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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Sun 01, 2018 11:56 pm 
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john8750 wrote:
I noticed there was a change in the NFG circuit from my 6V6 amp, which sounds very good. A .47uf cap was removed, and the resistance was changed from a 470K to a 51K.
But this amp sounds so good now, I will just leave the NFG disconnected.
Might have to do with the tone control circuit. With NFG connected, tone control was erratic.
I don't see how it could be caused by the NFB unless you've got something wired wrong. I could see it with the original arrangement because the FB was dependent on the source impedance, which was vanishingly small so any impedance on the input would have a significant effect. But the modified FB is totally isolated on the -input pin so nothing 'in front' of the + input should interact with it.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Mon 02, 2018 12:24 am 
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john8750 wrote:
I noticed there was a change in the NFG circuit from my 6V6 amp, which sounds very good. A .47uf cap was removed, and the resistance was changed from a 470K to a 51K.
But this amp sounds so good now, I will just leave the NFG disconnected.
Might have to do with the tone control circuit. With NFG connected, tone control was erratic.

When Flip redesigned the op-275 input to the non-inverting side he recalculated the feed back ratio
And it came out as 56k equals the same thing a the .47uf cap and 470k resistor somehow.
So 56k resistor works fine when you are using +/- 20v Vcc
And so on ... direct feed to grid and direct input from mp3 player.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Mon 02, 2018 5:34 am 
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Flipperhome wrote:
john8750 wrote:
I noticed there was a change in the NFG circuit from my 6V6 amp, which sounds very good. A .47uf cap was removed, and the resistance was changed from a 470K to a 51K.
But this amp sounds so good now, I will just leave the NFG disconnected.
Might have to do with the tone control circuit. With NFG connected, tone control was erratic.
I don't see how it could be caused by the NFB unless you've got something wired wrong. I could see it with the original arrangement because the FB was dependent on the source impedance, which was vanishingly small so any impedance on the input would have a significant effect. But the modified FB is totally isolated on the -input pin so nothing 'in front' of the + input should interact with it.



I will double check the wiring around the opamp. On my amp, there is a problem with that circuit. Bet I made a mistake somewhere.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Mon 02, 2018 5:43 am 
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Pbpix wrote:
john8750 wrote:
I noticed there was a change in the NFG circuit from my 6V6 amp, which sounds very good. A .47uf cap was removed, and the resistance was changed from a 470K to a 51K.
But this amp sounds so good now, I will just leave the NFG disconnected.
Might have to do with the tone control circuit. With NFG connected, tone control was erratic.

When Flip redesigned the op-275 input to the non-inverting side he recalculated the feed back ratio
And it came out as 56k equals the same thing a the .47uf cap and 470k resistor somehow.
So 56k resistor works fine when you are using +/- 20v Vcc
And so on ... direct feed to grid and direct input from mp3 player.



I just went through a detailed test. I have a problem in the feed back circuit. Has nothing to do with the tone board. I feed the MP3 straight in, and have the problem with tone board disconnected. I will double check wiring tomorrow.
But wilt NFB disconnected, the amp sounds just as good as my 6V6.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Mon 02, 2018 6:36 am 
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john8750 wrote:
I just went through a detailed test. I have a problem in the feed back circuit. Has nothing to do with the tone board. I feed the MP3 straight in, and have the problem with tone board disconnected. I will double check wiring tomorrow.
But wilt NFB disconnected, the amp sounds just as good as my 6V6.
If you have the feedback on the wrong side of the OPT secondary that would be consistent with the symptoms you're seeing. That would turn it into positive feedback, which has a much more 'noticeable' effect than the wanted negative feedback.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Mon 02, 2018 6:57 am 
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Flipperhome wrote:
john8750 wrote:
I just went through a detailed test. I have a problem in the feed back circuit. Has nothing to do with the tone board. I feed the MP3 straight in, and have the problem with tone board disconnected. I will double check wiring tomorrow.
But wilt NFB disconnected, the amp sounds just as good as my 6V6.
If you have the feedback on the wrong side of the OPT secondary that would be consistent with the symptoms you're seeing. That would turn it into positive feedback, which has a much more 'noticeable' effect than the wanted negative feedback.




