please explain this crossover...

[gcm908]

Can someone please explain the role of each cap coil and resistor on this xo.
preferably grouped by function...
thank you...

[Bill Fitzpatrick]

For the bass:

The 2.2mh and 3.9mfd compromise a second order low pass filter for the mid/woof. The 3.9 ohm resistor makes the 3.9mfd capacitor less effective thereby tailoring the roll-off.

For the tweeter:

The 4.7mfd in parallel with the 1mfd (totalling 5.7mfd) is the first series element in a 3 order high pass filter. The 10mfd in parallel with the 4.7mfd (totalling 14.7mfd) is the second series element in the filter. These paralled caps could be singles of 5.7mfd and 14.7mfd respectively and probably would have been if those values would have been available off the shelf. The 3.9mh is the shunt element of the filter. The 3.9ohm resistor reduces the drive to the tweeter so its output is more in keeping with the woof. The 33ohm resistor flattens the impedance curve of the tweet and makes in appear as a more resistive load on the network.

Each component value making up the electrical network is based on the response of the drivers used.

[Jonathan Bright]

Hi, gcm908, It is pretty standard. Working from leeft to right you have the following: The 2.2 mH coil will attenuate the higher frequencies entering the wooffer (bass speaker). The 3.9 ohm resistor and the 3.9 uF capacitor form an impedence compensating network. This is used to compensate for the rise in impedence that voice coils have. So this makes the impedence more resistive and less inductive. Therefore the low pass filter to the bass speaker has a "first order" or 6 decibel per/octave slope.

With the components that feed the high frequency unit there are again two functions being performed. The 4 capacitors make two separate components (5.7 and 14.7 uFs)(think of them as two caps not four. There are 4 units to make up the proper values) and with the .39mH coil form a "third order" or 18 db/oct high pass filter.ie it stops low frequencies entering the tweeter. The 3.9ohm and the 33ohm resistors are a simple network that reduces the voltage to the high frequency unit to bring its level down to match the bass speaker.

The diagram has not been drawn in the usually accepted way and so this makes it a little confusing at first sight but it looks to be an entirely conventional good quality Xover. The values, ratings and tolerances etc. are completely "normal" for a good quality unit.


I hope this makes sense to you. More details supplied if not.

[Jonathan Bright]

Sorry that the two responses doubled up. Bill got in while I was still typing. There is a slight difference in our interpretation of function but that is not critical and whether Bill or I am correct won't stop the xover working properly, okay?

[gcm908]

thanks...

would it matter that the tweeter is 4ohm and the woofer is 8ohm...

any way to determine cutoff freq.

also why different grade caps?

gcm

[Jonathan Bright]

Yes and No!
I don't know what impedences the units for the Xover you put up on your 1st post are but what ever they are (8 ohm or 4 ohm) they should not be changed........unlesssssssss! Unless you whan to alter the values of the cross over components. As a general rule when changing impedence from say 4 to 8 ohms then you half the value of the caps and double the value of the inductors(coils). And visa versa when going the other way; it is proportional. So you could have a mixed system but you would have to alter the Xover.

Point two.
It is not easy to get an accurate value for the xover frequency from the values on the circuit. That is because of the point Bill made earlier. No speaker has a flat acoustic response or a flat impedence curve. In addition the enclosure can alter the shape of the final response. The designer may also have subjective preferences. Now the cross over will have taken these variables into account and come up with values that you can just about guarentee (however you spell it!) are not the ones that would come out of a text book on "normal" cross overs. Sorry, but that's the situation in the real world.

Different grade caps.
Generally people put more money into plastic film caps for the higher frequencies. These are usually "poly...." something or other. There are frequently polypropolyene or polycarbonate. These are generally regarded as sounding better. But these are more expensive than electrolytics and the plastic film caps don't ccome in big values so they are much more expensive to use when the xover point gets into the 100s of cycles rather than the 1000's. The film caps also come in much tighter tolerances and are usually more stable over time. Also it can be a question of taste and what the desgner thinks sounds good.

Now you obviously didn't make the circuit up yourself so I would suggest that you stick with the drivers and construction advice that came with it in the first place. Can that be done easily? If not then put up the specific changes that you have in mind and see what people say.

Unfortunately while xover design is something that can be grasped theoretically it can also be a bit of a "Black Art' in practise.

Given the nature of your questions you will find that doing a bit of work on impedence and reactive components (ie caps and coils) will allow you to move around in this area with more confidence. Basic texts will cover this topic.

[cheesehead]

Aha, I knew I'd seen that kind of xover drawing...

audiocomponents.nl scanspeak A4 monitor

Xover for A4 monitor is the last drawing under the pic.

Always wondered how these designs worked...

cheesehead

[gcm908]

THANK YOU ALL VERY MUCH...

The cabinet blueprints are my missing link...

Thank You... gcm908