Crown Micro-Tech 1201 Informations techniques télécharger pdf (Page 17)

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Micro-Tech 600/1200/2400 Power Amplifiers

Reference Manual

1. Note the load resistance of the loudspeakers connected

to each channel of the amplifier. Mark this value on the

“Load Resistance” line of the nomograph.

2. Select an acceptable damping factor and mark it on the

“Damping Factor” line. Your amplifier can provide an excel-

lent damping factor of 1,000 from 10 to 400 Hz in Stereo

mode with an 8 ohm load. In contrast, typical damping fac-

tors are 50 or lower at these frequencies. Higher damping

factors yield lower distortion and greater motion control over

the loudspeakers. A common damping factor for commer-

cial applications is between 50 and 100. Higher damping

factors may be desirable for live sound, but long cable

lengths often limit the highest damping factor that can be

achieved practically. In recording studios and home hi-fi, a

damping factor of 500 or more is very desirable.

3. Draw a line through the two points with a pencil, and con-

tinue until it intersects the “Source Resistance” line.

4. On the “Two Conductor Cable” line, mark the length of

the cable run.

5. Draw a pencil line from the mark on the “Source Resis-

tance” line through the mark on the “Two Conductor Cable”

line and intersect the “Copper Wire” line.

6. The required wire gauge for the selected wire length and

damping factor is the value on the right-hand scale of the

“Copper Wire” line. For metric wire sizes, find the recom-

mended resistance in ohms per 305 meters (1000 feet) and

use this information to reference the correct wire size.

Note:

Wire size increases as the AWG gets smaller

7. If the size of the cable exceeds what you want to use,

(1) find a way to use shorter cables, (2) settle for a lower

damping factor, or (3) use more than one cable for each line.

Options 1 and 2 will require the substitution of new values for

cable length or damping factor in the nomograph. For op-

tion 3, doubling the number of conductors of equal thick-

ness will reduce the resistance in ohms per 1000 feet (305

meters) by half. When using AWG standards, you can esti-

mate the effective wire gauge by subtracting 3 from the

given wire gauge every time the number of conductors of

equal gauge is doubled. So, if #10 wire is too large, two #13

wires can be substituted, or four #16 wires can be used for

the same effect.

SOLVING OUTPUT PROBLEMS

High-frequency oscillations can cause your amplifier

to prematurely activate its protection circuitry. The ef-

fects of this problem are similar to the effects of the RF

problems described in Section 3.3.4. To prevent high-

frequency oscillations, follow these guidelines:

1. When using long cable runs, or when different

amplifiers share a common cable tray or jacket,

use tie-wraps to bundle individual conductors so

the wires for each loudspeaker are kept close to-

gether. Do not bundle wires from different amplifi-

ers. This reduces the chance of conductors act-

ing like antennas that transmit or receive the high

frequencies that can cause oscillations.

2. Avoid using shielded loudspeaker cable.

3. Never tie together input and output grounds.

4. Never tie together different amplifier outputs.

5. Keep output cables separated from input

cables.

6. Install an RF filter in series with each input (see

Section 3.3.4).

7. Install input wiring according to the instructions

in Section 3.3.4.

Another problem to avoid is the presence of large sub-

sonic currents when primarily inductive loads are

used. Examples of inductive loads are 70-volt step-up

transformers and electrostatic loudspeakers.

Inductive loads can act like a short circuit at low fre-

quencies. This can cause the amplifier to produce large

low-frequency currents and activate its protection cir-

cuitry. Always take the precaution of installing a sub-

sonic filter in series with each of the amplifier’s inputs

when inductive loads are used. A three-pole, 18 dB per

octave filter with a –3 dB frequency of 50 Hz is recom-

mended (some applications may benefit from an even

higher –3 dB frequency). Such a filter is described with

the subsonic frequency problems in Section 3.3.4.

Another way to protect inductive loads from large low-

frequency currents and to prevent the amplifier from

prematurely activating its protective systems is to paral-

lel a 590 to 708 µF nonpolarized motor start capacitor

and a 4-ohm, 20-watt resistor in series with the

amplifier’s output and the positive (+) lead of the trans-

former. This circuit is shown in Figure 3.14. It uses com-

ponents that are available from most electrical supply

stores.

Fig. 3.14 Low-Frequency Protection

Circuit for Inductive Loads

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