Active speakers using LM3886TF

[julDa]

Hello I´m rather new to this forum, my first thread so far, but I´ve been trying to make speakers for a while.

My next project will be a pair of active speakers, each containing two LM3886TF-chipamps, 4:th order LR-filters and some circuits for baffle step compensation.

Since this is my first try at making an amp I would realy like some comments from more experienced builders. I´ve been reading a few posts here to get started but I will probebly make some bad designchoices anyway.

Size is a factor since the amps will be built in to the back of a pair of small speakercabinets with a total of 7 liters volume.

I´ll post some pictures and hope that some of you is interested in helping me.

The first picture is my "starting plan"

David

[MJL21193]

Hi,
I did the same thing (minus the BSC) in this thread:
Active filter plus LM3886 - one board

Results are outstanding. I'm still working on it.

[julDa]

Hi John, I did read your thread and found it very interesting. My design will be rather similar but with a couple of differences.

First of all size does matter. The enclosure containing filter, two amps and the PSU should be one liter or less and airtight since it needs to be mounted on a speaker. I also want to build the filter-circuits on a seperate psb since I probebly will try to work with a dsp in the future.

Here is a more complete version of the filter-section followed be my first try at the psb.

[julDa]

Here is the psb, its one-sided and rather compact. I did try to make the singalpaths as short as possible. If anyone with more experience could take a look and comment on the design I would be most gratefull.

Regards
David

[Dag]

Excellent project!!! I think this can can end up in a very good speaker.

My amature comments:
1. Add a 1st order filter BEFORE the first op-amp (Just a cap) to remove RF signals.
2. The long wires to the speaker can pick up a lot of noise so a 3rd order bessel filte can be included in the BSC circuit without using any more opamps.
3. You probably do not need the 4th order electrical filter because the frequency responce of the speakers. With one op-amp you can make a 3rd order filter and this together with speaker will probably end up in a 4th order LR accoustical responce.
4. Add a notch possibility for the peaking of the midbass (can be important even with a 4th order filter if the peaking is large.
5. Fixed resistors will sound better than a pot.
6. Be VERY carefull with the grounding and the signal path.

Good luck!

[consort_ee_um]

The heatsink and transformer that you require will have a significant volume so you will need to make the enclosure bigger than you initially think. Or you mount the heatsink on the back in the manner of plate amps for subwoofers. The mechanical aspects of integrating active electronics are not easy, not difficult but need to be thought through!
As Dag said, if you are aiming at a fourth order Linkwitz-Riley acoustic response, the electronic filter to get this might be third or even second order.
I hope this does not put you off but gives you an idea of the issues involved.

[Dag]

Hypex is comming with a SMPS that might(??) be small and hopefully not too expensive. This could be perfect for an active speaker.

[julDa]

To explain the psb a bit more:

Every resistor and every capacitor is the same value (not the shelving bas-circuit thou) and the strange looking components are my trying to mount resistors verticaly to save space. The psb for the filter is only 3x8 cm and will, hopefully, be easy to manufactur at home, or in my case, at school.

The supply voltage will have one capacitor per rail and the OP:s will use +-12 volt. I will not however use a standard volume controll, only a simple resistor circuit on the poweramp-psb to cut down the output power. This can be done since the speaker will be connected to my PC most of time and further, I have yet to find a pot flexible enought to fit inside two amp at once =)

In the future I will probebly make the preamp and filter section digital and will then also put a digital pot in. The two sides will then be connected with a digital signal using led:s or something similar, much like an ordinary remote.

David

[julDa]

Thanks for the comments.

I will put a cap at the input and will also look into the bessel-filter to reduce noise, thanks.

The main reason for the 4th order filter is that I will need both the extra falloff and probebly an extra notch-filter at speakers as well. The loudspeaker will be using Seas Alu-drivers and the 5-inch has a nasty ring at 9kHz or so.

The grounding will be done via a "multicable" so each of the ground-points have its own copper to ground. Hopefully it wont be to much of a hazzle and with all cabels bundled, it will probebly look tidy to... =)

[julDa]

The layout will be similar to the layout of a std active woofer-amp.

My goal is to make the amp-box 20 cm high, 10 cm wide and about 5 cm deep. I´ve made rough scetches of the physical layout and it should be possible to fit all in with some room to spare.

The amp-chips and the voltage-regs will be mounted dirctly onto the backplate (using TF-package) to get good heat-bond and a realy compact design.

The heatsink will be mounted on the backplate and the backplate in itself will be made out of a 4mm thick alu-plate thats 15 by 20-something cm. The heatsink will be around 1.2 deg/watt so there realy shouldnt´t be a problem heatwise.

David