Alesis Amp board mods.
by Tom Whipple & Thomas Lane
1) To eliminate the requirement for two separate power feeds to the amplifier chips,
simply remove the LM2876 and replace it with an LM3886. Then just feed
both chips the same voltage according to the hookup directions (J4 connector
jump pin 1 to pin 5, pin 2 to pin 6 and pin 3 to pin 4). Additional LM3886
chips are available from a variety of sources for under $5 each. Go to
www.national.com and search for LM3886 for a source.
2) To remove the low-end rolloff, remove the plastic shield on the bottom of
the unit and solder in a jumper in parallel to C48 and C45 on the bottom.
There is no need to remove these capacitors from the board, just short them out.
To adjust the low end Fo frequency see mod #3 below - you can also use this
as a sub-sonic filter by moving the Fo to 10Hz or so. This change can easily
be done by adding caps in parallel to the existing caps. Parallel caps add
in value and will decrease the Fo point.
3) To change the crossover frequency of either the high or low pass filters use
the formula Fo = 1/(2*pi*sqrt(r1*r2*c1*c2)). The easiest way to find a
new crossover frequency will be to keep the capacitor values the same
they are now and play with different standard resistor values. This is because
capacitors cost about $0.60 each, while the resistors cost $0.08 each. Surface mount
resistors are available from Mouser, as well as other places. Be sure to change
all the high-pass or all of the low-pass filters to a similar Fo unless you
really know what you are doing :-)
4) Each individual low or high pass filter is second order. Both of the high and low pass
filters on this board are 8th order by design because four are placed in series.
If you want a lower order crossover you can disable one or more of the filters.
For the high pass filters, just short out the caps. For the low pass filters, just
remove the caps.
5) To change the gain on either the woofer or the tweeter output you will need to adjust
R14 to R16 ratio for the woofer or R3 to R4 ratio for the tweeter. The way the
voltage gain is determined is the ratio of these resistors plus 1. R3 is 10K and
R4 is 1K, therefore you have a gain of 10/1+1 = 11 on the tweeter. Change either
resistor to achieve the gain you need but keep the values somewhat reasonable.
It is possible to put in a potentiometer here for the larger value resistor
for adjustability. The data sheet indicates that the minimum gain should be 10.
The maximum practical gain is probably around 30. The gain of the woofer can
also be adjusted by changing R41, which controls the gain of the inverter
just prior to the woofer. The gain of this stage is R41/R44. Don't set the
gains too high, you will quickly create lots of distortion if you hit the
protection limits build into the LM2876/LM3886.