Crown Micro-Tech 1201 Informations techniques télécharger pdf (Page 15)

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Micro-Tech 600/1200/2400 Power Amplifiers

Reference Manual

SOLVING INPUT PROBLEMS

Sometimes large subsonic (subaudible) frequencies

are present in the input signal. These can damage loud-

speakers by overloading or overheating them. To at-

tenuate such frequencies, place a capacitor in series

with the input signal line. The graph in Figure 3.10 shows

some possible capacitor values and how they affect fre-

quency response. Use only low-leakage paper, mylar

or tantalum capacitors.

–

Balanced In

910

Ω

.003

.015

.018

1.8 mH

2.5 mH

.015

1.8 mH

Balanced Out

–

910

Ω

1.8 mH

2.5 mH

1.8 mH

–

Balanced In

Balanced Out

–

Balanced In

Balanced Out

–

Balanced In

Balanced Out

–

0.47 Film

Fig. 3.10 Subsonic Filter Capacitors

For balanced input wiring, use one of the examples in

Figure 3.12. Filters A, B and C correspond to the unbal-

anced filters above. Filter D also incorporates the sub-

sonic filter described previously.

Another problem to avoid is ground loops. These are

undesired currents that flow in a grounded system and

Fig. 3.12 Balanced RF Filters

usually cause hum in the output. A common source of

ground loop problems is the placement of input cables

parallel to power cables or near power transformers.

The magnetic field that surrounds these conductors can

induce the 50 or 60 Hz alternating current into your in-

put cables. To prevent this type of ground loop, it is al-

ways a good idea to locate input cables away from

power cables and power transformers. We also recom-

mend using shielded or twisted pair wire. With loose

wires, use tie-wraps to bundle together each pair of in-

put wires. This helps reduce magnetically-induced cur-

Input Wiring Tips

1. Use only shielded cable. Cables with

higher density shields are better. Spiral

wrapped shield is not recommended.

2. When using unbalanced lines, keep the

cables as short as possible. Avoid cable

lengths greater than 10 feet (3 meters).

3. Do not run signal cables together with

high-level wiring such as loudspeaker wires

or AC cords. This reduces the chance of hum

or noise being induced into the input cables.

4. Turn off the entire system before chang-

ing connections. Turn down the level controls

(fully counterclockwise) before powering the

system back up. Crown is not liable for dam-

age incurred when any transducer or com-

ponent is overdriven.

Another problem to avoid is the presence of large levels

of radio frequencies or RF in the input signal. Although

high RF levels may not pose a threat to the amplifier,

they can burn out tweeters or other loads that are sensi-

tive to high frequencies. Extremely high RF levels may

also cause the amplifier to prematurely activate its pro-

tection circuitry. RF can be introduced into an input sig-

nal by local radio stations or the bias signal of many

tape recorders. To prevent RF problems, install appro-

priate low-pass filters on the inputs. Examples are

shown below for unbalanced wiring:

4 kHz 10 kHz 40 kHz 100 kHz

Frequency

–10

–20

6 dB/octave

12 dB/octave

Amp

GND

Amp

GND

Amp

GND

Source

1.8 K ohm

.003

Fµ

.015

.018

Fµ

3.9 mH

5 mH

600 ohm

Source

600 ohm

Source

ote: A low source impedance (R) can be

ncreased to 600 ohms with an appropriate resistor.

Fig. 3.11 Unbalanced RF Filters

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 37 38

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Crown Micro-Tech 1201 Informations techniques Page 15

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