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Old 28th February 2011, 01:04 PM   #1
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Unhappy Subwoofer filter problem - Please help me

Hi
I recently have built a 250W mosfet amplifier and I need a very good Low pass filter ( Sub filter ) for it to drive my sub

I tried this one

Low pass filter - Subwoofer

Click the image to open in full size.


but I want something with higher performance
Please introduce me something that you've built and it did work
thanks

Please someone give me a reliable and tried schematic
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Old 28th February 2011, 01:53 PM   #2
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That's a standard unity gain Sallen-Key filter set up for LR2 which should work if properly implemented. An issue is that if the dual pot sections don't track well you could have some response irregularities. What are the performance issues that you would like to improve? What crossover frequency are you targeting? Are you high passing the mains? What is the mains F3?

Take a look at Active Filters for proven circuits. Use a decent regulated power supply and OPA2134 or better. Be sure to bypass the opamps power connections with 100 nF right at the chip.

EDIT: Actually for sub duty, you don't really need OPA2134, you can get away with even a TL072, although the cost differential is only a couple dollars.. Use better op amps on the high pass section if you build one.

I'd start with an input buffer like the circuit surrounding IC1 in your circuit, except use a fixed 47K resistor for R3. You need this to mix the two channels (assuming that this is a stereo project and not HT receiver is involved.)

Then add a standard Sallen-Key filter stage or two, depending on your target slope and an adjustable attenuation drive to the output buffer. The adjustable attenuation can be as simple as using your 47K pot as a normal volume control. You can also use it as a variable series resistor with a 10K resistor to ground across the input of the buffer. This would give you greater precision of adjustment over a +10to -5 dB range. (with R3 = 47K you will get 10 dB gain in the buffer. If this is too high, simply reduce it to 20K and you will have roughly 0 gain to -15 db.)

Last edited by BobEllis; 28th February 2011 at 02:02 PM.
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Old 28th February 2011, 02:37 PM   #3
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Thank you for your attention

my amp has these characteristics :

POWER RATING at 1KHZ with 0.1* THD = 200W/ 8 ohm, 260W /4 ohm
IMD= *0.008*
THD at 200W into 8 ohm *1 KHZ= *0.005*
THD at 200W into 8 ohm *20 HZ....20 KHZ= *0.05*
FREQUENCE RESPONSE [at 50W/8ohm, *0...-3dB]= 1.5 HZ.....125 KHZ
INPUT SENSITIVITY= 1Vrms
ΙΝPUT IMPEDANCE= 48K
SLOW RATE with input filter= 20V/uS
=SIGNAL TO NOISE RATIO [at 1W/8ohm] *99dBA
= DAMPING FACTOR at 8ohm [15HZ...25KHZ] 160


And I need a Sub filter for it
I tried upper circuit but it has a little noise on it ( I can hear a little voice of the singer on a music ( just a little ) when just subwoofer is working) and I think the output of this circuit is not enough for my input amplifier ( 1V rms ) do I need a preamp for this ?

I want something in the range of 30- 160 Hz

BTW is this circuit good for my purpose or I can get a better circuit with higher bass and better quality?

Sorry I'm not a professional, and I need more details for it
If you have a tried circuit diagram I'll be happy If you can link it for me or send it to my mail ( J_behzadi_5@yahoo.com )

Thanks a lot
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Old 28th February 2011, 07:03 PM   #4
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You will hear voices coming from the sub if the mains are off. Say you set the crossover for 80 Hz. As a Linkwitz-Reilly filter the low end of a typical male voice (85 Hz) will be attenuated a hair over 6 dB. A female voice low fundamental (160 Hz) will only be attenuated 18 dB. Both will be audible.

You might want to change your filter to 24 dB/octave. This will attenuate the higher frequencies more rapidly and make the sub harder to localize. However, you want to match the sub roll off to the mains. So we need to know your mains' low end capability and how you are rolling them off. You could string together filters and buffers as shown on the Linkwitz page I linked to.

The circuit is fine, although you should probably consider replacing R5 and R7 with fixed resistors once you determine the appropriate values.

