Type E crossover newbie questions

[folkdeath95]

Hello,

This is my first post here, even if I've been reading this forum for nearly two months. Since I decided to search a pair of Heresy 1. The forum gave me a lot of information and helped me to find my way in the Heresy jungle.

I finally find one that need a little work on the cabinet, because the previous owner painted them in green[:|], but that's not the subject.

I've studied the Type E schematic, but I'm a real newbie.

First, I considered the T2A as a kind of variable resistor (it looks like this on the schem), and so I thought that the crossover slopes was 6db/oct on all the speakers (low-pass on the woofer, high-pass on squawker and treble).

Then, I found a T2A datasheet:

And I realised that the autotransformer has an (variable) inductance. So, with the T2A as an variable inductor, the crossover slopes change. On the squawker, it becomes a second order crossover (a capacitor in serie, an inductor in parallel), on the treble, it could be a third order crossover (a capacitor in serie, an inductor in parallel then a second capacitor in serie).

I'm kind of lost... I don't know how to consider the T2A (variable resistor? variable inductor?) and so, which slopes are used on the crossover.

Does somebodu has any clues?

Some peoples are tweaking the Type E crossover by changing the medium/treble attenuation on the T2A. They also change the value of the capacitor just before the autotransformer to keep the right crossover frequency. That seems OK to me.

But if the inductance value of the T2A varies when changing attenuation, are we sure to keep the crossover frequency as it should be?

Thanks in advance for your answers and sorry if my questions sound silly.

[BEC]

The easiest way to think about how the T2A (autotransformer) works is to consider it to be an impedance multiplier. If a 16 ohm driver is connected to tap 4, it appears to be, at the input of the autotransformer, a 32 ohm driver. That is 3db of attenuation. Connected to tap 3, the impedance reflected to the input of the autotransformer is 64 ohms and that is 6db of attenuation. In the Type E crossover, the squawker is connected to tap 2 giving 9db of attenuation and reflecting 128 ohms to the input side of the autotransformer.

This reflected impedance then becomes the amount you have to consider to get the input cap the right size. And, any time you may want to change the tap for the squawker to make it more or less attenuated, you have to make a corresponding change to the input cap to keep the crossover frequency the same.

Bob Crites

[cfelliot]

I've often wondered about just putting the squawker cap on the secondary side of the autoformer. I know this drastically increases its size, but then you're free to move the taps anywhere?

[BEC]

That is correct. I often do that when working out a new crossover design to be able to quickly change attenuation.

Bob Crites

[folkdeath95]

Thanks Bob.

If I understand what you're saying, the inductance of the T2A should only been considered as a "side effect" and not a part of the crossover conception. So, this is a first order crossover, right?

[BEC]

Thanks Bob.

If I understand what you're saying, the inductance of the T2A should only been considered as a "side effect" and not a part of the crossover conception. So, this is a first order crossover, right?

You can consider the inductance to be so high that it is not a factor.

Bob Crites

[folkdeath95]

Thanks. Things become clearer in my head.