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Budget active subwoofer lowpass #2, buy, eval, mod
Budget active subwoofer lowpass #2, buy, eval, mod
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Old 16th March 2019, 03:48 AM   #1
DonVK is offline DonVK  Canada
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Default Budget active subwoofer lowpass #2, buy, eval, mod

I was looking again for another low cost subwoofer bandpass board. I've already modified one and I was about to modify it again when I decided to see what else was available and roll the dice another time. My previous one is at Budget active subwoofer lowpass, buy, eval, mod and required substantial mods and I'm not entirely satisfied with it.

I noticed a board that had stereo pots on it . Adjusting a 2nd order filter requires a stereo pot because 2 resistor values need to change at the same time. It also had enough caps to probably be a higher order bandpass and it's <$10 CDN delivered. Zero documentation is a standard for these products.
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File Type: jpg DSCN7562r.jpg (225.5 KB, 249 views)
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Old 16th March 2019, 03:54 AM   #2
DonVK is offline DonVK  Canada
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Default Layout and schematic

So here's the layout and schematic. All vanilla filters.

What's interesting is the one stereo pot is actually used to properly adjust a second order LP. The other stereo pot is sacrificed to volume control. The input summing circuit is also better than the previous one I tried. It comes with 12vdc regulators so I don't have to change them to use my +/15vdc supply. The volume control is unacceptable as the output impedance could be high and variable. Overall it seems alot better than the other model.
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File Type: jpg Subwoofer bandpass20001r.jpg (226.8 KB, 245 views)
File Type: jpg Subwoofer bandpass20002r.jpg (245.0 KB, 241 views)
File Type: jpg Subwoofer bandpass20003r.jpg (265.4 KB, 231 views)

Last edited by DonVK; 16th March 2019 at 03:57 AM. Reason: regulators
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Old 16th March 2019, 04:02 AM   #3
DonVK is offline DonVK  Canada
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Default Regulator issue

I powered it up and noticed a 2Vpp sawtooth wave @40Khz superimposed on the -12vdc. I thought it was the regulator so I replaced the LM7912 and same problem. Then I noticed the caps on the output were tiny (0.1uF) and lower than you usually use (> 2uF) so they (C2, C4 =2.2uF) got replaced and it fixed the problem. Its not a stuffing error, the silkscreen shows 0.1uf and its too small.
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Old 16th March 2019, 04:08 AM   #4
DonVK is offline DonVK  Canada
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Default First measurements

A few measurements to see if I need to change anything. I was surprised as its usable in stock form. There are some issues but it could be used as is.

Freq adj is independent of volume adj (unlike my other one). However it looks like the HP is crowding the LP because there's a knee near 30Hz and the amplitude increases a bit as you increase the cutoff point. It never reaches a flat top.
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File Type: jpg Subwoofer BandPass2 Stock Freq Adj2.jpg (142.2 KB, 54 views)
File Type: jpg Subwoofer BandPass2 Stock Vol Adj.jpg (143.0 KB, 39 views)

Last edited by DonVK; 16th March 2019 at 04:16 AM.
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Old 16th March 2019, 03:33 PM   #5
DonVK is offline DonVK  Canada
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Default HP changed to BW3

I changed the HP to a BW3 @ 20Hz. This flattens the low end of the curve alittle, and the higher end is still too rounded. There is a good filter calculator at Filter Design and Analysis that I used.

Changed resistors only R6=7K5, R4=39K, R7=180K.
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Old 16th March 2019, 07:18 PM   #6
DonVK is offline DonVK  Canada
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Default LP changed to BW2 + BW2

There are two 2nd order filters. One is fixed the other adjustable. With the current values they have Q=0.5 and I would prefer BW2 with Q=0.707. I would also like a flatter response when I change the cutoff freq.

Only needed to change two capacitors C8,C9 = 0.22uF. The fixed filter has been lowered to Fc=130Hz which is sufficient for me. The adjustable filter is now BW2 with adjustment range from 20Hz to 130Hz. At 130Hz the two filters becomes BW2 * BW2 = LR4.

Changed capacitors C8,C9 = 0.22uF.
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File Type: jpg Subwoofer LP BW2 + BW2.jpg (145.6 KB, 39 views)

Last edited by DonVK; 16th March 2019 at 07:20 PM. Reason: typo
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Old 16th March 2019, 09:35 PM   #7
KaffiMann is offline KaffiMann  Norway
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Budget active subwoofer lowpass #2, buy, eval, mod
So you're up to:
C2, C4 =2.2uF
C8,C9 = 0.22uF
R6=7K5
R4=39K
R7=180K

It looks much more useable now. What is your opinion on the circuit? Noise free and all that? Have you looked at any noise figures?
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Old 16th March 2019, 09:52 PM   #8
DonVK is offline DonVK  Canada
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Those changes are correct. I'm happy with the filter and cut adjustment now.

There's nothing special in this circuit, is all vanilla, and it works. I only made a few component value changes and those should have been done in the factory and it would not have changed the cost.

I still have to fix the output impedance and gain adjustment. That's fairly straight forward but will require 2 track cuts. Then I'll post THD and noise. Stay tuned.

Last edited by DonVK; 16th March 2019 at 10:01 PM. Reason: opinion
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Old 16th March 2019, 10:40 PM   #9
PRR is offline PRR  United States
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Budget active subwoofer lowpass #2, buy, eval, mod
Quote:
Originally Posted by DonVK View Post
...The volume control is unacceptable ...
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Old 16th March 2019, 10:46 PM   #10
DonVK is offline DonVK  Canada
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Exactly,
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