Words Of Wisdom by John Curl

[Wombat]

FYI only. Too many points to comment on.

http://jockohomo.net/data/johncurl-v.0.1.pdf 156 pages

 

[Blumlein 88]

I've owned a couple devices of his. I'd have no complaint with his work. It flat out performed in exemplary fashion at all times. In all ways.

But right at the beginning he describes hearing over a 45khz capable ribbon a difference in a 5 khz squarewave filtered to 100 khz response and one filtered to 35 khz response. Everyone heard it he said. More than FR they all seemed to hear rate of change he said. Well, okay, but they would have heard the 5 khz fundamental, and very likely the 15 khz 3rd harmonic. The filter change to 35 khz response would have lowered volume of the 5 khz a little, and would have lowered the volume of the 15 khz 3rd harmonic quite a bit. No surprise it sounded obviously different, and it wasn't about rate of change. It was just about a FR which inadvertently changed levels in what could be heard. Oops!

There would have been a .6 db difference in loudness in the two signals and a different level mix between the 3rd harmonic larger than this.

My words of wisdom: HiFi is 85% frequency response, and much of the rest is about level differences.

 

[Blumlein 88]

Okay then the odd even harmonic discussion most of which I've seen in one form or another before. Pretty much moot now. With decent gear none of the distortions rise to a level to matter. Speakers are in a good modern system your only source of distortion unless the recording has some.

 

[mansr]

But right at the beginning he describes hearing over a 45khz capable ribbon a difference in a 5 khz squarewave filtered to 100 khz response and one filtered to 35 khz response. Everyone heard it he said. More than FR they all seemed to hear rate of change he said. Well, okay, but they would have heard the 5 khz fundamental, and very likely the 15 khz 3rd harmonic. The filter change to 35 khz response would have lowered volume of the 5 khz a little, and would have lowered the volume of the 15 khz 3rd harmonic quite a bit. No surprise it sounded obviously different, and it wasn't about rate of change. It was just about a FR which inadvertently changed levels in what could be heard. Oops!

For a test like this, it's pretty silly to start with a perfect square wave and attempt to filter it. Better to synthesise the signal using only the harmonics falling below the desired cut-off. Then there can be no accidents like the one you describe.

 

[FrantzM]

Okay then the odd even harmonic discussion most of which I've seen in one form or another before. Pretty much moot now. With decent gear none of the distortions rise to a level to matter. Speakers are in a good modern system your only source of distortion unless the recording has some.

+1

I would add the room ...

 

[Sergei]

Interesting. Carrier of one of the previous design paradigms I guess: knowing a lot of schematic tricks and empirical rules of thumb is important, ABX tests aren't. "Pretty much moot now" is indeed a good phrase to describe it.

Not discounting his achievements during the last third of 20th century. The rules of thumb and schematic tricks were much more important back then. Today, in the era of precise lithography, laser wafer trimming etc. used for op-amps, many old analog circuitry tricks are no longer relevant.

 

[NTK]

... But right at the beginning he describes hearing over a 45khz capable ribbon a difference in a 5 khz squarewave filtered to 100 khz response and one filtered to 35 khz response. Everyone heard it he said. More than FR they all seemed to hear rate of change he said. ...

If the ribbon tweeter was "only" rated to 45 kHz, the ribbon would likely breakup when energized with signals above 45 kHz. Therefore, the listeners were possibly comparing sound from a normally operating tweeter to that of a broken-up one. With a broken-up ribbon, the tweeter's sound reproduction performance would have likely degraded globally. Anyway, how would the listeners judge which sound was closer to a true 5 kHz square wave? Different doesn't mean better or more correct. I wouldn't conclude anything from Curl's spiel.

A simple verification would be to compare a pure 5 kHz sine tone, to a 5 kHz sine tone with ultrasonic tones added to it. Anything differences to the sound would be "contamination" from the ultrasonic tones and therefore bad.

If you want to be "scientific" about it, use a high frequency bandwidth instrument mic to measure the square waves reproduced by the speakers (with different LP filter cut-offs). Look at the signal traces to see which ones more closely resemble a true square wave. You can also use a laser vibrometer to measure the ribbon movements to see how well the ribbon follows that of an ideal square wave.

 

[pozz]

What a bitter, indulgent text.

 

[JJB70]

What a bitter, indulgent text.

Sadly, this sums it up perfectly. Quite sad to see a fellow who clearly is talented and with a lot to say produce this.

 

[Sergei]

I wouldn't conclude anything from Curl's spiel.

After reading the whole thing, I think I discovered one of Curl's biggest design "secrets": using weakly-matched components (~1%) in the complementary parts of the amps, in combination with low levels of global negative feedback.

Naturally, this leads to plenty of distortions, which some listeners find pleasing on some music material. Contemporary op-amps, with their internal components, such as resistors, matched to 0.0001%, stand no chance of achieving that

I also found very illuminating Curl's recommendations, association with, and defense of, Bybee Quantum Purifiers. In my professional opinion - I worked in two major national High Energy Physics labs - the only purity there is the purity of unadulterated fraud exploiting the placebo effect.

 

[anmpr1]

Curl is at least pretty open and honest about his faith, and his ability. He admits that if you match levels and hide the brands you won't be able to tell any difference between his expensive preamp and a cheap circuit. But he then essentially says, "Trust me. Give me your money and I can guarantee that $5000.00 spent on my expensive design will sound much better." Of course he has no definitive way to test 'why' it sounds better... other than the dollars, it just does. LOL

 

[FrantzM]

After reading the whole thing, I think I discovered one of Curl's biggest design "secrets": using weakly-matched components (~1%) in the complementary parts of the amps, in combination with low levels of global negative feedback.

