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Bob Cordell
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The Halcro dm88 appears to miss its distortion spec

In his January 2001 discussion with Jon Iverson, Halcro

Jim Tavegia
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Re: The Halcro dm88 appears to miss its distortion spec

Halcro may have reached the level of where the test instruments are NOT the equal of the gear being measured. I do not know this to be true, but it might be a good guess. Also marketing folks have been known to push the envelope once in a while. I am not acusing Halcro of doing this as I would doubt there is any thing lacking in the Halcro measurable or otherwise. It is as usual in a class by itself. It is amazing Mr. Candy continues to even TRY to improve on his already spectacular products. He is so far a head of speaker design science and manufacturing, arguably the hardest of all the audio manufacturing disciplines, You wonder why he does it accept that he CAN.

Isn't the rub of high end is that there is always SOMETHING better than what you have now. If Halcro owners CAN'T sleep knowing they might not have reached the top of the mountain, how can any of the rest of us?

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

Ditto Jim's comment. I find nothing implausible in the hypothesis that many of today's super-low distortion amplifiers are better than the equipment used to measure them. I believe JA himself has questioned the ability of his measuring machinery to keep up with the latest technical achievements in amplifier and preamplifier design. The key, as you note, is how well the piece performs in real-world listening situations. And the latest Halcro apparently offers some slight sonic improvements over its predecessors. The point about getting one's 40 large worth seems inane, to me. As good old Ben Franklin said, experience runs a tough school, and the stupid will learn only after entrance. I am sure Mr. Candy will arrange for some kind of an exchange, if you bought one and are disappointed in the abstract specs...but don't ask for your money back -- school is now in session, and tuition is unrefundable. I'll get by with my used VTL, which sounded fine in my other system before I sold the Dynaudios. I bought gold and silver with the other 30 grand, a couple years ago, which has about doubled over the interim. I hope nobody bought this thing with a home equity loan. That would be graduate school.

This isn't going to be a replay of the Chord controversy, is it?

Jeff Wong
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Re: The Halcro dm88 appears to miss its distortion spec


Quote:
This isn't going to be a replay of the Chord controversy, is it?

Clifton - You know it's always possible to strike a Chord at these forums.

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

Hi Clifton. As I pointed out in my post, the dm88 appears to be a great performer both subjectively and objectively. In questioning its performance claims, I certainly did not intend to give the impression that it was not worth $40K. As a long-time amplifier designer I understand and appreciate the very high level of talent and effort that must be invested in a design that exceeds average performance by such a large margin, and at the same time the economics of providing such a high-performance product to a relatively small-volume market. That does not make the amplifier immune to discussion of its technical aspects, however. Some of us like to discuss what's under the hood.

Like many good hobbies, Audio has many different and fascinating aspects that are interesting to different participants in different ways. With vintage cars, some are more interested in what's under the hood, while others are more interested in the body lines, the paint job, or the history of the model. Audio is a big tent, and there is plenty of room for all of us. I think we all agree that the destination in audio is great sound, but some of us are just as interested in the journey as the destination. As one with an engineering background, the technical aspect of the journey fascinates me. You seem less interested in the technical aspects of these discussions, and I understand that my post may have been uninteresting to you and many others. Sorry to bore you. Talking about what makes an amplifier tick should never take away from the enjoyment of the subjective aspects. On the other hand, some of us really enjoy talking about the technology and the numbers and what they mean and don't mean. I have a tube amplifier that I built that has a hundred (maybe a thousand?) times the distortion of the Halcro, and I think it sounds great. That seeming contradiction doesn't bother me, but it does fascinate me.

Your point and Jim's point about possible limitations of the test equipment being responsible for the ostensible spec miss is a really good one. Although I think it is likely that JA's numbers are real (and that his Halcro did indeed miss its spec by a lot), I agree that we need to allow for the possibility that JA's equipment was reporting a "false positive" on distortion. Suppose, for example that a nonlinearity in a relay contact in his dummy load created a "floor" of distortion for any amplifier measured. That would also be of great interest (and might be something easily corrected for future measurements). I've learned by experience that we can't always predict where our technical curiousity will lead us. That's part of the fun.

