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Old 8th August 2006, 07:34 AM   #11
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Quote:
Originally posted by AndrewT
Hi,
is SKA by Ampguru a Sziklai connected FET?

Hi AndrewT,

The SKA is not a CFP or a SIZIKLAI type....Its simply behaves like a Common Source amplifier at its output...

K a n w a r
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Old 11th August 2006, 11:59 PM   #12
ingrast is offline ingrast  Uruguay
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Default Working MOS CFP IRF530/9530

Just came across this thread, think I may contribute a bit.

Attached is schematics of working MOS IRF530/9530 output stage, it may look complex but could not figure out a simpler approach to biasing control *and* gate capacitance issues at crossover. This design ensures both devices are allways conducting.

I had to remove some details since part of this amplifier is under patent registration process. Measured performance is comfortably below 0.001% THD.

Posted below is a photo of an assembled amplifier module.

Rodolfo
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Old 12th August 2006, 12:00 AM   #13
ingrast is offline ingrast  Uruguay
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Default Schematics

Don't figure what went wrong, here is the schematics.
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File Type: gif mos-cfp.gif (16.2 KB, 2626 views)
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Old 12th August 2006, 12:08 AM   #14
ingrast is offline ingrast  Uruguay
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Default Module

Here is one assembled module.

Currently there are 8 amplifiers working inside 4 biamped boxes, though the first 4 were earlier iterations. In all they have probably now over 400 hs. with no problems so far. Sound great for me but this is a biased opinion, others agree too.

Rodolfo
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Old 12th August 2006, 06:10 AM   #15
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Hi, Rodolfo,

Very intersting . Is it "current mirror with gain" like Blomley?
What is the bias of the output transistors, how many mA or A?
The signal input is 1 path, from the junction of R12-R13? How do you set the bias?
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Old 12th August 2006, 12:52 PM   #16
ingrast is offline ingrast  Uruguay
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Lumanaw:

Measured stand by current is about 80 mA. Simulations do not work very well I suspect beacuse of poor device modeling. Analytical solutions I found far too messy even with simplifying assumptions so I pushed them to later stages.

Anyway, it is not difficult to gather conceptual criteria for design. basically bias current is established for example for the upper half, by the ratio of T7 emitter resistor (R23) to T2 source resistor (R31) and by T7 emitter current. The circuit tries to equal voltage drop across both.
T7 emitter current in turn is set by R25 (T6 is normaly cut off, works for overload protection) and by T3 driver standby current.

Input is at the junction of R12 - R13 as you found.

Rodolfo
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Old 12th August 2006, 05:48 PM   #17
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Thanks for the schematic, I will study it.
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Old 12th August 2006, 08:19 PM   #18
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lumanauw:
" Is it "current mirror with gain" like Blomley?"

hello lumanauw,
no, itīs more like Hegglun (EW1995)
Regards
Heinz!
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Old 12th August 2006, 10:43 PM   #19
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by powerbecker
...no, itīs more like Hegglun (EW1995)
Regards
Heinz!

Interesting Heinz, I was not aware of this design. I notice there is no resistor in T3 (T7) emitter, so stand by current is not very well defined except perhaps - must study further - by T2 (T8).

There is no overcurrent protection provision either, but it is acceptable for a concept prototype.

Rodolfo
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Old 13th August 2006, 01:00 AM   #20
djk is offline djk
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There is another even earlier example of your bias scheme in Wireless World, I don't have a scanner hooked up though.

I built an amp like this (without your current limiting) in 1997. You have omitted a couple of caps for clarity I assume.

Rockford Fosgate sold CFP HexFet amps over 20 years ago. Their biggest at that time was 500W/ch. They used an ordinary bias circuit and a 5532 for the front end. Their 100W/ch unit was a huge seller.
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