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Transparent amplification

watchnerd

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At all levels above low wattage the fans in commercial amps like Crown can be quite obtrusive. I don't recommend them unless you can put them outside your listening room.

True, but they're so ugly you don't want them in plain sight, anyway. They're good for a rack/gear installation.
 

Jinjuku

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At all levels above low wattage the fans in commercial amps like Crown can be quite obtrusive. I don't recommend them unless you can put them outside your listening room.

That depends. Yamaha P2500S and Crown DriveCore XLS are silent. I can drop the hammer with the XLS and you can't hear the fan. At All.
 

watchnerd

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That depends. Yamaha P2500S and Crown DriveCore XLS are silent. I can drop the hammer with the XLS and you can't hear the fan. At All.

The upper end XLS series put out a huge amount of power...I can't imagine needing to drop the hammer on those in domestic setting.
 

CuteStudio

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Tricky to find a decent commercial amp, I would like to build them myself but there's little money in real HiFi.

If your speaker has a nice flat impedance curve (or you can flatten it with a zobel) then Class D is worth a look, ideally a TPA3116d2 unit with air cored inductors in the output filters.
Else:
Your choice of voltage amplification section (VAS) should be tube or transformer because tubes and transformers are the most linear devices. There's a good design that uses an input transformer to drive MOSFET followers that drives an OPT (output transformer), probably the best amp but complex DIY transformer winding only.

So you want tubes at least in the VAS and some class A method of driving the speakers: Class B has crossover distortion right where you don't want it, class AB has impedance issues and both class B and AB are hard on the PSU so you effectively end up listening to that too. Class A is the way.

So we're realistically now at a Class A Single Ended solution, but many SETs (Single Ended Triode) are low power, so I'd recommend a SEP (Single Ended Pentode) design but with a twist as espoused by Thorsten Loesch some time back on diyaudio which makes perfect sense and works very well. The twist is to avoid including the OPT in the feedback loop due to the OPT having bandwidth limits and phase lag - both of which ruin the feedback usage. Many tube amps get away with some feedback because they are so linear in the first place but it's entirely unnecessary as the feedback should be all in the tube-only section that drives the OPT.

GNFB (Global negative feedback) has plagued amps for years - since Williamson did to to his amps, it's a cheap way to reduce noise and tidy up some things but it's not free, the cost is in multiplied harmonics and a reduction of 'air' and 'space' so everything sounds a bit flat. This is why using harmonic rich devices like MOSFETs. BJTs etc and making them measure well with GNFB is a bad idea as well as particularly boring and ordinary.

The opposite SET tendency of no feedback is often however too low power - but secretly those SET amps do follow the Loesch philosophy because a triode has intrinsic internal local feedback, so a driver/output tube combo with feedback is merely an engineering extension of the SET idea but made usable for more power (and cheaper tubes!). I.e. it's like replacing the triode in a SET design with a super-triode.

I actually summarised amp design issues here: http://www.cutestudio.net/data/SeeDeClip4/playback/amplifiers/index.php a few weeks ago as it's a common question and the engineering is not new, it's just that not many people seem to apply the basic principle of using the best components with the best audio design for an amp which is why there are so few hybrid feedback class A pentode tube amps. DIY is not a bad way t get there though - that's how I build mine, by modding one that was close (a chinese single ended triode-strapped-pentode amp).
 

oivavoi

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I have not repeated that test with modern amps though I believe some of the better class D gear, like the Hypex or others might manage the trick or get close.
.

Members of LTS, the Swedish AES, have claimed on Swedish forums that they have done before/after blind tests on various well regarded class d amps, and that they still haven't found any d amps they deemed transparent enough to write about. They refuse to provide any other details though. I think that they want to do it thorouhly if they put a product down.

This surprised me, as objectivists in most other places seem to praise ncore etc. But I have no reason to doubt them. They ususally avoid audiophoolery. Would be interesting if others with the technical skills to conduct before/after tests could put the Ncore under the bench: is it transparent or not?
 

Blumlein 88

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Members of LTS, the Swedish AES, have claimed on Swedish forums that they have done before/after blind tests on various well regarded class d amps, and that they still haven't found any d amps they deemed transparent enough to write about. They refuse to provide any other details though. I think that they want to do it thorouhly if they put a product down.

This surprised me, as objectivists in most other places seem to praise ncore etc. But I have no reason to doubt them. They ususally avoid audiophoolery. Would be interesting if others with the technical skills to conduct before/after tests could put the Ncore under the bench: is it transparent or not?

Yes it would be interesting. I like their method. I too found actually only one amp that would pass that test years ago. Several good SS amps came close and weren't much off being transparent. Only one managed the trick. My guess on what might plague the class D amps is the output filter and interaction with speaker impedance. I would wonder if all that is needed is some EQ to fix that up to 20 khz for otherwise very good class D amps.
 

