Crown Audio STUDIO AMPLIFIER owner manual Input Wiring Tips, Solving Input Problems

Studio Reference I & II Professional Studio Amplifiers

SOLVING INPUT PROBLEMS

Sometimes large subsonic (subaudible) frequencies are present in the input signal. These can damage loudspeakers by overloading or overheating them. To attenuate such frequencies, place a capacitor in series with the input signal line. The graph in Figure 3.11 shows some capacitor values and how they affect the frequency response of a Studio Reference amplifier. Use only low-leakage capacitors.

dB
0
–5
–10
																									f
–15
			∝	f				∝	f			7	∝	f				∝	f			4	7	∝
		.7					.0				.4						.1				.0
	4					1				0						0				0
1 Hz					10 Hz										100 Hz											1 kHz	10 kHz

Frequency

Fig. 3.11 Subsonic Filter Capacitors

Another problem to avoid is large levels of radio fre- quencies 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 sensitive to high frequencies. Extremely high RF levels can also cause your amplifier to prematurely activate its protec- tion circuitry, resulting in inefficient operation. RF can be introduced into a signal by local radio stations and from the bias signal of many tape recorders. To pre- vent high levels of input RF, install an appropriate low-pass filter in series with the input signal. Some ex- amples of unbalanced wiring for low-pass filters are shown in Figure 3.12.

+	470 ohms	+
A Balanced In	.0056	Balanced Out
	∝f
–		–

470ohms

1.8mH

	+		+
B	Balanced In	.015	Balanced Out
		∝f
	–		–
		1.8 mH
		2.5 mH
	+		+
C	Balanced In	.018	Balanced Out
		∝f
	–		–

2.5mH

0.47Film 1.8 mH

+		+
D Balanced In	.015	Balanced Out
	∝f

– –

0.47 Film 1.8 mH

Fig. 3.13 Balanced RF Filters

Tip: The P.I.P.-FX has plenty of space on its circuit board for the addition of input filter circuitry.

Another problem to avoid is ground loops. These are undesired currents that flow in a grounded system and 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 input cables. To prevent this type of ground loop, it is always a good idea to locate input cables away from

Input Wiring Tips

910 ohms

		To
Source	.0056	Amp
	∝f
A		GND
	3.9 mH	To
R 600 ohm	.015	Amp
Source	∝f
B		GND
	5 mH	To
R 600 ohm	.018	Amp
	∝ f
Source		GND
C

4 kHz

Note: A low source impedance (R) can be increased to 600 ohms with an appropriate resistor.

dB

0

6 dB/octave

A

12 dB/octave

–10

B

C

–20

10 kHz 40 kHz 100 kHz

Frequency

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.

Fig. 3.12 Unbalanced RF Filters

For balanced input wiring, use an example from Fig- ure 3.13. Filters A, B and C correspond to the unbalanced filters shown in Figure 3.12. Filter D also incorporates the subsonic filter in Figure 3.11.

4. Turn the entire system off before chang-

ing connections. Turn level controls down

before powering the system back up. Crown

is not liable for damage incurred when any

transducer or component is overdriven.

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