3
West Mountain Radio
Operating Manual
Operation
Turn on the PWRspkr 15W using the Power/Volume knob . The green LED
will light. Adjust the volume control for the intended listening level.
Note:
The radio also has a volume control and it must be turned up for the
PWRspkr 15W to work.
It is possible to use either the radio or PWRspkr 15 W volume as the master.
It is recommended to set one volume control half way up. Then the adjustable
volume control can go both above and below the nominal listening level.
Circuit Description
The PWRspkr uses a 4-inch loudspeaker driven with a 15-watt RMS power
amplifier.
The power amplifier is a class–AB linear amplifier connected as a bridge-
tied-load (BTL). This allows the amplifier to produce 15 watts RMS into 4
ohms audio when powered by 13.8 volts without using an output transformer.
Note that both leads of the loudspeaker driver are “hot” and neither can be
grounded.
The power amplifier integrated circuit has internal protection for overheating
and a short-circuited loudspeaker.
Input audio is applied through a resistive dividing network and coupled to the
pre-amplifier via an isolating transformer. The input resistance is 100 ohms.
There are filter capacitors to remove any rf pickup from the audio line.
The internal pre-amplifier contains an active 300 Hz to 3.2kHz band–pass
filter. Note that the filter permits only “intelligence-energy” to reach the
loudspeaker driver. The “lows” and the “highs” are attenuated and not applied
to the loudspeaker. This is why the sound level can be very high without
damaging the loudspeaker from over excursion or excessive heat.
The loudspeaker sound pressure level (SPL) is 102.5 dB typical at 1 meter
before electronic clipping.
DC power is applied through a ferrite inductor and shunt capacitor to remove
any RF on the power leads. Large electrolytic capacitors further filter the
internal DC for any noise or ripple appearing on the DC power line.
