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Old 14th February 2013, 05:32 AM   #2601
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Quote:
Originally Posted by keantoken View Post
I think local error correction and positive feedback are key. Think Baxandall CCS and Tringlotron. Edmond Stuart's TIS implements the former, but does not eliminate the intrinsic logarithmic transfer curve distortion. Although in this case the output stage generates most of the distortion anyways. If we correct transfer curve distortion, then we don't need to make the compromise of degeneration, or bother with "swamping out" nonlinearities. I think the field of BJT analog computing may hold great revelations in this regard. After all they are useful for this purpose because of the reliability with which they conform to their logarithmic transfer curve.
Bear in mind that the cascomp developed by Pat Quinn at Tek deals with this issue in a rather elegant way. The cascomp is covered in my book. I also show how the cascomp principle can be applied to JFETs. I seem to recall that the cascomp has a 3-dB noise penalty associated with it, however.

Pat's mentor at Tek was Barrie Gilbert.

Cheers,
Bob
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Old 16th February 2013, 07:19 PM   #2602
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Originally Posted by Samuel Groner View Post
US patents 4,271,394 and 4,272,728 came to my mind, but there's probably prior art.

Samuel
Hi Samuel,

Thanks for posting these patent numbers. I finally got around to looking them up and checking them out. They are definitely pretty much the same idea, with the common mode signal for control of the two current source loads taken from the tail of the second diff pair instead of from merging resistors off of the emitter follower buffers as I did. Basically the same idea, and with the same advantages.

BTW, one can tell from the topology I used that I was involved in linear IC design, sometimes using more transistors than resistors :-). Small-signal transistors are cheap, and I think if they result in better performance their liberal use in discrete circuit design is just as valuable as in IC design.

I think that concerns that some have expressed about high-impedance nodes can be tempered by the fact that the usual feedback compensation circuits tend to make these nodes have a much lower impedance at high frequencies, where it counts. Of course, those who prefer amplifiers with wide open-loop bandwidth would justifiably disagree.

Cheers,
Bob
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Old 17th February 2013, 12:22 PM   #2603
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Originally Posted by Bob Cordell View Post
...I think that concerns that some have expressed about high-impedance nodes can be tempered by the fact that the usual feedback compensation circuits tend to make these nodes have a much lower impedance at high frequencies, where it counts...
I expressed concern a few posts back, partly because I don't plan to use the usual Miller compensation. For instance Miller Input Compensation would not lower the VAS input node impedance so much I think, but I haven't worked it out yet.
Any comments on how this worked out on your JAES amp?

Best wishes
David
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Old 17th February 2013, 04:52 PM   #2604
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Originally Posted by Dave Zan View Post
I expressed concern a few posts back, partly because I don't plan to use the usual Miller compensation. For instance Miller Input Compensation would not lower the VAS input node impedance so much I think, but I haven't worked it out yet.
Any comments on how this worked out on your JAES amp?

Best wishes
David
Good point, David. It turns out that the compensation loop in Miller Input Compensation itself needs to be compensated. In my MOSFET power amplifier with error correction, this is accomplished by the series R-C network across the differential collectors of the input stage. So this tends the lower the differential impedance at these nodes at high frequencies.

One other thing about using resistors instead of current sources, as Marshall Leach did, is that sometimes the presence of the load resistance makes the input stage work a bit harder at audio frequencies, increasing its distortion a bit.

In some no-NFB amplifiers, the gain and pole frequency of the VAS is established by a shunt resistance to ground from the high-impedance VAS output node. This also makes the VAS work harder and generate more distortion. An alternative to set the gain of the no-NFB to a defined value is to instead incorporate a local, flat feedback loop from the VAS to its input or back to an earlier stage, just like Miller compensation. That relives the VAS of the heavy burden of a load resistor. However, no-NFB purists would objevt to this, as it than constitutes a feedback amplifier where the output stage only is excluded from the NFB.

Cheers,
Bob
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Old 18th February 2013, 01:30 PM   #2605
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Quote:
Originally Posted by Bob Cordell View Post
... It turns out that the compensation loop in Miller Input Compensation itself needs to be compensated. In my MOSFET power amplifier with error correction, this is accomplished by the series R-C network across the differential collectors of the input stage. So this tends the lower the differential impedance at these nodes at high frequencies....

