jlsem said:
Respectfully, that is a rather sweeping statement to make, John (I did not see a smiley face, so I presume you are serious). You may be saved by the "necessarily" and "really", but since distortion is easily measured, practice appears to contradict your assumption in any normal circumstances.
planet10 said:
Again; Dave, I think it is rather a large step from "cannot actually replicate" - in practice it comes close enough - to "purely an intellectual exercise".
Larry Lomax has fortunately brought us back to reality somewhat (I wanted to post that, Larry!). Firstly it must be remebered that music mostly consists of periodic ingredients, one way or another, or there would not be a tune or at least a discernible pitch (which exists even in the case of cymbals). And even if Fourier analysis does not define music precisely, in amplifier testing we are not in the first place trying to analyse the music accurately, we are trying to analyse the amplifier.
So (to echo Larry), if we know that our hearing faculty can only hear stuff up to 20 KHz (or so) - there is a sharp decrease in hearing sensitivity above that, apart from many loudspeakers also not going much higher - then all we want to achieve is for the amplifier (well, the whole system actually) not to ADD anything to the original. Ideally, if a spectrum analysis shows no harmonics generated from any single sine input in the audio band, we have a perfect amplifier, whatever the music might consist of. Similarly if the spectrum analysis is clean enough, the amp will not add anything audible. For this the typical FFT route appeared to have been sufficient in the past, not just because of academic arguments but also as proved by listening tests at some prestigious research centres.
Since this is largely a tube thread, do we need anything more for amplifiers? (Sometimes the measurements may not be well interpreted, but they are there.)
Lastly a very important point Indm. Only reflections do not only cause standing waves, they also influence the direct (1st time) path to the ear and can be bothersome if several reflection pathlengths are such that interference (cancellation) takes place. This is especially so at high frequencies - for bass you are right.
Regards all.
I think this is perhaps getting carried away?.
Some people like valve amps, some like transistor amps, neither is 'better', it's purely a personal choice. Transistor amps can be measured as far superior, but measurements aren't what amplifiers are about - so everyone is welcome to their personal preference.
I wouldn't have a valve amp, but that's MY choice, everything I have works well and sounds great - but that's not to say everyone else would like it!.
BTW, I have owned valve amps over many years, and have also repaired and built them - and if someone would care to GIVE ME a large mains and output transformer, I'd build one again, just for the fun
Some people like valve amps, some like transistor amps, neither is 'better', it's purely a personal choice. Transistor amps can be measured as far superior, but measurements aren't what amplifiers are about - so everyone is welcome to their personal preference.
I wouldn't have a valve amp, but that's MY choice, everything I have works well and sounds great - but that's not to say everyone else would like it!.
BTW, I have owned valve amps over many years, and have also repaired and built them - and if someone would care to GIVE ME a large mains and output transformer, I'd build one again, just for the fun
Distortion of a "sine wave" is easily measured, not distortion of a complex music signal. Build a mathematical algorithm for a simple music passage and see if harmonics are mathematically canceled in push-pull scheme or a negative feedback circuit.
I'll admit to being an audiophile and music-lover. I grew up in a house with a grand piano in the living room and my mother and sister were fine classical pianists. I have attended the symphony weekly for many, many years and go to many piano, voice, and chamber music recitals. I own a simple single-ended amplifier I built myself. When I play a well-recorded piano cd or lp it sounds as if a piano is in the room - no distortion or syrupy colorations. Friends of mine who wouldn't know an amplifier from a toaster are astounded by the realism. I'm sure if I measured this amp with my HP signal generator and distortion analyzer I'd be disappointed and the electronics professor would laugh. So why is it when we go to listen to amps built by a local hero that have admirably low "distortion" specs and damping factors along with all of the other disireable aspects of a "well-engineered" product, we have to leave the room after a very short time because the thing SOUNDS AWFUL?!
John
Johan Potgieter said:
Belabouring the obvious, plus weighting of course. It's been a real learning experience. My first project was the venerable Mullard 3-3, a direct-coupled high feedback SE which as built had a measured THD of well under 0.5% at 'full rated power'. Two to three-and-a-half watts as I recall. The top end though was always just wrong, hard and abrasive. THD spectrum analysis revealed why, 10-20 dB below the second harmonic, which dominated the THD meter reading, every harmonic from the third up was present and strong past the limit of my sound card. Last night I put what I hope are the finishing touches on a new circuit around substantially the same components save for driver (EF86 changed to 6C45-pe.) Defectively designed output iron mandated global negative feedback again, but this time ~10 dB in total and a great deal of effort went into optimizing the front end for minimal distortion and biasing the output for minimum upper harmonics, ignorong the 2nd completely. The THD is 2-3 time the Mullard's at a watt, still dominantly second with third and fourth each another 20 dB down respectively and anything higher at or below the -125 dB limit of my test kit. It sounds worlds better.
Though I don't think it necessarily has to be that way I can certainly see why many are feedback-phobic. It took a lot of cuts at different circuits to get to a point where I'm happy with the sound. In my experience though if there's one consistent audible attribute to feedback, whether locally derived or global, it's a reduction in the sense of space. Ambience is diminished and images tend to pull closer to the speakers. I have no idea why and no explanation, and this last amp suffers least from it, but I hear it every time.
