Speaker Impedance; Speaker Polarity (Or Phase); Impedance Selector Switch - Mesa/Boogie Subway TT-800 Owner's Manual

against such faults, it's not good practice to test this protection on a regular basis.
Never connect the speaker output to anything except a speaker. This especially means not to a DI, even a speaker level DI
because the Subway TT-800 utilizes a BTL (bridged) output power amplifier. On a bridged amplifier, the minus terminal that is
normally at ground is actually a driven output that swings above and below ground at peak currents of up to 30 amps. Using a DI, the
normal shell or ground terminal of the DI is not at ground at the amplifier's end but may be connected to the console (mixer) circuit
ground bus at the console's end which would cause the amplifier to drive dangerous currents into circuitry that does not expect to see
such currents, nor is it generally protected from this fault. So, in addition to possibly damaging your amp, you could also damage
a (potentially) very expensive console.

SPEAKER IMPEDANCE:

that the amplifier is tasked with delivering. The lower the speaker's impedance, the greater the current that the power amplifier must
provide. The lowest impedance that the SUBWAY TT-800 amplifier is capable of driving safely is 2 ohms (with the impedance selector
switch in the 2 ohm position). Note that measuring speaker cabinets using an ohm meter will not give accurate results because ohm
meters read DC resistance not AC impedance (technically called "reactance"). AC impedance will always be greater than the DC
resistance, an 8 ohm speaker will typically measure between 5 and 7 ohms, a 4 ohm speaker will typically measure between 2.5 and
3.5 ohms when measured with an ohm meter (or digital multi-meter).

SPEAKER POLARITY (OR PHASE):

dustry standard will move forward when a positive DC voltage is applied to the positive terminal of the speaker. While there are well
established standards, there are also deviations from standards, either by legacy (for example early JBL drivers), by faulty repair
(incorrectly wired cabinet or defective recone parts) or a manufacturer choosing to ignore the standard. If a multi-driver speaker cabinet
or a pair of speaker cabinets does not appear to have the expected output or low end, it's always a good idea to double-check that
all drivers move forward with positive DC voltage (no more than a 9 volt battery) momentarily applied to the + terminal (this will be
the 1+ terminal on a SpeakOn™ connector, or the tip on a 1/4" (6.5mm) connector). If you find that on a multi-speaker cabinet, one
speaker moves out while the other does not move, it's likely that the non-moving driver has either failed or has become disconnected.
If one driver moves out while the other driver moves in, it's likely that the driver moving in is wired incorrectly (or in a sealed cabinet
is failed or disconnected and is merely being moved in the opposite direction by coupling to the air mass inside the cabinet itself).
Being aware of these possibilities can often help when troubleshooting something that doesn't appear to be performing quite right.

IMPEDANCE SELECTOR SWITCH:

the amplifier's drive capabilities to the connected speaker load. This is important for a couple of reasons, the first being that proper
matching increases performance and prevents damage to the amp. The second reason is that the power amp behavior, including
damping control, tracks the load matching via the impedance selector switch, which improves the feel and dynamics. The IMPEDANCE
SELECTOR SWITCH needs to be set to the combined speaker load connected to the amp.
The chart below will help you choose the correct switch position for any valid combination of cabinets you might want to use:
SPEAKER
COMBINATION
1 x 8 ohm cabinet
2 x 8 ohm cabinets
3 x 8 ohm cabinets
4 x 8 ohm cabinets
1 x 4 ohm cabinet
2 x 4 ohm cabinets
1 x 8 ohm + 1 x 4 ohm cabinets
2 x 8 ohm + 1 x 4 ohm cabinets
All speakers have a "rated nominal impedance". Impedance is the resistance to AC electrical signal
All speakers have "polarity". A speaker that is wired in accordance to the current in-
The SUBWAY TT-800 is equipped with an IMPEDANCE SELECTOR SWITCH to match
CALCULATED
LOAD
8 ohms
4 ohms
2.67 ohms
2 ohms
4 ohms
2 ohms
2.67 ohms
2 ohms
IMPEDANCE SWITCH
POSITION
8 ohms
4 ohms
2 ohms
2 ohms
4 ohms
2 ohms
2 ohms
2 ohms
PAGE 10
POWER DISTRIBUTION
PER CABINET
100% to the single cabinet
50% to each cabinet
33% to each cabinet
25% to each cabinet
100% to the single cabinet
50% to each cabinet
33% to the 8 ohm cabinet
66% to the 4 ohm cabinet
25% to each 8 ohm cabinet
50% to the 4 ohm cabinet