Quote:
Originally Posted by nhparrot
After trying 50Hz and 70Hz High Pass Harrison FMOD's in my system I have removed them from the signal chain. Testing with a SPL meter and the Rives Audio Test CD, I found that the FMOD's lowered the input signal to my amp at frequencies above the FMOD crossover point.
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The FMOD's are passive devices - they work by attenuating signals only. According to Harrison Labs they are 12dB/octave, though I have heard reports in practice they are closer to 6dB/octave.
In either case, these slopes are relatively shallow (i.e., they are not brick wall filters at 50hz or 70hz) and will offer a fair amount of attenuation above the knee frequency. Additionally, the actual corner frequency may differ from the nominal corner frequency depending on your amp's input impedance.
One "solution" is to remove them from the signal chain and your amp will again receive a full-bandwidth signal. Another option is to choose a different nominal frequency (e.g., 20 hz or 30 hz) to correct for your system differences. I think with devices like these, it is important to either measure or experiment with a number of frequencies. In the case of FMOD's, at $27.50 per pair I think we can easily swap them out.