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From: "Phil Allison"
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Tue, 19 Nov 2002 23:47:40 +1100
NNTP-Posting-Date: Tue, 19 Nov 2002 23:37:18 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Paul Burridge" wrote in message
> This isn't an attempt to build a hi-fi amp at all. The circuit is
> published in a book called the Art of Electronics by some guy from
> Harvard. It uses three compensating diodes to separate the transistor
> bases by 3 diode drops, although I found I had to take a couple out or
> there was far too much q. current.
** Three bias diodes sounds like a quasi complementary design - ie both
outputs are NPN and the drivers are NPN and PNP - is that the case ?
The thing actually worked best of
> all when *all* the diodes were shorted, but then of course there was
> no check at all on temperature increase in the output devices.
** This is sounding weird - with even one diode shorted there is no
> to say that for the sake of expediency, the output pair were *not*
> matched (75W and 90W) nor was it possible to select diodes with the
> appropriate characteristic curves to match those of the output
> trannies. The load was a 4ohm speaker. There was also a large
> bootstrap cap from o/put back to the middle of a split resistance that
> formed the driving stage's collector resistance. Supply was 30 - 0
> -30v. and the heat sink was just a sheet of alloy.
** The schematic has to be revealed and the type numbers too.
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