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mertol 4th December 2011 11:12 PM

My First Passive Line Level XO and active Speaker Effort
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I almost finished my first DIY speaker, an active design with an unusual passive line level XO.

I used a Westra 1 inch tweeter and a Philips made 4.5 inch mid/bass that I was able to source locally. Unfortunetly it's quite hard to find decent speakers in here, Turkey, as there seems to be hardly enough market.

I was quite lucky as they had very similar sensivity (90db), as smaller mid/bass drivers usually provide considerably less output. And I also recognized that I was quite lucky to find a driver with 4.5 inch cone and almost 4.6 inch mounting diameter, wasting no room for the frame basket, while also making it quite difficult to flush mount.

Two pairs of drivers cost me 80 USD, not cheap but they were one of the very few options.

I sourced the internal amplifiers from ebay and they are NE5322 + LM1875T based and they accept line level inputs.

While they sound OK, or in other words good enough, what ever I did, I was unable to completely get rid of the 50hz noise. Looking at the PCB, one of my friends told me that this could be the switching noise of the onboard bridge diodes as they were not by passed by a cap.

The pcb's were not designed to be mounted directly on the heatsinks and I had to solve a lot of problems to mechnacilay mount them to heat sinks. If I did build the speakers from scratch I'd certainly take the time to modify the boards to be used over the heat sink. Output is 2x30Watt per speaker and each driver has it's own integrated amplifier.

Pair of stereo integrated amps costed me only 20 use including shipment from HK.

Enclosure is made of 2 layers of 2/3 inch (16mm) good quality mdf. Instead of gluing I screwed the outer front baffle (the inner baffle is glued) as I might change the drives later. Inside, all 6 surfaces are textured with real wood triangles to both increase the amount of surface and to decrease the amount of standing waves. I used a thick layer of accoustic foam but I think that I may add some wool also.

Mdf for two speakers costed around 40 USD and the real wood triangles (bought from Bahous and hand cut in to small pieces) costed me 20 USD.

There is an air tight sub enclosure for the tweeter to keep it away from the pressure of the mid/bass driver, which seems to push a lot of air due to it's good X-Max

The rear of the enclosure hosts the heatsink, inputs and a switch to select onboard or external XO, so that I can try several different XO's.

Enclosure is sealed type and it took some effort to seal it well due to the heatsink. If I had to rebuild it from scratch I'd definitely put the amplifier and the power transformer in a sub enclosure.

By adding an other 2/3 mdf layer to the front baffle I think I aligned the acoustic centers of the drivers.

Power transformer is 160 Watts capable standard laminated type. That seems to be more than enough for the integrated amp. The cost for two was 40 USD.

Switchs, buttons and other stuff costed an additional 15 USD.

So total material cost was around 220 USD not including many tools I had to purchase to finish this thing and it's still not painted.

I opted for a line level passive XO due to several reasons
1) Amplifiers are integrated and will not be changed.
2) There is plenty of power to recover from the insertion loss of the line level XO
3) Mid bass is small enough and has a usable output to 10khz and thanks to it's paper cone cone breakup is not much, so I didn't need a 3rd or 4th order XO
4) It's very easy to design a PLL XO and fine tune it compared to other types.

I am currently running the system with a 6db XO at 3khz.

Each speaker has a weight of 18.5 kg (aka a lot )

I only did a 2 hr listening and here are my initial findings with the system. ( I haven't A B 'ed them with my other bookshelves)

There is significant bass dip around 250 hz, that can be measured but I don't think I can hear it. ( This can be the baffle step)

There seems to be enough bass from 80hz and up

Speakers image very well and disappear without effort even at close listening positions.

There seems to be a lack of openness in the upper end of spectrum ( tweeter quality??)

Midrange is open and transparent but can benefit from a better driver certainly.

Overall sound is quite good compared to the money I spent, actually far better than I thought it was possible with these drivers. I will post more after more listening tests

PeteMcK 6th December 2011 10:48 PM

the main benefit of going active is in the reduced intermod distortion between woofer & mid, & using passive line level xo you can compensate for baffle step at the same time.
Are you going to add woofers later?

mertol 6th December 2011 11:00 PM

Honestly I don't think that I will be adding woofers any time soon. How can I compensate for baffle step with the XO ?

PeteMcK 6th December 2011 11:16 PM

well, if you're adding woofers, just make the xover point at the BSC frequency, & adjust the woofer amp level.

You can also do it at line level this way: Baffle Step Compensation

mertol 7th December 2011 01:11 PM

Thanks a lot. I will be implementing it today, planning to implement it with an on/off switch.

As you can see from the images speaker has 2 baffle widths. Should I use the exterior one, that the woofer is mounted on, or should I use the average ...

best regards

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