LPF for one of two drivers in series

[RevMen]

If I have a pair of EL70s per box (wired in series) with both drivers firing forward and I want to roll off the lower driver with Fc around 700 Hz, is it as simple as putting a ~57 uF cap across the leads of the lower driver?

[ThorstenL]

Hi,

Originally Posted by RevMen

If I have a pair of EL70s per box (wired in series) with both drivers firing forward and I want to roll off the lower driver with Fc around 700 Hz, is it as simple as putting a ~57 uF cap across the leads of the lower driver?

Almost. The 57uF will resonate with both drivers voice coil inductances which can be fun, so you need to place a snubber/zobel across each driver to make it appear resistive.

I used this trick in the 80's with Schulze KSP215 (East German Full Range drivers made for Studio monitoring) to allow more power handling and loudness, withpot loosing this "single-driver" sound.

Ciao T

[RevMen]

Is the Zobel necessary, or just desirable?

So you're saying the upper driver should have a Zobel network (resistor and capacitor in series) across its leads, and the lower driver should have a Zobel network across its leads in parallel with a filtering capacitor?

[ThorstenL]

Hi,

Originally Posted by RevMen

Is the Zobel necessary, or just desirable?

With the drivers I used then (~ 25 Years ago) they where essential.

If your drivers have very low voice coil inductance (by virtue of having a copper cap over the pole piece) you may be able to do without.

Originally Posted by RevMen

So you're saying the upper driver should have a Zobel network (resistor and capacitor in series) across its leads, and the lower driver should have a Zobel network across its leads in parallel with a filtering capacitor?

Yes, plus the Zobel must be sized to oppose the Voice coil inductance, which as we know also varies somewhat excursion... If the manufacturer supplies reliable spec's for Le (Voicecoil Inductance) there are on-line calculators for the job...

Ciao T

[RevMen]

Great, thanks very much.

The drivers are MarkAudio/CSS/Planet 10 EL70s, with a reported Le of 0.256 mH.

For anyone finding this thread, here's the most to-the-point article on Zobel circuits I found: TA Speaker Topics - Neutralizing L(e) with a Zobel

[ultrakaz]

The EL70 impedance sweep looks a little funny as the fs appears incorrect. Still, even if it is shifted a bit the impedance rise does not look so severe. Please post your findings with and without the Zobel. Thanks.

[ThorstenL]

Hi,

Originally Posted by RevMen

The drivers are MarkAudio/CSS/Planet 10 EL70s, with a reported Le of 0.256 mH.

With 56uF this would give a resonance at 1.3KHz... So Zobel needed I think.

In addition, there is an important piece to note. Shunting one driver at MF/HF does not provide anything like baffle step correction, due the drivers being in series all that happens is that we slowly "belnd over" to single driver operation at higher frequencies.

So any baffle step correction still needs a series LR circuit in series with the Speakers!

What the Cap across the second driver does is to create a system that has greater LF power handling/displacement/SPL than a single driver, but at high frequencies acts as pure single driver system, with non of the comb filtering etc associated with multi-driver systems.

Ciao T

[planet10]

Originally Posted by ultrakaz

The EL70 impedance sweep looks a little funny as the fs appears incorrect.

The impedance curve is one i actually measured... LMS use a log vertical scale which was inconvienient. My kit will measure a higher Fs...

dave

[tuxedocivic]

I think he's refering to what we discussed here: EL70s have arrived!! starting at the bottom of the page. You corrected the data sheet, but Bob never did upload the correct sheet to the site.

This is the proper sheet: http://homepage.mac.com/planet10/CSS...ata-080909.pdf

[ultrakaz]

Originally Posted by ThorstenL

Hi,

With 56uF this would give a resonance at 1.3KHz... So Zobel needed I think.

In addition, there is an important piece to note. Shunting one driver at MF/HF does not provide anything like baffle step correction, due the drivers being in series all that happens is that we slowly "belnd over" to single driver operation at higher frequencies.

So any baffle step correction still needs a series LR circuit in series with the Speakers!

What the Cap across the second driver does is to create a system that has greater LF power handling/displacement/SPL than a single driver, but at high frequencies acts as pure single driver system, with non of the comb filtering etc associated with multi-driver systems.

Ciao T

Can bsc be addressed by moving the second driver to the rear or side? And what would be the sonic advantage (if any) to having both drivers forward firing?