Actually, if you look at JA's 19+20 kHz IM spectra for the Boulder 2050 and the Bryston 14B-SST, they are not that much different from those for the three Halcros reviewed, even though one would not expect those amplifiers to turn in IM performance quite as good as the Halcros. One might cite this as evidence that there is something going on that we don't understand. Who knows, maybe the Halcro does meet its spec and JA's measurements show distortion that is not there. In that case, I owe Halcro an apology for taking them to task on their specs. I'd love to know the answer, one way or the other.

Here's what I think is the bottom line. Bruce Candy is an engineer's engineer who sweated the details and measurements to achieve remarkably low distortion in his quest for the very best sonics, and he appears to have succeeded. That suggests to me that there is some value in performance metrics and measurements, even if we don't understand all the relationships.

Cheers.

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

Groan. Mr. Candy won't strike a bargain with anybody on this thread.

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

I think there is a lot better chance of correlating measurements of amplifiers with audible performance than is the case with speakers. Still, I'll just listen. I wonder if you could hook the Halcro up to some speakers, print the conventional response characteristics on a few graphs, do the same with my VTL (same speakers), and "see" any differences. I bet you could hear 'em!

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Re: The Halcro dm88 appears to miss its distortion spec


Quote:
if you look at JA's 19+20 kHz IM spectra for the Boulder 2050 and the Bryston 14B-SST, they are not that much different from those for the three Halcros reviewed, even though one would not expect those amplifiers to turn in IM performance quite as good as the Halcros.

I am beginning to believe that what is being shown in the IM graphs is the residual of the twin-tone source.

With the Halcro's harmonic distortion, Bruce Candy suggested that this could be due to a hitherto unsuspected non-linearity at very high power in my test load that had been unmasked by the dm88's very low distortion level. I have constructed another load, without switches or relays, to investigate this using the dm38 that I am following up on in the October issue. (The dm88 samples, unfortunately, had to be returned to the distributor prior to HE2006.)

To be honest, the very small shortfall in the dm88's measured performance vis-a-vis its specifications is academic.

John Atkinson
Editor, Stereophile

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

Clifton, you make a very interesting point regarding seeing amplifier differences when measured as connected to the speakers being listened to. Indeed, I have always wished that reviewers would at least measure and print the electrical frequency response of the amplifier under test, as seen at the speaker terminals of the speaker being used for audition. While certainly not all amplifier differences are attributable to frequency response differences under the actual listening conditions, a good many may be. This is so because of the wild impedance changes that many speakers present as a load to the finite output impedance of the amplifier. This is especially the case with tube amplifiers. The fact that JA tests frequency response with a simulated speaker load is veryhelpful and informative in this regard, but it would be even better to see the FR at the speaker terminals under the actual listening arrangement. Inexpensive PC/soundcard arrangements are all one needs to do this. It is not uncommon for a speaker's impedance to dip to 2.5 ohms and peakto 30 ohms. If driven with an amplifier with 0.25 ohms output impedance (corresponding to a damping factor of 32), this can result in a frequency response coloration of about one dB.

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

John, this is very interesting. If you are open to it, I would love to help you re-test the dm38. I could bring some specialized equipment that allows me to measure much lower THD and 19+20 kHz IM distortion levels than is normally possible with standard test equipment. This would also make it possible to do a sanity check on your setup. I would like nothing better than to be able to show that the dm38 actually does meet its distortion spec. Just name a time when it is convenient for you.

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Re: The Halcro dm88 appears to miss its distortion spec

Delighted to have posts from an Amp designer of the stature of Bob Cordell.

Also a pleasure to read his views.

I concure fully with Cordell's comment :


Quote:

That does not make the amplifier immune to discussion of its technical aspects, however. Some of us like to discuss what's under the hood.

I suspect that the vanishingly low levels of distortion on the Halcros is achieved by Distortion cancellation, from feedforward, rather than only feedback & ofcourse Very Careful Ckt design and layout.

