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Date: Mon, 19 Oct 2009 15:37:21 +0100
From: Brian G3YKB <g3ykb@onetel.com>
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Subject: Re: LF: AF Amps at 136
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Hi Chris -

I built a linear PA for 136 kHz (I wanted it linear so as to run=20
PSK31). It uses
four bipolar output transistors (two MJ15003 and two MJ15004) in complemen=
t
ary
parallel/push-pull class AB. I drive them with two other transistors (2SD1=
4
59
and 2SB1037) in darlington configuration - ie a simple "AF amplifier" type=
=20
of
circuit. The whole lot is fed from a TDA2030. It produces a peak output of=
=20
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=20
rate of
the power transistors).

"AF amplifiers" can work!

Regards - Brian (G3YKB)

Quoting Chris <c.ashby435@btinternet.com>:

> Hi All,
> Just started constructing TX for 136kHz, noticed Maplin do a Velleman=20
> audio PA kit that is rated at 200W (100W RMS into 4R). OK there's no=20
> mains transformer or heat sink, but reckon it's good value at =A314.99.=
=20
> Point is the specs also say the minus 3db frequency response is 3Hz /=20
> 200kHz.
> The RSGB books 'LF Today' refer to using audio amps as PAs at 136=20
> with 'a few modifications'. Does anybody know where these=20
> modifications are documented? I guess there's nothing major, reduce a=20
> few cap values? Remove Zobel Network etc.?? If anyone has experience=20
> with this approach I would appreciate some hints. Thanks.
> Chris, G4AYT, Whitstable.