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Mighty Atom D.C. 4 Amplifier (World's simplest valve/vacuum tube amp)

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Ash Small

Mon Nov 11 2013, 04:04AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This is not my work, but I've heard it and Chris is right, it's the best amp I've ever heard. Below is what Chris himself says about it. Any comments will be appreciated. I may even be able to get Chris to answer questions directly here, although there is an invitation to discuss it with him on FaceBook. I've not yet asked him for permission to post a link here to his Facebook page, but he did give me permission to post the circuit here for discussion. I do have a sketch of the power supply schematic, but I'll have to scan it before I post it:

"The Simplest Audio Amplifier in the World.
Complete wiring details and component values simply explained.
The way I have described my Mighty Atom D.C. 4 Amplifier in this diagram is so that even a ten year old could construct it if they sourced the parts. With modest output transformers it can be built for under a hundred pounds. The power supply required contains five more components. A mains isolation transformer two electrolytic capacitors a resistor and a bridge rectifier. In the usual configuration. No heater supply is required as the heaters are the same current as the amplifier circuit and replace the power resistor of my standard "Mighty Atom" circuit, and are therefore not only D.C. heaters but the heaters also supply good regulation to the rest of the circuit. This combination of the elements means greater speed, efficiency and smoother operation due to there being so few electrical time constants.
The Direct Transformer coupled variant approaches the ideal of an in phase amplifier, with less inherent losses which lead to even greater efficiency.
The performers really do stand up and come right into the room.
The D.C. Choke coupled variant negates the potential divider aspect of the power supply impedance. All three variants are superb sounding.
Parallel feed output can also be used if preferred.
The valves, Two UL84 and one UCC85 are still easily obtainable at a modest price.
Any one wishing to build it can speak to me on here with any queries (C:"

Ash Small

Mon Nov 11 2013, 05:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Here's a couple of shematics, one of which he scribbled put for me on a 'post it' note on Saturday. A few more 'words of description' from Chris, too:

"The Standard "Mighty Atom" Circuit. This is the Direct Coupled Stereo Amplifier Circuit I invented and developed in the late nineties, and is the circuit I use in every amplifier I build whether it be a Phono Pre-Amplifier a studio monitor Amplifier or a 500 Watt Push Pull Disco or Public Address Amplifier. The only thing which changes is the valves and their respective component values. The circuit remains the same. The circuit is neither Cascade coupled nor is it Cascode coupled. It is more like a combination of both and can only be achieved when the valves are coupled directly, negating the need for coupling capacitors. The circuit cannot be found in any of the literature or amplifier manuals. It is my sole invention and unlike other direct coupled amplifiers is a fail safe design.
A few years ago I learned that Alan Dower Blumlein had been looking for a third method of coupling before he died during World War II on 7 June 1942, aged 38, during the secret trial of an H2S airborne radar system.
The Power supply uses five more components as mentioned in my previous post.
Respect due to the shoulders of Ohm, Faraday, and Deforest, and not forgetting a few kind words from Albert Einstein (C:"

"You will notice that every component connects to one of only four points. Hence the name D.C.4"

1384189562 3414 FT158787 Mighty Atom Shematic

THE FOLLOWING DIAGRAM WAS NOT INTENDED BY CHRIS TO BE POSTED/PUBLISHED HERE, AND WAS NOT INTENDED FOR NOVICES. iF YOU ARE A NOVICE WITH VACUUM TUBES/VALVES PLEASE IGNORE THIS DIAGRAM, AND ASSOCIATED COMMENTS. THE OTHER TWO DIAGRAMS ARE THE ONLY DIAGRAMS SO FAR THAT CHRIS INTENDED FOR PUBLICATION

1384189562 3414 FT158787 Mighty Atom0002

radiotech

Tue Nov 12 2013, 06:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

You could make it simpler by grounding the grid and feeding the audio from the
low impedance source across the cathode resistor through a suitable (15 uF) capacitor.

Ash Small

Tue Nov 12 2013, 05:33PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radiotech wrote ...

You could make it simpler by grounding the grid and feeding the audio from the
low impedance source across the cathode resistor through a suitable (15 uF) capacitor.

I forewarded your comment to Chris, and he replied as below, although he also mentioned the expense involved in using a 'quality' 15uF capacitor, and something about requiring a 'very low' impedance source.

"I see what... (radiotech) ...means, however when you wish to make the amplifier stereo or multi channel then the component count goes up as each input section cathode then needs its own resistor in order to make each cathode the input, thus complicating the design further.
I have thought through all these possibilities and application versatility has shown itself to be the beauty of this design alongside its simplicity. The very fact that (under any conditions and no matter how many channels are employed) the cathodes are common and thus of a low impedance giving a high degree of regulation, gives the end product its distinctly superior sound stage."

Proud Mary

Tue Nov 12 2013, 11:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

EZ80: there should be safety limiting resistors in series with both anodes. For 300V, their value should be 215Î©.

Source:

Ash Small

Wed Nov 13 2013, 11:15AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

PLEASE SEE COMMENTS IN RED ABOVE

Proud Mary wrote ...

EZ80: there should be safety limiting resistors in series with both anodes. For 300V, their value should be 215Î©.

Source:

I've forwarded your comment to Chris, PM. I'm not sure he intended for me to post that schematic, it's one he drew out for me on Saturday on a 'post it' note. (I thought I'd post it anyway, though.) I'll post his replies as I recieve them.

He has suggested using silicon rectifiers in his comments regarding power supplies.

I imagine the 3H (minimum) inductor and 2uF capacitor provide some regulation, though. I believe chris uses inductors up to 8 H on some of his power supplies, but I'll wait until he replies.

(EDIT: I will be away overnight, as the RSC is 'broadcasting live' to selected cinemas around the country for the first time in history, but I will update as soon as I'm able to.)

Proud Mary

Wed Nov 13 2013, 12:34PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The function of the limiting resistors is to protect the valve from inrush current into the reservoir capacitor. If the size of the reservoir capacitor is increased, the limiting resistor must also be increased to take account of it, and vice versa.

I did not make my point about the limiting resistor value very clear, as the data sheet figures include transformer resistance and reflected impedance, which must be worked out first to find out how much additional resistance will be required to protect the rectifier valve from inrush surges.

Not using a limiting resistor will shorten the life of the valve, and is bad practice.

johnf

Thu Nov 14 2013, 05:55PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Proud Mary
I think that is why the first capacitor before the HT choke is 2uf ie around 800 ohms @ 100hz ripple

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