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Old 3rd March 2003, 11:26 PM   #1
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Default DIY subwoofer amp

I'm planning to build a DIY subwoofer amp. As the first step, I collected informations on active crossovers from the internet and came up with the following circuit. It consists of four sections: input buffer, mixer, two R-L filters in series, and output buffer with volume control. Because this amplifier will be used with my DVD player, which has only one pair of audio lineout (left and right), it needs to have a pair of buffered outputs which will be connected to my receiver. My reciever doesn't provide any lineout connection. The target crossover frequency is ~100 Hz which is pretty close to the value (109 Hz) from the resistors and capacitors shown in the circuit.
I will make a PCB for this amplifier and would like to make sure that this circuit will work before I print out the PCB layout. So, any recommendation and suggestion will be appreciated. Thanks.
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Old 3rd March 2003, 11:33 PM   #2
SY is offline SY  United States
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I think you mean "RC" not "RL."

In any event, the first thing that strikes me is that you've got the possibility for the opamps to lose their DC ground reference. There's also the potential for some major pops and bangs should some connections get loose.

Tie the buffer input to DC ground with a meg or so resistor. Tie the Xin inputs of the filter blocks to DC ground the same way.

A more global question is what acoustic crossover you're aiming at; it's rare that a desired electronic response will give you what you want acoustically unless you take the speaker's acoustic responses into account. What are you using for an upper-range speaker? Ported or sealed? f3?
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Old 3rd March 2003, 11:56 PM   #3
Diode is offline Diode  United States
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Jazz,

Dude, just go buy a cheap receiver that will do Dolby digital. Those have sub outputs. They are pretty cheap these days. I'm not trying to crush your project, just giving another idea..

With respect,

Chris
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Old 4th March 2003, 12:59 AM   #4
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Quote:
Originally posted by SY
I think you mean "RC" not "RL."
Thanks for the reply. What I meant by "RL" was "Linkwitz-Riley". I should have written "LR" instead of "RL".

Quote:

In any event, the first thing that strikes me is that you've got the possibility for the opamps to lose their DC ground reference. There's also the potential for some major pops and bangs should some connections get loose.
It's hard for me to understand why the opamps could lose their DC ground reference. Could you tell me more about it, please?
Actually, many part of this circuit came from Rod Elliot's website. This circuit is almost the same as the low-pass section in his electronic crossover design except for the summing amplifier which was copied from another project of his. One thing strange is that he didn't mention anything about "DC ground reference" problem. Maybe he's missing something? I thought his design is VERY reliable. Maybe not.... I don't know.
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Old 4th March 2003, 05:54 AM   #5
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Quote:
Originally posted by Diode
Jazz,

Dude, just go buy a cheap receiver that will do Dolby digital. Those have sub outputs. They are pretty cheap these days. I'm not trying to crush your project, just giving another idea..

With respect,

Chris
Thanks for your idea which is exactly the same as my wife's who is not an audiophile and doesn't want to be one either. The main purpose of my projects is just for FUN. "The JOY of building". If I had wanted the answer like yours, I wouldn't have post my message here. No offense to your idea.
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Old 4th March 2003, 02:16 PM   #6
SY is offline SY  United States
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Quote:
Originally posted by JAZZ2250

It's hard for me to understand why the opamps could lose their DC ground reference. Could you tell me more about it, please?
If your Xin is not connected to something with a DC ground reference (a cable not plugged in, for example, or a cable pulled out- these things happen!), the noninverting input of U3 will not have a DC return path.

BTW, I was somewhat unclear about the buffer- when I said to tie its input to ground via a large resistor, I meant at the input jack. That way, the end of the coupling cap isn't floating, a likely source of some nasty bangs.
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Old 5th March 2003, 03:42 PM   #7
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I'm wondering how much gain I should give to the subwoofer amp.
The input buffer and active crossovers will give a gain of 0, and the output buffer will give a gain of 6dB which can be adjusted by a volume pot. Higher frequencies (higher than the crossover frequency for the subwoofer) will be driven by LM3875 based amplifier with a gain of 27 dB. Of course there will be a volume pot at the inout stages of the LM3875s.
BTW, I'll be using PartsExpress 299-720 DVC subwoofer which can handle upto 50 watts/75 watt max.
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Old 5th March 2003, 05:07 PM   #8
miguel2 is offline miguel2  Portugal
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Hi Jazz2250,

I made an el-PipeO sub this weekend. For the electronics I used the passive crossover suggested in the article and then connected it to a 25W amp. I have to say that it goes loud but not that loud, so for home theatre you may need some extra power here. I don't know the sensitivity of the driver I used though.

Miguel
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Old 5th March 2003, 05:25 PM   #9
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Quote:
Originally posted by miguel2
Hi Jazz2250,

I made an el-PipeO sub this weekend. For the electronics I used the passive crossover suggested in the article and then connected it to a 25W amp. I have to say that it goes loud but not that loud, so for home theatre you may need some extra power here. I don't know the sensitivity of the driver I used though.

Miguel

Could you post more details about your project? What kind of driver and amplifier you used? Thanks.

According to the TI application note, +20 dB higher gain is given to the subwoofer section than the higher frequency range. I'm wondering if this is a general rule of thumb? I attached a figure from TI on the following post.
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Old 5th March 2003, 05:37 PM   #10
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Here it is...
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