One other thing about using resistors instead of current sources, as Marshall Leach did, is that sometimes the presence of the load resistance makes the input stage work a bit harder at audio frequencies, increasing its distortion a bit.
When I wrote that I wanted to lower the impedance I did not mean to imply the use of resistors rather than current sources, or the use of shunt loads.
That is precisely why I am interested in your differential current mirror to lower the common mode impedance without any sacrifice of differential impedance.
I want to take the rationale of your JAES amp one step further and compensate the MIC loop itself with nested Miller rather than shunt an R-C load across the IPS. This should offer similar benefits to the use of Miller rather than shunt in the overall compensation. But I suspect it lowers the impedance of the amp somewhat differently.
I am surprised that I can't find more theory for this, so thank you for the response.

Best wishes
David
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Old 6th April 2013, 11:38 PM   #2606
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Hi Bob,

I think the section on error correction needs revision to better explain how Hawksford's error correction circuit really works.

I also think the section on TPC and "TMC" lacks qualitative (algebraic) detail.
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Old 7th April 2013, 03:47 PM   #2607
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Originally Posted by michaelkiwanuka View Post
Hi Bob,

I think the section on error correction needs revision to better explain how Hawksford's error correction circuit really works.

I also think the section on TPC and "TMC" lacks qualitative (algebraic) detail.
Hi Mike,

Thanks for these suggestions. I had already planned to try to go deeper on the TPC and TMC matter, and with more results. This is obviously an area where more discussion is needed and where more proof of the pudding is needed. More guidance on choosing values for the networks is also needed. Finally, I expect to include results on an actual amplifier where ordinary Miller compensation, TPC and TMC are used, and compare the results.

I will also try harder to explain HEC. I already thought I did as best as I could explaining HEC, but I'll certainly try to improve it.

Cheers,
Bob
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Old 7th April 2013, 03:56 PM   #2608
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Originally Posted by Bob Cordell View Post
I will also try harder to explain HEC. I already thought I did as best as I could explaining HEC, but I'll certainly try to improve it.

You could include Hawksford's balance equations and their derivation.
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Old 7th April 2013, 06:35 PM   #2609
rsavas is offline rsavas  Canada
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Default DIY Ideas!!

Hi Bob & others,

As we all know, writing something in a book/design spec. and getting it to work in practice, is not always the same. Lots of variables.
I see this in Doug Self's books too, okay I realize that you can buy the stuff from Signal Transfer company. Is there a way to make it more available/affordable to the masses?
How about saying that it is made in North America for a change Too bad lots of my parts are made/assembled in Asia, but that I have little control over and learn to live with.

My suggestion is to actually take one or more ckts as described in the book and make them work. I know Bob has done his own implementations, but they are not generally available to people who just want to buy the pcb(s), parts & solder them up. Most people on this site can figure it out. I know chassis are available. If not I'll do it for you, since I am looking for work
Not sure if this a proper thing to discuss on this site, since they offer there own stuff?

What do you think Bob, I can offer to do a layout or two for you for $0, maybe some things for your web site. Maybe like what you did with VinylTrack, still gotta buy that copy of LinAudioSanta did get me that one.
I have basically done this of late, just have to fab/test:
1) Input AC assy with auxillary power(+5Vunreg/3v3 lin reg, for logic, soft-start relay, remote on/off.
2) AC rectifier assy, big ecaps, fusing.
3) Linear regulators (+/- 65 -75V),+/-15V, +5/3V3, PA protection ckts
4) LME49830 (WireAmp) clone. (Could be convinced to make this as a super gain clone)
I always like to have others review my stuff since I have no trade secrets with this design. Since the AC stuff is very important, this is where I would like to have a second eye.
I am sure that there are many ways to implements this stuff, but the idea is to make it small, not overly complicated/expensive, use easy to get parts, Mouser, Digi-key, Newark. Make it of quality construction
As it is, I have already bought all of the expensive parts for this PA project. As well I have got the radio/media player/pre-amp designed, working/tested, so I am going through with my stuff.
Listening to a Stones LP on the FM (Q107/Toronto) radio, "Going Home", first album "After Math", the record crackle was a welcome change.

Nothing like this PA
Prototype power amplifier
I'd like to see his invoices!

Cheers
Rick

Last edited by rsavas; 7th April 2013 at 06:59 PM.
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Old 11th April 2013, 02:26 AM   #2610
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Default Thermal Trak revisited.

The recent discussions over in the Self thread have made me revisit the Thermal Trak Vbe issue.

Bob posted thisThermal Trak Circuit before the book. It looks attractive but it never seems to have been followed up, I am curious to learn why.

Best wishes
David
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