Mofo

[Ray P]

fwiw I used split supplies for mine. A meanwell for the mofo output stage and a very heavily capped crc for the driver stage. It was adequate....but I'm sure Rolls Royce always thought there was room for improvement.

I was only suggesting the SLB supplies for the MoFo stage - can't imagine they'll be suitable for Steve's valve-based driver stage.

[steve s]

No problem Steve, it's good to see the progress you've made and you're clearly pleased with the results.

One interesting aspect of the power supplies I linked to is that they used switched mosfet rectification instead of conventional rectifiers - gets good reports.

Thanks ray it seems they do everything. Low ripple without using to many caps and in a small space.. I ant fi dthe boards in the shop.. There's an updated board due soon.

The driver valves are running on a lclc > interstage, split into two after the 1st choke

[ed]

did Mike ever run the mofo with an slb?

I was thinking at the time it might be nice in the F6 but i didn't pursue it . I wasn't getting anywhere near the ripple that people were reporting so I didn't bother. I think I was conscious of using up the 40 or so nichicon 15000uf that I'd bought super cheap when I did the F2.

I'm watching with interest

[steve s]

1st issue tonight..
The crackling came back so I was busy checking through the amp again
Carefully moving/rocking the valves as I've doubts about the contacts. It's usually the centre pin on b5s.
I'd turned the amp on and off 3 times over 10mins or so. The 3rd time there was a pulsing from one speaker.. I checked the amp over find the biasing had change dramatically, but rather than run really hot the power supply just gave in and was pulsing.. the bias voltage reading 0 to 4 ish volts as it pulsed
Quite a big adjustment was needed to correct it. But it rebiased striaght up and has been stable again.
The power supply giving up is quite a nice safety feature at this point.. I'm not going to rush in by building a beefier power supply until I've found out what happened and why and if it happens again...
Any ideas ?

Edit, the crackling was on the opposite channel to the one thats just cut out tonight

[ed]

Any ideas ?

if you are using Michael's circuit:

the fact that all you did in the end was re-bias lends pointing the finger at P1. The circuit R4-P1-R3-R2 would only mess the bias if there were a regular intermittent break. So, dry joint, P1, or the power supply itself was pulsing. I think, but I have been known to be wrong, as you well know.

[Ray P]

Any ideas ?

Steve, if you're a member, there's a wealth of experience, including the designer, of the MoFo on DIY Audio thread - I would be inclined to post the issue there.

https://www.diyaudio.com/forums/pass-la ... -mofo.html

Ray

[steve s]

Thanks gents, Ed I'll check the connections, but it's a strange one I have some spares for p1 I'll change it it happens a again
Ray I've spent another 2 hours reading that thread this morning... Its taking over my life... again il see if it reoccurs
Thanks both

[steve s]

I've had quite a time of it, what I thought was straight forwards build turned into a comedy of errors.
This is it playing and surprisingly sounding at least as good as it did on a board..I think it's better.

I originally started with new power supples, which turned into a disaster. I managed to inflict the amp with a hum.. I swapped back to the pc supplies and thought sod it.. next to no ripple anyway. Well not to bad!
The cooling seems to work a treat. I gave the fans a 7.5 volt supply and made that adjustable, the mosfets are virtually cold. I need to slow them a little once it's had a few weeks running.. at the moment it's comforting to hear that the fans are running.
The driver stage should have been straight forwards.. but I managed to connect the rectifier up wrong. Simple silly mistake, every pin was 90deg out, it took out the transformer, I had to find another and swap them over, and make a new mounting bracket for the 13 amp socket in there, so that was this afternoon's job.
Anyway at long last it works. The new boards I built are less than 30mm square, so I don't feel so bad having them mounted only on the mosfet.
I must say pieters interstage transformers are really good.
They can saturate if you overdrive them, but no danger of that with this amp.
There's much bigger output caps fitted. A couple electros and a 30 uf solen.
I'm impressed with all of it.
The mosfets in this one are vishay, Samsung's where in the 1st version.

I could do with more gain on the driver.
I'll rebuild the board version and test other drivers.. px25 may be good. Or even det 25 through suitable transformers. But may sound just fine with a small triode.
Let's see.

