Using variable resistors (Pots) in X-Overs

[Jeanrick]

Pardon my ignorance. I have learnt a lot on this forum over the past few weeks, but why doesn't anybody use variable resistors in crossovers, as this will alow one to fine tune the speakers from outside the box? Is it because of the power of the circuits or what other reasons are there?

[wintermute]

Hi Jeanrick, variable resistors as such aren't used, however variable lpads certainly can be used. They look like a big potentiometer but they are actually different in their construction.

Tony.

[Andy5112405]

The problem is power.

Most crossovers will use quite high power resistors. It is not easy to obtain high power variable resistors.

[AllenB]

When I adjust crossovers, I use a soldering iron. There is just as much chance that I will need to adjust a capacitor, or a notch filter, as a padding resistor. Having a variable L-pad would actually be inconvenient as the values aren't marked and an L-pad alters the impedance curve.

[digits]

So does alot of things, like heat....

[Jeanrick]

Many thanks guys - I didn't think it would affect the other components or curve too. It just seems a lot of work to play around with resistors and soldering them every time.

[AllenB]

It really depends on where you are with it, but once you get a crossover zoned in the changes become small and specific...nothing just an L-pad can fix.

I would suggest you design your crossover to exclude woofer cone breakup, tweeter resonance, roughly match levels and phase at the listening position, as well as your other system requirements. Then use an equaliser to develop the tone. It is much simpler and more accurate when you can make virtually instant changes. Then later you can translate that back into soldered components.

[Loren42]

Originally Posted by Jeanrick

Many thanks guys - I didn't think it would affect the other components or curve too. It just seems a lot of work to play around with resistors and soldering them every time.

If you are able to take good measurements of the SPL over frequency you can dial in the tweeter level very quickly.

[PeteMcK]

L pads in circuit are infrequently used, they oxidise & become scratchy, so they're better replaced with fixed resistors. I have one with the cover removed and alligator clips attached; this allows me to measure the values of it's arms when tweaking, and then replace it with fixed resistors

[Soundminded]

A crossover network is a filter network whose characteristics including crossover frequency depend on the resistance of the load. A potentiometer used for level control changes the crossover frequency besides changing the driver output level. When an L-pad is used instead, the crossover network sees the same load impedence so the crossover frequency does not change but the output level still changes. An L-pad is just two potentiometers, one in parallel and one in series. As you turn it, the resistance of one increases while the other decreases by the same amount. This is how the impedence on the crossover network is kept constant while adjusting the level of the driver.

Neither potentiometers nor L-pads should ever be used with woofers. Any resistor in series with a woofer reduces the effective electrical damping factor of the amplifier to surpress spurious resonances. Therefore, in designing a speaker, the woofer should be the least efficient (sensitive) driver in the group and the midrange and tweeters padded down with resistors or L-pads to match the level of the woofer to bring the system into acoustic balance.