Output capacitorless MOFO

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revintage
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#1 Output capacitorless MOFO

Unread post by revintage »

Since quite a while I have been "obsessed" with the idea, on how to eliminate the output capacitor, for using the MOFO concept for bass reproduction.

With the Hammond193U that I have, it should be possible to solve this with the help of a small servo and a 15-20W 3.3V SMPS below ground. As the choke DCR is 1.7ohm, you get Iq=3.3/1.7=1.94A . A choke with lower DCR will need an added series resistance. Off course a delay/DC protection relay circuit will be added.

A servo is not anything I have tried before, maybe it could be improved?

Will report back, after the smoke has dissipated :bom:.

About the damned "voltage controlled switch"/protection symbol, I had to fool it to stay open. Anyone who can solve this?

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Nick
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#2 Re: Output capacitorless MOFO

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I have to say that I have gone around the loop of thinking about getting rid of that output cap in the MOFO. Every time I run around the loop I stop after thinking "if its such a big deal, then why not a DC coupled push pull amp", and if you like the idea of a bit of second harmonic, then there are loads of low or zero feedback push pull circuits.

Not to say you should not see how it works, its just what I have gone thorough, and the conclusion I came to.
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simon
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#3 Re: Output capacitorless MOFO

Unread post by simon »

I've wondered about that cap too, really interested to see what you find.

OTOH a fairly cheap cap is a very simple and reliable way of solving the problem.
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Nick
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#4 Re: Output capacitorless MOFO

Unread post by Nick »

Given the servo action is to alter the current through the mosfet, would it not be better to just have an adjustment pot to see the offset?

The "better" solution would be to use a variable voltage source to adjust the voltage below the choke.

Also (and this is the sort of thing that made me stop thinking about it) you have removed a cap in series with the output and replaced it with a switch mode power supply in parallel with the output. Not sure which of those two I would prefer...
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
revintage
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#5 Re: Output capacitorless MOFO

Unread post by revintage »

Nick wrote: Tue Feb 04, 2025 1:34 pm I have to say that I have gone around the loop of thinking about getting rid of that output cap in the MOFO. Every time I run around the loop I stop after thinking "if its such a big deal, then why not a DC coupled push pull amp", and if you like the idea of a bit of second harmonic, then there are loads of low or zero feedback push pull circuits.

Not to say you should not see how it works, its just what I have gone thorough, and the conclusion I came to.

You don’t know, until you have tried it ;-) .

As I have all parts except the two 3.3V power supplies, it will be an easy task to breadbord them without spending much more than time.

I have this fix idea about every single stage of my new quad-amped system(except for the subwoofer PP amp) being SE. Must see how this turns out.

In the meantime I am using passsive filters for the speakers with my old 15W 6B4G PPP monoblocks, built by my late friend Erik Andersson(Audio Innovations, Audion)
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Nick
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#6 Re: Output capacitorless MOFO

Unread post by Nick »

You don’t know, until you have tried it
Indeed. Will be interested in your outcome.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
revintage
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#7 Re: Output capacitorless MOFO

Unread post by revintage »

Nick wrote: Tue Feb 04, 2025 3:04 pm Given the servo action is to alter the current through the mosfet, would it not be better to just have an adjustment pot to see the offset?

The "better" solution would be to use a variable voltage source to adjust the voltage below the choke.

Also (and this is the sort of thing that made me stop thinking about it) you have removed a cap in series with the output and replaced it with a switch mode power supply in parallel with the output. Not sure which of those two I would prefer...
Great solutions and suggestions, I really need to be questioned as to not fall into any pitfalls.

I was actually intrigued by the servo also taking care of the temperture drift, if it works IRL. About the switch mode supplies they will be very well filtered from HF. Will check out the possibilities with adjustable negative power source, question is how low you can set it?
Last edited by revintage on Tue Feb 04, 2025 7:56 pm, edited 1 time in total.
revintage
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#8 Re: Output capacitorless MOFO

Unread post by revintage »

Someone got there first 8) .

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