[steve s]

Forgot to say I built it with a volume pot on one input and straight though on the other input.
I fitted a vintage carbon pot that I had a job lot of.
At this point it seems to have been a mistake.. swapping inputs, it does the 'cover the speaker with a blanket ' trick. I need a volume control for the record player input. But it looks like I need something better than what I've fitted.
I've a few Elmer rotary switches that will suffice once armed with a few resistors

[Cressy Snr]

I've had quite a time of it, what I thought was straight forwards build turned into a comedy of errors.
This is it playing and surprisingly sounding at least as good as it did on a board..I think it's better.20200608_205950_copy_1024x768.jpg20200608_205939_copy_1024x768.jpg


I originally started with new power supples, which turned into a disaster. I managed to inflict the amp with a hum.. I swapped back to the pc supplies and thought sod it.. next to no ripple anyway. Well not to bad!
The cooling seems to work a treat. I gave the fans a 7.5 volt supply and made that adjustable, the mosfets are virtually cold. I need to slow them a little once it's had a few weeks running.. at the moment it's comforting to hear that the fans are running.
The driver stage should have been straight forwards.. but I managed to connect the rectifier up wrong. Simple silly mistake, every pin was 90deg out, it took out the transformer, I had to find another and swap them over, and make a new mounting bracket for the 13 amp socket in there, so that was this afternoon's job.
Anyway at long last it works. The new boards I built are less than 30mm square, so I don't feel so bad having them mounted only on the mosfet.
I must say pieters interstage transformers are really good.
They can saturate if you overdrive them, but no danger of that with this amp.
There's much bigger output caps fitted. A couple electros and a 30 uf solen.
I'm impressed with all of it.
The mosfets in this one are vishay, Samsung's where in the 1st version.

I could do with more gain on the driver.
I'll rebuild the board version and test other drivers.. px25 may be good. Or even det 25 through suitable transformers. But may sound just fine with a small triode.
Let's see.

It’s not just me and James taking out power transformers then.
They’re a pain in the backside to replace too, as they are usually the first to be wired in, then everything else goes over the top of the wires, so you can’t get at it properly. Took me best part of a day to replace mine.

Glad to see that the Mofo is sounding good.

[pre65]

There's much bigger output caps fitted. A couple electros and a 30 uf solen.
I'm impressed with all of it.

What size cap did you end up with Steve ?

[steve s]

Hi phill, there is around 11 ,000 uf comprising a cheap and a better electrolitic and the solen .

[simon]

when you've found your valve Steve I won't be able to ignore the thought that I wonder if it will compare to my mofo with a 3x gain opamp at the front...I'm getting right 'ornery in my old age.

Hi Ed, bit of an old one this that I've dug up but I'm interested in the 3x opamp you used and if you're still using it?

I finally got my MoFo going yesterday and had an hour or so listening (above 415Hz driving the Fostex 208s). It's early days and I need a lot more listening to understand what's going on but my first impressions are that the MoFo doesn't quite have the richness of tone of the 45SET. Or perhaps it doesn't have the distortion of the 45 amp? But 2 channels have cost about £150 compared to a whole lot more for the valve amp.

Anyway, what's interesting is that I'm not using a driver/voltage gain stage. It's probably just about loud enough without one, which took me by surprise a little.

But 2x gain might be useful, how to do this?

I'd originally planned to use 26s but that's a lot of effort for a little gain. So maybe a simple op amp/SS gain stage might do the job? But then why do people built significant complexity preamps? Sound quality? Vanity? Insanity?

The only experience of SS I have is this MoFo so I'd appreciate thoughts on ways of adding a little gain, and the pros and cons.

[Ray P]

Don't forget that the Mofo is nominally a unity gain power buffer though, in fact, there is a small loss so it actually has a voltage gain of something like 0.9.

[pre65]

But 2x gain might be useful, how to do this?

I'd originally planned to use 26s but that's a lot of effort for a little gain. So maybe a simple op amp/SS gain stage might do the job? But then why do people built significant complexity preamps? Sound quality? Vanity? Insanity?

The only experience of SS I have is this MoFo so I'd appreciate thoughts on ways of adding a little gain, and the pros and cons.

I'm interested in the same problem / solution.

What I was going to do is too big/expensive/time consuming.