Given the vanishingly low levels of distortion that the Halcros probably have even before Feedforward correction has a go, it seems that the feedforward really needs to be individually tweeked for each unit, to actually achieve the specs indicated.

I have little doubt that the ppb distortion levels CAN be met, but are not met, because in production, that individual level of attention is not given.... and would also be impractical to achieve, and probably also mess up the pcb with repeated component insertions and removals.

Mr Cordell, could you care to speculate on this ? ( Whether Feedforward needs to be individually tweaked on each unit, to achieve optimum results )

The obvious extrapolation of the above is that the new amp that JA receives WILL meet published specs, or bottom out the measurement capabilities of JA's equipment for not only the IM but other tests too.

Jeff Wong
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Re: The Halcro dm88 appears to miss its distortion spec


Quote:
I suspect that the vanishingly low levels of distortion on the Halcros is achieved by Distortion cancellation, from feedforward, rather than only feedback & ofcourse Very Careful Ckt design and layout.

I had thoughts along these lines when I read about the first Halcro amps reviewed by Stereophile. It does make one wonder.

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

Thanks for your kind words. Let me preface my remarks by saying that I don't know what circuits Bruce Candy uses in his various models. However, I am very familiar with his patents. His earlier patent #5,892,398 discloses what is essentially the same Hawksford error correction circuit that I first applied to vertical power MOSFETs in 1982 (published in JAES January 1984). Candy's version adds output-driven bootstrapped power supplies to the concept. If he is using a variant of that circuit, it is not feedforward but a specialized version of feedback that achieves error cancellation. As in my circuit, a pot can be added to actually null the distortion. It would take less than five minutes to make the null adjustment in production and should be quite stable. My circuit acheived 0.00025% THD at 1 kHz and 0.0006% THD at 20 kHz, both at full power, with 5% resistors. So, if he is using that circuit, it should not be difficult to get well below 0.0005% on a production basis, but more trimming might be necessary to get below 0.0001% to 20 kHz on a production basis.

On the other hand, Bruce discloses a different approach to low distortion amp design in his patent #6,798,285. For all I know, he may have abandoned the Hawksford error correction technique for this approach in his later designs. Indeed, in this patent, he cites as an advantage not having to make the adjustment I mentioned above. This patent basically relies on gobs and gobs of negative feedback around the output stage and everything else - NFB loops of high order. I don't know that there would be a distortion trimming adjustment in such a circuit. However, it would be very important in such a circuit to be certain that all of the multiple feedback loops are unconditionally stable in every production unit. A low-level, high-frequency oscillation might be undetectable from the outside, but might make very subtle degradations to the distortion performance. Mind you, I am not saying that NFB, when properly applied, is bad.

So, bottom line is that I don't think the Halcro uses feedforward, and in my experience the error correction circuit that he may be using is not difficult to adjust.

Cheers!

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

I might add that, to the best of my knowledge, the Hawksford-based error correction scheme was first implemented in a commercial power amplifier in 1986. That amplifier was the Meyer Sound MS-1000 professional power amplifier, which achieved over 500 Watts per channel into 4 ohms at less than 0.01% distortion. That amplifier employed lateral power MOSFETs.

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Re: The Halcro dm88 appears to miss its distortion spec

www.avahifi.com Dave Hafler was the FIRST to use MOSFETS in an audio amp back in teh 70's I think...Vanalstine continues teh Hafler tradition of top quality, high performance priced for mortals. An OmegaStar EX AVA in teh Hafler chasis I use a few of em, the P500 OmegaStar EX ckts are doing about 1200-1400W MONO into my speakers...speced at .01% or less distortion 800v/uS MOSFETS baby Exicon from England in the ckts. Faster more linear according to AVA. 12 in the DH500 12 in the P500 with a bigger mains transformer 16 in the XL-600 going down into 2 OHMS with stability and cranking them watts WATTS there is no substitute.Halcro costs how much? Chord costs how much? How much do they cost when meeting specs? Are they the hand selected ones that meet specs? McIntosh guarantees it meets specs don't they? When you pay alot you expect alot. When you pay less and get more....now that's called great design. John, maybe another measuring session, do some measurments of the AVA amps lets see how it meets specs?

