High Low Pass Filters Peaking????
« on: November 28, 2008, 07:25:50 PM »
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

Re: High Low Pass Filters Peaking????
« Reply #1 on: November 28, 2008, 10:06:32 PM »
Yes. Impedance is an issue. Re-measure with the correct load termination and you'll see what I mean.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

EmRR

Re: High Low Pass Filters Peaking????
« Reply #2 on: November 28, 2008, 10:24:06 PM »
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. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

clintrubber

Re: High Low Pass Filters Peaking????
« Reply #3 on: November 29, 2008, 09:07:20 AM »
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

Re: High Low Pass Filters Peaking????
« Reply #4 on: December 01, 2008, 10:11:31 AM »
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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