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View Full Version : 4 ohm high pass filter resulting in too much HF sound



robertbartsch
10-11-2016, 07:55 AM
This is a PartsExpress model that filters at 600HZ and is rated at 4ohms. I used this with a separate low pass filter which is unrated. Both drivers (Altec 802 and Altec 416 - 15") are rated at 8ohms.

Without any attenuation circuit, the HF driver overpowered the woofer. Since I installed the attenuation circuit, I have turned the HF down on the attenuator all the way, and now the system sounds much more balanced.

I don't recall crossover circuits (filters) being rated in ohms before now. Was the 4ohm rated high pass filter responsible for an overpowering HF driver?

grumpy
10-11-2016, 11:04 AM
I'm assuming this is a passive filter (no amp between the filter and the speaker drivers), ... if so,
google formulas or calculators for RC filters: you will notice there is an R value which directly affects
the effective crossover. For example, if you assume the driver is completely resistive (which very few can claim)
with a value of "R" and you place a capacitor "C" in series, the crossover frequency will be defined as f=1/(2*3.14159*R*C).

In other words, if you change R (or C), you change f (where the output voltage drops by a defined amount).
It works somewhat similarly for more complex crossovers... in that if you change the load, you change the response.

If it was a 12dB/octave (LC elements), a load mismatch is less simple, but it will not be what you expect if
you ignore the driver impedance rating. (a mismatch can cause less output or more -at- the frequency defined by
the LC pair)

Assuming your Parts Express xover looks something like this:
C1=47 F
L1=1.5 mH

Red=4ohm load (nominal)
Green=8ohm load (might explain extra emphasis from horn)
Blue=2ohm load

74012

Inserting an attenuator might mitigate (isolate somewhat, if you like) the change in response or crossover
frequency due to a mismatched load (8 vs 4 ohms), so I can envision that this might help your issue...BUT
you should think about getting a crossover appropriate for your driver. Depending on the specific models,
either of those Altec drivers could be 8 or 16 ohms... worth double checking... particularly the 802.