John Curl's Blowtorch preamplifier part II

[john curl]

Yes, good audio reproduction is a series of 'non-events' that when taken into consideration, build to a 'major event'. The CTC Blowtorch was a 'major event' for a select group of audiophiles, and it is composed of a series of 'non-events' that came together. That is the 'point' of this thread.

[Bonsai]

Originally Posted by john curl

I apologize to Walt for getting him involved in this 'discussion' but I want to also thank him for clueing me into Barrie Gilbert's article's more than a decade ago, and for his own summary of Barrie's work in the cite above.
It does get old, after awhile, and if each and every one of you has found an IC op amp that you find just great for your application, then go for it.
I will continue to 'muddle along' with my discrete designs when I can, and I will also select IC's when it is useful and necessary to do so. I sure would like a really quiet AD825, that would be VERY useful. I wonder why they made it so noisy?

It is quite noisy at 12nV per root Hz. But I think this is a nice device to put after a volume control. No coupling caps. I need to try it.

[john curl]

I guess that it is time to move on. What else could be important? I suspect that EMI proofing could be one of the next most important factors. For example, the CTC Blowtorch goes through: low leakage power transformer, high speed diodes, floating input cap, common mode choke, and ground setting caps, these three making a pi network, a traditional 317,337 regulator stage, all cap bypassed, shunt regulator, and finally a series mos fet based open loop multiplier for EVERY gain block, except for the phono stage which uses jfets instead of mosfets in the final regulator. (And people wonder why it costs so much?)
Overkill? I think not.

[jan.didden]

Hi Walt, nice to see you here!

Originally Posted by WaltJ

[snip]Jan's quote from the sid/tim AES paper needs to be taken in context. I think it is still accurate, insofar as all those measurements way back in the 1970's, i.e., that context was in terms non-linear distortions, NOT phase modulation.
[snip]wj

Noted. I was more trying to highlight your research on the link between OL BW and good audio reproduction. Fully agree with you that such a direct link is non-existing.

Wrt PIM, I believe Pavel's tests are interesting. If we take as a realistic value the 5mV pk-pk linear range for the input LTP, in a 100W amp (80V pk-pk Vout) at up to 20kHz I get a minimum required OL gain at 20kHz of (80/5)*1000 ~ 84dB. That seems excessive to me, even if that linear range can be doubled probably with nfb.

My conclusion would be that the avoidance of PIM depends on available OL gain at a particular freq and not on OL BW. Also, nfb or the absence of it does make no difference, as the OL gain and linearity of the basic amplifier circuit does not change with the application or not of nfb.

Is this correct? That 84dB above still bothers me; any comments on that?

jan didden

[PMA]

Jan, the 'bad' result example is for simple bipolar input LTP.

That means, in case you use more linear input structure, like JFET LTP or multitanh bipolar e.g., you immediately get into much more optimistic mood. Please see the result attached. (Distortion is in %)

[jan.didden]

OK, but still. Even with an 80mV linear input range you'd need 60dB OL gain at 20kHz to avoid gross non-linearity and thus PIM. Is that realistic? Or are we chasing a ghost? If we exceed the linear range by some amount, how much PIM is actually generated, does anyone have any numbers for that? Walt? Bob?

jan

[jcx]

assuming several mV of error V is the same as assuming limited loop gain, high feedback amplifiers can keep diff input V sub mV to beyond 20 KHz

this is a powerful means of linearizing the input stage which for bjt diff pair has predominantly 3rd order nolinearity - distortion to signal ratio reduces as diff input V**2 == 10x higher loop gain gives 100x less distoriton from input tanh gm

[jan.didden]

Originally Posted by jcx

assuming several mV of error V is the same as limiting loop gain, high feedback amplifiers can keep diff input V sub mV to beyond 20 KHz
[snip]

Yes but only if the amp itself has sufficient OL gain at that point, right? Suppose the amp has 30dB OL gain at 20kHz, you can put in all the nfb you want but for full power (80V pk-pk) you do need 80/30dB ~ 2.7V differential input signal at the LTP. I don't see that you can get around that.

So, are you saying that a good power amp has 80dB or more OL gain at 20kHz?

jan

[jcx]

I have built amps with 80 dB and more loop gain High loop Gain Composite Op Amp Circuits
the 1st post shows a 2-pole compensated composit amp with 80 dB gain @ 20 KHz

2-pole comepensation even allows high flat audio loop gain Bob Cordell Interview: Negative Feedback
which shows a direct comparison with huge IMD reduction over the normal single pole, "high bandwidth" compensation advocated here

[PMA]

Originally Posted by jcx

"high bandwidth" compensation advocated here

It is not advocated, I just wanted to show how it works. And as there were concerns about Gm variations with amplitude, I did show it as well. Remember, that I started with analysis of input LTP without feedback and such analysis is valid for ANY compensation scheme.

Regarding your 2-pole compensation, how about transient response. Is there a fast rise followed by slow, long time settling? How about settling time at 1% and 0.1%