Wednesday, August 9, 2017

Interesting Links

Here's Joe D'Appolito on loudspeaker measurements.

Here's Julian Dunn on Digital Audio measurements.

Here's Audiophile Nirvana's Resource Library.

Pictures and bios of people associated with the Boston Audio Society, including Hadaway (I've bought lots of stuff from him at dB Audio Systems, great guy), Meyer, Moran, and others.

Distortion Analyzer recommendations (from 2007, but I think John Curl still uses his modified Sound Technology).

Pictures and info about Sound Technology 1701A analyzer (just checked and mine is actually a 1700B, said to be the same as 1701A except for a few opamps were upgraded to 5534 and the oscillator changed).

Vintage audio publications, including 1700AB manual.

Repairing a 1700B, one person's experience.  He says the common view that LDR's and VCR's are what goes bad help up in his repair--once those were replaced everything else was fine.

Ethan Winer's book The Audio Expert (which I have just now purchased).

My current page at the Restaurant at the End of the Universe, more commonly known as the Blowtorch Forum Part II.

Soundoctor on Surround Sound.

Amazing history of Dynaudio (interleaved with many other speaker driver manufacturers).

Archive "A" of Dynaudio spec sheets.

Archive "B" of Dynaudio spec sheets.

My preferred supertweeter, the D21AF, has the most extended flat high frequency response (+/- 1dB to 40kHz) of anything shown, including the Esotar^2 110 (which sells for $1400).  After the D21AF and D21/2, Dynaudio has avoided building 21mm tweeters, basically specializing on 28mm tweeters which had started with the D28.

The D21AF does have an (proprietary) damping fluid (probably aka ferrofluid).  For supertweeter usage, the preferred approach would probably be no damping fluid.  Later models use ferrofluid also.

My sense now is that the Vifa NE19VTS is a decent substitute for D21AF in supertweeter applications, though the D21AF *is* better and looks to be far more rugged (my D21AF have survived over 18 years of triamplification...powered directly by amplifiers such as MC 225 with no protective capacitor, etc., because I was relying on electronic crossover.  But I am sure I pulled the audio cords by mistake at least once, sending large low frequency hum and/or transients into the D21AF and it always shrugged them off without damage.

Downloadable audible jitter tests.  The author produced elevated jitter levels high enough that they can supposedly be reliably heard.

Jitter comments by Steve Nugent of Empirical audio.  The thresholds he described were derived by computer models and not actual human testing.  Human testing has not shown jitter to be reliably audible until being many orders of magnitude higher.  Empirical was selling products with low jitter as an important feature.  So I don't trust what he says very much.

Another interesting discussion of jitter, with both viewpoints.  A graph from J. Dunn does show jitter less than 100 ps being audible!  However, that is only jitter above 3kHz.  There is an incredibly steep curve, with 20 Hz jitter being audible only above 1uS, to 20kHz jitter being audible down to about 30  ps (!!!).  This might be based on the same "simulation" Nugent talked about, but he didn't note the low audibility of lower frequency jitter.

The Richard C. Heyser memorial lecture published by Stereophile in 2011.  I notice that Heyser was unmentioned by The Audio Critic when it listed White and Black hats.  John Atkinson, who gave this lecture, was not so lucky (however, he Atkinson reacted very kindly IMO and said he was actually a Grey Hat).  Perhaps Heyser, an actual physicist, would be the quintessential Grey Hat.  BTW, John Atkinson, John Curl, and Bob Carver all have physics degrees.  Anyway, one more interesting fact about Heyser is that (at least according to a poster on the Blowtorch blog) never participated in blind testing.  He simply refused every time.

Refurbing a Parasound HCA-1000A amplifier.  I may be doing this before too long, because the 1000A powering my super tweeters does have significant power supply hum, which gets stopped now by a blocking capacitor, but really shouldn't be there in the first place.  The power supply caps, which were marginal because of space and cost at the time, apparently are the main thing to replace with something slightly better.

A poster in the blowtorch forum mentions a few other DBT protocols other than ABX: ABC/HR, PEAQ, ABC/HR, PEAQ, MUHSRA, duo-trio, triangle.  And he says there are more.

Funny way way back in the mid 1970's before I even worked at an audio store, I believed that determination of audible limits required forced-choice tests, rather than preferences.  I reasoned then that preferences could be more than one-dimensional, and could change over time, including during the test itself.

ABX appeared in the early 1980's, and it clearly was the kind of test I thought should be used.

However now (August 22, 2017) I am thinking that ABX is too cumbersome, difficult, and loaded with baggage that typical audiophiles won't like.  I'm thinking a simple blind A/B preference test, repeated to reach "decision" level probability.  The test could be relatively open ended...I think it's possibly the statistics can be made slightly more stringent (such as 0.02 instead of 0.05) to allow for early quitting or extended testing as desired.  The "proctor" tells the "testee" whether or not their decision has been made (meaning, enough trials and consistency at p<0.02 level) and they can keep going or not.  If consistency is not reached, a fair proctor will not reveal which devices was the "better" one.  Of course the level should also be controlled--and that is one of the biggest factors in the traditional lousy audiophile A/B test.  ALSO, the "testee" must reveal if he believes his "preferred unit" has changed.  Precisely at that point, the Proctor should reverse the previous entries.  However, the testee may also chose never to reveal which actual unit he prefers.

Discussion of the best sounding shortwave receivers.  Some say the famous Collins R-390A isn't good for station hopping as it's really intended for long term monitoring, though it is an engineering feat and blast to play with (others say the R390A is fine for band cruising if in good repair).  For listening to shortwave broadcasts (??? does anyone do that anymore ???) the Collins 75A4 is among the best.  People also like the Drake 4B but not the cheapened 4C unless it comes with all the options.  The Hallicrafters SX-115, SB-303, and Hammarlund HQ-215 are prone to overload now.

Here is a comparison of Drake R4B and R4C.

Now, however, I have learned that the R4B and R4C were only Ham receivers, receiving bands assigned to amateur operators, and NOT general coverage receivers.  You could convert them to general coverage using an external LFO (however, I wonder if that is really as good as having the general coverage built in.)

OTOH, the Drake R7, TR7, and R8 units all have general coverage receivers, with very good reputations.  The R7 and TR7 do not have identical receivers however.  The R7 receiver is triple conversion and it has more bandwidth selections on certain ranges, and may have a better blanker (there were apparently several used during the production runs of the R7).

Watkins-Johnson made some very nice looking VHF/UHF receivers including scope.  Audio quality out the back panel jacks said not always good because of digital grundge, but audio out to headphones said to be OK, and could be amplified.

Finding shortwave stations.

A discussion of loudspeaker measuring systems.  Speakers and the Room are the most important things, and I need to be getting back to loudspeaker measuring for my 3 way system. looks to be a useful resource.

Pano, a DIYAudio moderator, has had a level test inquiry.  He provides test signals to see how much voltage your speakers actually need.

A discussion of speaker driver materials.  It confirms (and corrects) what I have long said about the Kef B110 woofer used in LS3/5A.  The crossover uses a 6dB notch to correct the 1.5kHz resonance.  I discovered that the hard way, because for 18 years I used a standard electronic crossover with amplifiers driving the woofer and tweeter.  I begin to think there was some reason I could never get it to sound exactly correct, then I discovered the 6dB peak.  (Actually, I had recalled the peak being 12dB.)  The woofer has other resonance problems also and is very inefficient.  He says what I've long believed, that it is very strange that people still revere this design based on now very outdated drivers.

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