"Frank Gilliland" wrote in message
...
On Sun, 7 Jan 2007 02:04:05 -0500, "Jimmie D"
wrote in
:

snip

I must admit I never built a single transistor PA before SO I will have
to
look at the numbers again. Granted the push pull amp is more efficent than
the single ended amp


Not necessarily....


but Im still not sure about the gain. Il have to crank
the #s when I dont have a snoot full of cold medicine. Please correct me
if
I am wrong but you are saying that two amps biased for say classB
operation
that a 2 transistor anp will have 6 db more output than a 1 transistor amp
with the same drive power.


No, just 3dB. Double your pleasure, double your fun.

SO you are saying 3db more power with the same amount of input.



He probably does have an impedance matching problem ,


Yes he does. And his amp probably does, too.


almost all amps of
this type do to some degree. The impedance
transformation of the transformers can almost always be improved upon by
the
inclusion of an adjustable L or Pi network in the output of the amp.. I
know
the last amp I tested had an output impedance of about 35 ohms. A matching
network added to this amp increased power out, reduced harmonics and made
the transistors stop popping.


Impedance matching in an amp isn't a plug-n-play application. It's not
as simple as reading the specs off the data sheet and punching numbers
into a calculator. Since the big-signal gain of the transistors varies
so much, each transistor pair much be physically measured, matched,
and the impedance adjusted accordingly for maximum gain. The biggest
problem with these amps is that the transformers have so few turns
that they are very impedance-specific, as you discovered with your 35
ohm amp. There are only two solutions: either use transistors within a
very narrow range of gain -or- change the gain (and therefore the
impedance) of the transistors with negative feedback. I like the
latter because it increases linearity. It does require each amp to be
adjusted for any given pair of transistors, but almost all of the
transistors in stock can be used because the gain range isn't nearly
as limited.

Yes, you can transform the impedance with a tuning network prior to
the input transformer and after the output transformer, but it will
limit the frequency response of the amp to a narrow bandwidth. This is
how most of the so-called "broadband" CB amps are built. Most of the
people who have used them know what a bitch it is to retune the amp
every time you want to use it on a different part of the band. That's
not "broadband" -- it's false advertising.




The one I made covered the whole band pretty much, Pretty much meaning that
even though it wasnt a perfect match everywhere it was still a lot better
than what it was without it.