Rodolfo and Lumanauw, I think you are understanding what we are after, and why we design many circuits the way we do. Most of us use global negative feedback. Although sometimes, we experiment with open loop circuits.
Negative feedback may be necessary and even an improvement to a successful circuit, BUT it is not always so. IF we can reduce or remove global negative feedback, and still maintain fairly low distortion, we usually find that the open circuit sounds better. WHY? That is the question that we have been addressing for decades. Is is phase modulation? Is it harmonic order multiplication? Is it something else, like Hiraga distortion? Maybe. I don't know for sure, but I haven't given up.
To think that an IC like a 5532 or 5534 is all that anyone really needs, just isn't realistic.
Negative feedback may be necessary and even an improvement to a successful circuit, BUT it is not always so. IF we can reduce or remove global negative feedback, and still maintain fairly low distortion, we usually find that the open circuit sounds better. WHY? That is the question that we have been addressing for decades. Is is phase modulation? Is it harmonic order multiplication? Is it something else, like Hiraga distortion? Maybe. I don't know for sure, but I haven't given up.
To think that an IC like a 5532 or 5534 is all that anyone really needs, just isn't realistic.
Mr. Curl, I am quite surprised here. I really tought you have FOUND the answer to that kind of question.
You have studied the PIM family for decades, having lots-lots of measurement equipment, lots of reference, lots of quality discussions, but yet saying "I haven't given up"?
So how can a DIYer answer to "how to make a good sounding amp?"
The opamps that are considered better here uses folded cascode, or Current Feedback. Who invented folded cascode? While I'm searching here, I found out that one of the Guru's here was thinking to sue IC manufacturer for using Folded cascode for Opamps.
Last night I just re-read the DougSelf book. He recognize the OLBandwith controversy, between High OLBandwith VS Low OLBandwith. He even give example of an amp having OLB 20hz and OLB 20khz. He knew that people are talking about it, but he said that this matter "has no engineering justification".
Interesting he wrote that, because he writes his book based on meters.
lumanauw said:
Hi lumanauw,
Have you seen this article about op-amps? http://www.ee.bgu.ac.il/~angcirc/SomeStuff/History18opamp.pdf. In there on page 22 is the Harris HA-2539, which is a complementary folded cascode op-amp from 1982. I don't know who invented the folded cascode, but I have a book from 1972, "Applied Electronics" by Pierce and Paulus that mentions the folded cascode and has it in the index. So I bet it's a pretty old idea.
Re: beliefs===>reasoning
That's "simple", follow all known rules/theory, then adjust/try
different configurations... means experimenting.
Yes, you have an idea/believe, then you try to prove that it's correct
until someone else prove that it's wrong...
It's like in physics, the big problem starts if you can prove that
something is not correct and at the same time not incorrect.
Mike
lumanauw said:
So how can a DIYer answer to "how to make a good sounding amp?"
That's "simple", follow all known rules/theory, then adjust/try
different configurations... means experimenting.
Roushon said:
Yes, you have an idea/believe, then you try to prove that it's correct
until someone else prove that it's wrong...
It's like in physics, the big problem starts if you can prove that
something is not correct and at the same time not incorrect.
Mike
Quote #1:
Interesting, but certainly not a general purpose device. The linearity improvement could be great for audio, were it to be applied with a higher voltage process and perhaps less BW (so that it won't oscillate at the drop of a hat). Still, the combination of the linearized front end and the OL BW >20kHz should really offer superior results on the PIM issue.
Quote #2:
Still true, IMHO. But definitely it has been used in specialty parts, a long time ago. And, it is truly amazing how old some of the really fine treatment of these issues are! See for example, the original RCA app note on the CA3080 and CA3094 (late 1969 parts). http://www.intersil.com/data/an/an6077.pdf
They show an A/B comparison of the uncompensated/compensated xfer curves in Figs. 8 and 9 that should make believers out of skeptics.
Walt Jung
Interesting, but certainly not a general purpose device. The linearity improvement could be great for audio, were it to be applied with a higher voltage process and perhaps less BW (so that it won't oscillate at the drop of a hat). Still, the combination of the linearized front end and the OL BW >20kHz should really offer superior results on the PIM issue.
Quote #2:
Still true, IMHO. But definitely it has been used in specialty parts, a long time ago. And, it is truly amazing how old some of the really fine treatment of these issues are! See for example, the original RCA app note on the CA3080 and CA3094 (late 1969 parts). http://www.intersil.com/data/an/an6077.pdf
They show an A/B comparison of the uncompensated/compensated xfer curves in Figs. 8 and 9 that should make believers out of skeptics.
Walt Jung
Hey Walt,
I was a believer in this all along (since I laid my hands on those LM13700).
Interesting app note though; I saved it in my electronic archive. Shows again that those that don't know history are doomed to repeat it.... Or invent something better
Do you have any practical experience with that AD830 (or its successor, I believe the AD8030) [Edit: AD8129, AD8130]
Jan Didden
I was a believer in this all along (since I laid my hands on those LM13700).
Interesting app note though; I saved it in my electronic archive. Shows again that those that don't know history are doomed to repeat it.... Or invent something better
Do you have any practical experience with that AD830 (or its successor, I believe the AD8030) [Edit: AD8129, AD8130]
Jan Didden
PMA said:
Pavel,
What kind of link are you referring to? pre-amp out to power amp in?
For a 1V signal out of the preamp (which drives most power amps to deafening levels) into 10k input resistance of the power amp via a 1000pF cable capacitance gives a signal current of less than 200uA at 20kHz.
How is that stressing the opamp output/thermal/capdrive??
Jan Didden
Jan,
I do not know the exact reason, but the sound improvement of the buffered link output (preamp to poweramp) is dramatic. Just a result of more than 2 years lasting tests with many listeners. I do not feel like trying to find exact theoretical explanation, it simply works. See "Audio Buffer" on my web: http://www.pha.inecnet.cz/macura/buffer_en.html
The Audio Buffer is a link transfer via 50 Ohm + cable + 50 Ohm termination. The second way that works is buffer + low series resistance (5 Ohm e.g.) + cable + standard input (10k). Increasing series resistance at the preout leads to typical cheap "opamp" sound (unclear, grainy). Removing the buffer and using only opamp has the same destroying effect. It is not the case of THD distortion, you can visit my web and see the measurements (spectrum analysis): http://web.telecom.cz/macura/opamp_mer_en.html
Pavel
I do not know the exact reason, but the sound improvement of the buffered link output (preamp to poweramp) is dramatic. Just a result of more than 2 years lasting tests with many listeners. I do not feel like trying to find exact theoretical explanation, it simply works. See "Audio Buffer" on my web: http://www.pha.inecnet.cz/macura/buffer_en.html
The Audio Buffer is a link transfer via 50 Ohm + cable + 50 Ohm termination. The second way that works is buffer + low series resistance (5 Ohm e.g.) + cable + standard input (10k). Increasing series resistance at the preout leads to typical cheap "opamp" sound (unclear, grainy). Removing the buffer and using only opamp has the same destroying effect. It is not the case of THD distortion, you can visit my web and see the measurements (spectrum analysis): http://web.telecom.cz/macura/opamp_mer_en.html
Pavel
With the AD830, I have some experience going back about 10 years now. The concept is indeed elegant, and I, like you, don't completely understand why it never took root. I can only say that the AD830 needed some TLC to make it behave, at least in the circuits I tried (video).
But, that said, the concept is still very intriguing. I'd love to see it realized with a quad of FETs in the front end (2 FET diff pairs). No reason at all why this could not be done.
Walt Jung
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