RAS-300 Mosfet Amp.

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Hi. Could not find anything around here about this amp. Anybody have an opinion about it? Would it be OK to use mono as a bass guitar amp? I need at least 200W and this one looks easy enough and I have almost all the parts but would hate to buy the rest to find out it does not work.

Please check it out here:


That design is NOT so elegant.
First, it's not Nico Ras's design as he claims, it's a Hitachi AppNote amplifier - badly modified.
Then it won't give 240Wrms/8Ohm - 380Wrms/4Ohm. Nico, what about Ohm's law?!
Then the VAS current is 250uA! I't really not enough!
Then R3 causes decreasing of PSRR. Use CCS here, or a rail RC filtering in the + rail is needed.
A cap may needed in parlel with R8. How does the square-wave response looks now?
BD139/140 has low Vce0. Sorry, this is a foolish error on the circuit again.
D1-D2 makes me crying too. A trimmpot is needed here for bias adjustment. Now your amplifier is a true B, or even C class amplifier. That's crazy.
Anyway an input capacitor is always needed in a DC-coupled amp, and I'd add some protection in the OP stage (zeners on gates).

First and last I recommend NOT to build this amplifier.
And I'd ask the designer to redesign it. Now it's a bad joke, not a proper amp design.

I hope I wasn't offending. Sorry then.
But hey I hate bad amplifier designs!
I wouldn't call it "badly designed". I wonder though why Nico has so little current in the VAS stage? The drive capability for the mosfets will be very low meaning a slow amp.

The amp will also run in class B (possibly) since the D1 and D2 is not enough and certainly not trimmable. The Gate threshold voltage is normally aroung 2 volts. This means that the two diodes must produce 4-5 volts at least compared to 1,4 volts right now. It's worth mentioning that the gate voltage can be down to zero volts so in this case you'll have trouble with too much idle current.

See the original in the attached file.


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Rod Elliott to the rescue: http://sound.westhost.com/project101.htm

Elliott will sell you a PCB. Many members here are happy with their amps from him. It may not have Krell levels of distortion, but it's a proven design. It's also based on the Hitachi apps notes.

BTW - What are you going to do about getting a hi-Z input for the bass? Do you plan to use a standalone DI box, or are you making say, a FET-based buffer?

Hi Justin, thanks for the link. Well I just recently finished an Albert Kreuzer JFET bass pre-amp, the stand-alone one. It's working great and I'm looking to add a power section and turn it into an amp head. I'm also considering the BPA200 using LM3886 chips, couse it looks simple, but have not decided.

Thanks for the replies!

terranigma said:

Hi Andy, your allegations seem very serious. Can you discuss this with Ras via email?

That would be very instructive for us if you discuss here.

Sorry folks, I hope I didn't offended anyone.
Any engineer who is adept in amplifier design should agree that this amplifier has some serious design fault. I doubt it works fine.
Again, I'd never suggest anyone to build these amplifiers.
I'm not against Ras, I don't know him. I wouldn't offend him.
I claiming that his designs aren't good, and I can state why I think that.
Dear Andy,

I am not an expert on electronics, however, as far as I know from our experience with my friends, Nico's mosfet amps sound really very good, when we consider reproducing the original instrument sound.

I am not defending Nico, may be I should, I don't know :)

His designs may be based on Hitachi notes which sounds quite logical to me, but the result is quite good.

We built two prototypes, and impressed from how they sound.

And he is a perfect supporter, all about his designs.

Maybe those points are all emotional, not about the technical design, but as a DIY member, I am quite happy about the results.

However, I'd also like to learn your points about the design issues, which is an opportunity see the thing from another point of view.
Re: amp for albert kreuzer

engel dela pena said:
good day
we have a reverse problem i already have amp module looking for guitar amp,while you have pre amp looking for amp project,i suggest you build quasi's nmos 200 see his page

Hi Engel, I had missed this amp, looks very good!! Exactly what I was looking for, and looks easy enough. I am still pending on loading some pictures of the pre-amp, will do it soon.

you can build it but just know it will NEVER deliver the advertised
power it will give about 130 watts into 8 ohms and about 240
into 4. There is a thermal stability issue with those diodes
which will cause the quiecent current to vary consideribly they need to be replaced with a 470 ohm preset so the quiecent can
be set accurately. The distortion figures are also highly
exaggerated above 5khz the distortion in this circuit rises sharpley reaching almost 2% at 20khz therefore definatley NOT
hi fi good for guitar or subwoofer. Why don't you just build Rod
Elliot's guitar amp it includes a pre -amp and realistic power output figures and i.m.h.o. is easier to build with easily obtained bipolar output stage.
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