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Old 12th June 2004, 03:27 PM   #11
djk is offline djk
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Your circuit can only drive the output to the rails, less the Vgs of the FET.

With a 15V supply for most opamps that will be on the order of 10V peak-to-peak into the load, or about 6W at 8 ohms.

http://patft.uspto.gov/netacgi/nph-P...&RS=PN/4467288

Then click on 'images' (see the FAQ if you need a TIFF viewer)

This circuit only need provide the Vgs voltage to get the full V from the FETs, with 100V on the FETs you can get 500W/8R, 800W/4R, 1200W/2R, only limited by the current of your supply and how many FETs you use.

There are many fine opamps intended to drive 50R co-axial cable that will drive many pairs of FETs.

Only one 15V supply is needed for the opamps, it could also be shared with a preamp and tone circuits.

Each power amplifier output will require its own transformer winding.

Acoustat/Hafler/Fosgate are built this way.

QSC uses BJT with driver transistors and this same scheme.
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Old 12th June 2004, 03:30 PM   #12
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wouldn't it be simplier to use one of those power opamps as a high-voltage driver amp? like lm3875?
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Old 12th June 2004, 03:32 PM   #13
PMA is offline PMA  Europe
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FETs can be supplied from power supply with higher voltage than opamp. Most opamps can work up to +/-18V and great many of them up to +/-22V (OPA604 and others).

For most of IRF's, R5 and R6 must be reduced to 510R. The circuit was designed for 2SK413/2SJ118.
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Old 12th June 2004, 05:33 PM   #14
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Ron (bc),

I seems to me that your T1 is cut off on positive output swings. It takes some time for the opamp to travel back to the point that T1 starts conducting on negative swings. The result would be quite some cross-over distortion.
Some sort of biasing to keep the cross-over region as small as possible would improve this amplifier. For instance, a current source from T1 base to -supply to bias T1 at say .3VDC Vbe

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Old 12th June 2004, 05:56 PM   #15
fab is offline fab  Canada
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hi

The circuit attached on this thread:
http://www.diyaudio.com/forums/showt...308#post359308

uses an op-amp with +/- 15V supplies and complementary output mosfet (IRF540/9540) with any higher voltage desired (gain in the output stage). It can be used in class AB since it has bias adjustment. The resistor to polarise the driver bjt of the output mosfet can be replaced by current sources if required. I do not have THD info but I suppose it can be quite low. A "local" feedback could probably be added in the output stage to linearise it more.

The funny thing about this circuit is that it is one of our projetc in EE course back in 1987. I had a weirdy teacher close to retirement and I realised only many years after that he was very knowledgeable.

Fab
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Old 13th June 2004, 10:58 PM   #16
foo is offline foo  Tibet
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here u go
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Old 15th June 2004, 10:56 AM   #17
palesha is offline palesha  India
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Kindly give more details like supply, IC type, power etc. If u hv made it then let us know about ur experience.
Quote:
PMA said : So how about this one? I can guarantee function and high sound quality
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Old 15th June 2004, 06:21 PM   #18
PMA is offline PMA  Europe
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Quote:
Originally posted by palesha
Kindly give more details like supply, IC type, power etc. If u hv made it then let us know about ur experience.
Dear Palesha,

I have only simulated this amp. I have continued in simulation to display the circuit with component types and supply voltages. It is attached now. The output stage is in class A, every pair of the output devices has quiescent current of about 500mA. This can be changed by R5/R6. M1-M4 will need good heatsink. I did not listen to this amp as I did not build it. But according to my experience I assume this could be quite a good amp. Not the absolute top class, of course. Regarding the results of simulation, for simple circuits like this they correspond with real circuits well.
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Old 16th June 2004, 06:51 AM   #19
ronybc is offline ronybc  India
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Hey all...

Thanks for your responses and suggestions. Actually the non-availability of P-channel mosfets (locally) pushed me to attempt N-channels both side. I supposed that most of the problems including the biasing of T1 will be compensated by the opamp by negetive feedback from the output.

(PMA) your circuit is interesting.

(Fab) I tried disconnecting emitter of T1 from output and grounded it through a 100ohms. BUT the result was huge distortions in the output. I don't know what went wrong.. can you help me...

Thank you,
Rony B Chandran
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Old 16th June 2004, 10:33 AM   #20
amb is offline amb  United States
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Re: getting more voltage swing (and therefore output power) from opamp-driven power amps

See this article about a way to bootstrap the op amp's power rails to increase the voltage swing:

http://www.reed-electronics.com/ednm...=5%2F13%2F1999

Alternatively, you can add an additional VAS stage prior to the output MOSFETs to give the circuit voltage gain after the opamp, and supply the VAS and output MOSFETs with higher rail voltages. An example of this is found in Walt Jung's 3rd Edition of the IC Op Amp Cookbook, in the "Amplifier Techniques" chapter.

-Ti
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