john curl said:
Not so fast, John,
I measured PIM thoroughly 30 years ago, and did not have trouble doing it. This is nothing new (as you are so fond of saying). It just required a competent, detailed design of a custom piece of test equipment. It is fully described in the AES paper I wrote that is available on my web site at www.cordellaudio.com. You should give it a read. It's been done before, with great sensitivity.
Also take a look at my JAES MOSFET EC amp paper, where its PIM level is listed. It is extraordinarily low, in spite of a good measure of NFB (40 dB at 20 kHz, 80dB at 200 Hz).
I'll be interested to wait and see, both how the measurement is carried out, and how he measures PIM in amplifiers with and without high NFB or wide open-loop bandwidth. The instrument I designed and built to measure PIM in accordance with Otala's definition employs synchronous phase demodulation.
The nice thing about this sort of issue is that it does not rely on subjective listening tests to prove the point one way or the other. PIM is readily measurable with the right instruments, and if Otala or you assert that PIM is increased by large NFB or low open-loop bandwidth, the assertion can be checked readily by a fair apples-apples comparison of a common amplifier design with large and small amounts of negative feedback (as I did in my AES paper).
Cheers,
Bob
janneman said:
Thanks Jan!
Yesterday was indeed my birthday: after resistors burst in flame I've designed nice steroids for tube output stages. I can't reveal details because it is a gold mine and I am going to patent it, sorry. Even though I feel an itching to share what I made.
Speaking of PIM, they again are less audible when single ended amp produce them (think of Doppler!), and sound worse when very non-linear capacitances are allowed to impact on "what we don't measure". Designing my gear I was always aware of such things, though it is not my credo to write articles and compete for inventions. Who understands can see that on my schematics, who don't anyway would not value properly.
janneman said:
Seems first mention was 1970 in IEEE Transactions:
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1162117
Not saying I agree with his claims.
maxlorenz said:Happy birthday Wavebourn! (delayed)
Gemini, Polish: noted.
Thank you!
Actually, there will be one more, October, 30. This time I got a present from Jan Didden, and invented something that took my breath out. Something to make vacuum tubes 50 times more powerful, more linear, with smooth current limit and temperature stable, very simple!
john curl said:Good, as long is it is not negative feedback!
And a negative feedbacks as well, 3 of them, very sneaky. But they are very local, though, no need for any frequency compensation. You may count one more inside of the tube, if to think about the tube itself. Also, a feed-through approach that I always prefer. 3 MOSFETs total per tube, minimum.
