THAT4311 Datasheet, PDF (10/12 Pages) List of Unclassifed Manufacturers – Low-voltage, Low-power Analog Engine Dynamics Processor

Page 10
Low-voltage, Analog Engine^® Dynamics Processor
Preliminary Information
within the linear range of the THAT 4311. (Peak VCA
input currents should be kept under 250 mA for best
distortion performance.) The coupling capacitor (Cl,
47 mF) is strongly recommended to block DC current
from preceding stages (and from offset voltage at the
input of the VCA). Any DC current into the VCA will
be modulated by varying gain in the VCA, showing up
in the output as “thumps”. Note that Cl, in conjunc-
tion with R1, will set the low frequency limit of the
circuit.
The VCA output is connected to OA3, configured
as an inverting current-to-voltage converter. OA3’s
feedback components (R2, 20 kW, and C2, 47 pF) de-
termine the constant of current-to-voltage conversion.
The simplest way to deal with this is to recognize that
when the VCA is set for unity (zero dB) gain, the in-
put to output voltage gain is simply R2/R1, much like
the case of a single inverting stage. If, for some rea-
son, more than zero dB gain is required when the
VCA is set to unity, then the resistors may be skewed
to provide it. Note that the feedback capacitor (C2) is
required for stability. The VCA output has approxi-
mately 45pF of capacitance to ground, which must be
neutralized via the 47pF feedback capacitor across
R2.
The VCA gain is controlled via the EC- terminal,
whereby gain in dB will be proportional to the nega-
tive of the voltage at EC-. In this application the EC+
terminal is tied to VREF, though it could be the
driven port, or the control ports could be driven dif-
ferentially. The SYM terminal is returned nearly to
the EC+ terminal (which is in this case VREF) via a
small resistor (R3, 51W). The VCA SYM trim (R5,
20kW) allows a small voltage to be applied to the
SYM terminal via R4 (33kW). This voltage adjusts for
small mismatches within the VCA gain cell, thereby
reducing even-order distortion products. To adjust
the trim, apply to the input a middle-level, mid-
dle-frequency signal (1kHz at 200mV_rms is a good
choice with this circuit) and observe THD at the sig-
nal output. Adjust the trim for minimum THD.
RMS-Level Detector
The RMS detector’s input is similar to that of the
VCA. An input resistor (R6, 28.7kW) converts the AC
input voltage to a current within the linear range of
the THAT 4311. The coupling capacitor (C3, 47mF)
is recommended to block the current from preceding
stages (and from offset voltage at the input of the de-
tector). Any DC current into the detector will limit
the low-level resolution of the detector, and will upset
the rectifier balance at low levels. Note that, as with
the VCA input circuitry, C3 in conjunction with R6
will set the lower frequency limit of the detector.
The time response of the RMS detector is deter-
mined by the capacitor attached to CT (C4, 10 uF)
and the size of the current in pin IT (determined by
R7, 267 kW and VREF, 2V). Since the voltage at IT is
approximately 2V, the circuit of Figure 19 produces
7.5 mA in IT, The current in IT is mirrored to the CT
pin, where it is available to discharge the timing ca-
pacitor (C4). The combination produces a log filter
with time constant equal to approximately 0.026
CT/IT (~35 ms in the circuit shown).
The waveform at CT will follow the logged (deci-
bel) value of the input signal envelope, plus a DC off-
set of about 2VBE plus VREF or about 3.3V. The
capacitor used should be a low-leakage, electrolytic
type in order not to add significantly to the timing
current.
The output stage of the RMS detector serves to
buffer the voltage at CT and removes the 1.3 V_DC
(2 V_BE) offset, resulting in an output centered around
VREF for input signals of about 245 mVrms, or
-10 dBu. The output voltage increases 6.1 mV for ev-
ery 1 dB increase in input signal level. This relation-
ship holds over more than a 60 dB range in input
currents.
Control Path
The primary function of an audio compressor is
to reduce a signal's dynamic range. A 2:1 compres-
sor reduces a 100 dB dynamic range to 50 dB. A
limiter, or infinite compressor, is a special case of
compressor where the dynamic range is reduced to
the point where the rms level of the signal is con-
stant. This reduction in dynamic range is accom-
plished by a) raising the gain when the signal is
below some particular level -- often referred to as the
'zero dB reference level' -- and b) reducing the gain
when the signal is above that level. In addition, these
devices often have a threshold, below which the sig-
nal is passed unprocessed and above which compres-
sion takes place. This feature keeps the noise floor
from rising to noticeable levels in the absence of sig-
nal.
We previously established that the zero dB refer-
ence level of the detector is -10 dBu (zero dB refer-
ence level = 7.5 , R6 = 28.7 kilohms). Neglecting the
effect of the threshold control (R11 and R12), when
the output is below this level the output of OA2 is
driven high, forward biasing CR1 and reverse biasing
CR2. Since CR2 is not conducting, no signal is
passed to the VCA's control port by OA1. When the
signal level exceeds -10 dBu, the output of the RMS
detector goes positive, and CR2 begins to conduct. In
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
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