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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 offline 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 offline 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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Old 26th June 2007, 08:27 PM   #11
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Quote:
Originally posted by trats

- 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?
Hi Trats
My approach is to have pots to adjust the levels of all six channels, so that will cover baffle step (increase output to woofer)

The op-amps I have, so I'll stick with them. The NE5532 is good enough for this application anyhow - low noise.

Bypass to ground on both rails are in place (C13-C16) Could put one from rail to rail, say .1uF.

The values shown are temporary, as I haven't determined the optimal crossover freqs yet. The plan is for this to be modular,each board will be a self contained filter and amp for a driver. For example: the woofer will be driven by one with only the low-pass components on it. The midrange will use both high-pass and low-pass.
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Old 26th June 2007, 10:43 PM   #12
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A sample board layout, based on the schemetics posted earlier. Something I did before I started this thread to figure out the board size. Took a while to figure out how to get it to a postable format.
One sided, just 2 jumpers. Finished board is 2.5" x 3" which is the perfect size forthe chassis I've built.

Once again - thoughts, comments, like it, hate it, or just don't care. It would be nice for some of you out there to look it over with a critcal eye, point out my mistakes.
Like I said earlier, I want this right the first time.
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Old 28th June 2007, 01:50 AM   #13
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Quote:
Originally posted by MJL21193

Once again - thoughts, comments, like it, hate it, or just don't care.

Well, I'm just overwhelmed with the response!

Maybe it's all been covered before, but searching I found nothing like this to suit my needs.
Nevertheless, I'll press on. I'll go with the board layout above, for better or worse.
Thanks to those who contributed.
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Old 28th June 2007, 11:48 AM   #14
BWRX is offline BWRX  United States
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Hi John. You could compact the layout a bit, unless you find it harder to work on a board with closely spaced components. Also, since each of the 2 op amp filter blocks (high pass and low pass) have the same basic component configuration (just with R's and C's switched) you could come up with one good, compact layout of that block and then copy it twice. It's also considered good practice to have your connections/wiring points on one side of the board, but certainly not necessary. I do like the way you used separate pos_15 traces going to each op amp V+ pin. You should try to do the same for the neg_15 traces. Grounding is also suboptimal because you chose to do a single layer board. Have you thought about using a double layer board? You could use one side as a ground place and the other for the traces. It will be trickier etching that yourself though, but probably worth the extra effort.
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Old 29th June 2007, 03:27 AM   #15
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Hi Brian.
I'm working on a new layout, one that will increase the ground coverage, as well as widen the positive and negitive traces, and generally tidy things up.
I won't be etching these myself. I have found a place that will produce these boards for $15 setup plus $3.50/board. No silkscreen, but I can take care of that.
In the meantime, I have finished the chassis, except the power inlet (which I'm still working on).
Here's a couple pics, front:
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Old 29th June 2007, 03:28 AM   #16
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And back:
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Old 29th June 2007, 03:31 AM   #17
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Inside, no guts yet just heatsinks:
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Old 29th June 2007, 05:20 AM   #18
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Nice chassis! Got those heatsinks from barred_boss on ebay, huh?
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Old 29th June 2007, 06:19 AM   #19
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Aha! I was wondering how much fun you would have putting all those leds and sliders on an alu frontplate, but I see you used wood. Neat idea!
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Old 29th June 2007, 09:26 AM   #20
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Quote:
Originally posted by BWRX
Nice chassis! Got those heatsinks from barred_boss on ebay, huh?

Thanks. Yes, heatsinks from barred_boss = excellent deal. There is one full 12" piece of it in here cut up, with only a 1" strip left over. Not to waste anything, I used the that aluminum to make the slider knobs
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Old 30th June 2007, 12:34 AM   #21
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Quote:
Originally posted by gbyleveldt
Aha! I was wondering how much fun you would have putting all those leds and sliders on an alu frontplate, but I see you used wood. Neat idea!

Thanks. Actually would have preferred a solid aluminum face, but in the interest of saving money (and trying to find 3/4" ALU plate), I faked it.
Works just as good.
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Old 1st July 2007, 03:09 AM   #22
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Here's the revised board layout, still based on the previously posted schematics.
I took Brian's advice, and made the layout more compact. Also optimized the filter stages and made them alike, and increased the ground area.
Still single sided with one jumper. I have the board size down to 2.5" x 2".
Anyway, like before I need an experienced eye or two to look it over for bug/problems. Also general comments are welcome.
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Old 1st July 2007, 03:17 AM   #23
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Another view, the simulated copper side: (looks cool )
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Old 1st July 2007, 08:50 AM   #24
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Hi John,
I've had a quick look at your layout and noticed that the ground connection to C14 gets squeezed quite tight by R17. C14 will draw large pulse currents which could raise the ground voltage in the tight area by a few millivolts. This is the usual cause of hum coming from a speaker. I suggest rotating C10 clockwise to allow the ground fill to connect under the cap.

A better connection could be achieved if the order of C10 and R17 was swapped on the schematic then rotate R17 clockwise. This would improve the ground further because the pin spacing of the resister is wider than the cap allowing more copper area.

The output track might also benefit from a little thickening-up.

Finally, shouldn't R24 have a inductor wired in parallel?

Nice one,
David.
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Old 1st July 2007, 12:49 PM   #25
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Thanks David. That's what I'm looking for - an experienced eye to pick out the things I can't see.
I have made the changes you recommended - re-routing C10 and R17. Also I thinkened the output traces.
Although not shown, I will wind 10 turns of inductor wire around R24 attached to the leads (as recommended in Rod Elliot's design).

Any thoughts on power supply filtering? I have used large (8000 - 10000 uF) values for this chip before, but have seen others using as little as 1500uF on the rails - the idea being that large filter caps kill the midrange/treble. I have not tried this, so I don't have an opinion yet.
Perhaps, since the power supplies will be seperate - one for bass and one for mid/highs I could use different approaches for each - large caps on the bass for transients, small caps on the mid/high.
Best of both worlds?

Thoughts? Opinions?

Reworked board layout:
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