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Old 15th February 2012, 03:25 AM   #2371
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Quote:
Originally Posted by CopperTop View Post
In other words, could we take a snapshot of a test signal acquired by the AP2 and put it into a PC and devise an as yet unknown algorithm that could measure 'sound quality', or does the digital nature of the AP2 mean it simply can't be done, end of story?
The nature of reality tells us that qualities can't be reduced to quantities. End of story.

<edit> @dvv : Metal film resistors in SMT aren't particularly expensive. I bought a couple of reels recently, I think they were around $60 apiece. 4,000 on a reel. The ones I measured (LCR meter) were I think better than 0.05%. That was until I overheated them with my soldering iron set to too high a temperature
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Last edited by abraxalito; 15th February 2012 at 03:34 AM.
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Old 15th February 2012, 04:00 AM   #2372
benb is offline benb  United States
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Quote:
Originally Posted by CopperTop View Post
In this interview, Mike Albinson of Quad amplifier fame alludes to a "bridge test"

MikeAlbinson



Does anyone know the details of this test? It sounds like the sort of thing that could be more easily done in software with a generic 'AP2' using real music signals - if everyone agreed that an 'AP2' can acquire with sufficient fidelity.

Edit: I know he's talking about subtracting two signals - input from amp output most probably - but what were the test signals, pass criteria etc.?
Poking around with Google, I find an apparent description here:
http://www.angelfire.com/ab3/mjramp/distortiontest.pdf

It looks to be just what I thought, circuitry to take both the input and output and "cancel" them, leaving only the difference. A couple of R/C networks compensate for the high-frequency and low-frequency rolloffs of the amp. As one circuit shows, it's easy enough to do this with all passive components for an inverting amplifier, though apparently all amps thesedays are noninverting.

Of course, the AP2 uses the same or similar A/D converter chips as many computer audio interfaces, though presumably the AP's analog input and grounding design is as good as or better than most such interfaces.

There's that MSWindows .wav file diff program that does (almost?) the same thing digitally, mentioned earlier in this thread.
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Old 15th February 2012, 04:49 AM   #2373
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Hi,

Quote:
Originally Posted by benb View Post
Of course, the AP2 uses the same or similar A/D converter chips as many computer audio interfaces, though presumably the AP's analog input and grounding design is as good as or better than most such interfaces.
You really don't know anything about the AP2, do you?

Ciao T
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Old 15th February 2012, 07:06 AM   #2374
dvv is offline dvv  Serbia
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Quote:
Originally Posted by abraxalito View Post
The nature of reality tells us that qualities can't be reduced to quantities. End of story.

<edit> @dvv : Metal film resistors in SMT aren't particularly expensive. I bought a couple of reels recently, I think they were around $60 apiece. 4,000 on a reel. The ones I measured (LCR meter) were I think better than 0.05%. That was until I overheated them with my soldering iron set to too high a temperature
Point taken, but please bear in mind that I am the low in the "low volume", meaning that I make and sell 50...60 units per annum of the filter, which is the core of my operation. Headphone amps, to which SMD would be pertinant, will not sell much, if at all, simply because they came about 5 years too late.

They are definitely not portable at 20 lbs and in the 19" rack format, they are not WiFi, they are neither Apple nor Android compatible, you can't use them to chat with your friends, they do not deliver 3D pictures, and you can't use them to live your life on the Internet. Hence, they were practically stillborn.

I don't mind at all, I did them mostly for myself anyway, but I'll be lucky if I sell a total of 10.

Going the SMD way, with such low volumes, is simply counterproductive.

Also, with kind of bum luck I have, I would need like 50 of those reels of 4,000, and at $60 you mentioned, that works out at $3,000, to which I would need to add shipping costs, customs duties and VAT, making the final price more something like $4,260.

So you see, while your advice is good such as it is, it's just not applicable in my case.

Last edited by dvv; 15th February 2012 at 07:14 AM.
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Old 15th February 2012, 07:13 AM   #2375
dvv is offline dvv  Serbia
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Quote:
Originally Posted by tvrgeek View Post
How much factor can you give that if a change makes an audible difference by reasonable test methods, and you have invested considerable intellectual capital, that the change will be presumed positive when it may not be?

I would think that conformation of an audible difference is the invitation to find an objective measure of the difference. Without objective measurements, we are reduced to art. Not knocking art, but I like engineering too.
Good point, but also I think rather relative.

Remember, we cannot measure everything that is relevant, but not everything we can measure is relevant.

