Tascam PA-30 - Manual (Page 9)

The PA-30 rear panel includes a switch that selects
either normal stereo mode or a special mode termed
BTL (Balanced Transformerless). The setting of this
switch determines the maximum power available to the
load from the PA-30, and also decisively affects how
the load is connected to the amplifier. This section
explains the BTL mode, and describes how to use it.
Theory of Operation
BTL mode (also called 'bridged' mode) uses both
channels of the PA-30 to form a single monophonic
amplifier with significantly higher power capability. If
your loudspeakers can handle the increased power,
BTL mode will give you more headroom - resulting in
undistorted reproduction of program peaks and
significantly greater 'punch' from your system.
When the MODE switch is in the 'BTL (MONO)'
position, only the Channel A input is active; the
Channel B input is disabled, and signals present at
either of the Channel B input connectors are ignored by
the amplifier. The signal at the Channel A input is
amplified and sent to both outputs of the PA-30,
but the
polarly of Channel B is reversed with respect to
Channel A
. In other words, the two channels are
intentionally out of phase.
The single load (remember, BTL is a monophonic, or
single-channel, mode) is wired across the two channel
output 'hot' (red) terminals, and the black output
terminals are left unconnected. The minimum
allowable load impedance is 8 ohms (not 4 ohms, as is
the case in stereo mode), and the available power to the
load is 50 watts continuous (at 8 ohms).
BTL mode is electrically identical to push-pull
transmission at line level (the signal level is simply
much higher): the two channel outputs deliver the same
absolute signal voltage, but with opposite polarities.
So, when the Channel A output is at +10 volts, for
example, the Channel B output is at –10 volts. The
difference between the two (which is the total voltage
across the load) is then 20 volts.
The voltage across the load is thus effectively doubled
– which doubles the power.
Notice there is a crucial difference between BTL mode
and what is normally termed 'mono' in the HiFi world.
When we select 'mono' on a HiFi preamp or integrated
amplifier, what we intend - and what we get - is the
same signal from both speakers
with the same polarity .
It is very important to understand the distinction
between this and BTL mode, because if two speakers
are connected to the PA-30 in a normal stereo
configuration and the MODE switch is set to 'BTL', the
system still works but the two speakers are out of
phase. This will cause acoustical cancellations at low
frequencies, resulting in poor sound quality. In rare
cases, it can even cause a woofer to blow out!
Connections
Before changing the position of the MODE switch or
making any input and output connections, turn OFF the
PA-30. To select BTL operation, move the MODE
switch to the 'BTL (MONO)' position.
Figure 9 illustrates the connections for BTL mode. The
input signal is connected to the Channel A input, using
either the –10 dBV (RCA) or the +4 dBm (XLR)
connector. The Channel B input is left unconnected.
The load is connected across the two output 'hot' (red)
terminals, and the black terminals are left unconnected.
Be sure to observe correct output polarity: Channel A is
positive, and Channel B is negative. Note that the
minimum permissible load impedanoe in BTL mode is
8 ohms.
Connecting loads with a lower impedance will result in
current limiting, and may trigger the Protection circuits
to shut down the amplifier.
TASCAM PA-30
9
BTL operation
Figure 9. BTL Mode Connection
120V
NO CONNECTION TO BLACK TERMINALS