Another MoFo

[pre65]

I've decided to dismantle the 50W single ended mosfet amp, it's just too big and cumbersome.

Instead, I'm going to try a MoFo.

The power supply will be fairly simple, probably a pair of LT1083 modules as I already have them and a suitable pair of toroids also to hand.

Biggest problem is the choke/inductor/reactor as I don't have anything suitable. I'm tempted to go straight to the biggest option, but if I stick with 2A max the Hammond 193U at 200mH and DCR 1.7R might be better.

Other than that it's just suitable heatsinks, although I'm sure the ones on the 50W Mosfet amp would do, they are a bit big. I'll try and work out the heat rating of 1/2 of one as I do have one spare that could (with much effort, sweat and bleeding) be hacksawed into two pieces.

[steve s]

a good project Phil, the boards I got from ray are unused now if your stuck..
I used a couple of computer fans.. cheap and work a treat on around 5 volts, a small DC to DC adjustable converter about £2+ uk Ebay is useful
Make sure you use the correct diodes, with my last build, I've had problems from spikes.. order plenty of mosfets.
Ive ordered a couple of mains delay units to turn the mofo on after the valves are on.
From what seen the mofo is good but easy to spoil by what you plug into it.
I'm not using all the power available with the smaller hammond chokes, that reminds me I need to order another pair for my next build.
Best of luck..

[pre65]

Thanks Steve, but I've got two boards coming from Ray.

Can you elucidate on the diode question, as I'm not sure what you mean.

The only diodes I can see are the bridge rectifier in the power supply, and the optional 20v zener diode on the PCB.

If I go with the microwave transformers as chokes (MOTs) then I'll probably go with a 24v (ish) supply and bias it up to just under 2A.

I'd like to avoid fans if possible, but I have the means to do so if necessary. If I cut my spare big heatsink in half I end up with two 8" wide, 6" high and 3" deep. I'm looking for a way to calculate the c/w rating so I have a better idea of it's suitability.

[Nick]

Bolt a known metal cased resistor to it and connect to a known voltage. leave for a half hour, measure the temperature above room temperature and take it from there.

[Ray P]

Phil, I don't know if you have options for how you slice up your big heatsink but for a given area, a wide and shallow heatsink is better than a tall and narrow one. I used to have a bookmark to a page about it but it seems to have gone. As the air convects up the heatsink it warms up so towards the top of a tall heatsink the temperature differential is smaller and the air can remove less of the heat.

[pre65]

Thanks to Ray, and a small amount of beer vouchers ( ) I now have a pair of MoFo PCBs, and a pair of MOTs (microwave oven transformers) to use as chokes.

Now reading the DIY audio thread to see what R6 is, and to catch up on anything that I might need to know before starting in earnest (whoever he is ).

[pre65]

Do any of our MoFo builders use the optional 20V gate protection Zener ?

It seems the amp designer does not.

[steve s]

Good progress Phil, the diodes are there to protect against spikes, I would not say they are optional. I would use them. Plugging the power supply in can blow the mosfet with out them fitted. I put (not quite) the right ones in and lost a couple of mosfets..
See around page 100 on the DIY audio thread

Thanks Steve, but I've got two boards coming from Ray.

Can you elucidate on the diode question, as I'm not sure what you mean.

The only diodes I can see are the bridge rectifier in the power supply, and the optional 20v zener diode on the PCB.

[pre65]

Good progress Phil, the diodes are there to protect against spikes, I would not say they are optional. I would use them. Plugging the power supply in can blow the mosfet with out them fitted. I put (not quite) the right ones in and lost a couple of mosfets..
See around page 100 on the DIY audio thread

I'm only up to page 14 so far, but it was said the Zener was essential for a valve front end.

[steve s]

Around page 100 theres some using them to protect against power supply spikes as well Phil

[pre65]

Around page 100 theres some using them to protect against power supply spikes as well Phil

Up to page 27 now.

[Nick]

You could build the thing in the time it takes to read the thread.

[steve s]

That's true nick.. I looked in once I realised I'd got a problem.
The boards take about 15 mins each to solder up

[pre65]

You could build the thing in the time it takes to read the thread.

Indeed, but I'd rather be sure, and my time is not chargeable.

[Nick]

You could build the thing in the time it takes to read the thread.

Indeed, but I'd rather be sure, and my time is not chargeable.

The problem is you often end up with design by democracy, where the ones who have the strongest (but possibly wrong) opinion get heard and the ones that know what to do but don't care if anyone else knows it gets ignored.