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Which characteristics are more influent on the sound of SET amps?

Hi,

the title has been chosen to explicit that this is a pour-parler kind of thread, to better understand the preferences many people have on the SET amps.

I would like indeed to ask your opinion on which characteristics make SET amps so special compared to other kind of amps.
Is the predominance of 2nd (and then 3rd) harmonic distortion?
Is the absence of gnfb and so a different saturation behaviour and recovery after peaks?
Is the relatively low DF, generally between 2 and 4, and the consequent interaction with the speaker (more low-highs, even if less controlled)?
Others?

Thanks in advance.
 
zintolo,

Excellent Question!


Just my $0.02 off the top of my head, a few generalizations, and in no specific order of importance . . .

Single ended:
Assuming no negative feedback, and no significant 2nd harmonic cancelation of a 2 state amp . . .

Dominant 2nd Harmonic distortion (often with the 2nd harmonic distortion 15dB more than the 3rd harmonic distortion)

Low Damping Factor

Un-Symmetrical Damping factor, it is different for signal going positive versus signal going negative (*)
Consider how the loudspeaker 'sees' this Un-Symmetry.

A low(er) power amplifier does not significantly activate the loudspeaker's harmonic distortion, and the loudspeaker's Intermodulation distortion
(2nd order dBc distortion increases at the same rate as the signal increase; 3rd order dBc distortion increase at 2X the rate of the signal increase;
That is for well behaved amplifiers and loudspeakers, is worse for not well behaved loudspeakers and not well behaved amplifiers; well behaved is at lower levels, badly behaved is at amplifier clipping, and loudspeaker breakup and voice coils running into the stops, etc.)

The lack of negative feedback, which 'sticks' and takes time to correct and recover from overload.
No negative feedback present to make saturation Worse (negative feedback does not improve saturation).

Push Pull:
(*) The damping factor of a push pull amplifier is Symmetrical for Both the signal going positive, and for the signal going negative).
Consider how the loudspeaker 'sees' this symmetry.

My latest push pull and balanced amplifiers at 1 watt have 2nd harmonic distortion, 3rd harmonic distortion, 2nd order Intermodulation distortion, and 3rd order distortion that are all less than -50dBc, and the various orders are all within about 2 or 3 dB of each other.
(not anywhere near a 15 dB difference).
Oh, my amplifiers do Not have global negative feedback, no Schade negative feedback, no UL negative feedback.

Low power doesn't significantly tickle my loudspeaker's various types of distortion (which at some power level, all loudspeakers have).

Have Fun!
 
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Thank you @6A3sUMMER !

The asymmetric DF is something I now remember you pointed the attention to in the past, but I forgot while writing the thread.

It should create second harmonic due to speaker inertia on the “high plate voltage, low current” side that has almost no DF, while on the “low plate voltage, high current” side the DF is maximum and the speaker better follows the signal.
 
For an idea of the relation of 2nd harmonic distortion to 3rd harmonic distortion of a 300B in a single ended amplifier . . .

Look at page 851 of the document at this link:
300B.pdf
Notice the dB relation of the 2nd harmonic distortion to the 3rd harmonic distortion.
That is the "Bare Bones" 300B performance.

A two stage 300B amplifier will vary from those numbers, because of the input/driver stage's distortion, and how it adds or subtracts from the 300B's distortion.
Part of the 2nd harmonic distortion will be partially cancelled.
Third harmonic distortion does not cancel, but might be additive (increased 3rd harmonic distortion).

When the signal level is low, the 2nd harmonic distortion often is more dominant.
When the signal level is very large, the 3rd harmonic distortion starts to catch up to the 2nd harmonic distortion.

All Generalizations Have Exceptions.

Food for thought:
The natural 3rd harmonic of a clarinet is dominant; the natural 2nd harmonic of a clarinet takes second place.
You like, do not like, or are indifferent to the sound of a clarinet . . . which?
An amplifier adds its own harmonic distortion (that is not the natural harmonics of the instrument).
 
