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Old 11th September 2012, 05:49 PM   #891
bcarso is offline bcarso  United States
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Quote:
Originally Posted by homemodder View Post
What does one call that cascode used in the vas, first time Ive seen such a setup with BJTs. Ive been using composite cascodes for the last 5 years with mosfets for vas when seeking very low THD and it seems to me your cascode does the same function.
Depends on what newspaper you read

Scott refers to the approach as base current recapture, which is a nice neutral description that elucidates the functionality.

My money is on E. E. Aldridge, who showed it around 1962 with bipolars, in amplifiers for telcom etc.

Hawksford published unaware of prior art and the configuration has most frequently been associated with him.

By the way, the closely-related Boxall circuit from 1957, and which became known much later as the Baxandall-Swallow superpair, Walt Jung just noticed has propagated under a misspelling of the second author! It should be known as the Baxandall-Shallow superpair!
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Old 11th September 2012, 06:43 PM   #892
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Default further results for the post 135 schematic

Again with perfect buffer, 20kHz 7V rms out, closed loop gain of 4 (1k Rf, 333 ohm divider), harmonics to 5th: 3.6ppm, slight edge to 2nd harmonic. This is when driven from a zero impedance source.

Driven from 1kohm, the distortion at the input is 9.5ppm, at the output 13ppm. Undoubtedly, bootstrapped cascoding of the input devices would reduce this, while cutting into the common-mode range.

With the ideal output buffer, there is a latchup problem on turnon, but this is probably due to the unlimited output swing of the buffer. A real buffer run from the same rails (in my case +/- 15V) I doubt would have the issue.
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Old 11th September 2012, 06:58 PM   #893
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Quote:
Originally Posted by scott wurcer View Post
Dick, this discussion of non-switching class B might be of interest.



margan : non switching class B
That is very helpful. Especially re. thermals.
I'm having my friend Demian Martin out here do accurate backup base line measurements for me before any possible changes/tweeking. Do you have any other data from SIM? Thx. Dick

Last edited by RNMarsh; 11th September 2012 at 07:10 PM.
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Old 11th September 2012, 07:40 PM   #894
bcarso is offline bcarso  United States
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Quote:
Originally Posted by bcarso View Post
Again with perfect buffer, 20kHz 7V rms out, closed loop gain of 4 (1k Rf, 333 ohm divider), harmonics to 5th: 3.6ppm, slight edge to 2nd harmonic. This is when driven from a zero impedance source.

Driven from 1kohm, the distortion at the input is 9.5ppm, at the output 13ppm. Undoubtedly, bootstrapped cascoding of the input devices would reduce this, while cutting into the common-mode range.

With the ideal output buffer, there is a latchup problem on turnon, but this is probably due to the unlimited output swing of the buffer. A real buffer run from the same rails (in my case +/- 15V) I doubt would have the issue.
PS: just noticed (my printout had cut it off) that SW's rails are +/-30 V, not 15. So my results should be re-run for these values.

I did verify the absence of latchup with a real output buffer, and the enhancements with cascoding of the input devices. More at 11.
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Old 12th September 2012, 02:02 AM   #895
bcarso is offline bcarso  United States
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Originally Posted by bcarso View Post
PS: just noticed (my printout had cut it off) that SW's rails are +/-30 V, not 15. So my results should be re-run for these values.

I did verify the absence of latchup with a real output buffer, and the enhancements with cascoding of the input devices. More at 11.
Actually upon reflection, cascoding of the input devices is advised with 30V rails, as the 862 is rated for a maximum of 20V. A process 51 part sorted for ~50V breakdown would work fairly well, although not something one could count on for production. The NXP version of the J111 is rated for 40V.

Another alternative would be one of the Supertex deplection mode DMOS parts, the smallest of which is the DN1509, which has a specified 90V breakdown.
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Old 12th September 2012, 05:34 PM   #896
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Quote:
Originally Posted by bcarso View Post
Depends on what newspaper you read

Scott refers to the approach as base current recapture, which is a nice neutral description that elucidates the functionality.

My money is on E. E. Aldridge, who showed it around 1962 with bipolars, in amplifiers for telcom etc.

Hawksford published unaware of prior art and the configuration has most frequently been associated with him.

By the way, the closely-related Boxall circuit from 1957, and which became known much later as the Baxandall-Swallow superpair, Walt Jung just noticed has propagated under a misspelling of the second author! It should be known as the Baxandall-Shallow superpair!
I should have looked at that angle at the cascode. Composite cascode is very different.
I didnt use a perfect buffer so can understand the difference experienced in performance. Still boggled though why at 20Khz I got better figures without that cascode. Round 2 simulating this circuit needed.

Long before Hawksford published his paper the japanese companies were producing amps using the so called hawksford cascode, this in late 70s. Tanberg was also using it in their amps around 1982.
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Old 12th September 2012, 06:00 PM   #897
bcarso is offline bcarso  United States
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Originally Posted by homemodder View Post
I should have looked at that angle at the cascode. Composite cascode is very different.
I didnt use a perfect buffer so can understand the difference experienced in performance. Still boggled though why at 20Khz I got better figures without that cascode. Round 2 simulating this circuit needed.

