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Positive Feedback ISSUE1
A direct coupled, active loaded, parallel feed output SE 45 amplifier
(reprinted from Valve, October 1997)
Heres a truly delightful amp circuit for owners of truly efficient speakers (see the schematic shown below).
The protoype of this circuit, built at the first amplifier builders class after VSAC 97, left all in the room at its debut rather smitten, particularly John Tucker, who currently uses this circuit as his front line amp on his Exemplars.
The midrange is balanced, smooth, and very real. Bass articulation is phenominal, you would never expect this sound from a puny 45. Top end? How bout -1dB at 46kHz, and black as can be?
Downside? Well, its only 1.6 watts, so you better have some pretty efficient speakers (although Kim Jenkins seems happy with this setup on Whamos).
And its not a forgiving circuit, if your front end sucks, youll know it. This amp reveals a great deal of information.
Also, doing it right means it isnt a super cheap project. For best performance youll want to use nickel core parallel feed output transformers, and proper plate loading chokes. The choice of manufacturers for these items is pretty narrow (said with a big grin). The name of this amp gives about 90% of the circuits description. Note the use of an active load on the driver stage. This load is quite similar to the one used in the Afterglow circuit, and in concert with the parallel feed output arrangement, accounts for the amps superior speed and bass response.
You might try this load, with the proper adjustments as outlined in the note on the next page, as a load for your other projects. Try it on the first stage of the 5963 output stage in George Wrights line amp, or on top of the mu follower in the S.E.X. amp (put it in in place of the 20K resistor).
The 45 is running at a classic operating point, 250V plate-to-filament, with -50V grid bias, yielding a 34mA current draw. Note that these voltages are translated up by the plate voltage of the direct coupled driver stage, so actual measured voltage on the plate of the 45 is more like 400V above ground. Man, I get all tingly just writing about this amp.
Better get started on my own copy!
Direct coupled parallel feed output SE 45 amplifier and power supply - one channel shown.
Direct coupled parallel feed output SE 45 amplifier parts list (for two monoblocks).
Aside from the parafeed iron, most of the parts specd are fairly non-critical in terms of manufac-ture. I changed Smoothplates spec for the PS bleeders from metal film to carbon film, because Ive found they seem to hold up to big startup voltage swings a little better.
Note that the power trans specs are the bare minimum required. Bigger may work better.
Brooklyn BCP-15 plate loading chokes are available for $100 the pair, and Magnequest EXO-45 (5K:8) and EXO-46 (5K:16) parallel feed output transformers are available for $150 the pair with M6 steel laminations, and $250 the pair with Permalloy (nickel) laminations. For a real treat, we also have a couple of pairs of the Permalloy EXO-46s with pure silver secondaries and Teflon coated touched by God copper primaries, for $650 the pair. Call 360-697-1936 to order any of these goodies.