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Old 4th September 2006, 03:15 PM   #1
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Default irf640/9640 in amplifiers

can i use irf640/9640 in this amps with power supply of +/- 56v???

http://www.audiofanatic.it/Schemi/Ti...RF9540_540.jpg
http://www.audiofanatic.it/Schemi/Ti...530_2SJ201.jpg
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Old 4th September 2006, 04:34 PM   #2
ilimzn is offline ilimzn  Croatia
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Simple answer:

1st schematic: NO - other parts in that amp need to be changed for it to work at higher rails, and ITO220 MOSFETs are not robust enough termically for them. Also, in that amp it is very difficult to parallel MOSFETs.

2nd schematic: maybe - still, originals ate in a MT100 package offering 100W dissipation minimum, TO220 just can't do that (IRF specifies the dissipation for HEXFETs at very low temperatures which are practically unobtainable). Also, treshold voltage is higher for IRF parts, but a few resistors changed in the schematic should take care of that.

The better idea would be to use TO3P HEXFETs like IRFP240/2940. Actually, no IRFxxx - 9xxx are really complementary, just look at the numbers in thbe datasheet. I have only been able to find 4 combinations with good complementarity out of all IRF parts they make. You could try IRFP340/IRFP9240 (no, this is not a misprint, you have to use different voltage FETs to get all the other parameters to be complementary).
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Old 4th September 2006, 05:28 PM   #3
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OK....
in the 1st schematic the use of higher rail is not a chance....

but in the second, if the problem is only of didssipation i think that is possible because i have 4 pairs of irf... can i parallel them?? what value i should change for use this parts?
i want to use this amp as subwoofer amp, then, distortion at high frequency and small quality issues dont'make the big difference...

thanks...
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Old 4th September 2006, 06:14 PM   #4
ilimzn is offline ilimzn  Croatia
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In theory, Since A. Holton's design accepts IRFP240/9240, it should work without modification with the IRF640/9640 as they are very similar (except for the case and power dissipation limit).
I would still modify it a bit for use with IRF640/9640:
- Lower R37 to 1.5k
- Increase R24 to R31 to 0.33 ohms
- Increase C1 and C8 to 470uF (this may be useful for any type of output device for that design).
Since you are going to use it at lower rail voltage than the original, you may be fine with IRF640/9640 assuming you use a really big heatsink and mount the transistors on it properly.
In that schematic 4 pairs are already in parallel so that answers your question. You have not told us what load impedance this amp will be driving...
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Old 4th September 2006, 07:07 PM   #5
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thanks a lot for the suggestions... the amp will generally drive a 8ohm load... but is possible that i will'use it on 2 paralleled 8ohm woofers
tomorrow i will start to make pcb and buyng components...
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Old 4th September 2006, 10:00 PM   #6
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In Holton's you should decrease R34 or use a diode out of heatsink whereas keeping Q10 on heatsink.
regards
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Old 5th September 2006, 02:43 AM   #7
Leolabs is offline Leolabs  Malaysia
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Just keep an eye on thermal tracking stuff,esp dealing with HEXFETs.
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Old 5th September 2006, 05:38 PM   #8
borys is offline borys  Poland
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I have already build both of this amps. In mr Holton's design I have used irfp240/9240 (just like in project), but in mr Gibsters amp I have used irfp240/9140 and I have made some changes:
-C11/12 - 4x 2200uF (have been 10 000uF)
-100nF parallel with C11/C12 (on the bottom side)
-100nF parallel with C8/C12 (on the bottom side)
And amp has no oscillations.
You can also:
-increase R29/26 to 56-100R
-incresae R25/28 to 22-33R
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Old 25th March 2011, 12:39 PM   #9
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400w fet amp circute
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