Filter Envelope; Amplifier; Modulation Envelope; Output - Abstrakt Instruments Avalon Reference Manual

The frequency response of the audio signal to the final amplifier in
the TB-303 (and hence the Avalon w/ default setting) is limited to ~70Hz.
This was done to maximize the signal-to-noise performance of the TB-
303 by removing the low frequency component of the signal that would
drive it into clipping faster. If exact TB-303 clone behavior is desired this
switch should be left in he 'out' position. However, for a full-range
response, especially when using the sub oscillator, you may want to
extend the frequency response.

3.4 FILTER ENVELOPE

The filter envelope is a simple decay envelope. ENV MOD controls
the depth of the envelope modulation. ACC DECAY and ENV DECAY
set the decay time of the filter envelope for accented and unaccented
notes respectively. In both cases the decay time can be adjusted
between 200ms and 2s.
The implementation of the filter envelope and filter modulation in
general is non-standard. For specific details please see Appendix B.
The original TB-303 does not allow the filter envelope modulation to
be turned all the way off. The Avalon has an internal switch setting that
allows the filter envelope depth to maintain the TB-303 behavior. See
Appendix A for details.
When an accent has been programmed (or played) an additional
control voltage (envelope) is sent to the filter and final amplifier
(VCA). First, a copy of the filter envelope is sent to the VCA control
input to give a volume boost on accented notes. Second, a slewed
copy of the filter envelope is sent to the filter control input. This is
responsible for the characteristic wow sounds you can get with
accented notes. The character of this slewed version also varies with
the setting of the RESONANCE control, as increasing resonance also
increases the slew of this control voltage. The depth of this additional
accent modulation is determined by the ACCENT control.
For specific details on the behavior of accents on the filter and
amplifier see Appendix B.

3.5 AMPLIFIER

VCA DECAY sets the decay time of the final amplifier (VCA) envelope.
The control ranges from off to 4s decay time.
The VCA Envelope is identical to TB-303. While the decay time is
variable in the case of the Avalon, the attack is fixed to retain the
behavior of the TB-303. This envelope can be turned completely off by
setting VCA DECAY to the minimum setting.
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3.6 MODULATION ENVELOPE

The modulation envelope is an additional attack-decay (AD) envelope
that can be used to modulate the filter and the final amplifier (VCA).
In addition to the dedicated filter and amp decay envelopes, this
modulation envelope allows for more complex modulation. The
modulation is bipolar and the envelope can be triggered on any step
of a sequence and manually in REALTIME modes.
ATTACK and DECAY set the attack and decay time of the envelope.
The attack time ranges from 0.9ms to 6s. The decay time ranges from
1.7ms to 10s.
VCF DEPTH and VCA DEPTH sets the amount of positive (+) or
negative (-) envelope modulation to the filter and VCA respectively.
VCF DEPTH has been designed with a progressive voltage offset for
increasing (+) or (-) modulations amounts. When the control is
rotated clockwise for (+) modulation a negative offset is applied to
the filter control input. Likewise, as an increasing (-) modulation
amount is set by rotating the control counter-clockwise a positive
offset is applied to the filter control input. See Appendix B for more
details.
VCA DEPTH has been designed with a progressive voltage offset for
negative (-) modulation amounts. When the control is rotated
counter-clockwise for (-) modulation a positive offset is applied to the
VCA control input. See Appendix B for more details.
The VCA Depth implementation as described above and in Appendix
B allows the VCA to be opened (drone mode) by setting the control for
maximum (-) modulation.
A key feature of the Avalon is the triggering of the modulation
envelope via the sequencer. The term "MOD ENV" as used in this manual
refers to programming or playing a modulation envelope trigger. This
should not be confused with "ENV MOD" , which is the name given to the
filter envelope depth control.

3.7 OUTPUT

VOLUME controls the output level
There is a discrete "mixer" stage after the volume control. This is a
carryover from the TB-303 which has a Mix Input before the output jack.
In order to retain the same behavior the Avalon incorporates this same
circuit. This stage clips asymmetrically and can clip easily. If you hear
unwanted distortion in the main or phones outputs try turning down the
volume control.
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AVALON - Reference Manual