Home Theater subwoofer as bass amp? - diyAudio
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Old 4th April 2014, 05:15 PM   #1
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Default Home Theater subwoofer as bass amp?

Someone was throwing away a perfectly good Sony subwoofer, saw it sticking out of a trash can, so WTH took it home.... 75W RMS, 28-200 Hz range. Ran my SigGen into it (unit has an RCA for line level in), and boy that reflex port moves some air at 30Hz. Want to add a little preamp for guitar-to-line level, maybe give it a tone control.... whaddya think?
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Old 4th April 2014, 05:26 PM   #2
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What cab are you going to cross over to for the rest of a bass's frequency range? This is just for messing with at home, right?
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Old 4th April 2014, 05:57 PM   #3
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Yeah, you're right, I need to reproduce higher frequencies to sound anything close to good...I'm just excited about getting something to play with for FREE....Have perused this forum on bass guitar frequencies, lots of good info. This would be an amp for home or garage use, it does have 75W RMS, maybe a medium horn (not piezo) with a crossover. Stuff the preamp inside the subwoofer, tone knob out the back, horn on top. Do horns have to be enclosed? I think not...may be wrong....
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Old 4th April 2014, 06:02 PM   #4
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Quote:
Originally Posted by LATUBEGEEK View Post
Yeah, you're right, I need to reproduce higher frequencies to sound anything close to good...I'm just excited about getting something to play with for FREE....Have perused this forum on bass guitar frequencies, lots of good info. This would be an amp for home or garage use, it does have 75W RMS, maybe a medium horn (not piezo) with a crossover. Stuff the preamp inside the subwoofer, tone knob out the back, horn on top. Do horns have to be enclosed? I think not...may be wrong....
You might want to look at the whole fEARful world for inspiration, then try a cheap 6 or 8 incher with a proper crossover, or maybe just a guitar speaker on top if you have one kicking around.
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Old 5th April 2014, 10:32 AM   #5
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200Hz is waaaay to low for bass guitar amplification. A 4 string produces roughly between 40 and 300Hz base frequencies, so that's without the higher harmonics that actually give the tone it's character. IMO at least the 5th harmonic needs to be reproduced to give proper instrument definition. Placing an extra horn will leave a wide gap between lows and highs.

Perhaps the driver can produce much higher frequencies and it's just the existing low pass filter that needs some tweaking. If you can up the frquency to e.g. 600Hz, it might work.
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Old 5th April 2014, 12:09 PM   #6
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Yes, he'll need to change the lo-pass filter components - here's the circuit:
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Old 6th April 2014, 05:01 PM   #7
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1) that schematic has at least one gross error: it does not show any DC connection to pin 6 of IC 203.
Some must exist on the actual PCB but makes me suspect the schematic.

2) That said, and instead of messing with the SW, (you still have to make a Bass preamp, if possible add some speaker to carry the rest of the range, etc.) I would rather get as small cheap , typical "15 W beginner's/practice Bass Amp", which usually have small enclosures and a light 8" or 10" speaker , add a preamp out jack and drive the SW with that signal.
Instant killer amp !!!!!
And it might be usable in a Garage situation.

Remember that although the SW has a 75W amp , the speaker there is very low efficiency, precisely to get that low in such a small enclosure.
But it would perfectly match a regular 15W amp, which would sit on top of it as a "head" .
Good luck.
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Old 6th April 2014, 06:09 PM   #8
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I have been using a preamp to test SW with a bass guitar - gets loud, but my first problem with making a bass amp out of this is the frequency response of the amp (curve posted previously, but post was dropped when moderators moved me to this forum. I HAD wanted to talk to linear OpAmp guys....).Anyway, posted below is amp response (at spkr), jives with SONY specs (28-200Hz). Changed C204 from 0.1 to 0.01uF (per poster tip to LPF on IC202), but didn't work. Assumed RC LPF (lo-pass filter) was C204, R208 & R209. Cutoff freq calc'd out to 175Hz with 0.1uF, hoped to see 1750 Hz with 0.01uF....but didn't. Am I changing the right component? Should I lower the R (208 & 209)? Cant do much until I open up the amp response.... Thanks!
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Old 6th April 2014, 06:45 PM   #9
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Quote:
Originally Posted by JMFahey View Post
1) that schematic has at least one gross error: it does not show any DC connection to pin 6 of IC 203.
Some must exist on the actual PCB but makes me suspect the schematic.
Well it's the official Sony schematic, but as you spotted it has an obvious error on it

If I get time tomorrow I'll check the PCB layout.
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Old 6th April 2014, 07:33 PM   #10
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Pin 6 of IC203 only goes to C208....see pics. It is identified as a "buffer" not an "amp" in the schematic. Might this explain things?
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