Mofo

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Nick
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#136 Re: Mofo

Post by Nick »

Yep, I would (in reality) use something like a 25A 200V bridge, but MUR810 would be ok. I would ignore 1000v 1A rectifiers, fast or not. My normal suggestion would be big transformer, high current bridge, as many farads with as low a esr as I can afford. If course this is hifi, so who can actually say :-)

AFAIK, the B+ for the mofo just needs to be in the range that the mosfet is happy with and then you can adjust the current via the bias to give your operating points.
The benefit of the regulator is it's easy to set Vout
Never forget the option of adding bucking and boosting windings around the primary or secondary of a toroid to tweek the voltage. Or for that matter, get one wound to the voltage you actually want once you have narrowed it down, maybe with a variac.
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#137 Re: Mofo

Post by simon »

Yeah 19V@1.7A is a commonly used OP as it's a good balance of power with distortion.

Amending the ESR made a huge difference! This is a 22V 150VA secondary, with a 50,000uF/50mF filter with the 30mR ESR paralleled down to 6mR assuming 5 caps @ 30mR each.
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MoFo C1smaller.jpg
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The ripple is much better now but still 168mV. This would bother me with a valve power supply, but maybe it's okay for the MoFo?
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MoFo C2 smaller.jpg
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simon
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#138 Re: Mofo

Post by simon »

Nick wrote: Tue Jun 11, 2024 9:50 am Never forget the option of adding bucking and boosting windings around the primary or secondary of a toroid to tweek the voltage. Or for that matter, get one wound to the voltage you actually want once you have narrowed it down, maybe with a variac.
I rather like Toroidy audio grade as they feel nicely made and don't buzz! But the inside is epoxied so that stuffs that! Other toroids are available. But it's be a punt on how good they are as I don't have much experience of toroids.
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#139 Re: Mofo

Post by Nick »

Yes, I agree, Toroidy are very good. For less demanding use Vigotronix are fine, I use loads of them, and they have a open center. Though most of the problems go away with a variac for working out what secondary to eventually get.

I don't know what the PSRR for the MOFO is, the way to get it down is to use a high current hammond choke (or reactor as they call them) instead of adding resistance to the supply.
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#140 Re: Mofo

Post by Nick »

The mofo is a source follower, so I would expect the psrr is not too bad. trying it would be my suggestion.

Remember not to feel any safer with a low voltage power supply, it won't kill you if you touch it, but it will spoil your day if you short it out (with a ring for example).
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#141 Re: Mofo

Post by simon »

This is getting expensive :-).

A Vigortronix 300VA with 2x24V secondary is £45.76, 500VA is £65.84

https://uk.farnell.com/vigortronix/vtx- ... dp/2817701

A similar audio grade from Toroidy would be a fair chunk more when delivered I guess.

Bridges are cheap, a quid for a 200V 25A job. But are there benefits for more expensive fast recovery types?

Caps. I have 9 or 10 no. 10kF 35V caps which I could parallel up, but I'm a bit concerned about no-load fault condition when voltage would rise to 24V x 1.41 = 33.8V (minus diode drop) which feels a little close for comfort. Am I being overly concerned here?

For 50V rated caps, these look the cheapest options on Mouser:
Cornell Dubilier 22kF £5.87 each
https://www.mouser.co.uk/ProductDetail/ ... 3dso7ik%3D

63V rated:
Chemi-Con 22kF £4.60
https://www.mouser.co.uk/ProductDetail/ ... KrfoeSU%3D

Kemet 22kF £10.80
https://www.mouser.co.uk/ProductDetail/ ... KKZw%3D%3D

The large current Hammond chokes are another chunk:
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Hammond chokes.png
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Again at Mouser, each:
157D £20.18
156B £16.22
159ZL £30.32
154B £13.00
155B £15.39
157E £30.48
159ZJ £28.50

154B or 156B look the best compromise on cost and DCR. If needed.

So, around £50 if I use my existing caps and don't need chokes, or more like £100 if I go with two lots of 22kF-choke-22kuF.

Makes a £11 SMPS with chokes and 4700uF caps from stock look good value. But which sounds better?
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#142 Re: Mofo

Post by Nick »

Its worth than you think, the off load voltage would be higher than 24v, you would have to add the transformer regulation percentage to the 24v.

2A * 24v * 2 * 1.61 would make me think you could get away with a 160va transformer. Depends on how over the top you want to be.

Cant help with the "would it sound better". I do have a couple of unwanted 120va 25v transformers you could have for the cost of a car journey.
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#143 Re: Mofo

Post by simon »

Transformer primaries are specified as 230V but my mains is reliably 243V so there's probably another 5% on top too. 50V rating more comfortable I think.

Those toroids are tempting I must say. I'll ponder a little further.
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#144 Re: Mofo

Post by simon »

Playing with PSUD, taking the current up to the 2A rating of the Hammond 193T pulls the voltage of the 25V secondaries down to 21.3V from 22.9V at 1.7A with a CLC PS, so a window of adjustment. Looks like it needs to be a CLC though.

Are you going to the NW Audio show next weekend Nick? I'm planning on going again with Steve, if you are perhaps you could bring the transformers with you if it isn't too much hassle?
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#145 Re: Mofo

Post by Nick »

Yep good idea. Remind me next week.
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#146 Re: Mofo

Post by simon »

Excellent, it's a plan
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#147 Re: Mofo

Post by simon »

Nick wrote: Wed Jun 12, 2024 1:37 pm 2A * 24v * 2 * 1.61 would make me think you could get away with a 160va transformer.
Why 1.61 and not 1.41?
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#148 Re: Mofo

Post by Nick »

If you look here:

https://www.sowter.co.uk/rectifier-tran ... lation.php

You see Iac = Idc x 1.61 for a cap input bridge rectifier.
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#149 Re: Mofo

Post by simon »

So it does, the difference between full wave and a bridge
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