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Thread: 4 ohm high pass filter resulting in too much HF sound

  1. #1
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    4 ohm high pass filter resulting in too much HF sound

    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?

  2. #2
    Senior Member grumpy's Avatar
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    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 ĩF
    L1=1.5 mH

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

    Name:  Screen Shot 2016-10-11 at 3.28.44 PM.png
Views: 284
Size:  75.4 KB

    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.
    Last edited by grumpy; 10-11-2016 at 03:49 PM. Reason: Was working and not paying attention (to this)... plots/values fixed

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