BTL operation
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 ampli�er. This section explains the BTL mode, and describes how to use it. 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 ampli�er, 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 con�guration 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!
Theory of Operation
BTL mode (also called 'bridged' mode) uses both channels of the PA-30 to form a single monophonic ampli�er with signi�cantly 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 signi�cantly 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 ampli�er. The signal at the Channel A input is ampli�ed 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
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 ampli�er.
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.
