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Dusk 25th July 2005 02:05 AM

Still looking for loud mosfet power amplifier
Hi Again iam still looking for the monster of mosfet power amplifiers with an op-amp front end. Iwant to use this setup for my guitar rig that is i use a tube pre-amp in front of the power amplifier for a Blues-rock-jazz fusion type sound........still waiting for schematic/circuit.


sparkybg 25th July 2005 08:09 AM

Why opamp input stage?
Looking for balanced input or something like that?
Or high impedance input?

Dusk 25th July 2005 10:02 AM

Op amp front -end
Hi Sparkbq
I want a op-amp front-end so i won't have to worry about matching transistors.also less parts in power amplifier.

sparkybg 25th July 2005 10:34 AM

I am afraid that the output power will be about 25W RMS maximum on 4ohm since most opamps are designed for +-15V power supply.

It can be done with more power, but the complexity would increase even more than if you use classic scheme.

Maching of input transistors is not so critical, especialy if they have high gain (400-1000). And if you buy transistors from one serie (they have all text on them equal), the betas will be very close usualy.

Look there for example:

or there for MOSFET one:

I think these are simple enough, cheap, powerfull and high quality.

Craig405 25th July 2005 11:29 AM

maplin made a 150 watt RMS mosfet amplifier that would suit guitar amplification, there was a bridge circuit avalable too that could boost power to 400W RMS.

It was supposed to be very reliable and the circuit is pretty basic.
If you are interested i could have a look for my schematic. and post a pic.


palesha 25th July 2005 03:45 PM

"maplin made a 150 watt RMS mosfet amplifier that would suit guitar amplification, there was a bridge circuit avalable too that could boost power to 400W RMS."

How about Schematics & more details?

tlf9999 25th July 2005 05:23 PM

you can use one of those power ICs (audio or not. for example the 3875 or 3886 that has considerably higher rail voltage specs) or bootstrap an op-amp to increase its voltage output capabilities.

sparkybg 25th July 2005 06:19 PM

:idea: :happy1:

These LMs are good. And simple too. And have thermal and shortcircuit protection.

I think theese are most switable for you.

0.1% distortion on their top powers looks good enough.

There should be plenty of PCBs for them on the net.

And probably will run without a problem at the moment you switch the power on.

HIGHLY RECOMENDED!!!:yes: :nod: :up: :cool: :D

Craig405 25th July 2005 06:21 PM

ok here are specs:

+ and - 50V max supply
idle current 50-100mA
150 watts RMS into 4 ohm 100 into 8
THD 0.01% @1Khz
vin = 2.4v pk/pk for max o/p

Input is not op amp though, but its very simple to make.

Distortion is probably better in this circuit which uses doubled up MOSFETS. You will have to find modern equivalents for the MOSFETS and some of the bipolars but it shouldnt be difficult.
For guitar amplification such large supply caps wont be necessary if you want to skimp a bit, 6800uf per rail maybe?

I also have the bridge circuit schematic, and PCB layout.
The bridge can theoretically provide 650 watts RMS with 2 MOSFET modules attached.
But i cant be bothered to draw it up at moment :)

..............ok i cant attach a word document with a screen dump in it.
I need to make a picture out of my screen dump somehow first?


sparkybg 25th July 2005 06:24 PM

And Craig405, these amps are discontinued, but I am REALLY INTERESTED to see the schematics.

Please post them if you can.

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