Now that's a good tip. and very possibly the problem. Those durned Edcors have confused me every time. I will take a look at that. They might have the makings incorrect. I don't know of a way to test for that.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Tue 03, 2018 5:35 am 
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Try reversing either the primary or secondary winding polarity but not both windings


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Tue 03, 2018 5:58 am 
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I will reverse the secondary. That would be the easiest. Will report the results. Thanks for the help Men.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 2:37 am 
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I was gonna tackle that tonight. But run out a gas. I did look at it in the shop. Gonna be a bit of work. But will be well worth it. Then I will consider it hi-fi.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 9:38 pm 
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That made the difference. I transposed the secondary connection wiring. Now has great sound. Very low distortion. I think I still like it better w/out the NFB connected. So will leave it like that, remove the switch. It would just be confusing.
Now ready to present to my Cousin.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 9:45 pm 
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john8750 wrote:
That made the difference. I transposed the secondary connection wiring. Now has great sound. Very low distortion. I think I still like it better w/out the NFB connected. So will leave it like that, remove the switch. It would just be confusing.
Now ready to present to my Cousin.

That's how I've been enjoying mine... NFB switched off.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 10:30 pm 
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Pbpix wrote:
john8750 wrote:
That made the difference. I transposed the secondary connection wiring. Now has great sound. Very low distortion. I think I still like it better w/out the NFB connected. So will leave it like that, remove the switch. It would just be confusing.
Now ready to present to my Cousin.

That's how I've been enjoying mine... NFB switched off.



I agree Peter. On this amp I tried it disconnected. But had a problem, so couldn't really compare. Now that the secondary terminals have been corrected I can tell the difference. NFB might work better on a high powered amp.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 10:39 pm 
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john8750 wrote:
Pbpix wrote:
john8750 wrote:
That made the difference. I transposed the secondary connection wiring. Now has great sound. Very low distortion. I think I still like it better w/out the NFB connected. So will leave it like that, remove the switch. It would just be confusing.
Now ready to present to my Cousin.

That's how I've been enjoying mine... NFB switched off.



I agree Peter. On this amp I tried it disconnected. But had a problem, so couldn't really compare. Now that the secondary terminals have been corrected I can tell the difference. NFB might work better on a high powered amp.

I don't know what "high power" would have to do with it?
Why do you think that?

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 11:07 pm 
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Well Peter, what good would it be to have it if it don't help the sound of our low powered amps?

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 11:23 pm 
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john8750 wrote:
Well Peter, what good would it be to have it if it don't help the sound of our low powered amps?

It is "helping" to correct the "distortion" in the audio signal shape... but apparently the audio wasn't noticeably distorted bad enough before or ...not enough to notice.
Like THD ... or harmonic distortion is not necessarily "bad" to the ear... it is simply a harmonic overtone ... something which you may actually determine as an enhancement to the sound depending on what you like or your ear.

That's why I think THD should be redefined ... total harmonic (enrichment is a better term) instead of distortion.
Because, you like most folks think that "distortion" is a bad sounding noise. But harmonics are only called distortion because they are not part of the original; audio that came into the amp.
The amp helped harmonics to be created to ADD to the audio wave shape... and that is not a scratchy noisy distorted thing.
In fact harmonics are pleasant mostly.
So THD ... doe not mean "distorted" in the way you may have thought it to be a bad scratchy sound.
No.

So if your amp added some harmonics (thd) .... the NFB signal worked to help lessen or reduce those harmonics.

But when you dis able the NFB you may find that those harmonics that it was trying to fix actually were quite pleasant. So this is why you may find that turning off the NFB makes the signal "sound" nicer.
I know I do.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Fri 06, 2018 11:56 pm 
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john8750 wrote:
I agree Peter. On this amp I tried it disconnected. But had a problem, so couldn't really compare. Now that the secondary terminals have been corrected I can tell the difference. NFB might work better on a high powered amp.
The amount of power is irrelevant. Without NFB it's basically (push pull) 'radio' quality.

This amp has very little feedback and very low feedback can actually make an amp sound worse. That doesn't mean "no feedback" is 'Hi-Fi'. It means there's not enough negative feedback to get to 'Hi-Fi'. And then there's the fact of clipping and the opamps being incapable of fully driving the 6V6. While that might seem unrelated clipping introduces a bunch a higher order harmonics and the low feedback makes them worse.

