[FullRangeMan]

This is good news to users of high efficiency speakers.

If you intend to buy a $4000 First Watt amp first take a look at this small wonder, it have everything the First Watt amps have but at a much lower price and are sold as kit, it have no gain and need a pre-amp with some Voltage capability.

The Mofo power amp is a Mosfet Follower conservative 20W 8Ω per channel Class A, Damping Factor=40 created by Michael Rothacher and are a very simple amp with few parts, can be done as Stereo or Monoblocks, its a Single Stage + Single Ended circuit but have NFB (as some First Watt amps also have).

It have just 1 output Transistor per channel, the suggested output Transistor cost $3.76 each at Mouser, according the PDF datasheet the amp sound character are ''transparency, depth and ability to cast a great big soundstage''.
Thread:
https://www.diyaudio.com/community/threads/build-this-mofo.313649/

MoFo Board:
https://diyaudiostore.com/collections/power-amplifier/products/mofo?variant=5111698849826

Alternative Board:
https://www.ebay.ca/itm/224297548554?hash=item34392cfb0a:g:A8MAAOSwLq1f6064

MoFo PDF Article:
https://blog.audiomaker.tech/download/build-this-mofo/?wpdmdl=689&refresh=61b4f561f12a71639249249

Output Transistor :
Vishay MOSFET 200V N-CH HEXFET  $3.76 each at Mouser
https://www.mouser.com/ProductDetail/Vishay/IRFP250PBF?qs=%2FRKvNCQzLu1AE23xR%2FUypQ%3D%3D

Alternative Kit Parts:
https://qwertyl1.wixsite.com/diyaudio/mofo
Official Main Board:

[FullRangeMan]

Hi all, please note this board.
It is all the Mofo Amp, there is not much that can go wrong here

[Mike B.]

The only problem with the MOFO is it requires a preamp with lots of gain. Yes, I have built it.

[FullRangeMan]

The only problem with the MOFO is it requires a preamp with lots of gain. Yes, I have built it.

I had informed it in the first post, the Mofo amp have no input stage for gain, its a minimalist design.

[Mike B.]

It is not a good equivalent to a First Watt amp. It also requires significant expense for the Choke and a heatsinks.

[FullRangeMan]

It is not a good equivalent to a First Watt amp. It also requires significant expense for the Choke and a heatsinks.

Personal opinion stated as fact.
For more gain the Andrea Ciuffoli's stereo board have a tube gain stage.

[FullRangeMan]

This is a Zener diode from the Mosfet gate to source to protect the output transistor from inductor kickback.

[RDavidson]

If you intend to buy a $4000 First Watt amp first take a look at this small wonder, it have everything the First Watt amps have...

Also opinion stated as fact.

[FullRangeMan]

Also opinion stated as fact.

[FullRangeMan]

Here is a suited inexpensive heatsink for this project.
Heatsink, Aluminum  7 x 9.875 x 1.25,   $22.95 each
18 x 25 x 3cm.
https://www.rfparts.com/hs100-9-8-heatsink-aluminum-7-x-9-875-x-1-25-no-holes.html

[FullRangeMan]

Found an back support alu heatsink for the output Transistor:
Ebay 20pieces $1.74

[FullRangeMan]

M3-30 or 40mm thread lenght Bolt Hex Cap Flat head:
Ebay M3 Steel Bolt for Transistor

[FullRangeMan]

Aluminum heatsink 300x92x40mm black anodized $17.50each
https://www.goodcomponent.com/products/amplifiersspeaker-parts/heat-sinks/machined-large-aluminum-heat-sink-extruded-cooling-block-300x92x40mm-black-1508.html

[lokie]

following

[poseidonsvoice]

FRM,

I built a pair of MOFO monoblocks recently. Here is my writeup on diyaudio. This *is* a single ended Class A design, no Class AB here, it's impossible due to the design.

For the most part, it was a fun build, requiring some mechanical considerations I hadn't experienced before.

If you have questions - please ask.

Best,
Anand.

[FullRangeMan]

Nice congratulations, one great amp certainly a purist project 
Let us know your sound impressions when its on the run.

[FullRangeMan]

@Poseidon Voice
Certainly your fine amp put my builder on the shame.
What supply do you recommend?
What output do you recommend IRFP250 or other?
What effect a bigger choke will give on the music ?

[poseidonsvoice]

@Poseidon Voice
Certainly your fine amp put my builder on the shame.
What supply do you recommend?
What output do you recommend IRFP250 or other?
What effect a bigger choke will give on the music ?

FRM,

Sorry for the late reply, I don’t get notifications as often from AC.

The Mofo circuit is designed for 19V to 24V supplies. Since power is linear with the amount of voltage in the supply, obviously the 19V versions will have less power than the 24V versions. The power from 19V for example will be about 20% less than 24V. Of course, you can use even lower voltages than 19V but power output will go down linearly.

I do recommend biasing with as much current as possible, because in this single ended Class A circuit (and all Class A circuits in general), higher bias, reduces overall distortion. In Michael’s MOFO writeup he clearly shows lower THD at 2.5A versus 1.9A. I wouldn’t use it at less than 1A bias. Remember the amount of bias current also directly affects the ability to drive 4 ohm loads in a single ended Class A design. You can definitely use a linear supply instead of a switcher like I did - it just takes up more room in the chassis. I had the switcher in stock, it is of low noise design, so I used it!

At 24V and 2.5A, that is 60 watts of dissipation. So you need a good sized heatsink to dissipate that heat. The heatsink I used is about 0.5 deg C/watt which means a 30 degrees C rise over an ambient temperature of ~ 25 degrees C. I have a feeling that the heatsink as well as the clamping mechanism I implemented to tightly adhere the MOSFET to the heatsink has worked well. At various portions of the heatsink and also on the MOSFET itself, I measure anywhere from 40 degrees C to a maximum of 50 degrees C. You can place your hand on top of the enclosure for very lengthy periods of time.

Since the choke is in series with the MOSFET, it has to handle all the current as well. So you need to size a choke that is a minimum of 50mH (to get to ~ 25Hz bass) and at least 2A or more. Chokes get very expensive at higher currents. My LL2733 Lundahl unit is a DUAL choke, so I wired them in parallel so they can handle at least 2.5A current (they saturate at ~ 5 amps), while also keeping DCR low and the inductance in a good range. I measured the inductance (while having it run at 2.5A current) and it was close to 100mH which is plenty. You don’t want too high of a DCR because your output voltage will drop quite a bit. The DCR of the Lundahl LL2733 when the dual chokes are wired in parallel is ~ 0.9 ohms. Other chokes from Hammond, etc…will work - just realize the choke will be a LARGE size!

I haven’t played with other N channel devices in this circuit other than the IRFP 250. I do have some N channel SemiSouth JFETs which I might try in the future if I am curious. If so, I’ll have to drop the gate resistor to 50 ohms or less.

My link above gives all the details for my build. I wouldn’t recommend my method of building for a newbie though. There are a lot of elements that have to come together to make it work! And you should have an oscilloscope for testing, etc…

Best,
Anand.

[FullRangeMan]

Thanks a lot for your prompt reply, much appreciated your detailed report