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Re: Choke loaded VFET SET amp

August 11th, 2016, 4:44 pm

Wondering where you got the chokes? DIY?

One of the advantages of this kind of amplifier, that wasn't mentioned, is that the power supply gets essentially isolated from the signal path. If you use a single power supply, and capacitor couple the output with the other end of speaker tied to supply ground, then power supply noise and ripple is blocked by the inductor. And if you trace the output signal current paths, no speaker signal current flows through the power supply filter/reservoir capacitors (unlike with other schemes). The power supply outputs constant current, so the reservoir caps don't see any signal current. The inductor more or less takes over the "reservoir" function -- that's in the ideal case, of course, of the inductor being large enough that it acts as a current source for all the bandwidth... unfortunately it's not entirely the case at low frequencies. But the low impedance of a Sziklai pair follower, with DC supplied via the inductor, would still isolate well from the power supply. And if you use a regulated switching power supply (very cheap, use the ones made for LED lighting!), they have very little low-frequency noise, and their higher frequency noise is very much blocked by the inductor.

I've made several of a somewhat similar type amplifier, using (in most cases) Triad chokes. There is to be an article soon in Linear Audio magazine about a previous version (good for about 20W/ch). The latest (and probably last) amps I made that way do about 60W/ch/8 ohms, SE class A, no global feedback. Because of the choke, and some tricky circuit design, the efficiency is about 45%, quite good for a SE amplifier I think. The heatsinks are surprisingly small for a fully-class A of that power. The steady state dissipation is nearly identical to that of an old McIntosh MC60 amp in fact. The output devices are source (or emitter) follower configured Sziklai pairs ("CFP"), so the output impedance of the amp is still quite low, as is the harmonic distortion (under 0.1%, primarily 2nd order, with a little 3rd order, much like a triode's).

2nd SPRING schematic.pdf
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Sping Amp Pic BW.jpg

Re: Choke loaded VFET SET amp

April 29th, 2017, 9:22 am

The audioXpress article on the VFET amp is finally scheduled for publication as a two part series. It will appear in the June and July 2017 issues. Here is a view of the first page of the article. BTW, if you do not subscribe to AX, they allow purchasing of single issues as PDF files on their web site.

Tom
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Cherry Bomb page 1.jpg

Re: Choke loaded VFET SET amp

April 29th, 2017, 9:33 am

It sounds like a "new wheel." (No pun intended) But here is the thing -- amplifiers don't "amplify." They modulate the power supply however it's configured -- even solid-state equipment is a "valve."

Re: Choke loaded VFET SET amp

April 29th, 2017, 10:24 am

SoundMods wrote:It sounds like a "new wheel." (No pun intended) But here is the thing -- amplifiers don't "amplify." They modulate the power supply however it's configured -- even solid-state equipment is a "valve."


Depends on whether you are talking about the mechanism or the result. The mechanism is to have an active device of some sort to modulate the voltage of the power supply. However, the net result is that the input current and voltage from the source to the amplifier are amplified to provide the necessary power to drive the speaker. Most people look at it in terms of what happens to the input signal change and that is an amplification. It's all in how you view it.
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