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

From Hawksford and error-correction to MOSFETS to Hafler...to another Van Alstine commercial. DUP, have you no shame?

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Re: The Halcro dm88 appears to miss its distortion spec

Hi, Bob --

Actually, your posts are very interesting. An actual designer talking design theory (unlike a mindless statistician raving the superiority of abstract theory over the reality of listening) is quite fascinating. There were attempts, as I recall, by various reviewers a couple of decades ago (Julian Hirsch?) to measure amplifier performance through speaker taps, but nothing came of them. In today's oscilloscope-centric universe, it would be educational and fun to see those attempts revisited...especially since amplifier design has progressed sonically as well as theoretically. Amplifier/speaker interface is, after all, a trial and error, subjective inquiry as things now stand. And if the objective side of things has true real-world validity, one ought to be able to measure amplifier performance through speakers playing music. I suspect the results would be contaminated, initially, by the sheer variety and number of variables presented to the technician, but that has never stopped science before. I admit I have little patience with those "look" for music in a laboratory dominated by visuals, be they LED's, graphs, or blips, and draw sonic value judgments out of a non-aural matrix. But the design perspective is a different animal, as abstract theory must be documented with measuring instruments. Again, I enjoy your comments a great deal. The 40 grand? Discretionary spending is up to the individual. I can afford this for an amp, but shall not pay it, because there are other demands on my discretionary cash. Cheers, Clifton

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Re: The Halcro dm88 appears to miss its distortion spec


Quote:
Halcro may have reached the level of where the test instruments are NOT the equal of the gear being measured.

I always wonder when I hear something like that.

Mr. Tavegia, if what you postulate is true, then how would Halcro know the specs?

I'm a firm believer in "hearing is believing" but I am also opposed to bullshitting with specs.

A product should perform "as specified." Otherwise, "specify" it differently!

I don't have my issue handy, did Halcro address this in their Manufacturer's Response?

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Re: The Halcro dm88 appears to miss its distortion spec

HAFLER had on their XL-600 model I believe an internal trim pot for user adj distortion tweaking, this was back in the early 80's...HAFLER was probably as usual ahead of his time. He had an entire procedure et up in the manual for using one channel against the other as a reference. When ever I read, that the device under test is beyond the limits of teh test equipment, it means they have the WRONG test equipm,ent. You gonna try to convince me Fluke, Agilent, Tektronix don't have test equipment better than any audio product by any mfg? Surely you are deluded...how then as mentioned does teh audio maker know of what he makes? What is the slew rate I didn't cath it on the write up on the Halcro in this month's issue..should be something astronomical for this price....better be above 800V/uS damping factor should also be better than anything ever made As these amps get into the stratosphere in prices, it seems just because they can.....How do they get to these list prices anyway? That new McIntosh multi pice absurdity....looks great, but also seems like a bunch of nonsense to have to make so many different modules....I think it's just to impress someone, no logic to the design...Halcro looks like a monster, for what reason? I wanna see a water cooled amp with 10's of thousands of watts, running off 240V twist locks!!! For $40K..CROWN had them needed 240V supplys they where so potent!!! Ya can't get more than what teh line delivers either

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

Yo, Buddha --

Bruce Candy may have test procedures/equipment more advanced than other lab facilities. I don't know. I guess you have to make some "superlative" claim to justify the price. I would (as you suggest) be more comfortable if Mr. Candy had said, "...and you will hear less distortion in this revolutionary design than otherwise thought possible..." or some such. Why resort to the numbers as "proof," when the proof always has been in the listening?

Do you remember some of those 2nd generation transistor amps in the '80's claiming they had whipped "notch" distortion? The oscilloscope traces looked mighty impressive, especially compared with older models that typically topped out around 1% IM and 1.5% THD. But they still sounded like buzz-saws.

