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Boombox project, HPF question

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smithers View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote smithers Quote  Post ReplyReply Direct Link To This Post Topic: Boombox project, HPF question
    Posted: 19 March 2021 at 8:37am
Good morning all,

it’s been a while since i’ve posted, but i’ve been lurking in the shadows!

i’ve been pottering in the shed with 14 x 18650 batteries left over from a ninebot unicycle and built a 24v battery pack, one thing let to another and i’ve ended up with this test mule- 

<a href="https://ibb.co/rvwPp4Z"><img src="https://i.ibb.co/HXHvPnr/2-B7-C5-AB8-0-D4-D-48-BD-9-DEE-825-EB2-AFD5-F7.jpg" alt="2-B7-C5-AB8-0-D4-D-48-BD-9-DEE-825-EB2-AFD5-F7" border="0"></a>

using a TDA 2.1 amp powering 2 dayton audio 3” full range drivers and a tang band 6.5” subwoofer.
i’ve fabricated a 2” port from Pvc pipe with flared ends tuned to (i think) approx 38-40hz 

so far i’m pretty bloody happy with the sound, the dayton’s have a dip from 8k (the grill may also be causing some of that) so a small notch filter may be required to brighten up the top end, but at this point it’s more than acceptable.

the issue i’m having is to do with the driver unloading below the tuning freq and causing a fair bit of over excursion and port chuffing. above 38hz the port noise is pretty much none existent, but the noise below that is not acceptable.

the easy solution would be to fit a line level filter on the input to the amp, but in this instance the speaker will be used via bluetooth 99% of the time so this is not a viable option.

is a 30/35hz passive high pass filter on the sub output an option that could work?

i just need to figure out the best way to filter out the subsonic’s, the box is impressive output for its size, but you can’t get a pint from a shot glass so those pesky <30hz freq need to go.


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Peter Jan View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Peter Jan Quote  Post ReplyReply Direct Link To This Post Posted: 19 March 2021 at 4:24pm
Going passive in those frequencies will make for HUGE components, large coil and eventual capacitor (depending on the filter slope) and serious losses as a result of those components. Considerable weight added.... IF you can fit it in that box.
An active solution would be a lot more elegant here. Easy to go steep in filter slope.
Maybe change/add a few components on the amplifier PCB to get just that.
You have a schematic of it ?
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studio45 View Drop Down
Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2021 at 1:10pm
If it's one of the TPA3116 amp boards with built in Bluetooth, it won't be easy - but in theory, you could find the traces that pass the BT output to the amp, cut them and pick off the signal there, and pass it to an external HPF board before returning it to the amp. 
But probably easier, to get another BT receiver board (cheap), disable the onboard BT (cut power traces?) and use the line level input to get the signal in via a HPF board. I
t's quite easy to design a Sallen-Key highpass filter using an online calculator, and realise it on a scrap of Veroboard. For powering, there are nifty 5v> +/-15v converters available, you need to feed them from a 7805, which in turn is fed from your 24v rail.


https://www.ebay.co.uk/itm/Bluetooth-Audio-Receiver-board-Bluetooth-4-1-5-0-mp3-lossless-decoder-board-Wire/254801232701?_trkparms=ispr%3D1&hash=item3b5356433d:g:wrMAAOSwp19fzmqI&amdata=enc%3AAQAFAAACgBaobrjLl8XobRIiIML1V4Imu%252Fn%252BzU5L90Z278x5ickkrDx%252B2NLp21dg6hHbHAkGMT%252F%252BMCHE66ilKLUNme7UqLeFs3kiOk4JHMdRe1mP4ikmKJhyZy5KQ%252FbdxJa5nxBgPwKBcgs9Jh3qxcaMFM4EGGHsa%252Fd3JHT2XjFvPKR1u%252FkLvoxknuQBUig9gVroh6v0iRQ7zet70jP23QRXzNPeNowPs%252B3fvO4teSDWDRbRaewSOVN3NwTU%252BwdsaALmkD0HU%252BM7XKUfwns0CRk5At%252FF2o5jjI9pBEItmJiA48s8ttQ7Of8qN%252B1K1%252F51Ar5lJGPZoI38zqNpaxAlBb6%252B5TN2m%252BnTVbCEcwsUXuFSoRSRy0OfLLzdoMppjw17FkulROyU5o%252FrBtn6zpS5vKFANHiIKZ%252FIBC%252FPLfR%252FK5g8dN%252FiUc430DM2PUyx%252FGO1LbwCO6HaUB9VWoGEf97EDZtYmCKQBdCgRLYJIwdTxQeVaFY%252BkJ8UO%252F51fADGpL2KIfR6e1o9AZ9rsL%252FC%252F5Ny9tfUX%252BGjibf9aqwbreV3ldQ8mdNmW24zyVYYOg1NGptQ2WPlnUMB9%252FCmbscpMo6NBZ8obTZg8DfQ5mNw2Uwvj8dhEz%252B72M%252F35unHrYCPWUNcPz5srJWeLtaSL5tmDvv4YYWoYWbqYotAeKWEpE87Je65ahp5AUCDf2hi6rX5cNwRAyCxnotOXhWAJIBrkXgYy%252BEjWTZtteeMR%252FEL9o4uFnER8SE0AeQmMYL0CaOAFkhPhfcmL3o37OpmqC60igKvgdQ2KknGLtXdPdcQ4FgBq5N9Atf5ofthJ2Q9M70ySSTuSApDvcfhiI6jkGmxdc4bsyjdgJQHiIg%253D%7Ccksum%3A25480123270108ddf9fb817f41ca93d8b10706b0a8c3%7Campid%3APL_CLK%7Cclp%3A2334524

Studio45 - Box Builder Commotion Soundsystem -Mobile PA
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smithers View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smithers Quote  Post ReplyReply Direct Link To This Post Posted: 21 March 2021 at 5:50pm
apologies for the delay, cut my bloody finger at work pretty badly on friday 👎

many thanks for the replies, very useful info!

i think the best way forward is utilising a blue tooth pre amp and use that to feed the amp via line in as suggested by studio, great idea🤌
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smithers View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote smithers Quote  Post ReplyReply Direct Link To This Post Posted: 21 March 2021 at 5:51pm
apologies for the delay, cut my bloody finger at work pretty badly on friday 👎

many thanks for the replies, very useful info!

i think the best way forward is utilising a blue tooth pre amp and use that to feed the amp via line in as suggested, sometimes the easy solution is the most elusive👍
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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 23 March 2021 at 3:25pm
Sound, the only necessary component not shown in the filter calculator schematic is a decoupler for the chip - fit a 100nF film cap between +v and -v as close to the chip as possible. 

Apart from that, given that you're driving it from a buffered output into another buffered input, the required circuit is exactly as the generated schematic. 

If you find a 12dB slope isn't enough you can cascade two circuits to get a 24dB slope. If you experience signal level loss or the corner frequency is not as predicted, you may need another active buffer stage either at the output or input of the filter. 

You could also experiment with other filter types. For example a peaking highpass suits some boxes well as it gives a little gain bump just before the roll-off starts, this can firm up your bass if the alignment is good.
Studio45 - Box Builder Commotion Soundsystem -Mobile PA
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