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In Reply to: RE: Questions about high pass filter and balanced bridge posted by Tube747 on June 13, 2017 at 09:28:57
I believe the FMODS are single pole passive. Likely an RC. The impedance you need to give us to do the calculation is the amplifier output impedance not the input impedance. The output impedance is in series with the "R" in the FMOD and therefore will contribute to setting the frequency response of the network.
Output impedance? Since the FMOD is placed between preamp and power amplifier, are you talking about the output impedance of preamp?
First of all when designing antrhing you make sure the output impedance is much lower than what the load will be draining. One notable exception might be loudspeakers.
In general, this approach means you won't have problems with the load affecting the output and that can matter when it comes to crossovers. If you have a 47K input impedance, then go for a 100 ohm output impedance. It also helps to keep the noise out.
The simplest way to really do it right is with some OP AMPS and RC filters. If you want variable crossover you can do it with one double ganged pot as long as you stick with 6 dB/octave. If you want 12 dB/octave then you need either a four gang pot or a separate control for each channel. Of course you can go with other electronic options but none of them are as sonically pure.
But basically, you feed it a voltage. Impedance matching is for RF or back when transistors only had an Hfe of like ten or so. today it is east to feed a constant voltage at very little cost. So pretty much forget about it and feed it voltage. That's what everything else does, except tubes...
I thought you were referring to use these after the amp before the speaker, which would likely be a terrible idea.
In theory a single pole highpass and low pass of the same Fc should complement perfectly. The reason it might not happen in practice is if the impedance of the preamp output is significant with respect to the FMOD impedance. It could throw off the corner frequency of both the high pass and low pass in non complementary fashion and degrade the level of match.
That is what FMOD are trying to show with their graph showing different response with preamps of differing output impedance.
Unfortunately FMOD doesn't seem to give the schematic for what's inside. Without that it's going to be tough to get predict the response even if you know the preamp output impedance.
You might try emailing FMOD with the output impedance of the preamp and see what they say.
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