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LF: Re: MOSFET audio amp mods

To: [email protected]
Subject: LF: Re: MOSFET audio amp mods
From: "hamilton mal" <[email protected]>
Date: Thu, 1 May 2003 17:57:26 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 01, 2003 12:30 PM
Subject: LF: MOSFET audio amp mods

Dear LF Group,

I'm sending this again - it didn't work first time!

I looked up what I did to the Maplin audio amp to increase the slew rate
described yesterday - The mods should also be applicable to the BK amp
modules, judging by the circuit diagram on p4.23 of the "LF experimenter's
source book" entitled "BK Electronics OMP MF300 Module". The major
difference between the circuits is the  BK amp has pairs of output devices
in parallel (to drive a load of 4ohm instead of 8 ohm I guess), while the
Maplin amp has single devices, and the input stage of the BK amp is biased
by an active current source, while the Maplin amp has a simple resistor.
There are a few other minor differences in component values mainly. See
attachment on the basis that a picture is worth 1000 words!

In theory, to get an output voltage of 28V RMS into 4 ohms, giving 200W, a
minimum slew rate of 2.82.pi.f.Vrms = 34V/us is required at 137kHz. My
Maplin amp had a slew rate of 22V/us before the mods, and about 60V/us
afterwards. The small-signal bandwidth was more or less the same at about
1.5MHz. Full output could then be obtained on 136k.

Reducing the value of the 27p compensating capacitors will also increase
the slew rate, but will inevitably also reduce the phase margin, and
therefore increase the possiblity of the amp becoming unstable. It may or
may not work OK, depending on the individual amplifier, and the effect of
the frequency-dependent load impedance.

It seems these audio amps are not as popular for LF these days, presumably
because people are looking for higher power with better efficiency. But if
the US LF allocation arrives with a 100W power limit attached, MOSFET
amps could be a useful form of linear for the band.
Jim don't encourage anyone to use an audio amp of any sort. A proper LF RF
amp is the only way forward. In the past reading from here,  there were
bucket fulls of explode FETS, in spite of some sophisticated mods they
seemed to be unreliable, and using mains electric cable to the antenna
loading inductor did not help.I could fill another COMIC with the stories
that I have heard.

Cheers, Jim Moritz
73 de M0BMU

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