Mesa/Boogie Subway TT-800 Owner's Manual page 12

factor control that automatically tracks load impedance like an output transformer via impedance taps. These features are a continu-
ation of the research that led to the development of the Subway WD-800's well received power amp damping control. This feature's
contribution becomes apparent the closer to output stage limiting that the amp is played. It's somewhat subtle with as much of an
impact on feel as on tone. Just like on the bass guitar itself, perfect symmetry of the waveform is not necessarily a desirable attribute.
By decreasing the amount of symmetry in the overdrive circuitry, the overdrive character becomes more smooth, polite and forgiving
as well as allowing added interaction with the symmetry of the bass guitar's signal. Some players may perceive this as an added
richness or dimensionality to the tone and feel of the amp when driven hard.
Why would a player want to be able to adjust the symmetry of the output overdrive? Control over the symmetry changes how the
power amp enters and exits an overdrive event on a cycle by cycle basis. The greater the symmetry the tighter and more abrupt
the signal enters and exits overdrive. Perfectly symmetrical "clipping" appears to have the greatest "compression effect" and feels a
little stiffer and more "solid state like". As the overdrive event becomes less symmetrical, the signal enters the overdrive region more
gradually and with a progressive effect, which acts more like a soft knee compressor. It tends to be more forgiving, retaining a more
dynamic feel, with greater articulation.
POWER AMP DAMPING:
the primary differences between solid state and tube amps is how tightly the speaker is coupled to the power amplifier's output stage.
All amplifiers have some resistance (more correctly, impedance) between the power circuitry and the speaker. This impedance greatly
affects how tightly the amplifier can control the speaker. High damping means that there is very little impedance between the ampli-
fier's output circuitry and the speaker, the feel will be tighter and more controlled. Low damping means that there is more impedance
between the amplifier's output circuitry and the speaker, the feel will be looser and less controlled. Because a speaker has a complex
impedance, lowering the damping can be responsible for a bit more "bloomy", organic feel (especially in the low end).
Generally, tube amps fall into the medium to low damping category while solid state amps typically fall into the higher damping category.
Speakers have mass and the force required to accelerate the mass back and forth comes from the power generated by the amp and
delivered through the voice coil sitting within the magnetic gap of the speaker's motor. This means that to accurately accelerate and
decelerate the cone at each end of its travel requires efficient power delivery to the speaker. As the power delivery becomes less ef-
ficient, the force becomes less and the cone accelerates and decelerates more sluggishly, corresponding to undershoot and overshoot
of the cone. This, plus native voicing, are the most likely explanations for the "bloomy", slightly "round bottom" feeling reputation that
many tube amps enjoy.
In tube amps, the impedance between the output stage and the load is high (resulting in a low damping factor of approximately 20-50)
while in a solid state amp the impedance between the output stage and the load is low (resulting in a high damping factor of maybe
500 to 1000). These are inherent properties of the respective topologies. These damping factor numbers also vary with frequency,
as does the speaker impedance, so the interaction is quite complex actually. Since a tube amp matches the power tube plates to the
load through an output matching transformer, changing the tap automatically corrects for changes in the output impedance, therefore
the damping factor remains reasonably constant (though low) from 8 ohms down to 2 ohms.
What does this mean? With lower damping (compared with typical solid state, including class D), an amp will tend to feel a little looser
(especially on the low end), more bloomy, and a bit more organic. While there will be some tonal differences, it's really more of a
feel difference. With higher damping, an amp will feel a little tighter, dryer and more controlled on the bottom end. To some players
it may feel more "immediate", others may describe the feeling as "tight" or "dry". On the SUBWAY TT-800, the damping factor is
designed to be medium-low (at approximately 50, like the Bass 400+) and remains relatively constant no matter what the speaker
load is, emulating what happens through the output transformer on a tube power amp. This feature is controlled automatically by the
impedance selector switch located on the back of the amp, which should be set to the impedance of the connected speaker load.
POWER LED:
This blue LED indicates that the amplifier is connected to a power source and is switched on, operating correctly.
If this LED is not on, double-check the power source, and be sure the power cable is firmly inserted into the IEC power inlet socket.
PROTECT LED:
This red LED indicates that the amplifier has entered protect mode and is either protecting itself from an
external fault (such as blocked ventilation, shorted speaker cable or defective speaker), or that there is an internal fault within the
amp itself. Verify that it's not a fault external to the amplifier by disconnecting all cables (except the power cable) from the amplifier. If
the protect LED is no longer lit, the problem is most likely a bad speaker cable or defective speaker. Testing with a set of headphones
This is a "hidden feature" of the SUBWAY TT-800, first introduced in the Subway WD-800. One of
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