RAS-300 Mosfet Amp.

[Eugenio70]

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:

http://www.circuit-projects.com/300-...amplifier.html

Thanks!

[jacco vermeulen]

Why not ask Mr Nico Ras himself, he is sharing his enthousiasm here also.
I think he would suggest you to pick the updated version of his design.


hot line

[Eugenio70]

Thanks Jacco, I'l get in contact with him!

[Andy L. Francis]

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!

[peranders]

I have these app notes if anyone is interested.

I'll agree that some parts may be tweaked in his design.

[gnalbant]

Dear peranders,

I am quite interested in your application notes.

I am working building RAS250, as Nico stated in his website, but not RAS300, which is version uses BD series instead of MPSA types.

[terranigma]

Originally posted by Andy L. Francis
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.

....

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.

[peranders]

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.

[peranders]

Originally posted by gnalbant
I am working building RAS250, as Nico stated in his website, but not RAS300, which is version uses BD series instead of MPSA types.

The BD's are quite alright, possibly a bit low voltage rating if you'll have +- 55 volts.

[thermionic]

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?


Justin