Return-Path: Received: from rly-dg08.mx.aol.com (rly-dg08.mail.aol.com [172.19.151.92]) by air-dg05.mail.aol.com (v125.7) with ESMTP id MAILINDG052-5e94adc7a0d35a; Mon, 19 Oct 2009 10:39:17 -0400 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-dg08.mx.aol.com (v125.7) with ESMTP id MAILRELAYINDG088-5e94adc7a0d35a; Mon, 19 Oct 2009 10:39:12 -0400 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1MztMm-0005pb-HO for rs_out_1@blacksheep.org; Mon, 19 Oct 2009 15:37:44 +0100 Received: from [193.82.116.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1MztMm-0005pS-4c for rsgb_lf_group@blacksheep.org; Mon, 19 Oct 2009 15:37:44 +0100 Received: from out1.ip03ir2.opaltelecom.net ([62.24.128.239]) by relay1.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1MztLC-00020v-A0 for rsgb_lf_group@blacksheep.org; Mon, 19 Oct 2009 15:36:06 +0100 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: ApoEAHoW3ErUQ3lr/2dsb2JhbADbOIQxBIFb X-IronPort-AV: E=Sophos;i="4.44,585,1249254000"; d="scan'208";a="36874007" Received: from oasis-out-f.onetel.net.uk (HELO qm10.onetel.net.uk) ([212.67.121.107]) by out1.ip03ir2.opaltelecom.net with SMTP; 19 Oct 2009 15:37:25 +0100 Received: (qmail 49881 invoked by uid 80); 19 Oct 2009 14:37:21 -0000 Received: from 213-78-182-231.ppp.onetel.net.uk (213-78-182-231.ppp.onetel.net.uk [213.78.182.231]) by mailzone.onetel.net.uk (Horde) with HTTP for ; Mon, 19 Oct 2009 15:37:21 +0100 Message-ID: <20091019153721.iapc84ciw0owg8cg@mailzone.onetel.net.uk> X-Priority: 3 (Normal) Date: Mon, 19 Oct 2009 15:37:21 +0100 From: Brian G3YKB To: rsgb_lf_group@blacksheep.org References: In-Reply-To: MIME-Version: 1.0 Content-Disposition: inline User-Agent: Internet Messaging Program (IMP) H3 (4.0-BETA) X-Karma: unknown: X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,none Subject: Re: LF: AF Amps at 136 Content-Type: text/plain; charset=ISO-8859-1; format="flowed" Content-Transfer-Encoding: quoted-printable X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.8 required=5.0 tests=PRIORITY_NO_NAME autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-AOL-IP: 193.82.116.20 X-Mailer: Unknown (No Version) 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 : > 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.