The very low feedback is why having the OPT secondary just made the amp sound sort of bad. If it had anything close to that needed for 'Hi-Fi' it would have squealed like a banshee with reversed feedback.

When I redesigned the opamp section I kept it with the same opamp feedback, since Peter thinks it's 'super duper', but it should probably have the opamp feedback lowered and the extra gain go to global feedback where it could do some good.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Sat 07, 2018 5:39 am 
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Flipperhome wrote:
john8750 wrote:
I agree Peter. On this amp I tried it disconnected. But had a problem, so couldn't really compare. Now that the secondary terminals have been corrected I can tell the difference. NFB might work better on a high powered amp.
The amount of power is irrelevant. Without NFB it's basically (push pull) 'radio' quality.

This amp has very little feedback and very low feedback can actually make an amp sound worse. That doesn't mean "no feedback" is 'Hi-Fi'. It means there's not enough negative feedback to get to 'Hi-Fi'. And then there's the fact of clipping and the opamps being incapable of fully driving the 6V6. While that might seem unrelated clipping introduces a bunch a higher order harmonics and the low feedback makes them worse.

The very low feedback is why having the OPT secondary just made the amp sound sort of bad. If it had anything close to that needed for 'Hi-Fi' it would have squealed like a banshee with reversed feedback.

When I redesigned the opamp section I kept it with the same opamp feedback, since Peter thinks it's 'super duper', but it should probably have the opamp feedback lowered and the extra gain go to global feedback where it could do some good.



Flip, do you suggest a circuit change to get this amp closer to hi-fi?

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Sat 07, 2018 6:56 am 
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john8750 wrote:
Flipperhome wrote:
The amount of power is irrelevant. Without NFB it's basically (push pull) 'radio' quality.

This amp has very little feedback and very low feedback can actually make an amp sound worse. That doesn't mean "no feedback" is 'Hi-Fi'. It means there's not enough negative feedback to get to 'Hi-Fi'. And then there's the fact of clipping and the opamps being incapable of fully driving the 6V6. While that might seem unrelated clipping introduces a bunch a higher order harmonics and the low feedback makes them worse.

The very low feedback is why having the OPT secondary just made the amp sound sort of bad. If it had anything close to that needed for 'Hi-Fi' it would have squealed like a banshee with reversed feedback.

When I redesigned the opamp section I kept it with the same opamp feedback, since Peter thinks it's 'super duper', but it should probably have the opamp feedback lowered and the extra gain go to global feedback where it could do some good.
Flip, do you suggest a circuit change to get this amp closer to hi-fi?
I have some suggestions but can't simulated them fully for a myriad of reasons so it's sort of a hunt and peck thing, meaning you'd have to try, measure the feedback, then adjust as needed. Also, I'm not sure you're power supply can handle it.

First is to increase idle current to 35 mA, per the spec sheet. That alone cuts around 7 dB off the distortion figures. Then, as I said, reduce the opamp feedback (increase the resistor value) and increase global feedback (lower that resistor's value).

This sim gets it to 17.5 dB but that's with a 'normal' OPT and I don't remember if you're using UL. I sim'ed it with simply an R for bias but it works the same for your CCS. Also dropped the 'feedback pot' since it's basically useless.

Attachment:
6V6 bipolar rework more opamp gain 3.jpg
6V6 bipolar rework more opamp gain 3.jpg [ 119.18 KiB | Viewed 1348 times ]


