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an economic diy sub to compliment EV ZLX12P

Posted 17th August 2015 at 07:55 PM by colossusfan
Updated 9th September 2015 at 07:32 PM by colossusfan (Add mute function)

My first attempt at a blog so i hope i dont bore the pants off you all!!
I bought a 2 way so called 1Kw integrated speaker some time ago about which I have written various posts in the forums, management overview better value for money than DIY but vastly inflated specification as all similar units, actual measured output around 110dB max and LF rolloff starts at 80hZ!!
Soo it needs a sub!!
well if like me you like your low end forget the EV sub (ELX118p) and most of its compatriots for two reasons, one is rolloff starts at 40hZ or above, and 2nd is cost!!

Looks like i just use edit to add to the blog!

So to continue I used the UNIBOX spreadsheet (free) to model various drivers and box combinations, true to my namesake I started off with large Fane drivers but was scared of the price!! However I also discovered various nasties with ported or tuned enclosures most notably you tradeoff an extension in the 40-60hZ range for a much sharper cutoff below it and an inability to control excursion below 30hZ when compared to a sealed enclosure. I am particularly interested in the frequencies people say you cannot hear but to me inescapably add to the ambiance.

btw I settled on UNIBOX after trying many many other speaker design programs, I found many did not have a good library of drivers and were difficult to add new ones too, also they seemed to major on producing pretty diagrams of an enclosure rather than concentrating on the expected results, well thats my opinion!

So my search was on for a driver, I needed about 110dB@80hZ to match the ZLX12p with as much output as possible at very low frequencies (10-20hZ) as I say I found the professional type of PA speaker just too expensive, the only cheaper ones were the Chinese lookalikes that I wouldnt trust with a bargepole, no proper ratings and no TS parameters available.

In the end I did something against my nature and started looking at long throw car speakers, many of these fell into the extremely unprofessional category above, little more than toys but then i found the JBL GT5-12 ok ok i know its made in china and JBL is now owned by someone else BUT it does have published TS parameters, is rated in real watts RMS and does come from someone who has a track record!!

So thats my choice but in a large (~50Liter) sealed box!! To power it I have bought one of those Chinese IRS2092 amps simply because of the expense of home produced pcb's that I believe are essential for class D, I was prepared for it to be buggy and indeed it was but not to hard to sort out, I will definitely be running it with a small cooling fan to extend its life!!

So now I am on box construction, I chose 18mm MDF with a double thickness baffle, it uses half a sheet, I didn't want to go thicker in order to reduce weight as I want to be able to move it occasionally, hopefully it will come out as about 24Kgs all up. I am finding the box construction quite hard, getting all the edges perfectly square with simple tools is difficult, especially the hand held circular saw drifting off the last few inches when the wood disappears underneath it, if i had more space setting up some kind of saw table with a guide would be good. Fortunately I have an electric plane which helps to correct things. I have had no problems using #6 screws BUT i definitely recommend pilot holes, glue is PVA, loads of it!!
Today cut the baffle for the driver using a DIY jig for my router thanks to those who recommended this method here make a much neater job than jigsaw BUT at this depth of cut I lost control of the router once, not pushing down hard enough and the collet cracked from the load but managed to continue, definitely do lots of swipes at only 4-5mm depth increments at a time. now to see if i can add some pictures to a blog!!

Hmm not impressed just lost a blog edit because i realized after adding text i had to go advanced to add a picture grrrrrr so here goes again!
Completed the sub box and as the picture shows its sitting under the ZLX12p, the amplifier box is to the left on the bench under construction. To its left bottom corner is perched the ADAU1701 DSP PCB attached to its USB programmer that will become redundant once the DSP EEPROM is burnt with its final software (does this ever happen) hahaha ok hope this pic turns out after this text
Ohh it seems not, all pics are bunched together at end making it nonsensical!!
All done bar the DSP programming (waiting a forgotten cable to arrive by post....drat) built the auxiliary psu and balanced input, wired it all up, had some fun sorting out the grounding eliminating the hum caused in part by aux and main amp rails sharing the same transformer winding centre tap (something for the memory banks). The pics show the wired up guts and the complete unit already in circuit with the audio feed, just waiting to load the software so frustrating!! will publish performance as soon as i can measure it

Finally my programming cable arrived and I have been able to fire it up!! First I took some SPL plots, this suffers terribly from the room modes but at least shows roughly the SPL achieved and the low pass rolloff at 80hZ, yup I will have to haul it outside to plot it properly but in the meantime I have lots of development work to do and some real listening trials but from what I have heard so far ACE

Well before the rain hits the fan tomorrow I carted the whole lot outdoors and measured it, please see new plots that show both indoors and outdoors, to my relief the nasty peaks and troughs are indeed room resonances, the outdoor response being nice and smooth and 90dB SPL @ 10hZ exceeds my expectations nicely

I am still getting to grips with the DSP so I think the crossover knee is somewhat lower than expected (its supposed to be 80hZ but hey thats why I chose a DSP, endless tinkerabillity!! and correction for those nasty room modes.

So far with listening tests it definitely improves the ambiance dramatically on those recordings containing full depth (e.g. gaga/fame), I also have plans for dealing with those where some dumbcluck cut it out!!

I think I have shown youall whats possible without spending silly money and any further development of mine will be strictly personal rather than generic. So thank you for your patience in reading my little blog, I hope you enjoyed it and found some useful snippets of information

byeeee

PS I found a lot of my old vinyl LP's had lost most of the kick drum so I restored it using a sub-harmonizer within the DSP along with a 10hZ rumble cut, results amazing, new life to my old albums, fully justified the DSP effort, chart illustrates sub-harmoniser with 90hZ fundamental input.

PPS This thing is super sensitive to 50hZ (mains), I was considering a tracking notch filter but didnt fancy the outtakes from my music, in the end I devised a low signal mute, after all the hum is only noticeable when it's idle, does the trick nicely

Will I ever stop tinkering with the DSP ? don't know except its about 80% full now so eventually that will put a stopper on it

Just added a pic of the busy wiring as the DSP gets more controls and features.
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Attached Files
File Type: pdf JBL DSP V1 280815.pdf (5.5 KB, 138 views)
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Comments

  1. Old Comment
    colossusfan's Avatar

    experimental comment

    I wondered if comments actually worked! was gonna post another pic but see cannot do here.....
    permalink
    Posted 9th September 2015 at 07:28 PM by colossusfan colossusfan is offline
 
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