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Increasing resonance greatly decreases volumje. Normal?

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Increasing resonance greatly decreases volumje. Normal?

Postby Splashmash » Tue Apr 19, 2005 2:07 pm

Hi. I've noticed something with my new Voyager and I was wondering if it was a normal characteristic.

Increasing the resonance significantly decreases volume. This seems to occur but to a lesser extent on my Juno 106. I also have a Korg MS-20 filter clone and here you appear to be able to whack the resonance up as much as you like without a decrease in overall volume (in fact a limiter is necessary to stop it ripping the speaker to shreds!).

Is the behaviour of my Moog normal? If so, s'all good :)
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Postby Doki Doki » Tue Apr 19, 2005 2:24 pm

This is typical behavior for a Moog filter. When the resonance increases, other frequencies (usually lower frequencies) decrease. Its part of what makes the Moog sound!
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Postby Splashmash » Tue Apr 19, 2005 2:41 pm

Was hoping (and expecting) someone would say that. I'm new to Moog but I'm loving it so far :)
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Postby GregAE » Wed Apr 20, 2005 1:17 pm

Splash -

The Moog filter design provides a kind of automatic gain compensation (AGC). By this, I mean the as you raise the resonance level, you will notice that the output level of the filter is reduced. The idea is to keep the overall signal level constant. Without AGC, high resonance values could significantly increase the signal level at the filter output, causing an overload condition within the synth's signal path.

Not all LPF designs provide AGC.

AGC can be either a good thing or a bad thing - it depends on your musical situation. If you are trying to create a strong bass sound that contains resonance, for example, AGC could be working against you by reducing the bottom end.

AGC is not programmable and can not be switched off in the Voyager. Think of it as a design feature that creates the 'Moog sound'.
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Postby MC » Wed Apr 20, 2005 2:40 pm

When you turn up the resonance, you are increasing the negative feedback of the filter. Negative feedback meaning that the feedback signal is 180 degrees out of phase with the input signal. Everybody knows that adding two signals 180 degrees out of phase will cancel to nothing.

Like phase cancellation, the negative feedback will attenuate (not totally cancel) the filter response - most of the harmonics in the passband will drop in volume (which is the drop in volume you hear). The trick is that the harmonics that approach the cutoff frequency changes phase from 180 to zero degrees. The feedback changes from negative to positive around the cutoff frequency, so increased resonance does not attentuate but will boost harmonics.
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Postby Jiggz » Wed Apr 20, 2005 8:00 pm

Yep, this is the Moog - Ladder filter.

There are some Ladder filter designers that build Q compensation into the filter so there is no drop in level with high resonance values.
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