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Old 14th March 2006, 08:01 AM   #1
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Default From symmetrical to N-channel only

I have built 2 of a symmetric Mosfet amplifier but now I am left with just some N's. I would like to ask if changing the circuit to a pure N-channel is possible.

I have attached the output stage only and if any of you can help?
This is the original one
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Old 14th March 2006, 08:03 AM   #2
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Sorry cannot attach the file

Here is the original output stage.
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Old 14th March 2006, 08:04 AM   #3
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Here is the modified circuit. I am making a wild guess so laugh if you want to, but just tell me if I am going to kill my amp (or even myself) by doing this. Thanks for watching this comedy.
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Old 14th March 2006, 08:32 AM   #4
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You have not shown if there are drivers for the output MOSFETs in either schematic, but it appears they are probably not present. This is an important point as it will make it easier to convert to n-channel if drivers are present.

Your second schematic is incorrect, particularly the zener diode gate limiter.
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Old 14th March 2006, 12:09 PM   #5
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by bigpanda
Here is the modified circuit. I am making a wild guess so laugh if you want to, but just tell me if I am going to kill my amp (or even myself) by doing this. Thanks for watching this comedy.
In short: as darwn, it will definitely kill the fuse(s). If you short the fuses, YES you are likely going to make a lot of smoke - and kill the speaker attached to the amp for sure.
My point is that you cannot go modifying circuits without minimum basic knowledge of the operation of their parts. For instance: the original output stage is a follower, so gain is 1, i.e. at rst, you have 0V input and 0V output. To turn on a N-ch MOSFET to bias current levels, you need about +3V on it's gate compared to the source. This is done by the Vbe multiplier in the original schematic adding about 3V to the input for the top N-ch MOSFETs, and subtracting about 3V for the bottom, P-ch MOSFETS - for them everything is the same except with oposite polarity.
In your modification, for 0V input, the sources of your bottom MOSFETs are at -V rail (say -40V) and the gates are at 0V less 3V, since that has not changed from the PN version. This makes the gate at +37V WRT source. Disregarding for a moment the fact that maximum gate-source voltage for these parts is 20V and that with 37V it is only a matter of a short time till the gate is destroyed and the whole transistor turned into a crater, if 3V turns the MOSFET on to quiescent current levels, and about 6V turns it on all the way, making it essentially a short, what do you think 37V will do? Answer: for 0V in, your amplifier will produce a short to the -Vcc rail. I'm sure that was not so difficult to figure out? I mean, you would not go figuring out how a guillotine works by sticking your head in it, would you? Then why figure out how a N-ch MOPSFET works by putting it in place of a P-ch with D and S reversed?
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Old 19th March 2006, 05:29 AM   #6
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Quote:
Originally posted by bigpanda
Here is the modified circuit. I am making a wild guess so laugh if you want to, but just tell me if I am going to kill my amp (or even myself) by doing this. Thanks for watching this comedy.
Naw, you'll just fry the MOSFETs. You can't use the same drive circuitry with all NMOS outputs.

You want a quasi-complementary design. There is a long running thread on this very idea, because the designer had a bunch of N channel MOSFETs lying around. Go here:

Power amp under development

Here is the "final schematic" from that one.
http://www.diyaudio.com/forums/attac...amp=1099193939

He even published a board for the design:
http://www.diyaudio.com/forums/attac...amp=1099541934

But it might be cheaper just to buy some complementary devices, because most quasi amps I've seen either use boot strapping or auxiliary voltages to get rail to rail outputs. Good luck.
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Old 19th March 2006, 09:59 AM   #7
quasi is offline quasi  Australia
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Hi folks,

Slight correction. The final schematic is here; http://www.diyaudio.com/forums/showt...021#post601021 post 166.


And the 6 FET layout is here; http://www.diyaudio.com/forums/showt...264#post600264 posts 156 & 157.

Yes this amp was born because I had lots of N-channel FETs.

Cheers
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Old 19th March 2006, 06:13 PM   #8
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Tks guys,

Like I said : I have a bunch of the 240 at hand. Can they be used in Quasi's circuit. I attempt to change the original one to a N channel cause I was trying to make use of the old PCB board so that I don't have to lay the board all over again. Tks again.
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Old 19th March 2006, 08:39 PM   #9
ilimzn is offline ilimzn  Croatia
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Then you might want to have a look at passdiy.com, where you can find the MF12, Nelson's modified HK Citation 12 power amp. It is a quasicomplementary desigh using NPN BJTs originally, and Nelson modifies it to use two N-ch MOSFETs. Have a look at the output stage, it may prove interesting to you.
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Old 20th March 2006, 02:24 AM   #10
quasi is offline quasi  Australia
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Quote:
Originally posted by bigpanda
Tks guys,

Like I said : I have a bunch of the 240 at hand. Can they be used in Quasi's circuit. I attempt to change the original one to a N channel cause I was trying to make use of the old PCB board so that I don't have to lay the board all over again. Tks again.

IRFP240's are fine with the Quasi design.

Cheers
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