You have enough voltage output to drive your amp with this circuit. Most CD players have 1V RMS output (which is the input voltage for the filter) and the circuit can provide up to 4x gain. As long as you have a +/-15V power supply you can drive your amp to clipping and beyond. If you can't adjust your subwoofer loud enough with this circuit, you may have a very insensitive subwoofer and need more gain. You can increase R3 to a 100K pot and get more gain if you need it.

What are your main speakers and subwoofer's rated sensitivity? (xx dB @2.83V)
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Old 1st March 2011, 05:17 AM   #5
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Thanks again for your replies

I have two 8" and 8 ohm mini sub for now that I parallel them together to obtain a 4 ohm sub. I bought them about 8 dollars each that I don't think they are enough for my amplifier, am I right?
I want to buy a car sub woofer with a sub box for it ( Kenwood C10 12" -4ohm- 200W Rms )
Vendo mi subwoofer Kenwood 12" con caja

Click the image to open in full size.

Is it good for my purpose?

Only problem that I have with this filter is its bass
I think I can get more bass with another filter sub? is it right?
I also have these filter sub but I don't know which one is good or even Works?
you can download them from Rapidshare :

http://rapidshare.com/files/450364739/Sub_filter.rar

I want high power from this 250W amplifier.
can you help me to obtain to my desire?
I said this earlier that I'm not veteran at this but I have little experiences.
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Old 1st March 2011, 06:58 AM   #6
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BTW my filter's power supply is fixed -+12V



Click the image to open in full size.


Is the voltage enough for 1V Rms output?
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Old 1st March 2011, 01:39 PM   #7
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Other than in car use, 8" drivers rarely move enough air to be called subwoofers. You probably can't put anywhere near your amp's full power into them down low.

What do you mean by more bass? Are you looking for deeper or louder? That chest thumping feeling is usually not very deep.

If you are just looking for louder, change your filter's R3 to a 100K or 250K pot, that will give you the ability to dial in more gain.

Your power supply is OK. If you are going to add a high pass filter to the system, you might want to consider better regulators.

Quote:
What are your main speakers and subwoofer's rated sensitivity? (xx dB @2.83V)

... we need to know your mains' low end capability and how you are rolling them off.
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Old 1st March 2011, 03:26 PM   #8
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I need your help to find a sub woofer ( car sub woofer ) with its box for my amp
as I said before my two mini sub are not from a well-known mark and I just could read there watts and there ohms
Now I want to buy a sub woofer with a proper box
Can you help me to buy an excellent sub for my amp ( for my computer's speaker )
but I want it to have a powerful voice a please don't consider that this sub is for home and doesn't need high power.

and by more bass, I meant more chest thumping feeling ( as you said )
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Old 1st March 2011, 04:01 PM   #9
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Try increasing the crossover frequency to it's maximum (R5/7 to minimum) and turning up the gain (P3 set to it's maximum resistance) if you need more gain, and don't can't easily purchase a 100K or 250K pot, add a 39K resistor between R3 and the connection to R1 and R2. I assume that you had to buy more than two 39K resistors when you built the filter and have some on hand. If not, you can try 47K, or if that isn't enough 100K. This will make your 8" drivers play louder and hopefully have some overlap with the mains to get you some chest thump.

It's hard to recommend another driver without knowing your space constraints and budget. For small room/computer use, I'd normally say something like a 10-12" driver, maybe Peerless XXLS or similar. But since you're not really looking for deep bass almost any 12" driver that can go down to 60 Hz in a reasonable size enclosure should provide some thump. What drivers are available to you at reasonable cost?

What are your main speakers?
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Old 1st March 2011, 04:24 PM   #10
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Kenwood and pioneer and Jbl are plentiful here
If you know a specific type from these marks I'll search and see If we have that one here

I planned to buy Kenwood C10 12" 200W Rms
I'll expend about 80 or even 90 dollars for just sub and about 30 or 40 dollars for the Box

my main speaker are weak now and I'm going to raise them to an acceptable main speaker.
but my only problem is to build my subwoofer system and match it with a good sub and box
you count highest possible amount for room area
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