"Frank Gilliland" wrote in message
news
On Sat, 6 Jan 2007 15:40:50 -0500, "Jimmie D"
wrote in
:

snip
Two transistors in paralell only have the gain as one.


His transistors are in push-pull, not parallel.




Same thing no more gain. One would have to be driving the other to have any
more gain, It will just handle more power.

On Sat, 6 Jan 2007 20:35:49 -0500, "Jimmie D"
wrote in
:


"Frank Gilliland" wrote in message
news
On Sat, 6 Jan 2007 15:40:50 -0500, "Jimmie D"
wrote in
:

snip
Two transistors in paralell only have the gain as one.


His transistors are in push-pull, not parallel.




Same thing no more gain. One would have to be driving the other to have any
more gain, It will just handle more power.


I rushed the last answer because I was in a hurry to get out the door.
Anyway, your statement is true if you are talking only about current
gain. Power gain, OTOH, is double in either configuration.

For example, if a transistor with a current gain of 10 is fed an input
of 100mA the output will be 1A. Two transistors in parallel will split
the input current at 50mA each, with an output of 500mA each or 1A
total. No additional current gain.

But you are forgetting that when two transistors are in parallel the
input and output impedances are reduced by half. If you take the input
signal and convert its impedance to one-fourth of what is fed to a
single transistor you can maintain 100mA input to each transistor
using the same input power, with an output having -twice- the current
at half the impedance of one transistor. The result is double the
power gain.

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.

"Frank Gilliland" wrote in message
...

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.

Not the best, but if you look at the third image, you will see the
schematic... Or try this link, a little larger and clearer...

http://www.telstar-electronics.com/d.../image1462.png

"Frank Gilliland" wrote in message
...
On Sat, 6 Jan 2007 20:35:49 -0500, "Jimmie D"
wrote in
:


"Frank Gilliland" wrote in message
news
On Sat, 6 Jan 2007 15:40:50 -0500, "Jimmie D"
wrote in
:

snip
Two transistors in paralell only have the gain as one.


His transistors are in push-pull, not parallel.




Same thing no more gain. One would have to be driving the other to have
any
more gain, It will just handle more power.


I rushed the last answer because I was in a hurry to get out the door.
Anyway, your statement is true if you are talking only about current
gain. Power gain, OTOH, is double in either configuration.

For example, if a transistor with a current gain of 10 is fed an input
of 100mA the output will be 1A. Two transistors in parallel will split
the input current at 50mA each, with an output of 500mA each or 1A
total. No additional current gain.

But you are forgetting that when two transistors are in parallel the
input and output impedances are reduced by half. If you take the input
signal and convert its impedance to one-fourth of what is fed to a
single transistor you can maintain 100mA input to each transistor
using the same input power, with an output having -twice- the current
at half the impedance of one transistor. The result is double the
power gain.

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.



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 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.

He probably does have an impedance matching problem , 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.

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.


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.

On Sun, 7 Jan 2007 00:39:51 -0600, "PowerHouse Communications"
wrote in
:


"Frank Gilliland" wrote in message
.. .

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.

Not the best, but if you look at the third image, you will see the
schematic... Or try this link, a little larger and clearer...

http://www.telstar-electronics.com/d.../image1462.png


Not good; and it looks like his audio processor, not his amp.

"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.

"Frank Gilliland" wrote in message
...
On Sun, 7 Jan 2007 00:39:51 -0600, "PowerHouse Communications"
wrote in
:


"Frank Gilliland" wrote in message
. ..

Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.

Not the best, but if you look at the third image, you will see the
schematic... Or try this link, a little larger and clearer...

http://www.telstar-electronics.com/d.../image1462.png


Not good; and it looks like his audio processor, not his amp.

Yes, you are correct.... My mistake; just a little tired last night.

Frank Gilliland wrote:
Brian's amp should be pushing 18dB. If it isn't then he screwed up the
matching of the transistor input and output impedances. That's one
reason why I want to see his schematic.

Frank, wrong as usual. Here is an excerpt from
http://rfdesign.com/microwave_millim...fiers_improve/

"Consequently, the gain of a push-pull amplifier is the same as that of
an individual amplifier, where the output power is twice that of an
individual amplifier. Thus, push-pull amplifiers are frequently used
for combining power of individual amplifiers. However, there is more to
these amplifiers than combining power. This topology of amplifiers
helps cancel even harmonics and intermodulations and improves the
even-order intermodulation product."

Oh, let me guess... the man who wrote this article (director of
engineering at Mini-Circuits) doesn't know what he's talking about
either... LOL

Frank... you just keep getting better & better.

www.telstar-electronics.com

On Sun, 7 Jan 2007 09:53:10 -0500, "Jimmie D"
wrote in
:

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


That's the theory. Reality comes pretty close.


snip
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.


Without seeing it I can only guess that your tuning was pretty low-Q;
maybe not the best components, too large a coil, or perhaps some shunt
resistance.