SymiFet A 100% symmetrical mosfet amp

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Im pretty sure this have already been done many times over but i'll post it anyways.

I just came up with this based on five pairs of IRF540/9540 i have.

First a simulation: http://i.imgur.com/qwiho.png

And a readable schematic: http://i.imgur.com/hq1yk.png

Yes i know, the simulation doesent look all that impressive, but keep in mind i do all my simulations at 20kHz, most amps can do 0.005% THD at 1kHz and still exceed 1% at 20kHz.

However if THD can be knocked down to 0.05% at 20kHz, it should make for a really good amp in my opinion, even if it may be too perfect, making the sound lifeless and flat.
 
The input stage and the VAS in this amp are going to have problems unless the transistors are closely matched for beta. The problem is that the voltages at the collectors of Q1 and Q3 are not well defined. Since those same voltages are at the bases of Q6 and Q7, and thus determine the currents in the 330 ohm emitter resistors, the current in the VAS chain is also undefined. This is a common problem with this topology, using complimentary circuitry with the input stage loaded by an active source. Bob Cordell discusses this in his book, and I think there's some discussion of it somewhere in the book thread in this forum. Randy Sloan's book also has several designs using this topology, and there's been lots of discussion of those designs also in this forum.
 
This here i choose to call SymiFet Deluxe, based on the old Elektor HEXFET amp, using current sources for the input stage and beta enhanced VAS: http://i.imgur.com/fBNw2.png

And if the LED/diode type current source isent your flavor, the dual transistor one will also work just fine, if not better.

I do have an old transistor tester that measures the hFE so i can do matching if necessary, assuming that old thing still works and doesent use some long obsolete obscure unobtanium battery format.
 
And the verdict is: It was all just 100% waste of time!

It does not work at all, all it does is put the negative rail on the inputs of the ltp which puts the speaker output at either the positive or negative rail.

On the initial powerup i had wired the ltps wrong which smoked the current mirror resistors, but i unplugged power fast enuf to avoid burning them, but after fixing that i still had no luck, i replaced two transistors that measured dead short between B and E which did no difference at all!

Only thing that does anything is breaking the contact between the bases of the upper and lower LTP.

I highly doubt closely matched transistors would make any difference either.
 
This topology has been made to work, but it usually involves providing some sort of reference voltage to help define the operating conditions of the VAS. I'm sorry that I can't say much more than that, because I only glanced at the various discussions of the subject on this forum. I will say that those who have been able to make it work have been very pleased with the performance, so it might be worth while searching in the forum archives to find some of the solutions.
 
I did a quick search, here are a few threads on the subject:

http://www.diyaudio.com/forums/solid-state/133864-complementary-input-stages-current-mirrors.html

http://www.diyaudio.com/forums/soli...ror-image-topology-construction-troubles.html

http://www.diyaudio.com/forums/soli...one-amp-ie-yet-another-symetric-topology.html

http://www.diyaudio.com/forums/solid-state/16796-unstable-vas-current-amp-slone-book.html

As you can see, it's a problem that's been extensively discussed. I still think Bob Cordell had the best solution, but I couldn't turn it up with the simple search I did. His comments are worth searching for.

Mike
 
I've replaced all the input stage transistors and now it no longer does what it did before, now its showing signs of life as in a almost square wave oscillation which for me is a victory as yesterday was pretty much a day of fail.

Now to replace the 1.8 ohm resistors i accedently used with something bigger and see about adding some caps to the VAS's and the drivers.
 
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