OK. then decide what you want..a passive filter or an active filter

Again, I had said in one of my previous posts, I already have a passive filter, and I am trying to upgrade to an active filter.

 

First stage, use U1A in the circuit you supplied to sum R/L channels, but add a small cap across R3 (10pF). Then, second-stage, add a 3-pole butterworth (google it) at about 100Hz or lower (<20Hz for a real sub-woofer), followed by a simple non-inverting buffer output amp ...it doesn't hurt (too much) to play around cheaply.

 

I already have the driver, speaker box and amplifier, all of which I have previously stated.

But you do not have a sub-woofer. Instead you have an ordinary 12" woofer in an enclosure designed for a 3-way speaker. The woofer is probably flat down to 50Hz then its response drops at 12db/octave for lower frequencies. Its power handling is low so it cannot use bass boost.

You tested it down to 1Hz(!) but any speaker moves and produces noises at low frequencies but does not produce the wanted frequency. When you feed your woofer 30Hz then it probably produces 60Hz.

If the enclosure is sealed then the woofer in it resonates at one frequency. Its frequency response drops below the resonant frequency at 12dB/octave. It is easy to measure the resonant frequency:
1) Connect a 100 ohm resistor in series with the woofer.
2) Connect the speaker in series with the resistor to the output of a power amplifier.
3) Connect an AC voltmeter or oscilloscope to the speaker.
4) Connect a sine-wave generator to the input of the amplifier and keep the volume low enough so the speaker is not damaged and so the resistor is not too hot.
5) Vary the frequency and look for a peak in the AC voltage shown on the meter which will be from 30Hz to 100Hz. The peaked frequency is the resonant frequency.

 

I don't think he get's the Idea Guru.

 

This is exacly what I've been wanting to do.
I'm using two chanel set up in the basement, and just want to 'pick-off-
subwoofer frequencies from the line level.

Is there ben any more interest or progress on this idea.

I'll bet they would sell like hotcakes.

Does anyone have a proven schematic that works well, I'll build my own.
Please write back. Don

 

What is the idea? If the stereo does not produce deep low frequencies then you will not "pick" any to feed a lowpass filter and subwoofer amplifier and speaker.
My cheap clock radio sounded like all other cheap clock radios with no deep bass sounds from its little internal speaker. I modified the circuit (increased coupling capacitor values) so that now it produces deep low frequencies to an external speaker I made. My speaker also has a high quality tweeter so the clock radio now sounds like high fidelity.

 

Is it just me, or is the circuit in post #1 horrible? What I see are two high pass sections at 12.5 Hz and two lowpass sections at 16 Hz.

ak

 

The circuit in post #1 has two passive highpass filters at 12.5Hz plus two passive lowpass filters at 16Hz. Since they are passive and are not active then their frequency response around the cutoff frequency is very droopy. Since their frequencies are so close together then the peak at about 14.25Hz will be attenuated and will produce a one-frequency boom sound, if the signal source and woofer can produce sounds that low.

 

The circuit in post #1 has two passive highpass filters at 12.5Hz plus two passive lowpass filters at 16Hz. Since they are passive and are not active then their frequency response around the cutoff frequency is very droopy. Since their frequencies are so close together then the peak at about 14.25Hz will be attenuated and will produce a one-frequency boom sound, if the signal source and woofer can produce sounds that low.

 

What I'm simply trying to do, is let the 'pre-amp go to it's amplifier, and the Bass go into a powered sub, Without shorting to r & L chan together.

 

L and R are not shorted together, they are mixed together in the 33k resistors that feed the inverting input opamp, then the subwoofer can play low frequencies from either channel.
The passive filters in the first post of this thread are absolutely useless for a subwoofer. You need an active lowpass filter for the subwoofer and matching active highpass filters for the stereo signals.

 

U1A combines the two channels correctly. It has a high enough input impedance to prevent excessive loading, and the inverting summer circuit does not induce crosstalk between the channels.

Everything after that is terrible for the quality of the output audio. Some of that can be fixed by changing component values, although a true two-pole filter circuit would be better. What is the corner frequency you want for the sub-woofer channel?

ak

 

 

Thread #33 shows the idea we're trying to get at.
Any better sugestions. It would b using a Powered Subwoofer.

 

There must be tons of us two channel folks trying to do the same thing.
I want to buy or build something to that end, if I could get a proven schematic of what we are loking for.

 

Much better than post #1.

Instead of a single coupling capacitor for the two input channels, better to have one 1 uF capacitor in series with each input, and join the two 33 K resistors directly to the opamp pin 6. In this way there is nothing between the virtual ground and the input summing point to increase (slightly) crosstalk.

Calculate the cutoff frequency of the active filter and compare that to the bandwidth you want for your sub channel.

ak