LOW RES FREQ/Q/RESONANCE – Guitar loudspeakers have a low-frequency
resonance, typically about 100 Hz. This shifts up slightly when the speaker
is mounted in an enclosure and is typically lower for open back cabs. This
resonance causes an increase in the power amplier response due to the nite
output impedance of the power amp. The default LF Resonance is based on the
cab most likely to be used with that amp.
HI FREQ – Sets the “corner frequency” of the speaker impedance rise due to
voice-coil inductance. The speaker voice-coil presents an inductive load to the
power amp at high frequencies. This inductive load, in conjunction with the
output transformer capacitance, creates a high-frequency resonance. Typical
guitar speakers have a corner frequency between 1 kHz and 2 kHz. Lower
values give more midrange emphasis.
HI RESONANCE – Similar to HI FREQ but this control only changes the slope
of the resonance. Default value is consistent with typical “semi-inductance”
of speaker voice-coil. Varying this value will change the high-frequency load
presented to the power tubes.
The SPEAKER page is not an EQ. It allows you to adjust the impedance that
the virtual speaker presents to the virtual power tubes. For a guitar amp with
no negative feedback, the voltage frequency response of the power amp will
very closely match this since the power amp is basically a current source. For
a guitar amp with negative feedback, the resulting EQ is quite dierent than
the impedance curve since negative feedback attens the response. If you
turn NEG FDBK all the way down then the EQ will be close to the impedance
curve (but still inuenced by the transformer.)
Don’t be afraid to turn LOW RESONANCE close to 10. In fact, some Celestion
and Eminence speakers are equivalent to about 8-9 on LOW RESONANCE.
This will increase the interaction between the power tubes and the speaker
load.
XFORMER LOW/HIGH FREQ – These set the output transformer bandwidth.
XFORMER MATCH – One of the most powerful controls in the amp block. It
changes the turns ratio (and therefore the primary impedance) of the output
transformer, which controls how easily power tubes are driven into clipping.
Decreasing causes the power tubes to clip later, the phase inverter and grid
clipping become more predominant, and the speaker resonance will be more
pronounced. You also reduce the power tube compression of the lows and
highs. This control has more inuence with higher MASTER values and low
gain amps and less inuence with highly compressed amps. Increase MASTER
until desired amount of power amp distortion is achieved, then adjust XFormer
Match for sound’s character: higher = more compressed, lower = more open.
The LF/HF RESONANCE parameters interact strongly with this parameter.
XFORMER DRIVE – Sets the amount of core saturation in the output trans-
former, controling how hard the transformer is driven. Higher values simulate a
smaller, more easily saturated transformer.
SPEAKER DRIVE – Simulates distortion caused by pushing a speaker too far. It
interacts with the MASTER.
Amp Block
TOP ROW BOTTOM ROW
Use XFORMER MATCH to intentionally mismatch speaker impedance in order
to get a dierent tone. To simulate plugging an 8-ohm speaker into a 4-ohm
jack, set it to 2.0. For the other way around, set it to 0.5.
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