Looking For Cheap Crossover w/o any coils

[Mike]

Does anybody know of a low pass (for a sub) crossover that is easy to build and requires no coils? Thanks for your help.

[chessman]

Active or passive? Active = before amp, passive = after amp (usually)

[Mike]

passive

[russlk]

You can use a series capacitor high pass to the tweeter, but the only way I know to keep the highs out of the subwoofer is to use a large coil. However, the lows will bother the tweeter more than the highs will bother the sub.

[analogee]

Well, conceivably you could make a lowpass filter with a resistor and a capacitor. In other words, put a resistor in series with the speaker, and a capacitor across the speaker. At high frequencies the capacitor would shunt the current away from the speaker. Something like an 8 Ohm resistor and a 470 uF capacitor, say.<p>However, you would have to accept loss even in the passband because of the resistor (if the speaker is 8 Ohms, ignoring reactive parts, and the resistor is 8 Ohms, you'd lose half the voltage, or 6 dB). And subwoofers tend to want to take most of the power to begin with. In addition the resistor would have to be a high wattage resistor to stand all the heat dissipated. The capacitor should be low ESR to really do anything. Yechh! All elegance is completely gone.<p>So as Russ said, a coil would be preferable. Or, just leave the wideband signal to go to the subwoofer. It won't really respond to the high frequencies, and no harm will be done.<p>Todd

[chessman]

Yes I remember seeing the equation for the low-pass RC filter in one of my books.<p>We're talking a beefy resistor, considering it's a subwoofer and all.<p>Oh yeah, radio crap will sell crossovers and coils.

[Mike]

are there any Active crossovers that are for a sub?

[analogee]

Greg,<p>I took a quick look at your schematic. A 741 opamp will most definitely not drive a speaker. It MIGHT be able to handle a load resistance of 2k, but that's about it; 8 Ohms is right out. You could put this circuit ahead of a power amp dedicated to driving the subwoofer; it could drive the power amp's input.<p>Also, I'm sure it's a typo, but you would not want to return the feedback to the '+' input of the opamp. The '+' and '-' pins in your drawing should be reversed.<p>Regards,
Todd

[chessman]

Yeah....I'm not sure if an active filter exists after the amp, unless you created a discret op-amp with power components....but then it would just be another amp <p>Much easier to buy a coil...

[Dimbulb]

passive = resistors, capacitors and inductors have less effect generally compared to active that really have a dramatic effect on performance. I think you may find capacitors are not cutting it for subs. So Active is sub friendly.<p>I think lots of well planned op-amps used in a band pass mode will make an excellant means of getting the sub audio where they belong keeping right and left separate also.<p>While this is possible one concideration is that these filters need only a small input of audio but a stable amount so an attenuation scheme is prefered here so automatic adjustment keeps the audio to the op-amps always the same in case the volume of the stereo is changed. In this scheme the filtered output is then amplified and sent to the appropriate range R L woofers.<p>In this way the output from the woofers sounds more like being right there closeup to a real stomper percussion section rather than a just another drabby cheap monotone window buzzing sound<p>One method takes a small sample of audio from inside the stereo prior to the volume and attenuates it. This unit fits inside the stereo and more connectors are added to the back of the stereo. Only small speaker wire and a 2 gang volume control at this stage. All this can be mounted in a small box on the back. The power amp wires are big usually mounted in the trunk or near(background subs in house).<p>This active approach may be something to concider. You may be unhappy with expensive inductors or capacitors as it may not have the robust sound that the sharper filtering can produce. separating sub tones and dealing with them separate rather than conventional band aid approach.<p>[ March 13, 2003: Message edited by: 1206DX ]</p>

[Bob Scott]

Never insert a resistor in series with a loudspeaker. You lose a great deal of the amplifier's damping factor. The speaker will tend to flop aroung a lot more at resonances of the speaker and the box. Bass will sound a lot less tight (ie: flabbier, boomier).<p>Bob

[chessman]

Yeah that's why it's odd to see a request for that type of filter....<p>Mikea:<p>If you have the capability, use an active filter with an op-amp and run it through an amp. You'll get EXCELLENT results.

[Bob Scott]

Mike,<p>To answer your question, the simplest cheapest "crossover circuit" used by cheap speaker manufacturers for two way loudspeakers is this: Simply wire the amplifier directly to the woofer. The woofer does not respond very well to high frequencies, so there is a natural mechanical roll-off of the highs. The tweeter is also wired to the amplifier, but has a 4 microFarad bi-polar electrolytic capacitor in series with the tweeter.<p>Anything more complicated involves the use of a coil, which you apparently don't want.<p>Quality crossovers use film capacitors, not electrolytics. They also use air wound coils, not iron core coils.<p>We used to be able to pick up cheap Asian universal two way or 3 way crossover networks at a local electronics supplier for about $5 but that was back in 1984. They used electrolytics and iron core inductors.<p>Chessman, I agree with you about the op-amp circuits sounding better. No speaker damping is lost, the varying reactance of the speakers due to interaction with the baffle or box will not affect the response in any way, and there is a large POWER advantage.<p>This is not only due to using multiple amplifiers. A 1V 2,0000 Hz signal riding on top of a 1V 40 Hz signal requires double the total amount of specified amplifier power to produce from a single amplifier than using two separate amplifiers. ie: You can use two 1 Watt amplifiers or one 4 Watt amplifier for the same maximum sound pressure level.<p>Bob <p>[ March 15, 2003: Message edited by: Bob Scott ]<p>[ March 15, 2003: Message edited by: Bob Scott ]</p>

[chessman]

Mikea, I don't think you'd like the sound of the woofer wired directly to the amp. Sure, it boasts simplicity, but unless you have a $3000 car-audio sub (CAR AUDIO IS A JOKE!! NEVER HEARD ANYTHING WORSE IN MY LIFE) your woofer will make some very bad sounds.

[gadgeteer]

What's wrong with making an air-cored-coil? Just a few turns will do it, and it's easy to wind one. You can use a bobbin, or a simple cardboard tube. A bottle of nail polish and a couple of bread ties will finish it up just right...<p>You can probably use the "X(l) = 2 &#183 &#182 &#183 f &#183 L", set X(l) to your speaker resistance in ohms, "f" at your crossover frequency and solve for "L" (in henries; multiply by 1000 for microhenries). Simply wire the crossover coil in series with your woofer...<p>http://www.epanorama.net/documents/comp ... coils.html<p>[ March 17, 2003: Message edited by: gadgeteer ]</p>