Capacitor Crossover. Potential Problems

Hi, I'm about to connect the tweeters (Bose dome tweeters, the ones on Audis, etc. not very good) to the midranges.

The tweeter is crossed over at an unknown freq. by a 3.3 micro farad capacitor. I don't know much, but it seems to me it must a very high freq.

My question is: If I connect the tweeter in phase (+ to +, - to -) and I time-align both drivers, will the tweeter's sound be in phase with the mid-range's sound?

Is the capacitor going to change the tweeter's phase? (I read this first-order alignment would produce a 90 degree change in phase, what does that mean?) If so, time aligning would correct that?

Thanks
 

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BlueWizard

Member
2007-06-29 8:49 pm
The Tweeter is crossed over by a 3.3uF capacitor, but how is the midrange crossed over; bass to mid, and mid to tweeter?

If we assume the tweeter is 8 ohms, then the crossover frequency is about 6khz.

F = 1/[2(pi)RC]
F = 1/[2(pi)(8)(3.3x10^-6)]
F = 6028.6 hz

Using the same formula, if it is a 16 ohm tweeter, then the crossover is at 3014.3hz.

If it is a 4 ohm tweeter, then the crossover is at 12,057hz (not very likely).

For a three way speaker, that might be a reasonably crossover, though it is a little on the high side. Most commonly for a 3-way, you will have 400hz to 800hz for the low/mid and 3000hz to 6000hz for the mid/high.

Does that help?

Steve/bluewizard
 
Only the tweeter is crossed over by the 3.3uF. The mid-range does not have an inductor, it is depending on its mechanical behaviour at high frequencies (I guess)

The tweeter is 4 ohm, so, yes it must be crossed over at 12,057 Hz, which, I agree, sounds weird (and perhaps doesn't match the listening test) but I doubt the capacitor is wrongly labeled. I measured the tweeter and there's no doubt it is 4 ohms.

I want to know whether there is a change in phase in the tweeter's output caused by the capacitor. In order to know if time-aligning of the drivers is in order.
 

BlueWizard

Member
2007-06-29 8:49 pm
I assume you measured the tweeter with an Ohm-Meter. That can give you an estimate, but NOT an accurate measure of a speaker.

What you measured is the DC voice coil resistance. That is probably the lowest a speaker can every go, but when Dynamic Music is being played the voice COIL doesn't act like a DC resistance, it acts like a coil and is frequency dependent.

The ideal test would be to take an 8 ohm or 10 ohm resistor and put it in series with the tweeter. Then take two cheap voltmeters or a 2-channel oscilloscope, feed in a very low level 10khz signal, and quickly measure the voltage across the resistor and the tweeter. That should help you determine the real functional resistance of the tweeter.

Warning: Do not sustain a constant sinewave tone for any length of time. A constant tone like this will build up heat very quickly even at low levels, and could damage the tweeter. Keep the levels very low. Make the measurements quickly, and disconnect the tone right away.

Another possibility is to connect an 8 to 10 ohm resistor in series, add the capacitor to that combination and play music through it. Using two voltmeters compare the voltage across the resistor and the tweeter. Again, the ratio of that voltage will give you the ratio of the functional resistances.

Sorry, that's about the best I can do.

In your case, using speakers with such limited crossovers, I don't think you need to worry about phase shift. With a normal 12db/octave 3-way crossover, it is common to wire the midrange in reverse polarity (+ to -, and - to +). But as I said, in your case, I don't think it matters, just wire everything + to +.

Steve/bluewizard
 

Bobken

Member
2002-12-23 11:22 pm
UK
davidbosch said:
Only the tweeter is crossed over by the 3.3uF. The mid-range does not have an inductor, it is depending on its mechanical behaviour at high frequencies (I guess)

I want to know whether there is a change in phase in the tweeter's output caused by the capacitor. In order to know if time-aligning of the drivers is in order.

Hi,

The answer to this question is that there will be a nominal 90 degree phase-change when you use a capacitor in series with a tweeter like this.

If you were to use a differently arranged x'over (like a second order) where there is also another 90 degree phase-change in the other drivers circuit and these are cumulative to give 180 degrees difference, it is possible to do something about this by reversing the connections to one drive unit.

However, this only nominally corrects the phase at the crossover point, and at other frequencies which these drivers handle, there will be substantial phase differences, which I personally don't care for.

To correct for this 90 degree difference is not so easy in a x'over circuit, but "time-alignment" which you refer to can also be done by way of positioning the drivers relatively to each other, and locating them closer to, or further from, the listening position. However, as you needed to ask this question about phase differences in the first place, I think you may need to learn a lot more about this possibility before attempting anything like this, if you are to end up with a satisfactory result.

When measuring DC coil resistances of many different drivers, I find that almost universally the DC measurement will be in the region of 3/4 of the nominal stated impedance of the drivers. Therefore a nominal 16 Ohm driver will measure at around 12 Ohms, an 8 Ohm at around 6 Ohms, and a 4 Ohm maybe at about 3 Ohms, but this is only a general guideline. If your drivers measure up at 4 Ohms DC, as you suggest, then I would guess that they are probably in the region of 5.5 - 6 Ohms nominal, which is not unusual.

Regards,