THAT4305 Datasheet, PDF (13/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Document 600067 Rev 00
Page 13
The Sidechain
As noted earlier, for most effects compressors, it
is best not to linearly compress the entire dynamic
range of the audio signal. To this end, in the
sidechain of figure 16, we have added a threshold
amplifier with ±30 dB of threshold adjustment.
This restricts compression to signals above the
threshold, passing those below threshold without
any change. Additionally, we added a compression
ratio control adjustable from 1:1 to ∞:1. Finally,
we've added a means to apply up to ±20 dB of
static, or "make-up" gain.
The input signal is fed into the RMS detector
through C1 and R1. Like the VCA, the input to the
detector is AC coupled. This prevents any DC cur-
rent flowing into the detector's input from being
measured by the detector. If unchecked, such off-
sets would limit resolution at low levels, producing
an artificial "floor" to the detector's low-level re-
sponse.
As previously discussed (in the theory section),
the output of the detector is proportional to the log
of the RMS level of the input voltage. The output of
the detector will be approximately zero volts at the
"zero dB reference level" -- the point at which the
RMS value of the input current equals the timing
current (set at 7.5 µA for the 4305). We have chosen
a value of -10dBu, (245 mV_RMS) for the zero dB ref-
erence level. The required input resistor can be cal-
culated as
R=
245mV_RMS
7.5µA
= 32. 6 kΩ ≈ 33kΩ
.
Inverting threshold amplifier (U3) provides gain
of approximately -2 to the detector output signal
above threshold, and zero gain (A_V=0) to signals be-
low threshold. The change in gain is accomplished
by D1 and D2, which allow negative-going output
signals to pass but block positive-going ones. Be-
cause U3 is configured to invert, positive-going sig-
nals at the RMS output (indicating increasing ac
input levels) are passed onwards, while nega-
tive-going RMS outputs are blocked. By feeding
variable dc into this stage via R7 and the threshold
pot R10, we can vary the point at which RMS output
signals begin to be passed through to the threshold
amplifier stage’s output (at the junction of D2 and
R2.)
The scaling at the output of the detector is
+6.2 mV/dB, but because R2 is approximately twice
R17, the threshold amplifier (U3) has a gain of -2, so
the scaling at its output is -12.4 mV/dB. To swing
the threshold over ±30 dB, we can calculate the re-
quired value of R7 as follows:
15V(¹⁰_Rk7^Ω)
0.0124
V
dB
=
30dB
.
We can rearrange this to be
10kΩ
R₇
=
30dB
15V
⋅ 0.0124
V
dB
,
and
therefore,
R₇ = 403. 2 kΩ
We chose 430 kΩ for R7.
U4 is a variable-gain inverter that serves to
buffer the VCA's control port, ensuring a low-
impedance drive at that point. (High impedances,
even as little as 50 to 100 ohms, will increase VCA
distortion at high signal levels.) Above threshold,
when U4's gain is -½, the net gain of the sidechain
(from RMS output to VCA control input) is unity,
and the compression ratio is ∞:1. The network of
R3, R9, and R11 in conjunction with R18, allows the
gain of U4 to vary from 0 to -1/2, and simultaneously
shapes the (linear) pot's response so that 50% rota-
tion results in 4:1 compression. 4:1 ratio at 50% ro-
tation is often considered a useful target. Note that
with the more conventional approach of connecting
the undriven end of the pot to ground, the compres-
sion ratio at 50% rotation would be 2:1.
Finally, R8 and R12 provide the means for add-
ing static, or "make-up" gain. The control-voltage
sensitivity at the output of U4 is 6.2 mV/dB. There-
fore,
15V( ^5._R^18kΩ)
0.0062_dVB
=
20dB
.
We can rearrange this to be
5.1kΩ
R₈
=
20dB
15V
⋅
0.0062
V
dB
,
and
therefore,
R₈
=
5kΩ
20 dB
15 V
⋅0.0062_dVB
= 625kΩ
We've chosen 620 kΩ for R8 since it is the near-
est 5% value.
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