For about $100 you can add a DSP (digital signal processor) to your Zkit60 power amp.  This will give an unprecedented amount of customization capability to your audio.  Tune to your room, or just tune to what you like best.  Voltage gain (reduction) across the entire audio spectrum is also possible.  Parts Express sells the "MiniDSP" for $105 in a small plastic enclosure or $80 for a DIY unit without an enclosure.

The 2 input, 4 output DSP is very flexible as to what it can do.  I'm using it as a one to one pass through for a left and a right channel, to which I can add up to 10 programmable filters per channel to either cut or boost most any frequency band I define.  Rather than let the two other outputs go to waste, I've brought them out to the back panel as subwoofer outputs, which can be made stereo or mono at any frequency crossover and slope I choose.  They can even be set to two independent mono subwoofer outputs with different crossover frequency and slopes (don't know why you would want to do that, but you could).  The miniDSP connects to a PC / laptop through a USB cord.  All the programming of the DSP is done through the PC / laptop.  Once programmed, the DSP can run alone but it requires power - either through the USB cable or a user supplied 5v to 24v input.  I used a simple circuit using an adjustable regulator from Texas Instruments (TL783), that can tolerate up to 125v on the input, to take the 95vdc available in the zkit60 and convert it to 20vdc.  At these low current levels, no heatsinks are needed.  One thing to watch out for is the minimum current spec of the TL783 (hence the two extra 4.7k load resistors).  If you aren't drawing enough current through the regulator it won't regulate and the output voltage can go too high.

Ok, picture time:

Making sure it will fit:



Drill holes and install mounting bolts:



Drill the extra holes in the Zkit60 back panel - two rca jacks for the subwoofer outputs plus an existing hole for the USB cable to pass through:



MiniDSP bolts in.  I removed the RCA jacks on the miniDSP due to fit issues.  All audio connections to the miniDSP are made with the same coax cable used in the Zkit60 (Belden 9396):



MiniDSP power supply design and output voltage:






MiniDSP powered up and running:



Does it work?  You be the judge...the following spectrum analyzer graphs are a white noise source driving the system being picked up at the listening position with a microphone.  The speakers are a 2 way passive crossover design using an 8" full range Fostex and a 10" Dayton subwoofer crossed over at 200 hz.  The Fostex is then padded down about 12db to make its output similar in volume to the subwoofer speaker:

without DSP:




with DSP, set the way I like it (more bass please!):




Dan