Ashly PQX-571 Manuale operativo - Pagina 7

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Operating Manual - PQX 571 and PQX 572 Parametric Equalizer

2. With the entire PA hooked up and turned on,

slowly increase the sound level at the mixer un-

til feedback is heard, then lower the level by about

3 dB so that feedback does not continue.

3. Start with one of the PQX filters by setting

the level at 0, bandwidth set fairly sharp (about

.3 oct.), and adjust the frequency control to where

you estimate the predominate feedback frequency

to occur.

4. Push in the filter's EQ switch and increase its

level control by about +6 dB. Now "sweep" the

frequency around where you have estimated the

feedback frequency until feedback occurs. Once

you have induced the feedback by boosting its

frequency, quickly turn down the filter's level

control to about -6 dB to suppress or "notch out"

the feedback frequency.

5. Again slowly increase the master level at the

mixer until feedback is heard. If a new feedback

frequency is heard, then repeat step 3 to find and

suppress the new frequency. If the original feed-

back frequency is still heard, then adjust the first

filter's level even lower. The bandwidth control

may be adjusted full clockwise to produce a very

sharp notch so that a severe feedback frequency

can be attenuated by as much as 15 dB without

degrading the frequency response with noticeable

notches. Note: Very sharp bandwidth lowers the

maximum equalizer input level because of the

high filter gain necessary to obtain such a nar-

row bandwidth. Only use bandwidth control full

CW (.05 Octave) in severe cases.

6. Continue this iterative process of increasing

the mixer's master level and finding, then sup-

pressing feedback frequencies until a de-

sired sound system gain-before-feedback

level has been achieved.

6.4

Console Channel Equalization

Many mixing consoles provide only

simple equalization for individual channels. If

your console has channel inserts, you can patch

your equalizer into a channel that is used for some-

thing important, and use it to tailor the sound of

this channel exactly the way you want.

6.5

Large Room Equalization

Large rooms tend to suffer from multiple reflec-

tions with long time delays, long reverberation times, and

"ring-modes", all of which lead to reduced intelligibility

and a generally "muddy" sound. As sound travels long

distances through the air, high frequencies are attenuated

more than low frequencies. In general, large rooms ben-

efit from some low frequency shelf roll-off, high frequency

shelf boost, and attenuation of ring mode frequencies by

the parametric filters.

6.6

Small Room Equalization

Small rooms need less equalization than large

ones. However, with reflective surfaces so close together

it is more likely to encounter high frequency feedback

problems. Finding and critically notching out these of-

fending "hot spots" is precisely what the PQX equalizer

does best. Use the narrowest possible bandwidth, and as

always, avoid over-equalization.

THEORY

The heart of the PQX equalizers is a unique

bandpass filter circuit. Basically a "state-variable" type, this

filter is trimmed and optimized to provide excellent transient

response and a wide range of frequency and bandwidth ad-

justment. Each filter can be tuned over a 100:1 frequency

range (about 6.6 octaves) and a 70:1 bandwidth range with

no more than a 2 dB amplitude error at center frequency. At

its sharpest setting, the filter has a "Q" of about 35 and gen-

erates a response curve with 3 dB points only 1/20 octave

apart, making feedback control possible with no audible side

effects. Each filter is placed in the feedback loop of a sum-

ming amplifier to produce the desired frequency response.

Since a separate summing amplifier is used for each band,

no interaction between bands occurs.

+ +

BALANCED

INPUT

GAIN

DETECTOR

LOW

PAR.

SHELF

FILTER

20-20K

+/-

PQX Block Diagram

OUT

OUTPUT

CLIP

PEAK

PAR.

FILTER

20-20K

+/-

HIGH

SHELF

+/-