Purité Audio

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How did you know they weren’t transparent, how did you know that your reference was ‘transparent’.
I haven’t much if any difference between good measuring solid state amps if they are capable of driving the loudspeaker.
Keith
 

Cosmik

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Remember that to be officially transparent you also need to be able to demonstrate it over 120dB of dynamic range :)
 

oivavoi

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How did you know they weren’t transparent, how did you know that your reference was ‘transparent’.
I haven’t much if any difference between good measuring solid state amps if they are capable of driving the loudspeaker.
Keith

This method isn't an ordinary A/B test. The point is to insert an amp into the chain while keeping the original power amp - an amp before an amp kind of - to see whether the sound changes audibly. If it does change, it means that the amp isn't fully transparent. Doesn't matter if the rest of the chain is non-transparent. If an amp inserted into the chain changes the sound in any way whatsoever, it means it isn't fully transparent.

But one may debate how audible this will be with ordinary listening, of course.
 
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Purité Audio

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Hmm a power amp driving a power amp, I would stick to measurements, this is Serge’s territory,
Keith
 

Jinjuku

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This method isn't an ordinary A/B test. The point is to insert an amp into the chain while keeping the original power amp - an amp before an amp kind of - to see whether the sound changes audibly. If it does change, it means that the amp isn't fully transparent. Doesn't matter if the rest of the chain is non-transparent. If an amp inserted into the chain changes the sound in any way whatsoever, it means it isn't fully transparent.

But one may debate how audible this will be with ordinary listening, of course.

What? What's not to say the 2nd amp is transparent and it's the first amp that's changing.

That makes zero logical sense.
 

oivavoi

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What? What's not to say the 2nd amp is transparent and it's the first amp that's changing.

That makes zero logical sense.

They have been doing this for 30 years. I think @Blumlein 88 can explain better than me how it functions, since he has done it himself. The point is simple: Somehow they are able to insert an amplifier into a chain before the last amp that is driving the loudspeakers. If there is any audible difference between the two - the chain with and without the amp under test - it means that the amp under test is adding something to the signal. Nothing illogical about this.
 
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sergeauckland

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Hmm a power amp driving a power amp, I would stick to measurements, this is Serge’s territory,
Keith
You're thinking of the Straight-wire Bypass Test.

It's a relatively easy thing to do with line-level devices, a bit more complicated for amplifiers, but yes, still doable.

It requires the dummy load to be a good simulation of a loudspeaker, including RCL parameters, but also any back emf which is rather more difficult to do. Alternatively, a sound proof room to put the dummy load in, as this 'dummy' load needs to be another real loudspeaker if it's to be a real test of an amplifier driving a loudspeaker.

As I said, doable, but a bit of a faff.

S
 

iridium

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You're thinking of the Straight-wire Bypass Test.

It's a relatively easy thing to do with line-level devices, a bit more complicated for amplifiers, but yes, still doable.

It requires the dummy load to be a good simulation of a loudspeaker, including RCL parameters, but also any back emf which is rather more difficult to do. Alternatively, a sound proof room to put the dummy load in, as this 'dummy' load needs to be another real loudspeaker if it's to be a real test of an amplifier driving a loudspeaker.

As I said, doable, but a bit of a faff.

S

Faff:
to spend your time doing a lot of things that are not important instead of the thing that you should be doing: I wish you'd stop faffing about and do something useful!
faff about/around Definition in the Cambridge English Dictionary

iridium.
 
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Thomas savage

Thomas savage

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Fitzcaraldo215

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They have been doing this for 30 years. I think @Blumlein 88 can explain better than me how it functions, since he has done it himself. The point is simple: Somehow they are able to insert an amplifier into a chain before the last amp that is driving the loudspeakers. If there is any audible difference between the two - the chain with and without the amp under test - it means that the amp under test is adding something to the signal. Nothing illogical about this.
Hmm. I may be a total rube here. But, from what little I know, the test amp would only be able to operate at the extreme low power end of its output so as not to overdrive the following reference amp. Yet, most all solid state amps exhibit increasing noise and distortion as a % of signal as output level is reduced.

Do they instead run the test amp at higher power, but externally lower the output? If so, is the padding circuit itself transparent? It is not in the circuit when using just the reference amp. @sergeauckland may have answered this.

So, these tests, while interesting, require the test amp to commit an unnatural act, an extreme case, unlike their normal operation at normal output power directly into a speaker. So, what do these tests really mean?

I agree with Keith. I would rather trust the direct amp measurements, even into dummy loads.
 

oivavoi

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I really have no competence when it comes to assessing the validity of this test method. I just know that it's been widely employed in Sweden by their "AES", which comprises several engineers and researchers. So I don't think it's total bollocks. Here's an article written about it in English: http://www.sonicdesign.se/amptest.htm
 
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