Mofo

[Cressy Snr]

The empty case weighs about the same as our microwave before I start filling it !

[ed]

This is what I'm going to build, very slight variation on my previous version.

factored in the new choke, with OPA2134 or LM4562, spice says I'll get 4.5 watts into 8 ohms
biased the fet at 2.4 amps should dissipate about 53 watts...

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and

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[Nick]

You could always take the feedback from after the 6800uf cap instead of before the mosfet (just saying )

[ed]

You could always take the feedback from after the 6800uf cap instead of before the mosfet (just saying )

that's a tad unconventional, not to mention controversial, isn't it?

I'm assuming(risky, I know) that you're referring to the gain set on the opamp....I can't shake off my modular upbringing.

In my ideal world I would also like to take the output bias voltage from somewhere other that the output power supply, but that's one step beyond, but ultimate modularity.

[Nick]

Yep. Though there would be problems as the op amp would be open circuit at DC.

[pre65]

I will wait till the new chokes are delivered (no panic Ian) and set up and tested in the existing chassis (posh word for wooden lash up) before making a final decision on the version 2 setup.

I've been looking at some MDF 6U sloping front rack mount cabinets from a UK Ebay seller. They are made from 18mm MDF, I've done some cardboard cut outs to simulate the sizes, and they are looking like a possibility.

I've put a link up to show a picture. If I were more experienced in woodwork I could make my own, but I've got a bit of time to think about things.

My plan would be to mount the 300mm X 200mm heatsink on the sloping side, with what should be the back becoming the base. They would sit side by side with each heatsink facing sideways, necessitating a mirror image pair. That way they would easily fit on my rack. It would mean the heatsink fins are horizontal and at about 30 degrees tilt, but I don't envisage any heat dispersal problems.

https://www.ebay.co.uk/itm/202170361902 ... XQHPFRiiv6

[simon]

I'm not expecting the breadboard bit to work...

Well, rather surprisingly it worked first time. I was also surprised how little 2x gain added. And I'm not sure without some listening but my suspicion is it robs a bit of the life from the music. I've only tried it above 415Hz so far though.

But undoubtedly the £150 or whatever the bits have cost makes remarkably high fi. And I don't break my back hefting it about.

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[simon]

Mmmmmm. Just taken the op amp gain stage out and the sound is immediately better without it. More shine and sparkle, more detail, more space, more realism. More music. Interesting experiment.

[Nick]

Mmmmmm. Just taken the op amp gain stage out and the sound is immediately better without it. More shine and sparkle, more detail, more space, more realism. More music. Interesting experiment.

I would suggest trying with better metal film or carbon resistors, maybe tackman. Also try a simple 9v battery (or two if its split rail) I don't trust that SMPS and certainly don't if you have made a dummy split rail. Also if that ceramic cap in the feedback path? of so, replace with a polystyrene. Most ceramic (not so much C0G) introduce a shocking amount of distortion. Also need power rail decoupling, electrolytic and ceramic.

[simon]

Fair points, which weren't lost on me when I was breadboarding it, but I didn't have the experience to know what to expect.

To continue the experiment I might see if I can draft a simple PCB to be fabbed in China as the cost is so cheap. It would be a good excuse to see if I can get my head around DipTrace which I downloaded a while ago but never really got to grips with.

[simon]

I would suggest trying with better metal film or carbon resistors, maybe tackman. Also try a simple 9v battery (or two if its split rail) I don't trust that SMPS and certainly don't if you have made a dummy split rail. Also if that ceramic cap in the feedback path? of so, replace with a polystyrene. Most ceramic (not so much C0G) introduce a shocking amount of distortion. Also need power rail decoupling, electrolytic and ceramic.

(My bold highlights above)

Just to check - the two ceramics I've used above are power supply decoupling (didn't have any 10uF to hand to parallel). These aren't the same as if they were in the feedback loop I think? Do you mean a cap in parallel with the resistor between the inverting input and the output?

[Nick]

That's fine, I could only see one so wasn't sure if they were decoupling the supply pins. Ceramics are fine for supply decoupling.

Yes, I was referring to the cap in the feedback loop in ed's diagram.

[simon]

Brill thanks

[simon]

I took my MoFo (and D3a-2A3 SET) round to Bostod's earlier this morning, dodging the heavy rain. And I realised I never posted any photos of my "proper" build. There were a few MoFo threads at the time, I think this was the one I piggy backed, so I'll bung them here.

This was a cheap build - wood chassis, LED SMPS PS, PC CPU geatsink and fans rather than the big extruded heatsinks, and simple op amp gain stage.
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The base was a recycled bottom of an old OB speaker

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24V SMPS on the left with LC-CMC filter per channel. Op amp gain stage with very simple PS (one of Andrew's general 3 leg regs) in the middle. Space left for decent regs.
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Cover for the base from offcuts of mdf

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[simon]

Top on, marked up for air vents
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Router jig for the cutouts
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Finished
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