Hafler amps always sounded veiled to me, compared with the competition -- they lacked clarity and transparency, even though they sounded harmonically "right." DUP, are you sure you took your earmuffs off from last winter?? They put out a lot of veiled watts, though, that's for sure. Cheers, Clifton.

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Re: The Halcro dm88 appears to miss its distortion spec

That reminds me. Did you see that old W.C. Fields movie, where he is in a cabin in the frozen wastes of the Yukon? There is a howling blizzard outside, and he and his babe are trapped in some leaky cabin. He takes a zither off the wall, where it had been hanging on the end of a tenpenny nail, and begins to strum. He looks soulfully at the babe (Mae West??), saying softly, "Do you mind if I play with my mittens on?"

The Hafler -- mosfet mist? There is a reason why everyone who frequently heard live music preferred tubes, in those days, in spite of the relatively bad measured performance. Nowadays, the gap between tube and transistors has certainly been closed; even though there are still some slight differences you can sometimes hear, it is difficult to say one is better than the other, provided both types are top-notch designs. But the measured differences still remain -- whenever JA sums up a tube amp that he likes, he seems to always write, "these are excellent numbers, for a tube amp," or, "this product seems well-designed, for a tube amp." Still, most reviewers couldn't tell a VTL from a Krell in terms of the quality of the listening experience, although even I can say, "they sound a little different."

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Re: The Halcro dm88 appears to miss its distortion spec


Quote:

Do you remember some of those 2nd generation transistor amps in the '80's claiming they had whipped "notch" distortion? The oscilloscope traces looked mighty impressive, especially compared with older models that typically topped out around 1% IM and 1.5% THD. But they still sounded like buzz-saws.

Exactly!

So, why specify that way?

Why take relatively low priority "numbers" and "over-specify?"

The only reason to care about Halcro's over-specification is to wonder why they'd feel the need.
____________________________

I do figure that maybe they are that low and it's a question of measurement technique...

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Re: The Halcro dm88 appears to miss its distortion spec

Veiled? That's why Frank Vanalstine has upgraded from the original Hitachi MOSFETS in the Hafler DH500,XL600, P500..to EXICON from England faster, more linear according to Frank....If you ain't heard the latest OmegaStar EX ckts, you are missing a lot, ain't no veils, this ain't Mecca, or Baghdad, we don't cover it up to look like a ghost, we let it free to fly and sound REAL, LIVE with SLAM, IMPACT and REALISM. How can Halcro justify $40,000 price tag on those things, AND they appear to be like CHORD, don't meet advertised specs? FTC loves those kinds of bargains....If you bought a car advertised at 286 HP it only have 145...you would be pissed, and moving mighty slow ...same with WATTS, you be listening mightly congested, flat, lifeless, where's the SLAM, Dynamics, realism...where's MY WATTS. MOSFET smooth FAST WATTS....Didn't teh Phase Linears use Bi-Polar gritty transistors? The DH500 came out and smoked anything out there SS wise in it's time...

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Re: The Halcro dm88 appears to miss its distortion spec

Looks who back in town....cool, another inovative pioneer for lotsa good watts.. http://www.ampzilla2000.com/

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

I often cringe at terms like "MOSFET mist" that stereotype a technology and make generalizations that are often not justified. To me, it's almost as bad and as unfair as stereotyping people. I've heard great tube amps and TERRIBLE tube amps, for example. Same with bipolars and MOSFETs. We certainly have the capability to screw up any technology. Sometimes, often based on only one or two anecdotal data points, stereotypes of technology get passed around the audio community and often accrue more credance than deserved. The same sort of thing goes for negative feedback. It got a bad reputation in the 70's as a result of some bad applications of it, on the one hand, and some flawed theory on the other hand. Yet, when it is properly applied, there is no evidence that it detracts from the sound. Indeed, the Halcro has been blessed by many reviewers as one of the best sounding ampifiers, yet it employs substantial amounts of negative feedback (possibly much more than most amplifiers, see Bruce Candy's patents).

Cheers!

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Re: The Halcro dm88 appears to miss its distortion spec

I agree.