This get's it to around -73 dB (3'd harmonic only) at 1 Watt.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Sat 07, 2018 6:27 pm 
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I really appreciate the work you did on this Flip. Do you suggest doing away with the CCS? I do have UL hooked up. I went through your schematic, and can follow the changes. Is C2, 10pf added to stop oscillation?
If I decide to change the circuit, it will be easier to build a new board. Maybe work on it outside first.
My PT is 100watt, about 800ma. So far, it don't run too hot. I only need another 40ma.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Apr Sat 07, 2018 10:02 pm 
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Hi John:
Here's some more helpful explanation about SE & PP amps and how they relate to "harmonic Distortion" >
The second section Is all about NFB.
REF: http://www.x3mhc.no/dokumenter/SE-v-PP-Part1.pdf
PP vs SE
PART 1)
"Harmonic Distortion Series of SE and PP
SE and PP amplifiers naturally produce different harmonic distortion series. Different distortion series sound quite different to the ear. In general, the higher the order, the more offensive to the ear, and in correspondingly smaller amounts with progressively higher orders.
Also, odd order harmonics are more offensive up to a point in the series, where it all sounds ghastly horrible in very tiny amounts, whether odd or even order. For example, as much as 3% second harmonic is unnoticed by most people, while .3% (ten times less) fifth order is very noticeable to most people. Second order is manifest as being exactly one octave above the fundamental tone, and therefore it is not dissonant. In small amounts, it is not perceived by most ears as adding "sweetness" or "liquidity" to the sound.
Third order is a musical fourth above the fundamental tone, which is not terribly harmonious also but not terribly dissonant if held to low enough levels.
Fourth order is two octaves above the fundamental, and is not extremely dissonant but blurs focus. Fifth order and up just sounds very dissonant and nasty, period.
A SET (single ended triode) amplifier's main distortion product is second harmonic, which is relatively benign to the ear.
The single ended driver stages of a PP amp produce predominantly second order distortion, but the PP output stage itself produces mostly high/odd order distortion. Remember, the second harmonic created in the output stage is cancelled, but any distortion (whether second harmonic or not) produced in earlier stages is passed on through.
The reason why PP and SE output stages produce different distortion series is because they clip differently. This
"clipping" occurs when the tube reaches the maximum output attainable at the operating point chosen by the
circuit designer. The tops of the sine-wave form cannot go any higher (higher amplitude), because the tube is at it's
maximum, it cannot amplify any further. Still, on the oscilloscope screen the waveform amplitude tries to grow taller in response the increased input signal. Since it cannot, a flat line appears across the tops of the sine waveform at the "maxed out" point, giving them the appearance of having been clipped off with scissors, hence the name clipping.

A PP amplifier clips symmetrically, which means the opposing sides of the sine-wave clip alike and at the same time.
This produces high/odd order distortion products, which are more offensive to the ear than second order.

A SE amplifier typically clips asymmetrically (more on one side of the sine-wave), which produces predominantly second order as it's chief harmonic distortion artifact.

These different distortion series are one of the predominant reasons as to why PP and SE sound so very different."

Part 2: NFB

" Negative Feedback
To further widen the sonic gap between SE and PP, consider that most PP amps use negative feedback (NFB), while
very few SE amps use NFB nowadays. "Okay, that's nice" you say, "now, what is NFB?"
Negative feedback is a sample of the signal taken from the output of an amplifier stage and re-injected into the
front of that same stage (local NFB), or into an earlier stage (global NFB), where it is 180° out of phase with
the signal present at the re-injection point. In other words, "feeding back." Pretty crafty, eh?
The circuitry contained between the two points is called the "feedback loop." Usually, the global feedback loop in a PP hi-fi amplifier will run from the speaker terminal back to the amp's input stage, thereby encompassing the entire
amplifier circuit. Figure 6 points out the global feedback loop of a typical push pull amplifier schematic in bright
red.
Fig 6

Image

Global negative feedback loop
Your next question is probably, "Okay, now I know what it is. What does it do?" Good question. Well, it does a lot of good things.

(1) NFB reduces the distortion and noise in the stages contained inside the feedback loop. It cancels distortions
in the same way our PP OPT cancelled power supply hum and second order harmonics earlier.
(2) NFB lowers the output impedance.
This tightens and extends the bass.
(3) NFB flattens and extends the overall frequency response.
Most people at this point would be thinking, "Cool!

Bring on the negative feedback!" Not so fast. NFB also does some bad things.
(1) NFB kills the space and air between the instruments, parts, and voices, rendering music dull and uninteresting.
(2) NFB reduces the stability of amplifier circuits
(3) NFB reduces gain.

