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Old 30th September 2011, 05:45 PM   #1
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Join Date: Sep 2011
Default Looking for very small and simple variable bass boost

Hi yall, I am a DIY gadgetmaker and somewhat novice in electronics but I certainly understand the basics.

I am building a wacky device full of restrictions- its a boombox set to fit in a pair of old military headphones.

Due to my speaker size restrictions, the loudest speakers I could get in my pricerange were the Kenwood 835C 40W speakers. They are definitely loud but wayyy tooo trebly. I know they have really crappy frequency response 96hz-20khz, but its hard for me to find any under 3.5" that go much lower.

Anyway I am using the Sure 2x25W amp to power these bad boys and the circuit board itself is decides my maximum size restriction for the space I have for amps/preamps in this device.

So! I want a really simple and small (fits on a 1x1.5" board) way to ideally boost the lows (more like mids in my case 96-200hz) or if its the best I can do, cut the highs.

When playing around I just threw a 200uf cap between the hot and ground and suddenly it became a much more pleasant (though muffled) sound. In fact when I turn the volume up to compensate for the loss it actually sounds pretty damn good to me, considering how simple the circuit is. BUT I wanted my bass boost/tone control to be variable, so I tried throwing a pot between the cap and ground, and basically the pot was waaaay to sensitive, and acted more like a switch. So, I figured a pot with a much lower resistance would be better. I tried a 25ohm rheostat, and it worked!

Is my solution as hideous to you experts as I assume it is? What would you suggest instead?

Thanks!
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Old 30th September 2011, 06:28 PM   #2
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Join Date: Mar 2007
Location: California
Quote:
Originally Posted by homemadefrog View Post
Hi yall, I am a DIY gadgetmaker and somewhat novice in electronics but I certainly understand the basics.

I am building a wacky device full of restrictions- its a boombox set to fit in a pair of old military headphones.

Due to my speaker size restrictions, the loudest speakers I could get in my pricerange were the Kenwood 835C 40W speakers. They are definitely loud but wayyy tooo trebly. I know they have really crappy frequency response 96hz-20khz, but its hard for me to find any under 3.5" that go much lower.

Anyway I am using the Sure 2x25W amp to power these bad boys and the circuit board itself is decides my maximum size restriction for the space I have for amps/preamps in this device.

So! I want a really simple and small (fits on a 1x1.5" board) way to ideally boost the lows (more like mids in my case 96-200hz) or if its the best I can do, cut the highs.

When playing around I just threw a 200uf cap between the hot and ground and suddenly it became a much more pleasant (though muffled) sound. In fact when I turn the volume up to compensate for the loss it actually sounds pretty damn good to me, considering how simple the circuit is. BUT I wanted my bass boost/tone control to be variable, so I tried throwing a pot between the cap and ground, and basically the pot was waaaay to sensitive, and acted more like a switch. So, I figured a pot with a much lower resistance would be better. I tried a 25ohm rheostat, and it worked!

Is my solution as hideous to you experts as I assume it is? What would you suggest instead?

Thanks!
By adding the cap, you created a low pass filter. This partly does what you want - to reduce the highs - but it continues to roll off as the frequency increases, thus the "muffled' sound. By putting the resistor in series with the capacitor, e.g. between the cap and ground you have limited the attenuation in the upper frequencies - good.

Without knowing the input impedance of the amp (the resistance of its input pot) I can't give you exact values for the cap and resistor to use. But you are basically doing something like this:
Baffle Step Compensation

The only problem that I see is that you have reduced the input impedance to 25 ohms at high frequencies, and that is NOT good. You should have at least 10k input impedance. You need a different value cap (a much smaller one), then you can use a higher resistor.

-Charlie

Last edited by CharlieLaub; 30th September 2011 at 06:33 PM.
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Old 30th September 2011, 06:34 PM   #3
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Join Date: Mar 2007
Location: California
Follow up... if you can just rewire the pot, cap and resistor as shown in the Figure in the link I posted, you can then calculate the correct values to use for the added cap and resistor. But then your volume control will be changes to a baffle step control, and you will need something else (like a preamp) to adjust the input volume.

-Charlie
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Old 2nd October 2011, 05:01 AM   #4
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Join Date: Sep 2011
Thanks so much for your input!

First of all I checked the stats of the board (found in the link I used in first post) and it says the input impedance is 60K. Not really sure what to do with that number but you said it was something worth knowing.

I tried the setup you supplied in the Baffle Step Compensation link and it worked...But with the pot turned to full attenuation it still wasn't nearly as...bassy as it was with my ghetto 200uf cap. Also the circuit had a slight drop in the overall volume, even with the pot turned to 0, where as in my design the volume was full when the pot was at 0.

That being said, I did notice one disconcerting issue with my ghetto setup. I hooked it up with my friends mp3 player and with the pot turned so it was at 0k (full connection to cap) the music would play fine for 10 seconds or so then suddenly stop for a second or 2 and then continue randomly going out. When I turned the pot and the treble came back the problem went away. My only guess is that the mp3 player has some sort of safety where when it senses shorted output it stops....or, maybe the cap is reaching some maximum capacity and bouncing in and out of full connectivity.

So I guess my question now is, how would I make the circuit you supplied a bit bassier, either by increasing the frequencies being attenuated and/or the max attenuation of the dampened frequencies?

Also I'm not really sure what you meant by the last sentence. Would it help if I put the volume pot between the output of the circuit and the board?

Thanks!
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Old 18th October 2011, 05:20 PM   #5
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Join Date: Sep 2011
Default Attenuation Vs. frequency pass

I apologize if you feel your knowledge is being wasted on my electronically novice mind, but if you would humor me for just one more question I think this one will help.

If the capacitance of the capacitor determines the frequency range that gets attenuated ie. how far down from the highs it rolls off, then what determines the actual attenuation ie. how much this filtered range is lowered?

Thanks!
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