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Add a DSP to your Zkit60 (Read 440 times)
dank
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pair of dual 18
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Posts: 339
Add a DSP to your Zkit60
02/07/19 at 15:18:10
 
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



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dank
Seasoned Member
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pair of dual 18
Imperials

Posts: 339
Re: Add a DSP to your Zkit60
Reply #1 - 06/02/19 at 17:55:52
 
Just in case anyone reads this and decides to build something from it, I thought I'd update things and correct a problem.  Over the past few months the amp has failed 3 times.  When it fails, it produces a loud screeching sound out of both speakers until turned off.  Once turned back on, everything is fine.  It failed twice in 2 days after running error free for months, then ran for another month without error, and finally failed again yesterday.

The issue was the voltage regulator installed to power the DSP.  I had incorrectly decided I didn't need a heat sink on it.  I was close...I ALMOST didn't need a heat sink on it.  If none of you have ever seen a linear voltage regulator shut down because of heat, you have truly missed out on one of electrical engineering's gifts to mankind.  They get hot and shut down, so Vout drops to zero.  This results in the regulator cooling off a few tenths of a degree after which it turns back on.  A millisecond later its too hot and shuts off again.  The result is a nearly perfect sine wave on Vout.  In my case it was around 1khz with the frequency varying enough to make for quite a nice screeching sound.

A couple 1k resistors in parallel in front of the regulator to drop about 20v and a spare heat sink on the regulator has the part running quite cool now, never to fail again, hopefully.



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