this can be seen when they describe PP+feedback amps as being "clinical" or "boring", thus showing that they are not actually interested in hi-fi
7. a desire to show off the biggest valve in the street - SE amps rarely seem to have a top cover to hide everything and keep it safe

In many cases they will claim that their SE amp has low amounts of the distortion that matters (such distortion being unknown to conventional science and engineering) and so the easily measured (and sometimes heard) high amounts of conventional distortion (2nd, 3rd etc.) do not matter. Conversely, the PP+feedback amps favoured by others may have low amounts of 'conventional distortion' but the very techniques which ensure this (PP, feeback, careful design by real engineers) create extra amounts of the distortion which matters so this is why they sound so bad. All this is beasically a parallel belief system which can be quite unshakeable.
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Old 30th Apr 2019, 8:52 pm   #12
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Default Re: So what's the deal with single ended hifi amps?

Well, 7 for me is a valid reason - but then if I went pp, I'd have TWO of the biggest valves in the street, not just one.
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Old 30th Apr 2019, 9:52 pm   #13
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Default Re: So what's the deal with single ended hifi amps?

Four, if you listen in stereo, and a hole in your wallet to match .

Cheers,

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Old 30th Apr 2019, 11:35 pm   #14
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Default Re: So what's the deal with single ended hifi amps?

Quote:
Originally Posted by Studio263 View Post
It must be that beguiling combination of high distortion and low power output...
And high price! That's what seals the deal for many.

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Old 30th Apr 2019, 11:41 pm   #15
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Default Re: So what's the deal with single ended hifi amps?

But what about the argument that a push=pull amplifier is just two single-ended amplifiers?

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Old 1st May 2019, 1:12 am   #16
Herald1360
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Default Re: So what's the deal with single ended hifi amps?

Maybe...... if it's running fully class A. Perhaps like one of the more cunningly cost engineered stereo radiograms from before stereo FM.

Doesn't explain why P-P 300As aren't regarded as better than SE ones.
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Old 1st May 2019, 4:44 am   #17
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Default Re: So what's the deal with single ended hifi amps?

The resurgence or continuation of esoteric hi-fi seems to be akin to guitar amps, where distortion/"tone" is an aim of the design - how else can one wax lyrical about one amp being better or more sonic-ally enjoyable than another during a comparison.

There is also a pride-of-place aesthetic needed. You can't sell an amp unless it looks good. And you certainly don't hide it away in a cupboard if you bought it for lots of $. And if it looks a bit different from what others have, then all the better.
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Old 1st May 2019, 4:52 am   #18
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Default Re: So what's the deal with single ended hifi amps?

Quote:
Originally Posted by Radio Wrangler View Post
But what about the argument that a push=pull amplifier is just two single-ended amplifiers?

DAvid
This was the way push pull was initially designed by RCA and it was in their very first Radiotron manuals, both of the output valves were in push pull with what would be a class A bias and all the power and dissipation calculations based on that.

They had not cottoned onto the energy savings of increasing the negative bias for each valve and the reduced dissipation for each valve then the possible increase on power output (still while keeping the valve within its dissipation limits) afforded by class B or class AB1 or AB2, compared to class A push pull. Later, special valves that operated at zero bias appeared.
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Old 1st May 2019, 5:38 am   #19
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Default Re: So what's the deal with single ended hifi amps?

Been there!!! Done that!!

I used VT4C valves @ 1200 volts.
Output transformer was larger than my VERY old 135 amp arc welder.
3"x3.5" core, airgapped to prevent saturation, and 13 layers to distribute capacitance/leakage.

Admittedly it dint have a moveable "third leg" to control the current.
Output power was calculated at 30 watts, I made something less than 25 watts, BUT it could RMS 20 watts.
Sound quality ? Yes very nice, about the same as my push/pull 6BW6 valves. It did have better bass!!, BUT when I added negative feedback, the bass went away, about the same as my push/pull 6BW6 amp.
6BW6 stereo amp, with inclusive power supply weighs about 12 kilos. Each monoblock VT4C amp weighs about 30 kilos. Power supply another 20 kilos.
Lights dim in the house with switchon with VT4C, (even with current limiters)
the switch goes "klik" on 6BW6 amp.
Cost to me, 6BW6 amp ( mostly recycled iron, but all new passives and valves/bases) around $250.

Cost for VT4C monoblocs ( my cost!! NOT the customers) BARE iron alone for OPTs $600 INCLUDING freight. I had most of the wire on 30 KG rolls already in stock. Jensen PIO caps ( supplied by customer) for coupling, ( 4 off fo both channels) $250.
Three chassis $300.
My labour $500 ( design chassis, drill and cut holes, etch/undercoat/paint plus solder the eight or ten parts per chassis)
Two 640 watt toroids for PSU $1200
Four chokes for same PSU $350


6BW6 amp costs $17.86 per watt
VT4C amp costs $80 per watt

I havent factored in the power consumption from the electricity board, AND it was some years ago when SE power amps "erupted" onto the scene.
VT4C back then cost me $70 each
Go check the price on "that site" today!!!

Audiophool I be!!
Idiot Im not !!

Joe
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Old 1st May 2019, 5:51 am   #20
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Default Re: So what's the deal with single ended hifi amps?

Further to the above remark:

In RCA’s Radiotron handbook from the 1932, it was advised that two output triodes
could be run in parallel class A to double the output power over one tube. Or one could
boost the power to double in the push pull class A configuration. They stated that for the
push pull connection the power output was the same as running the tubes in parallel but
the drive voltage requirement was double. This made the push pull class A connection
seem unappealing to a designer reading the text, although there was a glib mention of
the fact that more than double the output power was possible by increasing the bias.
In the RCA handbooks to follow and by 1934 in the Radiotron Designer’s Handbook,
there was much more detail on the advantages of the push pull connection. They
outlined how to bias the tubes into class AB1 and AB2 or class B for substantially more
than double the output power from two tubes compared to what the same two tubes
could safely generate in parallel class A. It appears that these concepts took a few
years to gel in the minds of designers at the time.

The tube RCA designed for class B was the 49. It operated at zero grid bias. To get a pair of UX171's into that bias area was around 50V negative at the grid.
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