SymiFet A 100% symmetrical mosfet amp

[mjurban]

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.

[mjurban]

I did a quick search, here are a few threads on the subject:

Complementary input stages with current mirrors

HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles

Fresh fix for *that* Slone amp (ie yet another symetric topology)

Unstable VAS current in amp from Slone book

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

[Tekko]

Well the lower LTP putting out -40V on the bases seem more like a issue with the 2N5401's rather than the usual issues discussed about this topology.

[homemodder]

This amp will never work properly, like mjurban already mentioned the bias currents for the vas are not defined, rather ask Bob Cordell for his solution as published in his book. I dont think he has shown it here yet.

[Tekko]

Your talking about the first circuit i posted that just have the 18k resistors in place of current sources, right ?

[homemodder]

Tekko Im afraid the second suffers the same fate. Mjurban has shown the threads where the problem is discussed, Cordell has the simplest solution.

[Tekko]

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.

[homemodder]

Take a look at what happens with the output transistor biasing, first signs of problems.

[Tekko]

So i assume the issue everyones talking about is that i cant be biased, its either zero bias or near dead short bias.

Thats where im at right now, the amp works and play music but i cant bias it.

[Tekko]

So this is the actual circuit as it stands right now: http://i.imgur.com/96bKv.png

I ran out of 2N5551's so i dident build the current sources. The dc offset on the output is roughly 10mV accoding to the scope.

I have not yet decided if im gonna keep working on this or just scrap this circuit alltogether and stick to the blameless topology i've been using up to this point.