Re: Ultra-simple discrete 10W audio power stage for guitar use - would this work?

On Tue, 16 Dec 2008 11:52:52 -0800 (PST), morris.slutsky@xxxxxxxxx
wrote:


   And that's another thing, the current gains of these Darlington
pairs has to be carefully matched to be sure the positive and negative
half-cycles are the same at the output. On the other hand, such an
imbalance may simulate the sound of a single-ended tube output. :)

Like i said before, seriously Muntz'd.


   You can make this into a an equivalent but more
conventional-looking schematic by moving the ground to the 'usual'
place, swapping each Darlington pair of transistors to the other rail,
and driving the bases (which will be near their respective rail
voltages) from the input through capacitor copupling. I'm pretty sure
this would work the same way (except it won't have response to DC,
which you don't really care about, though this one doesn't anyway
because of that 'fake center-tapped' power supply), and the schematic
would 'look right' to everyone who has studied conventional audio
power amplifiers.



Graham

Yeah, I could move things to their 'usual' places and it would look
more normal. But then I'd need to use more components. Two more
capacitors? That'd be . . . more parts! This is a minimum components
idea. I'm willing to hand-select driver transistors, although yeah, I
don't mind a little bit of 2nd harmonic imbalance at all.

Voltage gain is about 4. That doesn't sound like much. But that'll
take a line level signal up to 9V RMS really easy, which is enough to
clip this thing out. Boosting the gain by changing the resistor ratio
would also push the bias way, way down in a bad way. But here's the
cool part - if I use a higher-voltage supply, then I can change the
resistor ratio for higher gain and keep the existing Class AB bias.
So it's sort of . . . invariant? Like, there will be enough gain from
line level, even if I use a bigger supply.

Right now I'm thinking that this could work nicely with a JFET preamp,
using J201's maybe. Yeah, the preamp will need it's own power
supply. But not a very powerful one. Heck, whatever wall wart I can
find in the sub-$5 range could donate it's organs to the cause.

Someone mentioned capacitor coupling - I like capacitor coupling for
guitar amps, I really do. Just like the tube amps with transformer
coupling - you can't put DC through it. But do you want to? Fact is,
a lot of musical waveforms (heck, even the human voice!) are
asymmetrical and can easily generate DC upon clipping. Speakers don't
like that DC. Sometimes it's nice to have an amp that won't put the
DC out. And if the same cap can be power supply filter and output
coupling, that's just cool!
.

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