Oh well...... Nothing's perfect, including NFB. Especially NFB.
This first brings us to the early days of telephone service, where long distance telephone transmission lines were powered by tube amplifiers. Due to parasitic losses in the long, cross-country runs of wire, repeater amplifiers had to be used at intervals. The cumulative distortion of several of these repeater amplifiers was horrid, and telephone communications were difficult to understand as a result. NFB was invented at Bell Labs to lower this distortion, and it indeed worked very well.
Without NFB, early long distance telephone communications across entire nations and continents would have been impossible.
Fast forward to the Golden Era of Hi-Fi, which was the original heyday of tube audio back in the 1950s and '60s, before mainstream solid state audio gear even existed. Just as it is in today's "big box store" world of mass
consumer audio electronics, specs on paper meant everything back then.
The lowest distortion at the highest power was what sold biggest. Lots of NFB was applied to everything, often even in multiple feedback loops.
This was done because NFB lowers distortion, and to the uninformed masses the lowest distortion means the best
sound. "Lower distortion has to sound better, doesn't it? I mean, distortion is a deviation from the original music
signal, so lowest distortion means the most "perfect" sound, right?" No, it definitely doesn't. We saw this in the last section, "HARMONIC DISTORTION SERIES OF PP AND SE."
The amp with the lowest distortion is not necessarily the best sounding. It more strongly depends on which orders of harmonic distortion are present, more so than how much. The human brain often disagrees with specs on paper as to what sounds best! Because NFB is taken from the very rear of the amp and injected back into the front, there is a frequency-

Single Ended vs. Push Pull:
The Fight of the Century 8 dependent time smearing effect that occurs, which the ear is extremely sensitive to. It blurs the focus and homogenizes the sound.
The excessive amounts of NFB that was used to get the best distortion specs on paper was what made many of these amplifiers of yore sound lifeless and uninteresting to listen to, and is a major reason why today's big box store electronics sound so bad as well. Both may have ridiculously low distortion, yet there are amplifiers with literally a hundred times more distortion that sound better. Human psychoacoustics do not obey the laws of science very often!
So why do most PP amplifiers use NFB and most SE amplifiers don't?
The main reasons are distortion and outputimpedance. Power triodes and triode-wired power tetrodes/pentodes were not used very much during the Golden Era of Hi-Fi, as a matter of fact almost never.
Triodes produce comparatively less power than tetrodes/pentodes, and remember, specs on paper were everything so the most power won the day (or at least the sale!) Practically 99.9% of amplifiers used beam power tetrodes or power pentodes.
As we saw in the last section, both of these tube types have very high plate resistance and therefore high output impedance. Neither is as linear as triodes; both types have much higher distortion.
A zero feedback amplifier built using these types has high distortion, with much of it being the high/odd order type, plus the very high output impedance means
the bass will be weak and flabby. It sounds bright, harsh, and thin, and literally un-listenable. So, NFB was a necessary evil with amplifiers using these tubes, both to lower distortion and yield an acceptable damping factor.
To make matters worse, many of these PP amplifiers had very complex, multi-stage signal paths to improve the specs on paper. This created more distortion, which of course required more NFB to correct! NFB reduces gain,
so higher gain driver stages were needed to recoup the lost gain. Higher gain also means more distortion is created than in a lower gain scenario, which means (you guessed it) more NFB is needed to lower the distortion!
It's like a dog chasing his tail, a vicious cycle to which there is no end, and nothing good can ever come from
it. Plus, the immense complexity of such amplifers also degraded the sound, due to the sheer volume of components, solder joints, and jumper wires the signal had to travel through.
Your music got ran through a huge network of sonically degrading components, and it sounded like it.
What's even sadder is that most of today's commercially produced PP amplifiers are still nothing more than
tweaked-over copies of these same vintage circuits! Fact is, many (most?) of today's companies producing these
PP amplifiers are still caught in this same "specs on paper" rut and can't seem to get out of it. I have this fancy
saying about this, "Test equipment ain't got ears".
The oscilloscope is not gonna be listening to it.
The harmonic distortion analyzer is not gonna be listening to it.
So who is? Humans!
We have ears and brains, not screens and displays. As I had said earlier, psychoacoustics usually doesn't obey numbers written on a piece of paper.
Now that we've beaten the high feedback PP horse to death, let's move back to triodes and single ended operation.
Triodes sound less bright, thin, and harsh when not using NFB, and have lower output impedance and
lower distortion.
A SE amplifier with a single, highly linear driver stage and a low plate resistance triode power
tube can usually get by without NFB, which means the average SET amp is usually more open, airy, and purer
sounding than an otherwise identical SEP (single ended pentode) amplifier using NFB to correct it's problems of
high distortion and output impedance.

In Summary
We've seen that single ended and push pull amplifiers both have advantages and disadvantages. It's difficult to make blanket statements about each, as there are many variations of circuitry and quality in each broad category, so keep in mind these statements are generalizations.
There are exceptions to the rule.

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