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Typically 2nd harmonic distortion of the fundamental creates a wave-shape that is long in one direction, and short in the other direction (speaker excursion directions; amplifier positive alternation versus negative alternation).
Depending on the phase of the loudspeaker connection to the amplifier output, you will have some cancellation of the 2nd harmonic distortion, or some addition of the second harmonic distortion (increase).
That is very similar to a push pull output stage that cancels 2nd harmonic distortion; but push pull does not increase the 2nd harmonic distortion.

Some general examples of no negative feedback amplifiers:

Single ended:
A triode that is near to cut-off has very little plate current, but even more importantly has much higher plate resistance. That means the damping factor is Low, and does not control the speaker.
When the triode is at high current, the plate resistance is low, and the damping factor is high.
Un-balanced damping factor.

Push Pull:
One plate is cut off, but the other plate has lots of current, and low plate impedance, damping is high, there is control of the speaker (*).
Both plates are not near to cut off, the plate resistance is low to medium, the damping factor is medium to high, there is control of the speaker.
Now, swap which tube is cut off, and you get the results of (*) above
Symmetrical damping factor.

Am I preaching to the choir?
Hello, is this microphone on?
 
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I don't think the single ended amplifier sound is so good because is somewhat distorded ... at low , normal levels distortion is minimal anyway
With negative feedback the sound is still very good .
Maybe it is important that has fewer parts , but is not made simpler on purpose to be crappy ...
 
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There is also a difference in the way the SET transformer works, with the current always flowing in one direction vs push-pull, where it flows in both, the hysteresis probably affecting the OPT performance. But I haveno idea whether that results in any audible difference.
 
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Depanatoru, (and other thread readers)

I think you and a lot of others need to attend at least one properly set up double blindfold test.

Good SE amplifiers, and good PP amplifiers. Average SE amplifiers and averagePP amplifiers.
The potential comparisons are endless.
Speaker Models (different model pairs to be fair)
Output tubes and circuits:
DHT
Indirectly heated (w/cathodes)
Triodes
Pentodes
Beam Power
Triode Wired Pentode and Triode Wired Beam Power tubes
Ultra Linear
With local negative feedback
With global negative feedback
With both local and global negative feedback
Schade negative feedback
Cathode negative feedback
With NO negative feedback
Interstage transformer
RC coupling
Phase splitting Auto transformer (no secondary)

Generalizations about SE sound, and about PP sound are just that . . . Generalizations
All Generalizations Have Exceptions.
Do not forget, the sound is dependent on all the parts in the system, including the recording that results in the signal source (CD, Vinyl, etc.),
and the rest of the way through the system, including the loudspeakers and the listening room.

If you have never heard a good SE amplifier that puts out medium to high power, then I have sympathy for you.
But, do not get me wrong. All 5 of my mono block amplifiers have been different SE configurations, but now all of them are one of 3 types:
Self Inverting output PP; Long Tailed Pair phase inverter & PP out; Balanced input & Balanced output; all of them either do not have any negative feedback, or only local negative feedback.
I am liking my latest designs, but I also liked my earlier designs too, including SE amplifiers.
Yes, I admit, I designed and built some "stinker" failures too.

If you have designed and built several of these amplifier types, then at least you have heard them in your own system.
Until then . . . How can you give a conclusion for anyone else.
Perhaps you have local friends that have some of these different designs, work with them.

The above are just my opinions and experience

I am only interested in helping others, not discouraging them, and opening up the possibilities for pleasing designs.
 
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sser2,

You are partially correct.
Both SE and PP transformers have Hysteresis.
The comparison ends there.
Why? Because:

SE Hysteresis is in One 90 Degree Quadrant.
There is no Crossing into or out of that quadrant.

PP Hysteresis is in Two 90 Degree Quadrants, in 180 degree oppositions.
There is a Crossing from one quadrant to the other quadrant.