Nigel Goodwin said:
Personally, I'd have said the other way around. What is your criteria for 'superior'. It's probably different to mine.
Dear jlsem, the answer to your question is easier than what you may mumble off.
I came of almost the same music experience you have.
But I'm also a "technician". I must recognize that measures are not very representative of the reality you are listening. The majority of time diyselfers, but not all, makes wrong measurements, or at last made up to demonstrate that their child sounds great.
Real professional world measures imply effort, knowledge, expensive instrumentation and impartiality.
You are right, harmonic distortion is not a paramount, neither is any other kind of distortion or "aberration" of the music, neither these odd qualities comes from the amp or somewhere else. Not at last till you can hear it or bother to.
Try listening an old fantastic record of music you love. Are you so ready to bother at the clicks, rumble and hiss is coming from the speaker or are you more interested in the music?
Best regards to all of you
I came of almost the same music experience you have.
But I'm also a "technician". I must recognize that measures are not very representative of the reality you are listening. The majority of time diyselfers, but not all, makes wrong measurements, or at last made up to demonstrate that their child sounds great.
Real professional world measures imply effort, knowledge, expensive instrumentation and impartiality.
You are right, harmonic distortion is not a paramount, neither is any other kind of distortion or "aberration" of the music, neither these odd qualities comes from the amp or somewhere else. Not at last till you can hear it or bother to.
Try listening an old fantastic record of music you love. Are you so ready to bother at the clicks, rumble and hiss is coming from the speaker or are you more interested in the music?
Best regards to all of you
lndm said:
As I said, 'personal preferences', but what measurements on a valve amp better the ones on a transistor one?.
Transistor amps have much lower distortion, wider frequency response, flatter frequency response, better damping factors.
I'll give you that the wide flat frequency response and damping factor and quantity of distortion (in most cases) measures as superior in ss amps.
Not far superior though. The numbers look far superior, but subjectively.....
The point I might disagree with you on is the spectra of distortion. I think that the quantity of distortion is of little importance compared to the nature of it.
Anyway, I'm sure we both enjoy good music at the end of the day.
Not far superior though. The numbers look far superior, but subjectively.....
The point I might disagree with you on is the spectra of distortion. I think that the quantity of distortion is of little importance compared to the nature of it.
Anyway, I'm sure we both enjoy good music at the end of the day.
My I give you a hint?
What about this question at which level you made the measurements? Is distorsion proportional to signal level in both ss and tube amps?
Which distortion prevails on solid states odd o even armonics?
And which one on tube amps?
What about transient distortion and overshoots?
And at what level of utput power measurements are made?
And most important, who made the tests?
Have you ever had in your hands a cheap amp from Taiwan? Have you noticed the so called "measures" made in that amp?
Ther's somenthing doesn't convince me...
Do you?
Regards
What about this question at which level you made the measurements? Is distorsion proportional to signal level in both ss and tube amps?
Which distortion prevails on solid states odd o even armonics?
And which one on tube amps?
What about transient distortion and overshoots?
And at what level of utput power measurements are made?
And most important, who made the tests?
Have you ever had in your hands a cheap amp from Taiwan? Have you noticed the so called "measures" made in that amp?
Ther's somenthing doesn't convince me...
Do you?
Regards
I have a nagging suspicion that the damping factor needs to be the optimum for the speaker concerned, which may not always be the maximum attainable. This may be why amps with bags of NFB are said to sound "dead". A more dynamic sound may be achieved by actually decreasing the damping factor in some cases.
In order to retain NFB's other advantages of constant gain, low distortion and broad frequency response, maybe the effect of a lower DF could be tried by the simple expedient of keeping the NFB as it is but introducing a low additional resistance in the circuit leading to the speakers, preferably variable to find the "sweet spot". I'd love to try this for myself but at the moment I'm away from home and I've only just thought of it!
Has anyone tried something like this?
In order to retain NFB's other advantages of constant gain, low distortion and broad frequency response, maybe the effect of a lower DF could be tried by the simple expedient of keeping the NFB as it is but introducing a low additional resistance in the circuit leading to the speakers, preferably variable to find the "sweet spot". I'd love to try this for myself but at the moment I'm away from home and I've only just thought of it!
Has anyone tried something like this?
Variable damping was a big thing back in the '50s. Generally, it was done by altering the ratio of voltage and current feedback, a method less wasteful of power than a simple series resistor.
lndm said:
That's why I very carefully (both times) said 'MEASUREMENTS', so subjectivity doesn't apply - and the numbers don't 'look' far superior, they ARE far superior - but as I also said, 'measurements aren't what amplifiers are about!'.
But again, that's not a MEASUREMENT, it's a purely subjective term, similar to oxgen free, one way, speaker cables? (but let's not go there!).
Anyway, I'm sure we both enjoy good music at the end of the day.
