Setting active crossovers with a DMM?

[cobra93 47]

I've searched for answers, but I've not been able to find anything definitive.

Sorry, this will not be a short question.

On most crossovers that we find on car amplifiers, there is a range silk screened on the crossover, let's say 50 hz. -500 hz. I don't know if the slope actually matters if you can get far enough away from the crossover point or disable the crossover.

I believe that the setting on the crossover is the -3bd down point, but there isn't a way to know what your setting it to by looking at the potentiometer and the numbers on the amp.

Allot of things I've read on this topic are extremely vague. Maybe I'm not asking the question correctly in my search.

Such as > "start with the crossover at **hertz and adjust from there until it sounds good."

I understand that I shouldn't be caught up with the exact frequency, but rather what sounds correct.

However, in the case of an infrasonic/subsonic crossover or a narrow band pass (mid bass) or playing a tweeter too low, I'd like to get the frequency as close as possible to start.

I've possibly been spoiled by JL slash amplifier's crossovers, they give you a chart that tells you the crossover frequency at every click in the crossovers range.

Is the information accurate, I don't know. That's the point of my question.

I've read reviews in different magazines before that stated the range the crossover covers is not accurate to what the numbers are telling you (overall span).

As I understand it, at a crossovers -3db down point the amplifier should produce 1/2 the target voltage with a test tone. I don't believe the crossovers slope is relevant if I'm high passing a component set for instance and can get far enough away from the target frequency. Correct me if I'm wrong.

Let's say I'm looking for an 80 hertz high pass on a set of component/coax speakers and the target voltage is 20 volts. If I measure a test tone at 1000 hertz (well out of the crossovers range) and have 20 volts output at the speaker outputs of the amp. Does that mean I should play an 80 hertz tone and adjust the crossover frequency until the measured output form the amplifier is 10 volts at 80 hertz?

If this is true, then I'd assume setting a band pass crossover for a mid bass speaker( 80hz.-250hz.) would be to find the target voltage with the high and low pass crossovers disabled and proceed to find the 1/2 voltage output for the high/low pass points.

Is this correct?

Please no short answers. If you can provide a link to answer my question I'm happy to read it.

Thank you for your time.

Edited August 22, 2010 by cobra93

[Sir-Lancelot 216]

No idea on all that man, sorry.

IMO, you are just going to have to do it by ear. Turn off all the other speakers and listen for stress.

[cobra93 47]

No idea on all that man, sorry.

IMO, you are just going to have to do it by ear. Turn off all the other speakers and listen for stress.

I'm looking at your sig., You're running 2 18"'s in a ported enclosure.

How did you set your infrasonic/subsonic crossover?

Did you just play test tone while watching the subs excursion?

Thanks for your reply.

Does any of what I said make sense to you?

[Impious 1,476]

As I understand it, at a crossovers -3db down point the amplifier should produce 1/2 the target voltage with a test tone.

Not half of the voltage. That is incorrect.

You would multiple your "passband" voltage by .707, and that would give you the target voltage for the crossover, or the -3db point of the signal.

We can verify this with Ohms Law, which I'll show down below.

Let's say I'm looking for an 80 hertz high pass on a set of component/coax speakers and the target voltage is 20 volts. If I measure a test tone at 1000 hertz (well out of the crossovers range) and have 20 volts output at the speaker outputs of the amp. Does that mean I should play an 80 hertz tone and adjust the crossover frequency until the measured output form the amplifier is 10 volts at 80 hertz?

You could just start with an 80hz tone and leave the crossover off (assuming it has an "off" switch or the range is adjustable enough to leave the initial signal unaffected).

But you wouldn't have a target voltage of 10V for the -3db point. The target voltage for the -3db point, based on a 20V starting voltage, would be 14.14V. We can verify this back with Ohms Law. For example purposes, we'll pretend load (speaker) is 4ohm.

20V^2/4ohm = 100w

14.14V^2/4ohm = 49.9w (we'll just round this up to an even 50)

10*log(100/50) = 3db

So, we just verified the difference in voltage is 3db

The reason the target voltage for the -3db point of the signal is .707 and not half is because the decibel difference for voltage is a 20*log function and not a 10*log function like power output differences are.

20*log(1/.707) = 3db

Half voltage would instead be -6db.

[Sir-Lancelot 216]

No idea on all that man, sorry.

IMO, you are just going to have to do it by ear. Turn off all the other speakers and listen for stress.

I'm looking at your sig., You're running 2 18"'s in a ported enclosure.

How did you set your infrasonic/subsonic crossover?

Did you just play test tone while watching the subs excursion?

Thanks for your reply.

Does any of what I said make sense to you?

Yes Sir. Test tones, eyes, ears, and patience.

[cobra93 47]

As I understand it, at a crossovers -3db down point the amplifier should produce 1/2 the target voltage with a test tone.

Not half of the voltage. That is incorrect.

You would multiple your "passband" voltage by .707, and that would give you the target voltage for the crossover, or the -3db point of the signal.

We can verify this with Ohms Law, which I'll show down below.

Let's say I'm looking for an 80 hertz high pass on a set of component/coax speakers and the target voltage is 20 volts. If I measure a test tone at 1000 hertz (well out of the crossovers range) and have 20 volts output at the speaker outputs of the amp. Does that mean I should play an 80 hertz tone and adjust the crossover frequency until the measured output form the amplifier is 10 volts at 80 hertz?

You could just start with an 80hz tone and leave the crossover off (assuming it has an "off" switch or the range is adjustable enough to leave the initial signal unaffected).

But you wouldn't have a target voltage of 10V for the -3db point. The target voltage for the -3db point, based on a 20V starting voltage, would be 14.14V. We can verify this back with Ohms Law. For example purposes, we'll pretend load (speaker) is 4ohm.

20V^2/4ohm = 100w

14.14V^2/4ohm = 49.9w (we'll just round this up to an even 50)

10*log(100/50) = 3db

So, we just verified the difference in voltage is 3db

The reason the target voltage for the -3db point of the signal is .707 and not half is because the decibel difference for voltage is a 20*log function and not a 10*log function like power output differences are.

20*log(1/.707) = 3db

Half voltage would instead be -6db.

I believe your math to be correct, but I haven't done anything relating to logarithms since high school (I'm 38 now). I'll have to do some brushing up.

Since I can't reproduce the math (10/20 log) I'm lost, but I'll figure it out.

Thank you for a definitive answer to my question, and by the way, that was fast.

[cobra93 47]

O.K. , thanks to both of you.

Isn't the internet a wonderful thing.

[Sir-Lancelot 216]

Yes Sir, you really can find everything on the internet. lol

[cobra93 47]

Yes Sir, you really can find everything on the internet. lol

That's awesome, what a way to start my day.

[Tirefryr 1,742]

That could be deemed a vioaltion of the TOS, but that's just so not right, I can't fault you for that.

[Impious 1,476]

I believe your math to be correct, but I haven't done anything relating to logarithms since high school (I'm 38 now). I'll have to do some brushing up.

Since I can't reproduce the math (10/20 log) I'm lost, but I'll figure it out.

Thank you for a definitive answer to my question, and by the way, that was fast.

I cheat and use MS Excel to calculate just about everything math related