KLF-20 Minor External Mod for Alternate Voicing

[Pete B]

A friend has a pair of KLF-20's that he has fixed up, reglued the boxes and woofer
spiders, recapped the crossover. He felt that they sounded forward and bright and
obtained a schematic for the crossover from Klipsch. It did not show his version
that included biamp posts so I edited it as shown below:
We decided to try padding the top range with a resistor. We used a resistor switchbox
to select the series value, the shunt was fixed at 20 ohms. I liked an 8 ohm pad and
my friend liked 5 ohms. We A/Bed against Celestion SL-700 mini monitors as a reference.
The 8 ohm resistor was closest to the SL-700s, but my friend said that his ears were older
so he went with 5 ohms.
The mod with resistors is shown in the second diagram, nothing was changed inside
the speakers. They sound great with the mod:



Diagram with external resistors shown:

 

[efzauner]

you are changing the source impedance by adding those input resistors. My understanding is that crossovers are expecting to be driven by voltage source with next to zero impedance. The new R in the crossover now has an R in series with it equivalent to the combined parallel input resistors. So that would also change the crossover frequencies... if my circuit theory is correct. Not just reducing overall power to the mids and tweets.

 

[Pete B]

You are correct that the crossover frequency is dependent on both the load and
the source impedance, however the degree of sensitivity is dependent on the
crossover parameters.
I believe that the tapped inductor on the "squaker" acts as an autoformer so that
it presents a very high impedance to the 1.25 uF capacitor.
I don't believe that the crossover points move much, but if a person is concerned
then they shouldn't try the mod.

It sounds excellent, by the way, and I could live with these speakers with the mod.

 

[efzauner]

um.. an autoformer has the same basic voltage/current/impedance model as a normal transformer. The only difference is that the autoformer does not isolate the primary and secondary. Source and load impedance would be transformed as well based on the square of the turns ratio. Normal electrical circuit analysis applies. You can wiki this, pretty basic stuff. It is a fun exercise actually. If you do the math, based on the 10dB attenuation of the Autoformer, the turns ratio Primary: Secondary is 3.16:1 The impedance ratio is therefore 10:1 . As seen from the secondary side, the source impedance attached to the primary is 1/10 So your 5 or so ohm resistance in series would look like 0.5 ohms.. So yes your assumption that the midrange crossover frequency does not change much is correct as you are only adding half an ohm to the voice coil impedance. That is what? 0.5 ohms to 4 ohms? 12%? That may not be insignificant... that is like 100Hz change for an 800Hz Mid crossover frequency. On the other hand, the tweeter is not connected to the autoformer and sees the entire added resistance.. I do not wish to analyze that... Oh you can spend lots of time doing this stuff! enjoy your KLFs! I should listen more and type less!

 

[Pete B]

I am an EE and I do know how an autoformer works thanks.

 

[efzauner]

Great! That makes two of us!

 

[Pete B]

Interesting that the tapped inductor (autoformer) and high Z tweeter eliminate the
the need for resistors at least how Klipsch voiced them.