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Old 9th December 2005, 10:32 PM   #1
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Default Non (global) feedback amp/hawksford

Has anyone actually buildt an amp based on the hawksford cascode VA-stage? And if so, how did it sound?
(J10 REDUCTION OF TRANSISTOR SLOPE DISTORTION IN LARGE SIGNAL AMPLIFIERS, M.O.J. Hawksford, JAES, vol.36, no.4, pp.213-222, April 1988).

I got curious and had to try it out in the simulator, and the simulator showed exactly the same distortion numbers as measured in the hawksford publication. Well, I couldn`t just sit and stare at the curcuit (wich I didn`t come up with), so I did some modifications. For better or worse is yet to see...:-)

(Everything I come up with it simulations as usual, since I haven`t got any equipment and components...yet -see signature . Please don`t consider me an idiot for that reason, eh)

I changed the input (CB), into a diff stage, recalculated the currents, and got even less distortion (0.006%). I probably could have used another topo for the output, but I just wanted to see if my circuit would work.

The circuit uses 50V rails, and delivers 100W@8 ohm.
Distortion 1kHz@100Wrms is 0.084%.

The circuit is shown with single ended input, but 100% balanced input is possible (of course)
Put additional 560 ohm resistors in the right legs of the diff-stage, with additional VA and OPS, and you will have a balanced output too.

I wonder if this thing would work, and if so, would this sound better than an amp with voltage feedback?
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Old 9th December 2005, 10:50 PM   #2
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The current sources are there but not in the picture, I used symmetrical wilson with 56 ohm degeneration resistors. One of the resistors is "replaced" with a 100 ohm potmeter so one can adjust the offset.
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Old 10th December 2005, 01:31 AM   #3
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Hi,

just want to ask if you are going to use exactely those components stated in your schematic, in that case I would look for more linear output transistors than those outdated MJ transistors, I think so at least as the circuit is non FB.

I would suggest for instance 2SA1943/2SC5200 which are one of the best BJT's around at also unbeatable prices.

Cheers Michael
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Old 10th December 2005, 10:20 AM   #4
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Thank`s for the advice...any good places to buy these...?
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Old 10th December 2005, 03:18 PM   #5
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Hej Panzerlord,

you can buy them at: Porfusion PLC
They are O-grade, meaning highest hfe class for these.

Cheers Michael
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Old 11th December 2005, 05:55 AM   #6
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M.O.J. Hawksford, JAES, has authored some of the finest
papers on amp design, i have run them through extended
simulation and the results i have had are in agreement with
the papers, however i have extended his work with much
better understanding of the interrelational mechanisms of
which they are defined (classified)

regards
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Old 11th December 2005, 12:23 PM   #7
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You`re right, they are very cheap. Thanks again!!!

Quote:
M.O.J. Hawksford, JAES, has authored some of the finest papers on amp design, i have run them through extended simulation and the results i have had are in agreement with the papers, however i have extended his work with much better understanding of the interrelational mechanisms of which they are defined (classified)
I`m currently studying his paper on error correction in output stages, but the stuff is a bit over my head..at least for now.
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