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Old 4th March 2011, 01:55 PM   #1541
Macleod is offline Macleod  France
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Quote:
Originally Posted by Olivier73 View Post
Off-course, anyone is free to comment on the clamps, but on the whole circuit too !!!

For now simulation shows 1.6ppm THD20 at full swing - not loaded (only vas & ips, no ops!).
Quite complex. No?

Is that complexity to have low distorsion at 20kHz? But your figure is without OPS. So...
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Old 4th March 2011, 09:38 PM   #1542
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Default thd

Hi Macload,
I am aware of the relativity of THD figures, especially if it is w/o OPS. Maybe a bit surprisingly, I am not specificaly searching for THD world records, far from it. So why the the complexity? Well because I like the Mirror Image topologies with current mirror loaded LTP's combined with a Cascoded darlington VAS. Off-course there is the VAS Iq issue, and I see it as a challenge to solve it (even if I have not invented the solution for it - my capacities are too limited for that). As a matter of fact I started out with this circuit reading Randy Slone's book. I lost much time with, but I learned a lot of things while burning down too many transistors... So I have a strong will to make it work...

My concern for the moment are thos clamp diodes in the VAS and in the IPS. I have seen so many different options ... is this a good one?

Cheers

Olivier
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Old 5th March 2011, 12:42 PM   #1543
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Originally Posted by dadod View Post
Hi Bob,
Thank you for your comments.
I wanted to keep drain - source valtage constant. Luxman amp did it simple with resistor devider. Roender is using something similar in his one stage amp.
I don't understand what you mean by using emitter follower. How to connect it to feedback signal? If you could put some diagram here, as I did not find, how to best bias a cascode, in your book.
C7 is suggested by Samuel Groner in his comment on D.Self. It enhace slew rate. And indeed in my simulation it increased dow slope from cca 70V/us to cca 90V/us. Comments on Audio Power Amplifier Design Handbook by Douglas Self
R42 is used to adjust the bias. I used your suggestion for ThermalTrak from the tread Biasing/thermal compensation of Thermal Trak transistors
I included a new LTSpice with simpler cascode bias, TTP tail current of 6mA.
Damir
Hi Damir,

I guess the use of C7 is a way of getting the VAS to act in a push-pull sort of way at HF without going to the trouble of a complementary VAS and complementary VAS drive arrangement. It may increase the slew rate, but I just would not go there. Whether it increases slew rate depends on where the slew rate limitation in the amplifier without C7 is occurring.

The emitter follower I was mentioning would have its base connected to the tail of the LTP and its emitter would be used to drive the common mode signal to the bases of the cascode. This buffers the tail from having to be loaded and to have the extra bias current injected.

Cheers,
Bob'
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Old 5th March 2011, 01:52 PM   #1544
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Bob Cordell View Post
Hi Damir,

I guess the use of C7 is a way of getting the VAS to act in a push-pull sort of way at HF without going to the trouble of a complementary VAS and complementary VAS drive arrangement. It may increase the slew rate, but I just would not go there. Whether it increases slew rate depends on where the slew rate limitation in the amplifier without C7 is occurring.

The emitter follower I was mentioning would have its base connected to the tail of the LTP and its emitter would be used to drive the common mode signal to the bases of the cascode. This buffers the tail from having to be loaded and to have the extra bias current injected.

Cheers,
Bob'
Hi Bob,
I hope I understand how to do that. Something like this?
regards Damir
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File Type: jpg cascode-bias.jpg (99.7 KB, 382 views)
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Old 7th March 2011, 11:06 AM   #1545
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Originally Posted by Olivier73 View Post
Hi Bob,

Reading about Bakerclamps in your book I implemented some in the joined circuit. Are these well placed?

For information : OPS is still not implemented nor is the bias circuit.

Off-course, anyone is free to comment on the clamps, but on the whole circuit too !!!

For now simulation shows 1.6ppm THD20 at full swing - not loaded (only vas & ips, no ops!).

Cheers,

Olivier
Hi Olivier,

You've got a lot going on in this circuit. It looks like you are using clamp diodes that effectively feed back from the output of the VAS to the input of the VAS to keep the VAS out of saturation. This is not the usual use of Baker clamps in a power amplifier. The usual approach is much simpler, and was used, for example, in Tom Holman's APT-1. The clamp diodes go from the output of the VAS to a fixed voltage reference, preventing the VAS output from going more than a diode drop beyond the reference. The arrangement assumes a current-limited VAS.

Cheers,
Bob
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Old 7th March 2011, 11:08 AM   #1546
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Originally Posted by dadod View Post
Hi Bob,
I hope I understand how to do that. Something like this?
regards Damir
Hi Damir,

Yes, but you may want to replace R26 with a zener diode. As shown, it looks like there will be attenuation of the common-mode signal before it reaches the bases of the cascodes.

Cheers,
Bob
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Old 7th March 2011, 08:28 PM   #1547
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Bought my copy today at lunch. Nerdbooks.com in Richardson, TX.
Weird place, four large dogs follow you around. Then you have to
buy online from a Mac terminal, won't take cash. But also did not
have to pay shipping...

Also bought a book by Douglas Self right next to Cordell's. Couldn't
find Morgan Jones Valve Amplifiers. But they said they could get it.
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Old 7th March 2011, 09:39 PM   #1548
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Originally Posted by kenpeter View Post
Bought my copy today at lunch. Nerdbooks.com in Richardson, TX.
Weird place, four large dogs follow you around. Then you have to
buy online from a Mac terminal, won't take cash. But also did not
have to pay shipping...

Also bought a book by Douglas Self right next to Cordell's. Couldn't
find Morgan Jones Valve Amplifiers. But they said they could get it.
Hi kenpeter,

Thanks for buying my book and I hope you enjoy it.

I recently bought Valve Amplifiers by Morgan Jones from Amazon and I have given the whole thing a first pass look. I really think it is a good book.

Cheers,
Bob
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Old 8th March 2011, 02:43 AM   #1549
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The two things that drew me to the book were EKV models and KK.
But there is wayy more other good stuff here than I was expecting.

I am trying to understand how to hybrid model the IRFP240/9240?
Gather would have to be a .subckt, [CTRL+RightClick] and change
prefix field from MN or MP to X. Would you be OK with discussing
specific book given models outside the book?
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Old 8th March 2011, 06:47 PM   #1550
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Quote:
Originally Posted by kenpeter View Post
The two things that drew me to the book were EKV models and KK.
But there is wayy more other good stuff here than I was expecting.

I am trying to understand how to hybrid model the IRFP240/9240?
Gather would have to be a .subckt, [CTRL+RightClick] and change
prefix field from MN or MP to X. Would you be OK with discussing
specific book given models outside the book?
Hi kenpeter,

Yes, I would be happy to discuss models, but I have to be careful what I promise because models can be a real time sink.

I'm still trying to finish up the new group of model updates I will be posting on my website. Almost every time I look at modeling I get angry with the manufacturers because their models are so awful. Look at the MJE340/350 spec sheet - they don't even quote ft, much less provide a graph of it.

Cheers,
Bob
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