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Old 15th September 2012, 12:19 PM   #7171
DF96 is offline DF96  England
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Quote:
Originally Posted by nigel pearson
The test has to stand examination if it is to succeed .
That depends on how you define success. For a marketeer, success means increased sales rather than peer-confirmed truth.

A standard technique is to put out an article which looks vaguely technical (with impressive graphs of 'before' and 'after'), but doesn't say exactly what was done and hides technical things like numbers. The marketeer may genuinely believe that he is making techy stuff accessible to the masses, but what he actually does (accidentally?) is make it harder for technical criticism to take place.

One way to get an impressive graph is to (accidentally?) exploit numerical coincidence. A sharp downward spike on an FFT (below the noise floor) must be an artifact of the calculation (e.g. windowing, rounding errors). It can look impressive when this 'detail' is washed out by the test. The skirt response on an FFT tells you very little about the signal being analysed; it is set largely by the windowing function used and the time period for the sample. People who buy (and sell) cables probably won't know this.
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Old 15th September 2012, 01:20 PM   #7172
tvrgeek is offline tvrgeek  United States
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DF96 points out what it all comes down to. They are a business. You run a business for the reason of making money. We should admire a company with the creativity to show "tests" that prove snake oil works. Everyone needs to eat.
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Old 15th September 2012, 01:41 PM   #7173
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I have asked Dr Gareth Humphries-Jones to give his views and a link to our forum . I imagine he might see it on Monday ?

I am not sure if it is the same gentleman I met ? If so I expect it to be a proper discussion far removed from hi fi hyperbole .

I remember null tests were rejected by many . I never understood that . In that case it was perhaps the subjective people who didn't like them .

I am sure penicillin seemed to be snake oil at first , it wasn't easy to produce . Certainly it was ignored for years .
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Old 15th September 2012, 03:08 PM   #7174
DF96 is offline DF96  England
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Perhaps what they should have done is find a suitable frequency standard and use that with a phase detector to measure the medium-term (secs->mins) stability of the CD player crystal clock. Essentially they have done this, but in the wrong way, so their results are almost meaningless.

Having got their results, they could then do some double-blind trials to see to what extent, if any, they correlate with sound perception. Note however, that the likely effect of wobble in a decent crystal osc could be the same as the listener moving his head a few mm forwards or backwards. Quartz crystals have much higher Q and better frequency stability than musical instruments, human voice, turntables etc. so even a measurable wobble is IMHO unlikely to be audible.
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Old 15th September 2012, 03:35 PM   #7175
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I think the double blind test very useful . Proper double blinds should always include playing the same thing twice . It was found many years ago that an identical thing sounds brighter the second time around . Doubtless the brain has learnt and is focusing differently . By the third try the effect has past . It is subtle .

Some people can not do blind tests . They switch off for whatever reason ( fear ? ) . Other people are not gifted even though they might think they are . I would imagine a professional conductor not a bad choice ( Andre Previn even though now 83 would be my choice ) . I would recommended Bill Low of Audioquest if he would ( cables ) ? From analogue Hi Fi Frank Schroeder if he would ( sorry all my German friends , Frank seems the most trained spokesman of us all ) . 20 total ?
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Old 16th September 2012, 02:45 AM   #7176
jned is offline jned  United States
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Default Pics, took me longer to be able too!

SteamPunk pictures by mrqueue - Photobucket

First is the speaker that I built with my Grandfather in high school, That is half of one, I cut it in half and made two, I gave the other two to a friend. The original drivers where 12" coax from Lafayette Radio.

What's in there now is from Pioneer and Audio Concepts with some U-Do-It from Needham MA

I originally planed on using the cabinets to test drivers but wound up with what is there now due to 'weighing anchor' so to speak, and she doesn't like the look of them btw! Perhaps some wood veneer and a grill would fix that?

I have these situated in a nook up stairs, I'll post pics of that, but the area is not ideal for listening, so I have to make it work with the new fan-geld digital realm, that I had sitting on my desk for quit some years when I noticed it said, crossover! Light bulb picture goes off!

The amp is an old Crown I bought off of craig(s) list, total water damage but after bias adjustments, sounds fine, it was repaired before I got it and shows. I now know why the older guys like analog Simpsons multimeters, these digital ones throw you off when the battery is low!

What I've had fun doing is going to the source of what music I like and listening to it played by the Artists, then trying to find recordings that are good and recreate the performance the best I can, to the way I like to listen!

So, over all, just have fun!


Quote:
Originally Posted by jned View Post
What ever happened to the direct feedback of the speaker to the amp?

I remember reading this long ago, a company patented it then it disappeared?

I might actually have the speaker builder article.

I will take pics of my cabinets, heavily damped/stiffened that I built with my Grandfather, he made the cabinets, I still have and use them! I moved them recently, wow they weigh a ton!

Damping factor, the more the better, this is a physical piston moving, so if you can move it with more control, the better, sound wise? Well, that is up to the listener!

The whole system, needs to be taken into account, not just one component!

I think that is the fun of all this, to experiment!!!
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Old 16th September 2012, 10:05 AM   #7177
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The speakers look great . Some Soviet ones I took apart recently had a similar look and sounded great .