A third possibility:
A Parafeed amplifier that uses a Plate Choke, Coupling Cap, and Output transformer (non air gapped to take advantage, but it can be air gapped if you do not have a non air gapped output transformer) . . .
Guess what?
It has a single quadrant for the Choke
It has Two quadrants for the output transformer.
Now you know something some fans of Parafeed have never realized.

Many SE, PP, and Parafeed circuits work Very Well
(in spite of some pre-conceived notions, and personal thoughts).

Relax, everybody, Enjoy the Music!
 
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zintolo,

A couple of typical 300B examples, for every B+ voltage.

2nd harmonic -20dBc (10%), and 3rd harmonic -30dBc (0.32%).
Notice the 2nd and 3rd are 10dBc apart

2nd harmonic -26dBc (5%), and 3rd harmonic -35dBc (0.18%).
Notice the 2nd and 3rd are 15dBc apart
Oooh, the often quoted "classic" number, 15dB apart.

Just consider the tradeoffs of:
B+ voltage
Grid bias voltage
Output transformer primary impedance
Output power
Plate Watts dissipation (Heat and tube life)

Have fun!
 
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sser2,

You are partially correct.
Both SE and PP transformers have Hysteresis.
The comparison ends there.
Why? Because:

SE Hysteresis is in One 90 Degree Quadrant.
There is no Crossing into or out of that quadrant.

PP Hysteresis is in Two 90 Degree Quadrants, in 180 degree oppositions.
There is a Crossing from one quadrant to the other quadrant.

A third possibility:
A Parafeed amplifier that uses a Plate Choke, Coupling Cap, and Output transformer (non air gapped to take advantage, but it can be air gapped if you do not have a non air gapped output transformer) . . .
Guess what?
It has a single quadrant for the Choke
It has Two quadrants for the output transformer.
Now you know something some fans of Parafeed have never realized.

Many SE, PP, and Parafeed circuits work Very Well
(in spite of some pre-conceived notions, and personal thoughts).

Relax, everybody, Enjoy the Music!
Both are two 90 degree quadrants.

You can look at it like this. Hysteresis is due to re-orientation of magnetic domains. In SE transformer core, a fraction of domains is permanently oriented by DC magnetization. Orientation of these domains does not change with applied AC signal, so they can be disregarded. Whatever domains remain behave as if only AC magnetization is applied. Same as in a PP transformer.
 
Output tubes and circuits:
DHT
Indirectly heated (w/cathodes)
Triodes
Pentodes
Beam Power
Triode Wired Pentode and Triode Wired Beam Power tubes
Ultra Linear
With local negative feedback
With global negative feedback
With both local and global negative feedback
Schade negative feedback
Cathode negative feedback
With NO negative feedback
Interstage transformer
RC coupling
Phase splitting Auto transformer (no secondary)
Super triode connection
 
Long time ago I add another transformer after the main OT in SE amp.
Additional transformer has 1:1 ratio very small gap, and very low Rdc of windings. Tick wire.
This was experiment with added no-DC magnetizing current transformer and significantly lower damping factor
The sound improved a lot as I am remember now?
So maybe it is not bad idea, as experiment, to use 2 output transformers, connected in point of
Geometrical mean of current transformation ratio. Which is converge to about 200 ohms for many primary values.
For instance
5000ohm : 200 ohm + 200 ohm : 8 ohm
expressed in ratios
25 : 5 + 5 : 1
 
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Zoran,

Another way to use 2 output transformers:

Build 2 SE amplifiers.
Drive one amp in-phase, and one amp out-of-phase (the second amplifier).
Disconnect the 2nd amplifier's secondary common lead from ground.
Connect that un-grounded common lead to the first amplifier's 8 Ohm output.
Connect the 2nd amplifiers 8 Ohm out to the 1st amplifier's common output.
You now have a push pull amplifier, that does not use a push pull output transformer, instead it uses 2 SE transformers.
I did this, thinking I had created something new.
It sounded real good.
Then I found out that the French had done it before I did.

You need 4 transformers for Stereo.


Question:
Super Triode. Is that when you drive the Screen of a Pentode, or a Beam Power tube?
 
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