THAT Corporation Design Note 122
5.1 Channel
Volume Control
The circuits within this application note feature THAT218x to provide the essential
function of voltage-controlled amplifier (VCA). Since writing this note, THAT has introduced
a new dual VCA, as well as several Analog Engines®. Analog Engines combine a VCA and an
RMS detector (RMS) with optional opamps in one part. With minor modifications, these
newer ICs are generally applicable to the designs shown herein, and may offer advantages in
performance, cost, power consumption, etc., depending on the design requirements. We
encourage readers to consider the following alternatives in addition to the 218x:
• Analog Engine (VCA, RMS, opamps): 4301
• Analog Engine with low supply voltage and power consumption (VCA, RMS, opamps):
4320
• Analog Engine with low cost, supply voltage, and power consumption (VCA, RMS): 4315
• Analog Engine with low cost and power consumption (VCA, RMS): 4305
• Dual (VCA only): 2162
For more information about making these substitutions, please contact
THAT Corporation's technical support group at apps_support@thatcorp.com
45 Sumner St, Milford, MA 01757-1656 USA; www.thatcorp.com; info@thatcorp.com
Copyright © 2000 - 2010 by THAT Corporation; All rights reserved.
Document 600160 Revision 01
5.1 Channel Volume Control
THAT Corporation Design Note 122
Newer 5.1 channel home theater systems will benefit from a rethinking of existing
approaches for controlling volume. In the past, ganged, audio taper potentiometers
controlled the stereo (2 channel) signal, and the mis-tracking between channels was tolerated, since it could be easily compensated for with a balance control. This situation no
longer applies, since the digital stream in which the audio is encoded is directly decoded into
the left, center, right, left rear, right rear, and sub-woofer channels, and there must be
independent control of each of these signals.
Voltage controlled amplifiers (VCAs) provide a superior alternative to a six section
ganged potentiometer. The exponential control input of log/anti-log VCAs is considered by
many to be a superior control function in comparison with a ganged, audio taper potentiometer. The audio taper potentiometer uses concatenated linear sections to approximate
logarithmic volume control. A linear control voltage applied to the VCA’s control port results
in volume control that is linear in decibels. Additionally, tracking between VCAs is superior
to that of ganged potentiometers, being essentially limited by the thermal mismatches
between devices.
C8
a) Left Channel Gain Control Block
Signal In +
+5A
C1
10n
to digital section
to digital section
to digital section
to digital section
Master Volume
U1
DAC1
Vdd Ref
Data
CH1
Clock DAC
CH2
LDAC
Dgnd Agnd
Center
Channel
Rear Channel
6k81
C6
2u2
U2
DAC1
Vdd Ref
Data
CH1
Clock DAC
CH2
LDAC
Dgnd Agnd
Sub-woofer
+
R18
3
6k81
2
R25 R24
200k 2k49
R17
R16
6k81
R6
6k81
5
R14
6k81
R3
+
R9
133k
-15
-15
R13
R12
6k81
R2
6k81
+
U5C
8
R11
2k61
14
NJM2060
R7
133k
Control Port
d) Left Rear Channel Gain Control Block
Control Port
U5D
13
32k4
+
C10
470n
Control Port
e) Right Rear Channel Gain Control Block
12
R1
Signal Out
b) Right Channel Gain Control Block
NJM2060
R8
133k
1
5532
c) Center Channel Gain Control Block
9
32k4
C11
470n
U4A
3
Control Port
7
R23
2k61
10
2
5k1
5%
NJM2060
32k4
+ C9
470n
R21
20k0
-15
U5B
6
R15
32k4
C3
2u2
1
+15
2
U3
4
EC+
7 2180A
SYM
R20 1
V+
8
IN
OUT
V20k0
GND
5
EC6
3
R22
NJM2060
V-
6k81
R4
+
47u
U5A
R5
32k4
+
C5
2u2
32k4
C4
2u2
+5A
C2
10n
R19
C7
22p NPO
Control Port
f) Sub-woofer Gain Control Block
R10
2k61
-15
5.1 Channel Volume Control Circuit (see page 6 for a larger version of this diagram)
The attached schematic show how VCAs facilitate remote gain control without the use of
a motorized potentiometer (though a single, linear, motorized pot could be used as a master
volume control), and simplify the implementation of independent balancing of center, rear,
and sub-woofer level. This circuit uses four, linear 8-bit DACs to control the Master (overall)
volume and to provide greater than ±12 dB of fine adjustment to the Center, Rear, and
Sub-woofer channels.