Naturally, this leads to plenty of distortions, which some listeners find pleasing on some music material. Contemporary op-amps, with their internal components, such as resistors, matched to 0.0001%, stand no chance of achieving that

I also found very illuminating Curl's recommendations, association with, and defense of, Bybee Quantum Purifiers. In my professional opinion - I worked in two major national High Energy Physics labs - the only purity there is the purity of unadulterated fraud exploiting the placebo effect.

You're a gentleman Sergei ...

Let's call the Bybee Quantum Purifier by its real name : " B.S". and anyone defending it, is a manure peddler ...

 

[anmpr1]

I really like his take on silver wire: Silver wire works great if it is pure, and if it is broken in before installation. Um...John. How pure is pure, and how the hell do you 'break in' wire? Yes. This is definitely the guy I want to give five large to so he can build me a preamp. Unbelievable.

 

[anmpr1]

Here's another hooter: The circuit [described] that came from the JC-2 was originally designed for the big 'Wall of Sound' system used by the Grateful Dead. Mark adopted it for the JC-2 because of its success with the Grateful Dead in sounding better than [other] op-amps.

So let's get this straight. John and Mark are at a GD concert, analyzing op-amps blasting through the 26,000 watt PA system? Making quality value judgements right then and there? I mean, does that even make sense to anyone? I can just imagine how this went down: "My god, John, that circuit really sounds so much better than the others. Just listen to the front to back depth coming from Jerry's Fender Twin. I'm tellin' you brother, you gotta put that in my next preamp. And now that that's decided, don't Bogart that thing anymore. Pass it on over to me, eh?"

 

[Blumlein 88]

What a bitter, indulgent text.

Now you know why his final designs were from his company named Vendetta Research. He named it for just those reasons of bitterness born of a sense of betrayal.

 

[Blumlein 88]

If the ribbon tweeter was "only" rated to 45 kHz, the ribbon would likely breakup when energized with signals above 45 kHz. Therefore, the listeners were possibly comparing sound from a normally operating tweeter to that of a broken-up one. With a broken-up ribbon, the tweeter's sound reproduction performance would have likely degraded globally. Anyway, how would the listeners judge which sound was closer to a true 5 kHz square wave? Different doesn't mean better or more correct. I wouldn't conclude anything from Curl's spiel.

A simple verification would be to compare a pure 5 kHz sine tone, to a 5 kHz sine tone with ultrasonic tones added to it. Anything differences to the sound would be "contamination" from the ultrasonic tones and therefore bad.

If you want to be "scientific" about it, use a high frequency bandwidth instrument mic to measure the square waves reproduced by the speakers (with different LP filter cut-offs). Look at the signal traces to see which ones more closely resemble a true square wave. You can also use a laser vibrometer to measure the ribbon movements to see how well the ribbon follows that of an ideal square wave.

I'll cut him a little slack. What you are describing is something any of us could whip up on the sound editor on the computer in a few minutes time. Things weren't that easy back then. People working on designing gear would have had a good quality function generator which meant a square wave was there to be had, and altering the bandwidth was a reasonably easy thing to do. He just drew unfortunate conclusions from it. Putting together various mixes of the 5 khz tone and harmonics could be done, but not at all conveniently. He wasn't doing university level research. So his method is understandable for the time. That makes his Words of Wisdom not age very well however. There were no laser things generally available in those days and high frequency instrument microphones were scarce on the ground as well.

 

[anmpr1]

I'll cut him a little slack. ... He wasn't doing university level research.

But then he whines about not being invited to write for the Journal of the Audio Engineering Society. Says they have 'something' against him. I just don't get that. Shoot from the hip about silver wire and comparing op amps at a Grateful Dead concert, and then angry that JAES has a different agenda?

 

[NTK]

I'll cut him a little slack. ...

You are right. It was a different time.

Having read more of his "... Words of Wisdom" and posts in this thread, I start to feel sorry for the guy. Feeling of being betrayed is tough to handle, regardless of how it happened (whether it's true betrayal or not).

 

[anmpr1]

Here's how I see the JC thing within an historical context. You had the LNP-2 (not a Curl, but a Dick Burwen design) that was a legitimate professional grade, low noise preamplifier. It looked twice as expensive as it was, and it was about the most expensive preamp out there. Maybe it was the most expensive. It immediately gained a reputation, and was pushed in the press, especially some write ups in Audio magazine. I don't recall an actual review of it in Audio, but write ups.

With a dealer network and word of mouth reputation now established, Mark's JC designed 'budget' preamp was cited in the 'underground' press as pretty much the next coming. It was the solid state wunderkind. The first solid state preamp to make silicon haters love silicon. Or that's how it was presented. Particularly important in creating the mystique was Harry Pearson's magazine, but mostly Peter Aczel, who couldn't heap enough praise on whatever Mark was selling, in those early days of 'high end'. [FWIW, I know for a fact that Peter later considered his words to be something of an embarrassment, and he attempted a rectification during his 'resurrection' issues. But by then it had all gone down.] To most high enders, John's reputation as the man with the magic was firmly established. How could anyone take that away?

For whatever reasons, Curl split the scene, Mark changed JC to ML, and the wand was passed to Tom Colangelo. In the fickle press, Tom's creations 'blew away' whatever John had rigged up, hitherto. Could this have sat well with John? It doesn't matter, because such is the fate of all high-enders... to be blown away by the next new and expensive thing to hit a reviewers desk. If you designed it, fine. If not, too bad. You live by the high end, you die by the high end.

 

[pozz]

Now you know why his final designs were from his company named Vendetta Research. He named it for just those reasons of bitterness born of a sense of betrayal.

Really. What a strange piece of history.