Sold Out--------Alesis Amplifiers

We just received a supply of Alesis Amplifiers and Amplifier modules. This aquisition represents the best value we have seen since we ran out the beloved ParaPix amps. Hurry at these prices they will not last long.


Alesis small amp board momo amplifier module. Similar to the modules seen above. Just add a power supply. Module includes hookup info thanks to Mike Neary and Rudi Blondia contains 1 LM 3886T 68W amp and a LM 2876T 40W ampalong with op-amps ........More info to follow soon..........

Sold Out

    Now in stock Connector set for the Modules.... 3 - 6 pin connectors 2
large and 1 small and 2 - 2 pin all with pins $2.49/set

Hook up information thanks to Rudi Blondia .

The main tap on the Signal BL1733A @ Apexjr for $9.95 is perfect for the +/- 34 supply (48VCT, 7A). It also has 28VCT that will give +-20 for the LM2876. Even better, replace the LM2876 with the LM3886 per the modification suggestions so that all the power amps use the same +/-34 supply. Apexjr has those computer grade 10,000uF caps for $1.50—should be easy to make a low-cost supply. The preamp voltages just use the LM7815/7915 regulators that are cheap and easy to find. I don’t know why Steve@Apexjr doesn’t put together a kit with this stuff—good opportunity for him. The other low-cost route is to salvage power supply parts from an old stereo receiver—usually available for cheap at yard sales and thrift stores (that’s what I’ve done). This usually gets you some useable heatsinks, which are often tougher to find than power supply parts.

Those $5 Alesis amps at Apexjr are cool. In addition to the two power amps (3886/2876), they have a total of 10 Sallen-Key topology filters that can be modified rather easily, just by changing the capacitors or resistors. If you use two modules per speaker, there is more than enough circuitry to have an active 3-way or 4-way system, using a mix of 2nd order, 4th order or eighth order filters. The steep slopes of the higher order filters make the driver response mostly a non-issue--the only tweaking is to adjust the gains for drivers. The image shows one of the two 8th-order sections. With the current values they are configured as Linkwitz-Riley filters (cascaded Butterworths). The two sections yield an 8th-order low-pass and high pass with a 1600Hz crossover frequency. However, by changing the Q values of the filters, these filters can be reconfigured to give you 4 4th-order Linkwitz-Riley filters, which can be cascaded to make the bandpass filters for an active 3-way or 4-way system.

I prefer changing the capacitors rather than resistors, since the capacitors are through-hole rather than surface mount (maybe 20 years ago I'd go for the resistors). I compiled the capacitor values for a range of crossover frequencies--see the schematic at the Apexjr website and the spreadsheet in the link. The first table in the spreadsheet shows the values needed to change the 8th-order crossover frequency. The second set of tables shows the values needed for making the 4th-order bandpass sections. The capacitors can be ordered from Digikey: the Panasonic "P" series polypropylene are mostly 2% and go up to fairly large values without exceeding the 5mm lead spacing that's on the board. You may need to use the metallized polystyrene for some of the larger values. One set of the filters (High Pass, sections 2&4) requires a resistor change to achieve the Q needed for a Butterworth (the resistor values are in bold font), but if you have two modules per speaker to get the four power amps, you will probably be able to avoid this circuitry, anyway.

The two "odd" filters that Alesis uses for RF rejection and low-frequency roll-off can be put to work as a crossover to a subwoofer or bass driver. They are configured as a 2-pole Bessel filters, which give reasonable summed response.

This spreadsheet was hacked together rather quickly, and no guarantees are made for its accuracy. But it's free, and there are no restrictions on its use--in fact, I'd be quite happy if someone took this and made it "pretty" and checked it over for errors. I haven't done any actual modifications on the boards yet, so I can't provide any war stories on making the mods. You can get the equations for the Q and F0 for the Sallen-Key filters and the coefficients for higher-order filters from a number of application notes that are on the web: do a Google search on Sallen-Key filters.

Posted by Neil Davis

Also available LM 3886T power chips at $3.95ea

We have a
schematic available in Pdf format and circuit modifications thanks to Tom Whipple & Thomas Lane .

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