Quasi Complementary was Phoo-phooed for years, once matched pnp power transistors became available.

Now we have the darTZeel power amp.....

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

Exactly. The proof is always in the listening (after all, the reason for the amplifier's existence), in real listening rooms, through real speakers, and with real-world recordings of music played by real musicians. Most of the transistor amps of the '70's and early '80's sounded quite unrealistic, whether MOSFET designs or bipolar designs. I remember. I heard most of them. That's what drove me to tubes, even after consideration of the typical tube amp's own bag of sonic compromises. I suppose you could argue that tube design was a mature technology, building on 50 or more years of trial-and-error and innovative progression, and that transistor design was still in its infancy, with the errors dominating the various solid state design approaches. Currently, solid-state (and hybrid) design has certainly evolved to the point where many of these amplifiers are preferable to tubes. I had a Steve McCormack amplifier for about 5 years (the DNA-1), with Steve's own upgrades added in as they became available. It was a MOSFET design, as I recall, but had none of the flaws I remembered hearing in earlier MOSFET amps. It was superbly transparent and smooth. I suspect design will always triumph over materials.

You shouldn't cringe at the fact that labels inevitably accrue when the product doesn't perform. The bad reputation may be wrongly attached to the designer's materials, instead of the designer, but that's merely language at work. Today's designers have plenty to emulate on the positive side of the evolutionary curve, and many negative stereotypes are, indeed, labels that are time-specific, to the past, and no longer applicable in the present. I was speaking of a '70's amplifier (the Hafler) that was veiled, compared to the best tubed designs of its era. And, yes, "mosfet mist" was the tag that summed up this situation. Somewhat analagous to the choice between AR-3's and JBL's, on the speaker side of things, the choice between "mosfet mist" and "bipolar spit" was Hobson's choice. But, fortunately, things have improved.

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Re: The Halcro dm88 appears to miss its distortion spec

If all it matters is listening why are there specs? Cus' specs matter, how else do you have any idea what the thing even starts to do? any product that is so overpriced and so under performing, would indicate a defect? How's that CHORD doing at less watts than ya paid for? and ya be paying an awful lot for nutin'.

Bob Cordell
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Re: The Halcro dm88 appears to miss its distortion spec

DUP, I agree with both you and Clifton. Specifications, and adherence to them, are important, but are not all-important. Neither so-called subjectivists nor so-called objectivists advance the state of audio by going to an extreme and throwing the baby out with the bath water. I listen to the music and enjoy it, and also do critical listening, but I also value the objective measurements and maintain a healthy degree of skepticism toward every new tweak that appears not to have a basis in science. I don't instantly reject such a tweak, but if it does not have a reasonable basis in known science, I do raise the bar a bit for believing in it. Perhaps that makes me a subjectivist with a healthy dose of skepticism. I am intensely interested in the matter of correlation between subjective results and objective pereformance, and do not believe that all of the conventional lab tests done under nominal conditions adequately predict the sonic quality. Indeed, that correlation is the Holy Grail of enabling us to design new audio equipment that has a higher probability of having great sonics.

Specifications and adherence to them are important because they also relate to matters of credibility and trust. You may not smoke, but if the used car dealer tells you the cigarette lighter works, and then you discover it does not work after you have bought the car, you begin to wonder what else he did not accurately assert.

Cheers!

Jim Tavegia
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Re: The Halcro dm88 appears to miss its distortion spec

At the point of the Halcro I think the specs only matter to the designer who is trying to go better than the DM 38. I am amazed at those who can hear the difference between the resolution and presentation of the 38 to the 88.. God love'em. At my age of 59 this month...that is not happening for me.

