John Curl's Blowtorch preamplifier part II

[john curl]

I am talking 'HISTORY' here, I have not gone any farther than 1980. Let's talk about the time and place, NOT your 'hindsight' today. We have worked on plenty of other stuff, since 1980, but not relating to TIM. What we ALSO found as important in audio design is beyond discussion at this point, on this thread.

[john curl]

Now, we are at 1980 +/-. TIM has been 'put to bed' so to speak, Japanese jfets are becoming popular, and I am working for myself, with a minimum of lab equipment, so no more direct research from me, for awhile. In this time period, I designed 3 master tape recorders, 2 eight channel mixers, and 2 moving coil pre-preamps, two power amps, and started on the Dennisen JC-80 all fet preamp with phono and added switching. Ultimately, this preamp made 'preamp of the year' in Japan.
Here, we had the ability to make the best preamp ever, so we put a lot into it. It was dual mono, used P&G linear volume controls (just like the pros), hermetic relay switching, an open loop phono input stage, direct coupling, servos, ground planes, etc. We made a few mistakes too, and hindsight (like what Scott likes to use) would be applicable here. Still, it competed with the best of them for about 10 years.
Vendetta was not started as yet, and it basically obsoleted the JC-80 in subsequent years. (more later)

[john curl]

Now what did we learn TIM that can be used today?
First, 5V/us for preamps and 50V/us for a 100W power amp was found to be a reasonable guideline.
Second, there was lots of ultrasonic garbage, mainly from MC phono mistracking, but other sources were possible, and this had to be passed cleanly, and this is what raised the TIM guidelines.
Third, that was NOT all there was to making amp and preamp nirvana. We were missing some factors: PIM, high peak current, very low level higher order distortion? All three, probably.
For example, I made a power amp with over 1000V/us, 30 amps peak output, and 64W class A operation and 240W class AB operation, YET it failed to satisfy the client over a long period of time. What was wrong? What could I have changed? People here would have thought my effort, exotic, and over the top, BUT it still was not completely successful. (more later)

[john curl]

In the 1980's much of effort in audio design went to digital. You know, 'Perfect sound forever', and not much improvement was sought in analog design.
Still, we had some sidelights, one of which was CAPACITOR 'error' both linear and non-linear, and self resonance. This led to direct coupling of almost all designs, sometimes using servos, and sometimes using close matching to achieve this. In any case, we found that coupling capacitors, mostly, were a problem. A great deal of controversy was started between Doug Self and me over this, that went on for years. Ultimately, Walt Jung and I published a practical measurement of linear distortion in caps, actually being partially helped at the time by Scott Wurcer and Bob Pease, due to their own research. By the end of the decade, it was pretty much settled, and polypropylene, polystyrene or Teflon caps were chosen where they were necessary. This was true with the JC-80 and any following preamps I have ever designed, ever since. (more later)

[john curl]

Where were we? Oh yes, capacitors. Even today many engineers use problematic caps with impunity, but they can be measured in several different ways, and found to be almost laughable in some cases.
Going back to the early '70's, and before, almost all audio designs used lots of coupling caps. It didn't matter: Ampex, Studer, Revox, Marantz, Dyna, etc all used electrolytic coupling caps. First, to support a single voltage power supply, and later, to avoid servos. Often, Solid Tantalum caps were used then, as they were considered available, cheap, and small. Sometimes 2.2 uF ceramic caps were used, as they were bipolar, small, often rather attractive looking. However, they were relatively expensive at the time. While certain aspects of capacitor limitations were deemed important in some applications, such as sample and hold circuitry, it was generally acceptable to use just about any cap that did not show an overt problem when measuring SMPTE IM distortion of the entire assembly. The lessons previously learned by Analog Computer Engineering were soon forgotten, and cheap and small became an overwhelming choice. (more later)

[PMA]

Repetition is the mother of wisdom.

[john curl]

Repetition is necessary when professionals and amateurs alike appear to have little or no regard as to how much sonic damage they are doing by using inappropriate capacitors in audio projects.
In all fairness, I was 'clueless' for the most part, 40 years ago, yet my design topologies were very advanced.
I used ceramics, Mylar, mica, tantalum, aluminum caps, for the most part. YET, sometimes they seemed to be part of the problem, but HOW? That was the question. It was NOT on the data sheet.
In 1974, the breakthrough came for me, when Tektronix showed me the 'transfer function' of a typical ceramic cap. It was amazingly lousy. Kind of like the 'transfer function' of a 741, open loop! Once I found out how to measure caps the way Tek did, I got Mark Levinson to remove all ceramic caps where we were using them in the JC-2.
I went on a campaign against these caps and pushed for mica or mylar, instead. It wasn't until 1977 or so, working with the Symmetry Xover, that Noel Lee insisted that he could hear the Mylar coupling caps in the design. I didn't believe him, at first, but he was right. What was going on? This is when Dick Marsh started talking about dielectric absorption in audio caps, and this seemed to fit. The cap article in 'Audio' by Jung and Marsh settled it, no more coupling caps, if possible.
Independently, in 1978, I published low frequency distortion in Tantalum caps when used as feedback caps in preamps and power amps. This also pointed to using servos, but this still was relatively impractical. (more later)

[john curl]

What else did we look into? Well, resistors, connectors, circuit board material, layout, shielding, grounding, etc. That's what!
We attacked each and every of these topics, trying EMI line filters for example, trying different brands of resistors, using better than FR4 circuit board material, HEAVY gold on gold connectors, advanced circuit layout, heavy aluminum boxes, etc.
When it came to commercial EMI line filters, we found that they were as problematic as the problem they were designed to solve. Ceramics again? I'm not sure.
Resistors: Several good 1% resistors were found, Corning Grey, Resista, Dale (in some cases), Vishay bulk metal. I settled on Resista.
As far as RCA connectors go, the Tiffany RCA connectors could not be beat. They were thick gold plated, and tight fitting, beautifully made. You could have a tight, secure connection and not worry about 'breaking' the RCA connector away from its mounting.
Later, they made an even more exotic RCA connector with thick gold plating directly over OHFC copper, with NO nickel interface. I shudder to think what they cost today, even OEM.
We also got into the physics of solder, it's optimum configuration and how to use it. We started using special solder 'servers' that kept human fingers off the solder, when making a connection.
Etc, etc. (more later)

[Bonsai]

Originally Posted by Bob Cordell

Hi John,

Just writng an article in Audio is certainly not a "takeover" of the subject, nor was it intended to be. It is silly of you to imply that five guys would feel sole ownership of the subject, and you should only speak for yourself in this regard. I have both the original from you and the rebuttal from me, and maybe I'll get around to PDFing it and putting it up here so people can see what is there without your distortions. As with many other things where you choose to re-write history, your characterization of Gene Pitts' actions and views in that matter are quite distorted.

Cheers,
Bob

If people feel the need to 'own' a subject, then DONT PUBLISH. Its as simple as that. Your work(s) then remain concealed from the public domain, and people that don't have that knowledge have to battle through until they develop it or discover it.

One of the reasons papers or articles are written in science and engineering is so that they are open to critique, interpretation and further discussion . . . thats how knowledge is shared and expanded.

Ottala (spelling?) et al published his work in the 70's, Cordell had a different interpretation and published his work in the early 80's. There is a difference of technical opinion. No amount of discussion about the historical chronology will change that!

[john curl]

Wow, and I have been friends with Gene Pitts for about 40 years! I apologize to him, if I am in error, I just tell it as I see it, or was told by others AT THE TIME. Tell you what, Bob, let's get together, sometime with Gene and find out what REALLY happened.