Silicon Chip publishes SET amp

Aug 03, 2005 8 Replies
P
** World comes to an end this month .....


The guys at SC magazine have broken one of their longest held vows - never to publish the design of a valve amplifier.



Worst of all, it is not a hi-fi amp but an SET ( single ended triode) design, beloved by the miniscule 1930s retro-brigade among "tube heads".



As if to spit in the eye of such Luddite wankers, it sports back to back wired toroidal transformers in the PSU, 100 volt, PA line transformers for output, common guitar amp valves wired to work as power triodes and a



*transistor* buffer stage driving the two ( paralleled ) output valves ????

Good grief !!!



No seriously exotic, obsolete or hand made components to add mystery value at all !!!


.......... Phil


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M

These are the same people who thought they'd set a new standard with a 12AX7 peamp with 0.1% distortion.

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T

**Indeed. SC go a step further to eliminate the most insidious distortion in SET amps, by capacitor/inductor coupling to the output transformer. Quite clever.
Trevor Wilson www.rageaudio.com.au
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T

Definition: "peamp, a piss-weak preamp"

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P

"Trevor Wilson"

** What utter rubbish.

Distortion generated by that 9H iron cored inductor goes directly into the output signal - at 30Hz it has an impedance of only 1700 ohms while the output transformer primary is nominally 2000 ohms - so it winds up becoming the dominant source of non linearity and low end response roll off.

The OT is a 100 volt type intended for PA use - so poor to no low frequency preformance.

.......... Phil

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T

**Good point.

**I am not attempting to defend what is, essentially, a complete waste of valuable paper and ink. The output topology is, however, considerably better than regular SET output stages. BTW: I agree that the use of a crappy output transformer is a big mistake. Leo claims that the thing has a decent frequency response, despite the use of a crappy transformer. I can't confirm, because I have not measured it.

Personally, I don't see the point.

Trevor Wilson www.rageaudio.com.au
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P

"Trevor Wilson"

** Saying the same dumb thing it over and over does not make it any better.

Using a choke plus cap plus OT ( called para-feed) is inferior to using a well designed OT alone.

For the damn choke to NOT spoil the results, its inductance value has to be huge - so it must be huge - and it must be made with the best grade of transformer iron to avoid generating THD.

Easier and cheaper to design a OT with the desired performance.

.......... Phil

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