Crown Audio STUDIO AMPLIFIER owner manual High frequency oscillations can cause your amplifier

Studio Reference I & II Professional Studio Amplifiers

1. For loads connected in parallel, use the equation that	amplifiers share a common cable tray or jacket,
follows to calculate each channel’s total load resistance.	use tie-wraps to bundle individual conductors so
Substitute the rated impedance of the connected loud-	the wires for each loudspeaker are kept close
speakers for the Zs in the equation. When finished, mark	together. (Do not bundle wires from different
your answer on the nomograph’s “Load Resistance” line.
	amplifiers.) This reduces the chance of conduc-
Total Load Resistance in Ohms = (1¦Z1 + 1¦Z2 + 1¦Z3…)–1	tors acting like antennas to transmit or receive the
2. Select an acceptable damping factor and mark it on	high frequencies that can cause oscillation.
	2. Avoid using shielded loudspeaker cable.
the “Damping Factor” line. Your amplifier can provide an
phenomenal damping factor of 20,000 from 10 to 200 Hz	3. Never tie together input and output grounds.
in Stereo mode with an 8 ohm load. In contrast, most	4. Never tie together the output of different amplifiers.
other amplifiers have a damping factor rating of 200 or
less. Higher damping factors yield lower distortion and	5. Keep output cables separated from input cables.
greater motion control over the loudspeakers. To give you	6. Install a low-pass filter in series with each input
a basis for comparison, effective damping factors for
	(see Section 3.3.4).
commercial applications typically run between 50 and
100. Higher damping factors may be desirable for live	7. Install the input wiring according to the instruc-
sound, but long cable lengths often limit the highest	tions in Section 3.3.4.
damping factor that can be achieved practically. (Under	Another problem to avoid is the presence of large
these circumstances, Crown’s IQ System is often used so
amplifiers can be easily monitored and controlled when	subsonic currents when primarily inductive loads are
they are located very near the loudspeakers.) In record-	used. Examples of inductive loads are 70-volt step-up
ing studios and home hi-fi, a damping factor of 500 or	transformers and electrostatic loudspeakers.
more is very desirable.
3. Draw a line through the two points with a pencil, and	Inductive loads can appear as a short circuit at low fre-
	quencies. This can cause the amplifier to produce
continue until it intersects the “Source Resistance” line.
	large low-frequency currents and activate its protec-
4. On the “2-Cond. Cable” line, mark the length of the
	tion circuitry. Always take the precaution of installing a
cable run.
	high-pass filter in series with the amplifier’s input when
5. Draw a pencil line from the mark on the “Source Resis-	inductive loads are used. A three-pole, 18 dB per oc-
tance” line through the mark on the “2-Cond. Cable” line,	tave filter with a –3 dB frequency of 50 Hz is recom-
and on to intersect the “Annealed Copper Wire” line.	mended (some applications may benefit from an even
6. The required wire gauge for the selected wire length and	higher –3 dB frequency). Such a filter is described with
	the subsonic frequency problems in Section 3.3.4.
damping factor is the value on the “Annealed Copper Wire”
line. Note: Wire size increases as the AWG gets smaller.	Another way to protect inductive loads from large low-
7. If the size of the cable exceeds what you want to use,	frequency currents and prevent the amplifier from pre-
(1) find a way to use shorter cables, like using the IQ Sys-	maturely activating its protective systems is to parallel
tem, (2) settle for a lower damping factor, or (3) use more	a 590 to 708 mF nonpolarized motor start capacitor and
than one cable for each line. Options 1 and 2 will require the	4-ohm, 20-watt resistor in series with the amplifier out-
substitution of new values for cable length or damping factor
	put and the positive (+) transformer lead. This circuit is
in the nomograph. For option 3, estimate the effective wire
	shown in Figure 3.15. It uses components that are
gauge by subtracting 3 from the apparent 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,	4-ohm, 20-watt
or four #16 wires can be used for the same effect.
	Resistor
SOLVING OUTPUT PROBLEMS	+	+
High frequency oscillations can cause your amplifier	590 to 708 ∝f Capacitor
	120 VAC, N.P.
to prematurely activate its protection circuitry. The ef-	From		Inductive
	Amplifier
			Load
fects of this problem are similar to the effects of the RF	Output
		–
problem described in Section 3.3.4. To prevent high-	–
frequency oscillations, follow these guidelines:
1. When using long cable runs, or when different	Fig. 3.15 Inductive Load (Transformer) Network

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