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Old 9th October 2003, 09:06 AM   #1
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Default Choice of MOSFET, and SoZ question

Hi everyone, I'm new here. I have lurked for all of about 5 to 6 minutes, so really, I'm very new.

Lately I've taken interest in the DIY projects offered up by Pass Labs, and I think it's a great thing.

It seems that Nelson Pass, Pass Labs, and their customers pretty much keep International Rectifier in business

I was wondering what other brands of MOSFETs people have tried, and if IRF really is the preferred vendor. Fairchild (formerly Harris I think) has some decent looking MOSFETs... SGS has a few too.

Also... the Son of Zen concept rocks because it is about an inefficient as you can get. It is completely impractical, and is a space heater, but I've just got to have one! I'm not kidding... It's off tha chain, fo shizzle... or something like that.

Anyway, what kind of turn-on pop (or perhaps more accurately: turn-on explosion) does that circuit exhibit, especially the OCL version?

I would assume that the balanced/differential light bulb circuit (which is so freakin cool!) is even more violent when turned on. It's probably worse than me in the morning!

Anyone have any experience with that?
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Old 9th October 2003, 03:07 PM   #2
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Default Re: Choice of MOSFET, and SoZ question

Quote:
Originally posted by JFETglare

what kind of turn-on pop (or perhaps more accurately: turn-on explosion) does that circuit exhibit ?
If you mean turn on bump on the speakers , well SOZ has NOT turn on bump!


[QUOTE]Originally posted by JFETglare

I would assume that the balanced/differential light bulb circuit (which is so freakin cool!) is even more violent when turned on.
[/QUOTE

same thing

You can choose pratically what mosfets you like for the SOZ project , certainly some types/brands are noisier and with greater distorsion
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Old 10th October 2003, 03:07 AM   #3
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JFETglare,

just be sure you have a speaker that can live with the SOZ.
Go with IRFP240's, that's what i used.
And get big heat sinks - separate the FETS from the Resistors
on the heatsinks... and have fun. I mention speakers, because
i think it's more practical to keep the SOZ below 10 watts...
unless you like living dangerously
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Old 10th October 2003, 05:11 AM   #4
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Yeah, I agree about limiting a resistor-driven design like that to about 10 watts.

I worked for Cary Audio back in the mid-late 90s, and of course acquired some tube gear during that time. A pair of single ended 300Bs make really nice space heaters... I'm sure a pair of MOSFETs and 8 ohm power resistors make good space heaters as well!

I also thought about incorporating some kind of "stand by" mode that would lower the current across the MOSFET to keep it "electrically warm", but prevent it from cooking the house while I'm not at home.

I'm concerned about the sound quality a little. I always thought the SET-II single ended MOSFET contraption we had at Cary sounded dull, but atleast it wasn't fatiguing. And, you needed oven mits to move the thing around on the burn-in rack. Hahaha The sound might have been the fault of the Hitachi MOSFET, who knows...

I have hopes that this resistor-driven circuit will sound nice. It seems to have quite a following, at least on this forum it does. I'm also impressed that Nelson Pass himself participates.

The balanced nature of the SoZ will have a tendency to cancel some 2nd harmonic, which may not be a good thing... But it will probably help to give it a cleaner more detailed sound--sort of "unclogging" the midrange and presence bands a bit. I think I might like it!
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Old 10th October 2003, 05:54 AM   #5
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Living dangerously is what DIYers do. That's the point.

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Old 10th October 2003, 06:09 AM   #6
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Default Explosion

Amplifier without delayed output relay is crime, minimaly in case of safety and fire hazard.
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Old 10th October 2003, 02:35 PM   #7
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Default Re: Choice of MOSFET, and SoZ question

Quote:
Originally posted by JFETglare

I was wondering what other brands of MOSFETs people have tried, and if IRF really is the preferred vendor. Fairchild (formerly Harris I think) has some decent looking MOSFETs... SGS has a few too.
Do you have any requests?

Right now, I'm building a smallish Aleph amp with the intention of investigating the effects of different output FETs and other circuit parameters.

I'm afraid I'm going to confine my results to strictly measurable parameters like THD, frequency response, power output and stability. If you want to know whether a particular configuration suffers from excessive "glare" or insufficient "sweetness" , I'm not your man...

Cheers
IH
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Old 10th October 2003, 09:19 PM   #8
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I was looking at the Fairchild FQA70N15... The reverse transfer capacitance is pretty low compared to several other MOSFETs, and it looks to have a nice amount of thermal inertia. Junction temp is up to +175C. Dissipation is 330w.

There's another one, the FQA48N20, which has lower reverse transfer capacitance (like 75pF). The thermal inertia graph looks similar, but it has a lower dissipation (280w).

I was going to use the FQA70N15, because it is more "diesel" than the other.
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