unusual passive x-over question

[amp88]

ok here goes

i have 4ohm mids, and 4ohm subs, i need to build a passive x-over that will allow me to wire the speakers in series to present the amp with a 8ohm load on each channel. Also the other reason i need the x-over is to keep the highs/mids from going to the subs.

ps this is my first post


thanks in advance to anyone who can help me.

[bunkie]

This simply isn't going to work. First, if you wire the two drivers in series, the moment you add the crossover components, you will have created a blocking filter. The low-pass section will attenuate the signal to the mids and the high-pass will attenuate the signal to the subs.

It must be stated that despite the fact that drivers are rated at a specific impedance (4 ohms, 8 ohms, etc.), this is a *nominal* value. What this means is that impedance varies as a function of frequency with all drivers having an impedance peak at their resonant frequency. When you introduce a crossover, the amp only sees the impedance for the driver within the passband of the crossover. Here's a little though experiment to illustrate this point.

Let's assume that you could create a perfect crossover that limited the passband to 1/12 octave. Let's also say that you have 120 8 ohm drivers and you wire them all up in parallel using one of our magic crossovers tuned to its specific frequency for one of the 120 segments of the audio spectrum. What would the nominal load be for the amplifier? The answer is eight ohms.

To get back to your question, if you really need an eight ohm load, either use 8 ohm drivers or wire identical pairs of drivers in series to achieve the 8 ohm load.

[amp88]

well, is there some way to create a lpf to put on the speaker wire going to the sub? the reason i ask is, i have car speakers that i am no longer using, and a home stereo receiver with no speakers. I will not have the money to buy speakers anytime in the near future, so i was hoping to find a way to use the ones i already have.

[bunkie]

Sure, you can create a low pass filter for your sub. All you need is a coil in series with the sub for a 6db/octave (also known as a first-order) filter. The value of the coil is a function of the impedance of the sub and the desired crossover frequency.

But I have to ask, why it's so important to have an eight ohm load? Very few amps have trouble driving four ohm loads.

And, finally, if cost is an issue, I recommend that you check out the Parts Express sale flyer (Parts-Express.com sells Speakers, Replacement Speakers, Speaker Building Parts plus HDMI Cables, Home Audio and Video, Pro Audio and Commercial Sound. We offer services for Speaker Reconing, Speaker Refoaming, Speaker Repair. Great selection of Elect). They often have blowout deals on all kinds of drivers and it's not uncommon to see, for example, woofers for six or seven dollars, tweeters for $1.99, etc. It's a cheap way to experiment.

[amp88]

Originally Posted by bunkie

Sure, you can create a low pass filter for your sub. All you need is a coil in series with the sub for a 6db/octave (also known as a first-order) filter. The value of the coil is a function of the impedance of the sub and the desired crossover frequency.

But I have to ask, why it's so important to have an eight ohm load? Very few amps have trouble driving four ohm loads.

And, finally, if cost is an issue, I recommend that you check out the Parts Express sale flyer (Parts-Express.com sells Speakers, Replacement Speakers, Speaker Building Parts plus HDMI Cables, Home Audio and Video, Pro Audio and Commercial Sound. We offer services for Speaker Reconing, Speaker Refoaming, Speaker Repair. Great selection of Elect). They often have blowout deals on all kinds of drivers and it's not uncommon to see, for example, woofers for six or seven dollars, tweeters for $1.99, etc. It's a cheap way to experiment.

Can you please specify what kind of coil you are referring to? and if possible please provide a link, if not i can google it. The back of the receiver says 8ohm minimum, so there for running it below 8 risks burning up the amp.

Also is there a coil that would provide a steeper rolloff like 12db/octave?

thanks in advance

edit: not having much luck finding one on google lol according to the calculator on the12volt.com 4ohm/80hz i need 7.95798 Millihenries (mHy)

[bunkie]

To get a 12db/octave slope, you need a second-order filter. That has a series coil and a capacitor connecting the output of the coil to ground. The capacitor provides a low-reactance (AC equivalent of resistance in this case) path to ground for the high frequencies, this sharpens the slope. For a second-order filter, you will have a different value for the coil.

I recommend that you google to get the formula for second-order speaker crossovers to calculate the correct values.

[amp88]

Originally Posted by bunkie

To get a 12db/octave slope, you need a second-order filter. That has a series coil and a capacitor connecting the output of the coil to ground. The capacitor provides a low-reactance (AC equivalent of resistance in this case) path to ground for the high frequencies, this sharpens the slope. For a second-order filter, you will have a different value for the coil.

I recommend that you google to get the formula for second-order speaker crossovers to calculate the correct values.

ok, if i do the calculation can u direct me to a site that has the parts?

again this is from the calculator at the12volt.com

80hz at 4ohms 12db/octave

Coil: 11.25259 Millihenries (mHy)
Capacitor: 351.64332 Microfarads (µfd)

[bunkie]

One of the biggest problems with doing passive crossovers at extreme low frequencies is that component values become very large. The capacitor value specified means that you will have no choice but to use a non-polarized electrolytic capacitor. All electronic components have a tolerance, typically 5-20% meaning that the actual value can vary within that range. The closest standard value of the cap is 330 uf. You can add capacitance by wiring additional capacitors in parallel, so to get close to 351uf, you could add a 22uf cap in parallel. Use caps of at least 250V ratings.

As for sourcing these parts, I would try the usual sources (partsexpress, mouser.com, digikey.com, madisound.com or allied).