AL type crossover transformation.

[Danieln]

This is a followup to my previous post  "La Scala impedance curve vs Tube Amp"

Having designed and built a pentode based SEP amp, I discovered that it's high output impedance is not a good match to the AL crossover.

I also read it is one of the worst crossover used in the LS.

 

My first  goal is to have a constant impedance network and secondly to made the midrange a true passband.

I completely dismantled the AL cross over and ended up with a big collection of parts, mainly from the infamous notch filters.

Looking at this kit, it became obvious that a redesign was possible.

A lot of the inspiration was from discussion and schematic revolving around ALK work.

So the end product is mainly inspired from the ALK universal.

 

A first order filter for the lower crossover.

A combination of second and third order for the high transition.

 

i modeled and Spice simulated many versions of the design.

I ended-up with the following:

1) A 1.25 mH inductance to the woofer

2) A 38uF going to the midrange filter.

    These two first elements defines the lower crossover.

 

3) A 0.3mH, 2uF and 10 Ohms connected to  T3a (-6db)

4) A 2.4uF, 0.2mH, 8uF tweeter filter (Including protection diodes)

     These two  elements defines the higher crossover.

 

Simulations shows a lower crossover at 450Hz  @ -3.1Db and a high crossover at 5900Hz @ -2.8db.

 

As a proof of concept, the AL provided all the parts to assemble this design.

Use two 2.5mH in parallel to built the 1.25mH

Use the 30uf + 8uF to built the 38uF

Modify the AL  .5mH and .125mH to .3mH and .2mH (By basically transferring the right amount of wire from one to the other coil)

 

So I built this thing, installed and measured its performance.

 

I measured the low transition at 440Hz @-2.9db and the high transition at 6000Hz@-2.88Db....basically matching the model.

 

Of course, this is just an ugly proof of concept. Better component can be used.

 

 

 

 

 

[Olomana]

I'm interested as I have a set of old ALs sitting around now.  How do these modified ALs sound?  Did you have another crossover to compare them too?

[Danieln]

I modified and measured the first one in order to compare the results side to side.

It positively corrected the midrange shooting.

I'm very careful to measure things since a few db can easily be lost using hearing only.

But this mod left no doubt in the hearing test department :-)

I then modified the second one and I'm now very happy with my Amp/ LaScala combo.

Note again that my amp has a 20 Ohms output impedance.

 

First step for you is to measure your amp output impedance.