Only one channel is shown in the VCA section. The remaining 5 channels are identical to
the left channel gain control block, and consist of a THAT 2180A VCA and one half of a 5532
op-amp. The THAT 2180A provides the best distortion performance of any pre-trimmed VCA
available on the market today. The 5532 is a good audio op-amp, though any number of
Copyright © 2000 - 2010
by THAT Corporation
All rights reserved
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; info@thatcorp.com
Design Note 122
Page 2 of 6
Doc. 600160 Rev. 01
THAT Corporation Design Note 122
5.1 Channel Volume Control
amplifiers including the OP-275 and the LM833 could be used in its place. Layout for this
VCA is relatively straightforward, and there are only two basic issues to be concerned with:
1. The node connected to the output of the VCA (and the inverting input of the 5532)
should be as small as it can reasonably be made. This will help minimize channel to channel
cross talk and maximize “off isolation”, which are both due to signal being capacitively
coupled from a channel input to the output node of the VCA. Also, with the 5532s and their
associated de-coupling capacitors placed close to the VCA, there should be no need for
additional de-coupling around the VCA.
2. The packages of the VCAs should ideally be arranged in a line with the bodies actually
touching. This arrangement helps keep the VCAs at the same temperature, and minimizes
temperature dependent gain variations between devices. These VCAs are superior to ganged
potentiometers with respect gain linearity and tracking, and when we do see problems,
thermal mis-tracking is usually the problem. A benefit of this arrangement is that the
layout is made very simple. With the VCAs in-line and all in the same orientation, GND, V+,
and EC+ of all of the VCAs can be connected to each other with straight traces on one side
of the board. The traces connecting EC- and V- can be on the other side of the board and
run between the VCAs. The input and output are at the ends of the package and should not
be difficult to route.
This design uses four linear 8-bit DACs to accomplish both overall gain adjust of all six
channels and fine trim of the center, rear, and sub-woofer channels. The master volume
signal is injected into all of the channels, and center, rear, and sub-woofer controls are
superimposed onto the master volume signal.
The exact derivation of the component values used in these summing networks is rather
tedious, and not particularly instructive. It is useful, however, to understand the actual
functions of these components. In the left and right channel control path (which is buffered
by U5A), the master volume signal is injected through R18 and R19, and the actual control
signal develops across the parallel combination of R24 and R25:
R eq R24,R25 =
1
2.49k
1
+
1
200k
= 2.45k
And
R eq R18,R19 = 2 % 6.81 k = 13.62 k
With a DAC reference of 5V, the control port voltage can swing
CV swing =
5 V % 2.54 k
2.45 k + 13.62 k
= 0.762 V
which, in dB, is
CV dB =
0.762
6.1 mV
dB
Copyright © 2000 - 2010
by THAT Corporation
All rights reserved
= 125 dB
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; info@thatcorp.com
Design Note 122
Page 3 of 6
Doc. 600160 Rev. 01
5.1 Channel Volume Control
THAT Corporation Design Note 122
Since this voltage connects to EC-, this swing is from zero dB to -125 dB. R25 serves to
offset this swing into the usable range of the VCA. The current through this resistor develops across the parallel combination of R24 and R18 plus R19.
R eq R18,R19,R24 =
1
2.49 k
1
+
1
13.62 k
= 2.11 k
Since R25 is connected to -15V,
15 V % 2.11 k
2.11 k + 200 k
CV offset =
= 0.157 V
which, in dB, is
CV dB off =
0.157
6.1 mV
dB
= 25 dB
resulting in a gain range of -100 dB to +25 dB.
The fine trim of the center, rear, and sub-woofer channels is accomplished in a similar
manner. The center channel trim is injected through R5 and R6, and developed across the
parallel combination of R9, R23, and R16 plus R17. R9 serves the same purpose as R25 in
the previous discussion, but it must now offset both the master volume signal and the
center channel trim, and its value has been reduced accordingly. R23 serves the same
purpose as R24 in the discussion above, but since the master volume signal now develops
across the parallel combination of R23, a reduced R9, and the series combination of R5 and
R6, the value of R23 has been increased to keep the scale factor of the master volume
constant.
