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 as
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 the
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 audio
amps could be a useful form of linear for the band.
Cheers, Jim Moritz
73 de M0BMU 
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