That's what it's all about 😀
BTW, you might be interested?, back in the 1970's I delivered a new Hacker stereo record deck/amplifier (Hacker were probably the BEST quality radio manufacturer in the world). The lady I took it to had a homemade stereogram (I presume her husband had built it?), and keep the speakers (I forget what they were, but were nice ones). But she wanted to sell the stereogram, so I bought it off her, for not much money.
It had a really well made cabinet, a Goldring record deck (the variable speed type), a Leak valve preamp, and two Leak valve power amps. I kept it a few months then sold it, I wonder what it would be worth now?.
Hi Nigel,
Hi ray_moth,
Yes. I don't know about various speakers, but when designing an amp there is a sweet spot for NFB. Too high and the sound is "lifeless" and too low the sound is uncontrolled. Especially in the bass region (not surprising) but also everywhere else.
This is a variable that would be interesting to explore (different speakers). If the value of NFB didn't change the gain, a limited range control might be the ultimate "tune". Of course it would be misused out in the wild.
-Chris
Ouch! Not so much the value even, but just to have this old gear.
Hi ray_moth,
Yes. I don't know about various speakers, but when designing an amp there is a sweet spot for NFB. Too high and the sound is "lifeless" and too low the sound is uncontrolled. Especially in the bass region (not surprising) but also everywhere else.
This is a variable that would be interesting to explore (different speakers). If the value of NFB didn't change the gain, a limited range control might be the ultimate "tune". Of course it would be misused out in the wild.
-Chris
Yes, I know that but the technique you refer to was generally used to increase damping still further. What I had in mind was to be able to keep NFB but "jetison" some of the damping, if that would make your speakers sound better.
True, it would sacrifice a proportion of the power, (approx) 1/DF = (added + existing OP impedance)/(load impedance), but that's what a low DF does anyway.
I realize that the load impedance is anything but constant with a speaker, which we blithely assume to have its nominal impedance, e.g. 8 ohms; however, since I'm talking about tuning the thing by ear, rather than calculate a value, maths doesn't enter into it.
jlsem said:
John,
Not to belabour this, but I don't know if we are talking about the same thing. I tried to indicate in the 3rd and 4th paragraphs of my post #81 (am not going to waste screen space by blocking all), that an amplifier's distortion does not really concern the music complexity. If ideally the distortion is zero for any frequency in the audio band, then it generates no extra harmonics irrespective of what you throw at it in that band. I do not see where the algorithm of a musical passage comes into the equation; it is what the amplifier generates. If it cancels something or does not generate it at all (in the limit), that is valid for all frequencies that your music might ever consist of.
Also, be assured that I respect your music experience; this is not about your personal background. I only wish that I can have more opportunities to do the same!
It is possible that loop phase differences between 2 stereo channels can influence spatial definition. With feedback amplifiers, when the loop characteristics differ because of a spread in components this could do the above though the response amplitude may be similar (although it does not happen often).
I am always careful about "good specs" and then the result is "awful". What specs? As Rdf indicated it is well known that an amplifier can have a low thd, yet sound poor as a result of high order harmonic products, even of low amplitude. I don't want to stick my head out, but within my limited experience I have thusfar always been able to find by investigation why an amplifier sounds bland or whatever unsatisfactory experience has been reported.
Lastly not to open a can of worms, but regarding damping factor: I hope it is realised that there is no such thing as a high damping factor, as is suggested by the popular definition. The damping (braking action) of a driver is dependant on the total resistance in the circuit. Since this includes the voice coil d.c resistance, in practice the real damping factor can be no more than about 1,6. Thus, decreasing the amp output impedance from 1 ohm (a "DF" of 8) to 0,1 ohm (a "DF" of 80), makes a real difference of only 1,6 to 1,33 (taking an 8 ohm voice coil d.c. resistance of 5 ohm). This is borne out by practice. (Unless as said before elsewhere, a negative amplifier impedance is generated, effectively cancelling the loudspeaker d.c. resistance.) I am not a loudspeaker expert, but I have it that most of the damping in a modern loudspeaker is governed by the enclosure design.
We seem to have wondered a long way from rectifier choice, but I find the discussion interesting.
Regards all.
Nigel Goodwin said:
I took distortion spectra to be a measurable thing. IMO not worth squabbling about though. 🙂
I do believe in consistent dynamic distortion. An amp that sounds the same during clipping as at lower levels (only more, if you take the meaning). This is probably less of a valve issue, probably more of a simple circuit issue.
And damping factor, I think the cure can be worse than the problem. Personally, I spend my time creating speakers that don't need to rely on the amp.
The point of all that was that an amplifier with apparantly high measureable distortion sounds less distorted than one with low measured distortion.
John
Johan Potgieter said:
Likewise! Dick Pierce's analysis, posted in another thread, provides a handy rule of thumb. He suggests if the damping factor hits 2 digits the amp's done its work. Which to bring it partially back around, is it possible to achieve a damping factor of 10 with a single tube, non-feedback SE amp? SE AmpCAD suggest crazy combinations like a KT100 into a Bartollucci 16K transformer gets close (~9.6, 6 watts out.)
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