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PostPosted: October 23rd, 2017, 8:46 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 211
Just a quick posting with some pics on the First Watt F4 clone I just built. The F4 is a MOSFET source follower power buffer (i.e. no gain) that provides 25W Class A in to 8 ohms. Because it has no voltage gain, it needs a pretty high gain preamp or small power amplifier (like a low power SET amp) feeding it. There are a couple of threads on DiyAudio.com that discuss this amplifier and the project.

http://www.firstwatt.com/f4.html
http://www.diyaudio.com/forums/pass-labs/97540-f4-power-amplifier.html
http://www.diyaudio.com/forums/pass-labs/234355-guide-building-pass-f4-amplifier.html

The F4 boards are available at the DiyAudio Store, as well as the universal power supply board, for a total of $50 for the PCBs for the project. I also bought the Deluxe 4U chassis with pre-drilled heatsinks, which saves alot of headaches with drilling and tapping all the holes needed to secure the MOSFETs and the F4 boards to the heatsink. I looked at chassis from China, and honestly, considering the work involved drilling the heatsinks, the price was close (once you take into account shipping) for a 4U sized chassis, and the Italian chassis that the DIYAudio Store sells, post paid, arrives more quickly (FedEx vs slow boat from China), goes together nicely and is attractive.

I bought a set of matched LSK170/LSJ74 from DIYAudio Store, but sourced my MOSFETS from an Ebay dealer who sells closely matched devices (IRFP240/IRFP9240), since DIYAudio store was out at the time.

I did depart slightly from recommended CRC power supply by using a slightly higher voltage (20V) and VA rating (500VA) on the transformer (Antec AS5220), and a pair of Hammond 10mH 5A chokes with the same voltage drop for a CLC supply.

All the rest of the parts came from Mouser, or stuff I had already in my parts bin. The hardware I sourced as needed on Amazon or McMaster-Carr (standoffs & 3mm metric cap screws).

The project came together quickly, probably a 20 hours of total time.

Since my intent of this amp is to feed it off various SET amps I have built, I made a cable with a banana plug to plug into the output terminals of the SET amp, and an RCA for the F4 input. I am using the F4 to power my bass cabinets on the Basszilla, while feeding the midrange/tweeter baffle panel off the SET amp. Using the speaker passive crossover of around 200 Hz. Long range plan is going to electronic crossover.

Only a couple of hours on the F4, and my initial impression is that it really provides alot of control of the bottom, I finally getting impactful bass out of the cabinet. The soundstage seems to open up more, and there is less strain as the SET amp is not having to drive the bass cabinets (just providing voltage swing as input to the F4). I want to try the F4 full range to hear what it can do, but so far I like what I am hearing.

David


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