Another MoFo

[Ray P]

When I finally do a MoFo build (I still have many of the parts and have only sold on surplus items I don't need) I have a pair of single rail SLB power supplies. SLB (Smooth Like Butta) is a cap multiplier supply with an active rectifier - got them on a DIY Audio group buy.

As I mentioned previously on Steve's thread, my driver stage will be based on a pair of 4P1Ls. One day!

[pre65]

Just out of interest Ray, what were your thoughts on the heatsinks ?

[Ray P]

Just out of interest Ray, what were your thoughts on the heatsinks ?

They're essential Phil!

[Ray P]

If on the other hand you're asking about the sizing of the heatsinks, you need to be more specific as the amount of heat to be dissipated will depend on your power supply voltage and what standing current you'll be using.

If you read Michael Rothacher's original document he tells you his recommendation for heatsinking based on the voltage/current parameters he used - page 17.

Ray

[pre65]

Yes, in general I appreciate all that, and I have at least 5 different options to think about.

I just wondered if YOU had settled on a particular way to go in that direction.

If not, don't worry.

[Ray P]

I saw your post on the DIY Audio MoFo thread,,,

You should keep R6 as it gives an easy way to measure your standing current - Ohms law again, handy little beggar! If you omit it you must put a link in its place otherwise your MoFo will not be connected to the power supply.

Regarding the MOTs, these quotes are from little eddy's MoFo thread, Regarding the MOTs (same as the ones you have);

I went and tested a few and these had the lowest DCR of what I tried, around 2R. I'm sure these tested at above 100mH so no issue with bass response.

The one thing I would suggest is that the MOT eats up voltage and available power so if you want more than say 12W, I would suggest a transformer higher than 18V if conventionally aspirated.

and using those same MOTs;

I have a supply voltage of 21.7V to the pcb, so running with 1.7A I have just under 13W output. Dissipation is theoretically 31W. I would really like to run at say 2A so should really have bigger heatsinks than the 0.53 C/W ones I bought, hence me looking at the cpu heatsinks with fans.

[Ray P]

Yes, in general I appreciate all that, and I have at least 5 different options to think about.

I just wondered if YOU had settled on a particular way to go in that direction.

If not, don't worry.

No, not got that far with thinking about it.

[pre65]

Thanks Ray.

I'd been thinking about a 24V supply and biasing around 2.5A.

I've ordered a 5W R6 now, although I will probably use the MOT terminals to set the bias, now I know how to do it properly.

I have two huge heatsinks 300mm x 200mm x 70mm, and I have no doubt they would cope. BUT, because of the directions of the fins they have to sit 300mm high (as they did on my 50W single ended Mosfet (puck) amp) and that is not conducive to good design.

What I had considered was cutting each one in half, then they are 200mm wide and 150mm high, and using one each side of each monoblock connected by a cross piece of 100mm x 100mm x 6mm alloy angle. The cross piece would have a 6mm plate welded each end so as to bolt to the heat sinks with a 100mm x 100mm x 0.5mm heat conducting pad. The Mosfet would sit on the crosspiece.

https://www.ebay.co.uk/itm/1pcs-100mm-x ... Swo4pYlcjL

Before I go to that extreme I'll use my 3rd (slightly damaged) full size one on a breadboard mock up.

That's the thought for today (it's Sunday now) but as ever with me, things may change.


*

[pre65]

Other options could be one, or other of these two, but I'd rather not use fans.

https://www.ebay.co.uk/itm/Heatsink-Uni ... 1438.l2649

https://www.ebay.co.uk/itm/Heatsink-Uni ... 1438.l2649

[Ray P]

That's the thought for today (it's Sunday now)...

My, how time flies when you're having fun.

[Paul Barker]

On fans, if you run the fan at lower voltage you can get them quiet.

[Ray P]

Give me passive cooling everyday, it's simple, it works, it's silent and there's nothing to go wrong.

[steve s]

I have pc fans on my motor home fridge and cupboard enclosure, they have run continuously and have proved very reliable over the last few years. I'm not into pcs but I don't remember lots of fan failures when I was.

The pc fans on my mofo are virtually silent at just under 5volts, take no space (they are mounted over a hole in the chassis) the circuit board is mounted directly on top of them, so the heat flows vertically, mine where around a fiver each, the amp is cold, even after last night's 6 hours running

In my limited experience so far, theres alot to like about them

My adjustable dc to dc converter, about £2 ebay, makes adjusting them to 'just enough draught' is great
The power supply is a miniature wall wart that plugs into a small caravan type 13amp socket mounted internally, I had all the parts.
Works a treat.

[Ray P]

I was just expressing a preference, not criticising, Steve...

[pre65]

On fans, if you run the fan at lower voltage you can get them quiet.

Indeed Paul. I had fans on the 50W Mosfet amp, and running from a bench power supply I could turn the voltage down so they ran quiet but still did a good enough job.

I went for a good quality (Arctic) fan quite big ,(5.5" square) so I got enough volume of air without high revs.

IF by any chance,(when on test) the big heatsink proves to be inadequate then I still have the fan option.

The huge 50W amp is being broken up so things like heatsinks, fans, inductors in the power supply etc can, and will be, reused.