Determining the frequency response of an audio crossover (something seems off)


I'm in the process of putting together an audio crossover. I finished assembling the components, and I though it would be neat to do a frequency sweep on the filters. When I inspected the tweeter filter, I got some weird results. I assume I somehow mis-wired something, so I decided to go through the math to figure out what I should be seeing.

In any case, here's the circuit (not my design):


simulate this circuit – Schematic created using CircuitLab

Below is my analysis of the filter in the S-domain (I hope I did my math right):

enter image description here

In any case, I threw this transfer function into MATLAB and it generated this Bode plot:

enter image description here

The phase plot is fine in my opinion, but the magnitude plot seems amiss. One, the magnitude of attenuation seems ridiculous, and two, there's no knee. From the circuit, it's obvious that it's a high-pass Chebyshev filter. It should be attenuating only the lower frequencies and letting the high frequencies through. So, it seems like I did something wrong? Is it my math or something else?

Best Answer

I ran the circuit through Proteus simulator and the frequency sweep gave me these results:


No load response


6 Ohm load response

Both graphs run from 1k to 20k, and the key figures are as follows:

            fo         Go        f(-3)        Gpb        G(3.5)
NoLoad    5.9 kHz    - 3.3 dB    4.6 kHz    - 4.4 dB    -15.0 dB
Load      5.0 kHz    -10.0 dB    4.2 kHz    -12.4 dB    -20.9 dB

    fo     = Peak frequency
    Go     = Gain @ f0
    f(-3)  = -3dB point
    Gpb    = Passband gain
    G(3.5) = Gain @ 3.5kHz (cut off frequency of tweeter, from manufacturer)

For the LOAD graph, the 'passband gain' is read at about 8.1kHz; after this point it climbs back to -10dB at 20k.


Check both your maths and your soldering.

I say the above with all due respect - I'm too lazy to do the maths these days so I'm definitely not one to judge. However, it does apear that you've 'got your wires crossed' in the assembly process and that there's some breakdown with the maths/MATLAB combo.

Check ground connections, and perhaps consider laying out the components better than that horrible spider-web monster on page 5 of the supplied datasheet!