The equivalent resistances used when calculating the center channel trim sensitivity are:
R eq R9,R23,R16,R17 =
1
2.61 k
1
+
1
133 k
+
1
2 % 6.81 k
= 2.15 k
And
R eq R5,R6 = 2 % 32.4 k = 64.8 k
With a DAC reference of 5V, the control port voltage can swing
CV swing =
5 V % 2.15 k
2.15 k + 64.8 k
= 0.161 V
which, in dB, is
CV dB =
0.161
6.1 mV
dB
= 26 dB
Since this voltage connects to EC-, this swing is from zero dB to -26 dB. R9 serves to
offset this swing and make the adjustment range symmetrical. The current through this
resistor develops across the parallel combination of R23, R5 plus R6, and R16 plus R17.
R eq R6,R7,R16,R17,R23 =
Copyright © 2000 - 2010
by THAT Corporation
All rights reserved
1
2.61 k
1
+
1
2 % 32.4 k
+
1
2 % 6.81 k
= 2.11 k
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; info@thatcorp.com
Design Note 122
Page 4 of 6
Doc. 600160 Rev. 01
THAT Corporation Design Note 122
5.1 Channel Volume Control
Since R23 is connected to -15V,
CV offset =
15 V % 2.11 k
2.11 k + 133 k
= 0.234 V
which, in dB, is
CV dB off =
0.234
6.1 mV
dB
= 38.4 dB
25 dB are used to offset the master volume as was the case in the left and right channel
control path, while the remaining 13.4 dB (approximately one half of 26 dB) offset the center
channel signal.
The same control path is used for the rear and sub-woofer channels. C3 through C6 and
C9 through C11 are used to limit the slew rate of the control port voltage as well as serving
to absorb any glitch energy from the DACs.
This approach satisfies the basic requirements for a 6 channel volume control. Using
linear 8-bit DACs to generate a control voltage for the exponential control port of a VCA is a
low cost way to control volume over a 125 dB range, and the use of DACs makes remote
control easier to implement.
Copyright © 2000 - 2010
by THAT Corporation
All rights reserved
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; info@thatcorp.com
Design Note 122
Page 5 of 6
Doc. 600160 Rev. 01
Copyright © 2000 - 2010
by THAT Corporation
All rights reserved
to digital section
to digital section
to digital section
to digital section
U1
DAC1
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; info@thatcorp.com
Data
Clock
LDAC
C2
10n
CH1
6k81
R2
R13
+
6k81
R12
C10
470n
U5A
U5B
R7
133k
7
U5C
14
R10
2k61
NJM2060
13
12
U5D
NJM2060
9
10
8
NJM2060
6
5
R8 R11
133k 2k61
-15
1
NJM2060
2
3
R9 R23
133k 2k61
-15
-15
32k4
R1
C11
470n
32k4
32k4
C4
2u2
6k81
R3
6k81
R4
+
R14
R15
+
R5
V-
32k4
+ C9
470n
6k81
6k81
R6
32k4
+
C5
2u2
R16
R17
32k4
+
Sub-woofer
C3
2u2
U2
DAC1
DAC
CH2
Dgnd Agnd
Vdd Ref
+5A
Rear Channel
Center
Channel
R18
6k81
6k81
+
C6
R25 R24
2u2
200k 2k49
Master Volume R19
Vdd Ref
Data
CH1
Clock DAC
CH2
LDAC
Dgnd Agnd
C1
10n
+5A
47u
+
20k0
R20
1
U3
2180A
V+
8
OUT
V-
7
-15
5k1
5%
GND
5
EC6
3
R22
IN
4
EC+
SYM
+15
f) Sub-woofer Gain Control Block
Control Port
e) Right Rear Channel Gain Control Block
Control Port
d) Left Rear Channel Gain Control Block
Control Port
c) Center Channel Gain Control Block
Control Port
b) Right Channel Gain Control Block
Control Port
Signal In
C7
2
a) Left Channel Gain Control Block
3
2
5532
1
U4A
20k0
R21
22p NPO
C8
Signal Out
THAT Corporation Design Note 122
5.1 Channel Volume Control
Design Note 122
Page 6 of 6
Doc. 600160 Rev. 01