[tardishead]

I have some vintage HPF LPF units - RCA, Cinema, Langevin and Allison Labs. They are all awesome have a great sound but they all tend to boost the cutoff fequency. I thought this would be impossible being passive units - but I had a look on the Spectrum Analyser and sure enough there is a peak at the cut off frequency on all units. Can someone explain what is going on. I usually use them on inserts on my Neve desk. Is this environment causing a impedance issue which is contributing to this effect???
 Should I terminate the inputs and outputs with 600 ohm resistors.

[SSLtech]

Yes. Impedance is an issue. Re-measure with the correct load termination and you'll see what I mean.

Keith

[EmRR]

Yes, it's a termination issue.  If you want to know flatest for a given interface mess with some pots as series input and shunt output loads while measuring. 

[clintrubber]

They are all awesome have a great sound but they all tend to boost the cutoff fequency. I thought this would be impossible being passive units -

L & C can do that.

Regards,

  Peter

[clintrubber]

They are all awesome have a great sound but they all tend to boost the cutoff fequency. I thought this would be impossible being passive units -

L & C can do that.

Regards,

  Peter

To make this more clear (and please allow me to cut a few corners w.r.t. the textbook version):
passive means there can't pop up power out of nowhere (no power-gain).

This doesn't mean that a signal voltage at the filter-output can't be larger than at the input,
this because of resonance.

Just like a transformer: passive component, yet V_out can be larger than V_in. Voltage is just one of the contributors
to power.
Might look at filter-resonance as an impedance-transformation as well (at that freq).


Bye,

  Peter

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