Heavy L-pad attenuation, what are the things to consider?

[rhythmdiy]

I have a tweeter and midbass with 94 and 91.5 db respectively to be coupled with the 10 inch woofer with 87db sensitivity can anybody tell me If I would like attenuate the woofer and tweeter sensitivity to 84db then what are the effects that I will be facing ? any suggestions please tell me what are the sensitivities should be?

tweeter 94db to 84db?
midbass 91.5 to 84 ?
woofer default 87db?

any idea please....

[sreten]

Hi,

You can use something like Basta or the FRD tools to see the
basic effects. In a nutshell and ignoring the c/o the L-pad
creates a source resistance which is the series and parallel
resistances in parallel. This interacts with the driver impedance.

This is not necesarily a bad thing, it just needs considering.
A typical error for attenuated midranges crossed over low is
the effect of the source resistance on the sealed alignment.
Dome midranges often need impedance compensation.

Significant attenuation also implies driver EQ is fairly easy.

Your numbers are ~ correct for 3dB of baffle step compensation
assuming all the drivers responses are flat, they may not be.

/sreten.

[purplepeople]

This assumes that the baffle step frequency matches the crossover point between the woofer and mid.

:)ensen.

[rhythmdiy]

so what should be done now? can I use such high impedance but in general what can happen? what about heat dessipation? and I heard somebody stating about Qt problem by using heavy L-pad attenuation? what about that?

[rhythmdiy]

any adverse effects? like heating up of the resistors or degrading the sound quality?
please please help me ....

[Gary P]

For attenuation figures as high as these, look into autotransformer attentuators like used by Klipsch. You can get high attenuation without high series resistance.

With autotransformers the source impedance as seen by the drivers is kept low.

One of many sources for autotransformers for crossovers

[Pano]

Hey Gary,
Doesn't the autoformer "magnify" the impedance of the driver? At least that's what I've measured with autoformer volume controls for speakers.

As the attentuation rises - so the driver's impedance curve gets magnified. Isn' that right? Would make it tough to calculate a crossover. Maybe a swamping resistor across the driver would help.

[Gary P]

Hi Pano,

Yes, as the attenuation goes up so does the impedance seen looking into the autotransformer. Conversely, the driving impedance as seen by the driver remains low, which is quite different from resistive L pads. The percentage change of impedance vs frequency does not change much though.

What is commonly done is to put a resistor across the input side of the autotransformer. The resistor swamps out the impedance change vs frequency of the driver and makes a nice flat impedance for the high pass filter to work into.

Sound wise using an autotransformer attenuator has a much more direct and dynamic sound than resistive attenuators.

Here is the ALK upgrade crossover for the Klipsch Cornwall and Heresy showing the resistor on the input of the autotransformer.

[HK26147]

any adverse effects? like heating up of the resistors or degrading the sound quality?

Any resistor that has current flowing through it will heat to some extent.
From a minimalist viewpoint a resistor in the signal path is a potential degredant.
One of the advantages of active crossovers, is matching sensitivity via amp gain vs burning off current as heat in a resistor, and elimination of a passive component in the signal path.

If I had to I would use the transformer approach that Gary P provided vs resistor.

[Pano]

Thanks Gary - that makes sense. I see the damping resistor. In effect, it's the same thing I do with my transformer coupled DAC.

But looking at the circuit all I see is that the series R of the L-Pad has been replaced with an autoformer!

Of course, it probably sounds different. Clever, too, to driver the hi and mid of the same transfo.