alternative mosfets for Zen amp

hi everyone,
I want to build the zen amp, but in my area (north of Italy) it is quite difficult to find IRFP140, IRFP9240 and IRFP9140. (in fact, I found irfp9140, but they are too expensive 10 U$ each or so...)
I'd like to try IRF9540 + IRF540; they are really cheap and I can get them at my local store. I just need some suggestions, maybe other people tryied them out.

Do they sound well? Are there any electrical "troubles" or things I should know about these MOSFETs?
Looking at the datasheets, I think I probably can put them into the circuit and they'll work just fine without changing other components.
Please send me your comments and suggestions !

P.S.: I've been building every kind of loudspeakers for many years but just one (bad) MOS amplifier !

Mad K

2001-09-26 2:18 am
irf 540 _ ZEN


I've used the irf540 in my first version of my SOZ, and it worked quite well. I guess it would work in the zen also. BUT, the irfp 140 is by far better sounding in my opinion. If I were in your position, I would toss out the extra $, knowing that it would work, and that it sounds better. It will be cheaper/easier to buy the best ones first, than to substitute later. Remember, the type of mosfet you use in this designs play a significant role in the performance/sound quality of the amp. I don't think this is the place to save money!

good luck!

ps! the to-247 casing on the irfp140 is by far superior to the to-220 casing on the irf540...
Hi Mad K,

Can you please explain why the TO-247 casing is better than the TO-220 casing?

Also I am looking at parts to built a Zen amp. The IRFP140 FET is too expensive (AUS$19) compared to the IRFP140N (AUS$5.40) . On the Fairchild data sheet for the IRFP140, this product is not recommended, possible substitute IRFP140N.

Have you or any one out there used the 140N instead of 140. What is the main difference between the two?


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
The choice of device for the P channel current
source does not have much effect on the sound,
but the best choice for the N channel device in
my experience is the IRFP040, which is no longer

The 040 is doped a little differently than the 140
or 240 and the lower withstand voltage comes with
higher transconductance, which accounts for the
better performance.

I have a very small stash of them, but most of
the time I use the much higher voltage IRFP240 as I
have lots of them laying around.

Mad K

2001-09-26 2:18 am
irfp 140 /to-247

the to-247 casing is better because it makes a lower Rth (thermal resistance) with the heatsink, pluss you don't have to use a spacer on the mounting bolt (plastic casing). you still have to insulate it from the heatsink with mica+grease/or sil-pad.
read this article, it explaines a little more:

-use the irfp 140N, it's great! I don't actually know what the difference between the irfp 140 / irfp 140N, but I think it is just minor (maybe an improvement? N for new or something? someone else who knows?)

PS! When I ordered irfp150's, I actually got irfp150N instead...


2001-02-04 4:23 am
In general we don't want the 'N' versions for audio.The 'N' stands for new and improved.For switching use.The original 540 was 125W, the 'N' is only 94W for the same Rds and drain current.Improved process allows for less area for the same switching performance.But less area means less Pd.The Logic Level 540 is only 42W.So far the P channel devices have not been 'improved'.
hlvan said:
On the Fairchild data sheet for the IRFP140, this product is not recommended, possible substitute IRFP140N.

I believe that Fairchild are recommending the newer IRFP140N instead of the IRFP140 because it has the same switching performance on a smaller chip and so is cheaper. But this is not helpful to us so it is better to use the older IRFP140.......

Does anyone know if the IRFP140 is a relyable part? should i just get the IRFP240 from IRF and be done with it?


2002-02-07 2:35 pm
IRFP054 IRFP150 and IRFP250 for Zen ?

Hi !
Wich one is the best for a 10 watts Zen used as a guitar amp ?
( i will need a RIAA preamp, i'm tuned 2.5 steps down)

Type Volts Amps Watts Casing Price Euros
IRFP054 MosfetN 60 70 230 to247 3.8
IRFP150 "" 100 41 230 top3 5.3
IRFP240R "" 200 20 150 to247 3.8
IRFP250 "" 200 30 190 top3 3.8
BUZ14 "" 50 39 125 to3 12