To: <[email protected]>
Sent: Monday, October 19, 2009 3:37 PM
Subject: Re: LF: AF Amps at 136


Hi Chris -

I built a linear PA for 136 kHz (I wanted it linear so as to run
PSK31). It uses
four bipolar output transistors (two MJ15003 and two MJ15004) in complement
ary
parallel/push-pull class AB. I drive them with two other transistors (2SD14
59
and 2SB1037) in darlington configuration - ie a simple "AF amplifier" type
of
circuit.
What is it ? its either an AF AMP or RF AMP.
G3KEV


The whole lot is fed from a TDA2030. It produces a peak output of
150
watts (from a dual 32 volt supply) and has withstood all sorts of abuse.
However it is only 40% efficient (I suspect this may be due to the slew
rate of
the power transistors).

"AF amplifiers" can work!

Regards - Brian (G3YKB)

Quoting Chris <[email protected]>:

Hi All,
Just started constructing TX for 136kHz, noticed Maplin do a Velleman
audio PA kit that is rated at 200W (100W RMS into 4R). OK there's no
mains transformer or heat sink, but reckon it's good value at £14.99.
Point is the specs also say the minus 3db frequency response is 3Hz /
200kHz.
The RSGB books 'LF Today' refer to using audio amps as PAs at 136
with 'a few modifications'. Does anybody know where these
modifications are documented? I guess there's nothing major, reduce a
few cap values? Remove Zobel Network etc.?? If anyone has experience
with this approach I would appreciate some hints. Thanks.
Chris, G4AYT, Whitstable.

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