Many who looked at Motional feedback ( Philips , circa 1975 ) also looked at negative output impedance . Some 1950's RCA amps had it . Professor Korn of Belgium had an amplifier called Servo Sound slightly before Philips . Meridian M series speakers might have it if a breif conversation with their rep was correct .

At college we had a Servo Sound where incredibly the germanium transistors survived and the cones didn't . It was said better quality speakers did not suit it and TV types were used as the servo liked them . No idea if true .

From what I understood the box might halve in volume for a given output if using Motional feedabck . One suspects it would be partly due to active crossover which helps .

The Linn Isobarik although not a servo speaker looked also to get more out of a smaller box . It has a pressure sealed extra drive unit inside the box . It seems to work . The latter Linns had external crossovers . I saw a pair for 350 recently . I was very tempted . I would love to build them a nicer crossover ( active ) . Also to try a bit of negative output impedance just to know if it is a no no .

I can see an argument for critical under damping ( 8 R resistor to speaker reducing to zero ) . One could split that into 4R + 4R and get the error signal . I dare say even 0R1 + 0R1 as an op amp will amplify it nicely . If one finds the error disappears at 0R1 then output impedance is doing the job instead ( I am sure it does ) . In the 1950, Wireless world estimated by experiment a damping factor of 3 to be critical . Speaker cones/surrounds were stiffer then . If we said 2R + 2R we might be in interesting territory . It could be switched off to have pseudo valve sound . Sid Smith of Marantz was convinced much of it was that ( the output resistance ) . The Marantz had low distortion yet still was valve in it's virtues . Another has said that divorcing the amplifier from the speaker a little bit is no bad thing . I could well believe some bandwidth limiting of the feedback could offer many opportunities . One could argue that some of the virtues of current drive might be tested and a sweet spot found .

Variable Amplifier Impedance

DVV you use servo's . Why not do both jobs in one ? It could be called variable damping . It will fine tune a speaker to a room ? We all need a unique selling proposition . Some rooms are so dry , a bit of bloom might do magic . It needs very fine control is my guess . Oh if I could dial my suspension of my car ( some can ) . Lets be fair VW it is not bad ..
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Old 16th September 2012, 12:32 PM   #7178
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I think this is a modern version of Motional feedback . They look interesting , especially the tweeter .
https://www.google.co.uk/search?q=Ph...w=1280&bih=885
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Old 16th September 2012, 01:13 PM   #7179
a.wayne is offline a.wayne  United States
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I have always found servo speakers to sound unnatural , attack and decay of the note ...That is ..
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Old 16th September 2012, 05:11 PM   #7180
dvv is offline dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
...

DVV you use servo's . Why not do both jobs in one ? It could be called variable damping . It will fine tune a speaker to a room ? We all need a unique selling proposition . Some rooms are so dry , a bit of bloom might do magic . It needs very fine control is my guess . Oh if I could dial my suspension of my car ( some can ) . Lets be fair VW it is not bad ..
Quite simply, Nige, because I don't believe in it. Not saying it's all wrong, just that I don't believe in it. I believe in naturally (i.e. without global NFB) obtained good damping factors, one of the reasons why I prefer to use 4 pairs of smaller power trannies rather than two pairs of higher power trannies. That's why I like to use parallelled capacitors of decreasing values, and why I prefer to have no inductors at all in my signal path.

This DF control switch was built into some integrated amps in the early-mid 70ies (e.g. Accuphase), and I tried it then and wasn't impressed with the results.

Philips' Motional Feedback is quite another matter. It's not much on lower cost versions, but on the biggie model of the time (12" bass driver), I did hear some rather amazing bass notes, in clarity comparable with any of the time for a whole lot less money than the passive speakers, given that their speakers were fully active and thus required only a preamp.

In addition to which I happen to have some oddball ideas which are out of synch with the times, which naturally limits me to a series of 1, all the more so since I do not wish to get into the general electornics market, feeling that it's already so crowded that it's bursting at its seams.

While I really love to read comments posted here by various members, and am grateful for it, I nevertheless do have my own experiences and views, all of which boil down to a single word - moderation. Zero global NFB amps sound loose and not quite precise to, while large global NFB amps all too often sound cold and lifeless to me, too "mechanical". Experience teaches me that a balance between zero and "much" global NFB is usually the best all around solution. So, I will have neither 0 dB nor 60 dB of global NFB, but will aim for say 26 +/- 1 dB as what I feel is a reasonable compromise.

At that value, no matter what else I do, and given that I hardly ever use less than 3 series/parallel pairs of output devices, often 4 pairs, I will have what I feel is a high damping factor anyway, it's not likely to be less than 30:1 into 4 Ohms and at 20 kHz. My feeling is that I simply don't need any more.

My favorite NFB loop consists of a simple resistor voltage divider, I do my very best to keep it as simple as possible. I try to avoid parallel capacitors as much as I can, but I will not sacrifice overall stability for anything.

In fact, to put it simply, I agree with John on all points except one, he once wrote that he tries as hard as he can under the circumstances to avoid any global NFB - I do not, as I said, I feel a reasonable amount of global NFB actually improves the overall results soundwise.
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