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Old 20th June 2008, 06:47 PM   #1
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Default LME49830 And Parallel IRFP240 9240

Hi All,

Not sure yet if this sort of question belongs here or in solid state. Or if I should just keep it to myself.

Anyway, I'm wondering if the LME49830 can drive 5 pairs of the IRFP parts? I'm afraid I'm very unexpert in the requirements for driving MosFet amplifiers.

For background I have an old amp with one channel burned up that has +/- 85V rails and was rated at 400W/ch at 4 ohms in the original quasi-complimentary mosfet configuration. I would like to use that as the basis for an as simple as possible power amp capable of the same sort of power output.

Thanks,
Michael
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Old 26th June 2008, 08:57 PM   #2
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I don't know where the LME49830 runs out of steam (as in drive current) but I have driven 2 pairs of IRFP240/9240 just fine. I had 2 boards, one with an additional drive current gain stage and the other using the LME49830 driving the FETs directly. There was no benefit in slew rate or drive ability with the extra stage. But what the limit is for the LME49830, I don't know. You might have to just try it or design your PCB with space for an additional buffer stage or maybe someone with more knowledge than me can help.

-SL
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Old 26th June 2008, 10:11 PM   #3
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The chip has a current limit at about 50mA. It should be up to the job.

5 pairs on +-85V sounds a little bit on the edge though - but that depends on how stiff the supply is. It might be ok as long as you don' t use 2 ohm loads or use it for PA. What was originally used as output devices in the amp and how many?
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Old 28th June 2008, 04:36 PM   #4
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You are going to apply to what purposes this amplifier? Entrance capacity IRPF9240/240 is great enough. Ciss=1300. 1200pf. These transistors are good for Subwoofer. On LME49830 it is possible to hang up no more than two pairs such transistors. If it is necessary for you more it is necessary to apply the pretarget cascade (for microcircuit unloading on a current). But there are transistors with many smaller entrance capacity 2SJ1058/2SK162. They are good for the broadband amplifier. The maximum food, for these transistors no more +-75. 80V. They can be used and for a subwoofer in class B, but it already for the sake of universality of the amplifier. Here my universal scheme on LME49810, but it is quite fair and for LME49830. The scheme contains still pulse protection on a current.
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Old 28th June 2008, 05:00 PM   #5
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Sure, the laterals will have much better slew rate with thier lower capacitance. Voltage is too high for 2SK1058/2SJ162 though. There are a higher voltage version but I don't remember the part number.

But in this case 1200pf per device is not what will be seen - Cgs is mostly bootstrapped away. 500pf is more like it. 2 pairs (4pcs) will do about 20V/uS, 5 pairs -> 8V/uS. This is enough for 60V pk sine wave at 20kHz.

It is a bit on the edge - if you need full output at HF...

I'd use BJT outputs instead with drivers - but not just because of this. Easier to have them share current for example.
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Old 28th June 2008, 06:05 PM   #6
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It is the scheme universal high-quality PROAmp. For broadband and SubAmp applications (class AB-B). These laterals quite confidently hold +-75V, is checked up by experiment. More high-voltage laterals to me are not known. If you know, inform. Very much it would be desirable to apply more high-voltage laterals. It will allow to increase power of the given amplifier, at high sound characteristics.
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Old 29th June 2008, 12:22 AM   #7
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I found them - 2SK2220, 2SK2221 and 2SJ351, 2SJ352. Don't know where you can get them though. There are both 180V and 200V versions.

http://www.datasheetcatalog.org/data...or/mXwuzrz.pdf

Semelab has a 200V version too - BUZ901 & BUZ906 for TO3. BUZ901P & BUZ906P for plastic package.
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Old 29th June 2008, 10:58 PM   #8
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Default Thanks for the education

Hi,

Thanks for the info.

A little more background. I was looking for something simple to build, simple enough that I could, hopefully, try making my own boards or build on perfboard. It is imperative that it be inexpensive. It would be nice if the design was tough enough for PA use, but not required. It would be nice if it was full power full range, but may not be necessary.

The original circuit was rated at maximum average power output of 550W at 2 ohm, 475 W at 4 ohm, and 300W at 8 ohm. FTC continuous power was rated at 400W at 4 ohm, 250W at 8 ohm and 800W at 8 ohm in mono bridged mode. The original outputs were Exicon 10N16 in quasi-complementary mode. There were 10 outputs.

I chose the IRFP parts because they seemed to meet the necessary spec for the power supply and they are available very inexpensively. I would also consider BJT or whatever, if it could be built with similar complexity and cost.

What is the relationship between gate charge and input/output capacitance in MOSFETS?

Thanks,
Michael
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