The question is what are you going to use to measure the improvment in the DM88 to the soon to come DM99!!!!!! Or what ever is coming next from Halcro. Until some speaker can do what the Halcro does I am not worried at all.

gkc
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Re: The Halcro dm88 appears to miss its distortion spec

I like the cigarette lighter analogy: we all want everything to work as advertised, even if we will never use the lacking feature (or, in this case, hear the missing spec). And, since you are a designer, and thus fussy about everything you put your name on, you are going to be a bit hyper-critical about something designed by one of your peers. I like to think I can still hear the distortion that matters, and I don't really care about the number of zeroes behind the decimal, as long as I don't hear any grit. But I don't have the professional perspective that you have...and professional pride in the details is as much responsible for progress, in audio products, as gross and subtle improvements in the perceived sound. So, yes, Mr. Candy should clean this up, even though I don't really care about this particular product. Cheers, Clifton

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Re: The Halcro dm88 appears to miss its distortion spec


Quote:

I am beginning to believe that what is being shown in the IM graphs is the residual of the twin-tone source.

I would believe this. I have always encountered problems when tried to generate ultra-low distortion differential tone test signal. The best results were obtained by summation of 2 single frequency sine signals on resistor network.

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A question to John Atkinson

Is there any special reason that you usually use 50Hz sinus for THD spectral measurements? I would guess that 1kHz or even higher fundamental frequency (up to 10kHz) would show more of the spectral behaviour of the amplifier, especially the one as good as Halcro.

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Re: A question to John Atkinson


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Is there any special reason that you usually use 50Hz sinus for THD spectral measurements? I would guess that 1kHz or even higher fundamental frequency (up to 10kHz) would show more of the spectral behaviour of the amplifier, especially the one as good as Halcro.

I do perform FFT analysis of higher-frequency tones, but don't publish them unless they are different from the LF tests. I use 50Hz as it is close to the 60Hz AC supply frequency and at high powers will reveal power supply deficiencies, an idea first proposed by Martin Colloms in the early 1980s.

John Atkinson
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Re: A question to John Atkinson


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I use 50Hz as it is close to the 60Hz AC supply frequency and at high powers will reveal power supply deficiencies, an idea first proposed by Martin Colloms in the early 1980s.

John Atkinson
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It may probably mask the power supply residual spectral lines due to finite PSRR (multiples of 50Hz or 60Hz), which can be clearly seen in case that we use 1kHz tone.

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Re: A question to John Atkinson


Quote:

Quote:

I use 50Hz as it is close to the 60Hz AC supply frequency and at high powers will reveal power supply deficiencies, an idea first proposed by Martin Colloms in the early 1980s.

It may probably mask the power supply residual spectral lines due to finite PSRR (multiples of 50Hz or 60Hz), which can be clearly seen in case that we use 1kHz tone.

Maybe, but you can can clearly see spectral lines developing at 70Hz and 190Hz with some amplifiers (120Hz

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Re: The Halcro dm88 appears to miss its distortion spec

I have not had this problem, but one does have to be careful with layout and which op amps to use. In my case, I use two Wein bridge oscillators, one at 19 kHz and one at 20 kHz, each employing a single 5534 op amp and carefully-designed FET-based oscillator agc. These oscillators are very good (about 0.01% THD), but not astounding (they don't need to be for generating the component frequencies of a twin-tone IM test). Believe it or not, although no longer regarded as sonically superior, the old 5534 op amp produces vanishingly low THD and IM distortion. With respect to the two oscillators, it is important that they not be in tight proximity physically or electrically. This prevents them from each influencing the other in such a way that would produce IM products. As you know, for example, oscillators can try to "pull" each other. My two oscillators are in the same box, but on separate boards (each about 2.5 X 2.5 inches) that are separated by about three inches. The boards are mounted only 3/8 inch above the aluminum chassis. The critical summer is mounted on a third small board. It is an inverting "virtual ground" summer implemented with a 5534. That is followed by a level control whose output is then buffered by another unity-gain non-inverting 5534. I cannot see any IM products of the output of this twin-tone IM source on my spectrum analyzer.

By the way, this source includes a third oscillator so that I can generate the three tones of the MIM test I developed back in the '80s (9.00, 10.05, 20.00 kHz). That test is handy because it generates both even and odd-order IM products down around 1 kHz +/- 50 Hz, where one can use a combination of LPF and BPF filters to measure the products without resort to an expensive spectrum analyzer.

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