Increments in rising quality are, in my view, even more subjective than the basic design. Not many users will know the device as well as its designer does, so in most cases, the improvements would be actually known only to the designer. The designer is, objectively speaking, the man with the best historical perspective of the design, he knows best where he started from and where he is now with it.

Some improvements may be rather obvious to one and all, such as, for example, suddenly doubling your filtering capacity, but then this is hardly a minor change.
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Old 15th February 2012, 07:24 AM   #2376
dvv is offline dvv  Serbia
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On a general note, while I am not a regulation freak, it seems to me that much of the debate about measurements does not address some of what I see as key issues.

A good example is the work load for amps. We are still using, at least offcially, simple lab resistors, when we all know only too well just how sadly inadequate that is.

An official standards (say, IHF) measurements should use a rough approximation of what might be called an "average" speaker, both as 8 and 4 Ohms. By this, I mean a simulation of an actual speaker crossover, say 3 way, with resistors, inductors and capacitors all mixed in so they give us a say -45 degree phase shift as a realistic, real world case.

Something along those lines, but surely a little more real life like, because a simple resistor sure as hell is NOT real life like at all. Letting things go on as they do simply prolongs our misery, these people determining the standards seem to have stopped evolving them something like 30 years ago.

Not that the Chinese care, mind you, they will still claim that a 12 Volt power supply for your PC speakers will deliver "1,200 Watts of S.M.P.S." power, whatever that means.

How can we expect our mesurements to more faithfully describe our products when our benchmark standards and values are so far removed from anything barely resembling real life?
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Old 15th February 2012, 07:55 AM   #2377
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If the input and output of a hypothetical amp were compared (with any real speaker load on the amp), and found to give a difference of zero with any and all test signals, including real music, to the limit of an AP2's resolution, could we declare that amplifier to be 'perfect', for all practical intents and purposes?
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Old 15th February 2012, 08:05 AM   #2378
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A null <-110dB? Good luck with that
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Old 15th February 2012, 08:32 AM   #2379
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Quote:
Originally Posted by abraxalito View Post
A null <-110dB? Good luck with that
Well I did say "hypothetical". And yes, the question is precisely that: could a null at -110dB, as measured by a state of the art piece of equipment be considered 'perfect', or would there still be room to quibble?

My main concern is that I suspect there are people who would not trust an amplifier that had op amps and digital elements in it, on principle. (I'm not completely immune to those thoughts myself!). If the test equipment itself is built like that, could they accept its findings?

And another question I'm trying to tease out, is that some people think that an audio system's job is to actually add something that makes the final sound at the speakers more like listening to a live performance than the recording itself - many threads contain such suggestions e.g. the measured distortions of vinyl and tubes serendipitously enhance the sound. A true difference of zero would mean no added 'musicality'. Is this 'perfection' in such people's eyes (ears...)?
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Old 15th February 2012, 09:35 AM   #2380
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Hi,

Quote:
Originally Posted by CopperTop View Post
If the input and output of a hypothetical amp were compared (with any real speaker load on the amp), and found to give a difference of zero with any and all test signals, including real music, to the limit of an AP2's resolution, could we declare that amplifier to be 'perfect', for all practical intents and purposes?
The first pitfall is the comparison. How will you compare? Can you make sure you comparison system does not add or subtract something?

And of course you must use the actual system, mains cables etc. included, testing the Amplifier in isolation removes too many variables that can influence the outcome.

You may as well leave an AP2 out of this, as this is not a function it provides. It may be useful to inform yourself at AP's site as to how these devices work and what they can do and what they cannot.

If you instead suggested to use a system involving common DS A2D and D2A converters with normal (<= 200KHz) sample rates I would suggest that they are incapable in aiding such determination as you wish to make (and are unsuited to use in high quality audio).

On to perfection.

Now if an Amplifier (or a piece of wire) reliably does not alter any signal under any "designed for" conditions the Amplifier or Wire would be indeed 'perfect'.

Such a device does not exist and cannot exist, we would require to establish in what ways the amplifier (or wire) may alter the signal to remain:

1) Inaudible

2) Audible but not objectionable

3) Audible and objectionable

Then we could agree that an amplifier (or wire) may be regarded:

1) sonically transparent

2) sonically not transparent but offering good sound

3) sonically not transparent and offering bad sound

Having determined from the above which technical parameters actually matter and how (e.g. not THD but instead the best available distortion metric in terms of audibility research) we can measure how far devices deviate from 'audible perfection' and if the deviations are objectionable or not.

This would be what is called in other disciplines an "evidence based" approached. As we can observe no such approach is undertaken anywhere in audio.

Ciao T
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