Balanced Connection Notes/Amplifier Gain Measurement - Infinity IRS Epsilon Owner's Manual

Epsilon

Balanced Connection Notes/Amplifier Gain Measurement

Many electronic products, however, follow no convention about signal polarity; some invert phase, and some don't. If
you care about absolute polarity, many test CDs include impulse signals which are unipolar, that is, they are only
positive-going when referenced to ground.
An oscilloscope connected to the outputs of each electronic unit in the signal chain, including the CD player, can verify
whether absolute polarity is being maintained. On balanced connections and a positive unipolar pulse signal, pin 2
should have a positive going signal, while pin 3 should have a negative going signal. Correctly interpreting the 'scope
display often requires expertise, depending on the nature of the pulse waveform.
Reversed polarity in the power amplifier(s)
If the amplifiers have reversed balanced connections, then as far as the SCU is concerned, they are changed from non-
inverting to inverting, or vice-versa. For example, if a non-inverting woofer amp has reversed balanced connections,
then from the SCUs perspective, it is an inverting amp and the SCU's phase switch must be set to the 180 position.
Furthermore, if you have amps which have mixed reversals, that is, one reversed and one not, refer to page 12.
Correcting a balanced connection polarity reversal on any piece of electronic equipment is a simple matter of
unsoldering and resoldering two wires on the XLR connector. This is an easy task for any technician.
AMPLIFIER GAIN MEASUREMENT/SETTING
Power amplifiers do just what their names imply: They take small, low-power audio signals and amplify them, increasing
their power so that they drive the loudspeakers to audibly useful output levels. The amount of amplification is measured
in decibels (dB) and is called gain.
Many power amplifiers are fixed-gain units with no input level or sensitivity adjustments. Others have switches and/or
potentiometers that adjust the input sensitivity or overall gain. As noted in the section Epsilon Servo Control Unit Set-
Up, the Epsilon Servo Control Unit has a dB gain control to optimize the servo system for the gain of the woofer
amplifier. When the amplifier has no input level control or gain adjustment, the SCU dB gain control is adjusted as
previously described.
Measuring amplifier voltage gain
If you do not know the gain of each of your amplifiers with a 4 Ohm load, and cannot tell what it is from the
specifications, you must measure the gain or have it measured. Then, you can use the gain or level controls, if any,
along with the controls on the SCU, to match all of the amplifier gains.
To measure the effective voltage gain:
1. Connect a high-wattage 4 Ohm load resistor (25 Watts or more) between the amp's "4 Ohm" and "common" output
terminals.
2. Apply a 1000HZ sine-wave input signal of known voltage, as measured with an AC voltmeter. The input level
must be low enough so that the amplifier is not close to overload (clipping) at the output; an input level of 100mV
is a good starting point. If possible, monitor the amplifier's output with an oscilloscope to verify linear operation.
3. Set all gain or level controls or switches to maximum, and measure the output voltage across the 4 Ohm load
resistor.
Note that maximum gain means highest sensitivity; for example, if the amp has an input switch labeled "1 V" and "2.5V",
the 1V position results in the highest sensitivity and gain. If the AC voltmeter does not have dB scales or readings,
calculate the gain as follows:
dB gain = 20 log (Vout/Vin)
4. Check the gain of each amplifier with an input signal of 100Hz, as well. It should be virtually identical to the gain
at 1000Hz; if not, note the two different readings, and use the 100Hz reading for the woofer amp and the 1000Hz
reading for the mid/high amp.
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