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Old 25th June 2007, 01:56 AM   #1
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Default Active filter plus LM3886 - one board

Hi,
I have been working for the past several months on a new three-way speaker system that will be powered by 6 LM3886 amps.
Originally I thought it would be best to put the active filters for each channel on a separate board, then do 6 amp boards.
Recently I changed my mind and decided to incorporate an input buffer, a high and a low pass filter, an output buffer with volume control and finally the amp circuit all on the same board.
The reasoning behind this is that It will be cheaper to have 6 small boards produced that are identical, and it makes things much neater.
Components can be left out of the portions of the board that will not be used. Only the high pass component need go in the high pass board, and so on.

I have adapted Rod Elliot's P09 24 dB/octave filter and used his implementation of the LM3886 (P19). I have come up with the following schematics, the first is the filter section.
Thoughts and comments either positive or negative are welcome. I want this to work right the first time.
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Old 25th June 2007, 01:58 AM   #2
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And the amp section:
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Old 25th June 2007, 05:59 AM   #3
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Keep in mind that the LM3886 is itself an op-amp so you can wrap a 2nd order filter around it and eliminate an op-amp -- the fewer the better... with a little finessing you should be able to build a 3rd order filter around the power op amp....

dave
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Old 25th June 2007, 04:45 PM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
if you can arrange the components R3 & 4 and C2 & 3 to fit a DIL header and arrange the PCB with a DIL socket, you can swap out different headers with alternative frequencies and or Q values.
You can also swap C for R in each header by simply turning it around to convert from low pass to high pass (but this screws up the Q if you go for unity gain filter topology). If you go for equal value S&K then Q is set by the opamp gain and the Rs&Cs control just the frequency and not the Q.

This arrangement is very easy to change and saves a lot of PCB space by avoiding the need for the extra dual opamp and it's components.

Have you read the filter threads for the previous group buys? There is a lot more than just turn-over frequency and slope to consider when activating your drivers.
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Old 25th June 2007, 06:03 PM   #5
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I went further and put the bridge rectifier and storage capacitors on the board as well! Needs careful layout though.
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Old 25th June 2007, 06:34 PM   #6
AndrewT is online now AndrewT  Scotland
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Hi,
forgot the other part of your question,
You appear to be building monoblocks dedicated to each driver.
If at some later time you change the driver(s) then the more flexibility you put into your crossover and equalisation then the more chance you can reuse the monoblocks with minor "software" mods rather than a complete rebuild.

I like the idea of each amp dedicated to the driver. But suggest you build one and get it working (with the driver) to your satisfaction before laying out all the copies.
Gives much scope to meeting my big advice:- use VERY SHORT speaker cables.
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Old 25th June 2007, 06:39 PM   #7
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Quote:
Originally posted by planet10
Keep in mind that the LM3886 is itself an op-amp so you can wrap a 2nd order filter around it and eliminate an op-amp -- the fewer the better... with a little finessing you should be able to build a 3rd order filter around the power op amp....

dave

Excellent idea Dave, I hadn't thought of using it.
The other filter sections are unity gain, the amps at 27dB gain - any adverse effects?
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Old 25th June 2007, 06:52 PM   #8
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Quote:
Originally posted by AndrewT
Hi,
if you can arrange the components R3 & 4 and C2 & 3 to fit a DIL header and arrange the PCB with a DIL socket, you can swap out different headers with alternative frequencies and or Q values.

Another good idea. The use of sockets would vastly simplify the "tuning" process. The component values in the schematic are not the actual ones I will use, as I haven't determined the ideal crossover frequencies yet.
I want the board as simple as possible though, with the shortest runs and single sided. The DIP sockets might mean a lot of re-routing to accommodate.
As for changing the speakers some time in the future...without a doubt I will, as I get tired of things eventually. Chances are I would re-do the active amp at that time also.
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Old 25th June 2007, 07:04 PM   #9
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Quote:
Originally posted by consort_ee_um
I went further and put the bridge rectifier and storage capacitors on the board as well! Needs careful layout though.

I have started to build a new chassis that will house the power supply seperately, along with the power supply for my sub amp and anything else I wish to add in the future. It will have:

1/ 50-0-50 800VA (+-70V)
1/ 30-0-30 300VA (+-42V)
2/ 25-0-25 120VA (+-35V)
1/ 12.6-0-12.6 60VA (+-15V regulated)

In the chassis will be the bridges and filter caps. Also should have a soft start circuit capable of handling 1500VA.

Pic of partially complete power supply chassis:
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Old 26th June 2007, 05:01 AM   #10
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Hi MJL21193,

The ones that I've built were LR4 @ 100 and 2k, modified from Siegfried Linkwitz's phoenix xover. A few things came to mind about your design:

- baffle step compensation
- use lm6172 or opa2132 opamp instead of NE5532, and include PSU cap bypasses from +ve to ground, -ve to ground, and +v to -ve

A question though about your schematics which show LP 300 and HP 3k, were you also intending to add LP300 + HP 3k for the midrange drivers in the 3-way?
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