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Old 19th January 2002, 05:24 AM   #1
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Join Date: Nov 2001
Location: Virginia USA
Question EQ boost and Q......help

A little help please for an electronically challenged individual....?

(Must admit the whole Q concept is still a bit foreign to me - Any good primers out there on basic circuits, filter orders etc.?)

Main question:
What's the practical effect of the filter Q of a sub EQ circuit?

For example the "Adjusting EQ bass boost" notes for the Parts Express 250W sub amp lists a "pre amp filter Q" for each of the possible boost settings.

What does this do to the total system Q, and what is the effect on dampening and transient response?

Thanks for any help.
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Old 20th January 2002, 02:15 PM   #2
Wizard of Kelts
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Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Javaman:

What the preamp does will NOT affect the speaker's Q at all. It has no effect on speaker damping, since there is no passive component, (inductor, capacitor, etc.) between the speaker and the amp. If there is any effect on transient response, it will come from the filter itself-that is, the transient response of the signal will be changed BEFORE it gets to the speaker. The speaker itself will have the same transient response as before

About Q. It measures the size and shape of the impedance rise of a filter. As regards speakers, the speaker's Q tells you high high below-or above-midpoint the speaker is around the resonance point.

The midpoint is the SPL the speaker usually plays at. For most non-PA speakers, it usually around 87-90 dB.

A) In a closed box speaker system with a Qtc of .5 and an Fc of 40 Hz, the response will be 6 dB down from the midpoint at 40 Hz.

B) If the Qtc of that speaker system were .7 instead of .5, the response would be 3 dB down from the midpoint at 40 Hz, (Fc).

C) If the Qtc of that system were 1.0. the response would be exactly at midpoint at 40 Hz (Fc).

In all cases, the response would drop after Fc.

Preamp filters have their own Q's and resonances as well. For instance, a preamp filter for example A might be 6 dB UP at 40 Hz, to compensate for the speaker's response being down by that same amount.

To repeat, the speaker's Q, in both sealed and ported systems, is not affected by any preamp filter. The preamp filter exists only to "fill in" the response where the speaker droops or falls off.
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