Long before Hawksford published his paper the japanese companies were producing amps using the so called hawksford cascode, this in late 70s. Tanberg was also using it in their amps around 1982.
Is it possible there's a fortuitous cancellation of distortions between the non-cascode stage and the buffer?

Another inventor in and around the various cascodes, whom Walt J. uncovered and with whom he's been communicating, is a modest gentleman named Csanky. We shall hear more about him

Scott is doing at least a couple of things in this circuit that are important for distortion reduction. One of them is the translinear drive to the gm transistors, the other the base current recapture.

Where one ought to see the cascoding improvement in the gm stage is in the appreciable reduction in output capacitance, which in turn reduces its propensity for variation with voltage and hence distortion generation. The buffer following can spoil this in a hurry. I spliced in the diamond buffer I showed a while back in here to get additional results, and it does limit the performance a bit even though its input devices are bootstrapped.

Another of the problems with no cascode in the output gm stage is severe swings of dissipation for a non-cascode part as well as significant Early effects on both beta and Vbe. The thermal stuff is tough to model so tends to become the elephant in the living room for evaluating sweet Spice lies.

The input stage also sees thermal swings with signal, but device matching takes almost all of this out, especially with high loop gain. If, in addition, both the input devices and their current sources are cascoded, this all gets even better. As well the input capacitance drops more and the distortion at the input for a more-than-zero Z source gets very small indeed.

Some sim results using an ideal buffer, for cascoded BF862-J111 pairs at the input: 1k source R, 20kHz to 5th harmonic, 20V p-p out (feedback 333 ohm divider R, feedback R 1k): output distortion 242ppb 20V p-p, 24.7ppb at the input. Again the internal gain-setting R is 100 ohms and the compensation C is still 30pF. Pretty impressive for the component count!
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Old 12th September 2012, 06:58 PM   #898
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Well, I might as well interject something more about HK, just for 'completeness'.
My experience with HK goes back to 35 years ago, when Sidney Harman was not directly with the company, and HK Intl consisted of HK,JBL, and Ortofon, primarily. In 1977, I was hired by HK Intl to be a consultant for both HK, and JBL. I spent about 1 1/4 years there.
They apparently were looking for 'new blood' and had a succession of 'gurus' in which they hoped to help to successfully produce new product ideas.
In previous years (decades) HK had made some noteworthy audio equipment, such as the Citation A preamp (germanium), and huge, high bandwidth, tube power amplifiers.
In 1977, they were making the Citation solid state series of audio products, including quasi-complementary output stages, and discrete preamp designs that showed a lot of potential. Somewhere in their history they became enthralled with 'wide bandwidth' extremely wide bandwidth, in fact, due to some listening tests that they made in the past. (quite a departure from today)
The place was run by middle managers, who seemed to be immune to layoffs, or any specific task. The engineers, techs and production workers were the 'expendable' ones, apparently, and while they were competent engineers, etc, they were NOT dedicated to audio quality in any significant way, and ultimately they moved on, mostly to aerospace.
In any case, they were always looking for a competitive edge, in this case, it might be my contributions, if any, to give them an edge on the competition. They were completely opposite of objective testing, at the time, including even personal persuasion as to what was best to build, instead of what sounded best, measured best, or had the most hope to make into a successful product. In a way, it was 'subjective testing' run wild, without any real conclusions as to what was the best design approach.
The company, the president of HK told me over dinner, one evening, at a restaurant that I could have never afforded, was losing $100,000/mo and with little hope of change, so eating out, etc at the best places seemed a good idea at the time. So much for company spirit. (more later)

Last edited by john curl; 12th September 2012 at 07:02 PM.
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Old 12th September 2012, 07:02 PM   #899
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Quote:
Originally Posted by bcarso View Post
PS: just noticed (my printout had cut it off) that SW's rails are +/-30 V, not 15. So my results should be re-run for these values.

I did verify the absence of latchup with a real output buffer, and the enhancements with cascoding of the input devices. More at 11.
The 30V rails are a holdover from some power amp experiments, 18 max or maybe in some cases 24 would make more sense for line level stuff. The 4401/03 have large capacitances there should be better choices in some places (just have them lying around in case I want to actually build one). In 2009 I posted a crazy ultra symmetrical circuit that had Aol and offset trims (Aol trims to "infinite" and Vos to 0) for which I bought bags of SK170/74 and 4401/4403.
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Old 12th September 2012, 10:32 PM   #900
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Can we then use /-15 and +/-24 volts as rails for simulation and circuit performance reporting then? If someone has a desire to see low voltage performance perhaps +/-8V would be reasonable?

This is not to limit any rail voltage usage or application but to make compairsons easy between designs for conversation sake. These two nicely cover integrated op amps and the pro audio modules so they are typical usage.

Dave

Last edited by fitzfish; 12th September 2012 at 10:36 PM.
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