Download THAT Corporation Design Note 120

THAT Corporation Design Note 120
VCAs in a Pan Potentiometer
Application
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:
y Analog Engine (VCA, RMS, opamps): 4301
y Analog Engine with low supply voltage and low power (VCA, RMS, opamps): 4320
y Analog Engine with low cost, low supply voltage, and low power (VCA, RMS): 4315
y Analog Engine with low cost and low power (VCA, RMS): 4305
y Dual (VCA only): 2162
For more information about making these substitutions, please contact
THAT Corporation's technical support group at [email protected]
45 Sumner St, Milford, MA 01757-1656 USA; www.thatcorp.com; [email protected]
Copyright © 2001 - 2008 by THAT Corporation; All rights reserved.
Document 600130 Revision 02
THAT Corporation Design Note 120
VCAs in a Pan
Potentiometer Application
The pan potentiometer (pan pot) allows users to steer an audio signal between two
channels and, unlike standard potentiometers which have a linear taper, or audio potentiometers which have something like a log taper, the elements in a pan pot will have something
approximating a sin vs. cos taper. The intent is to yield a constant power sum of the two
output channels as the pan control is swept.
The common method of implementing a pan pot, using a single linear potentiometer with
the wiper grounded, and resistors of approximately equal value to the pan pot connected
appropriately, approximates the sin vs. cos taper described earlier. While this is certainly an
elegant solution, there is no easy way to automate this arrangement, short of a motorized
potentiometer, for pro-audio applications. "Digi-pots" would introduce significant amounts
of zipper noise, and have limited adjustability as well.
The following schematic shows how an "electronic pan pot" can be implemented. The
cosine law of the pan pot is implemented by a nonlinear circuit with two break points.
U4A&B are the control voltage buffers for the VCAs, U1 and U2 respectively. One VCA is
driven via its Ec- control port while the other is driven via its Ec+ port. This allows a single
polarity of control signal to increase the gain of one VCA while decreasing the gain of the
other. U3A&B are the respective output amplifiers for U1 and U2. U5A&B are used for
breakpoint generation.
The VCA and control voltage buffer portions of this schematic are similar to other application circuits, and the usual precautions apply; use a low noise, wideband amplifier for the
control voltage buffer to minimize noise modulation and the potential for instability, etc.
U4A scales the 0-5 V control signal so that at zero volts, U1's gain is zero dB, and as the
control voltage increases, the gain goes down. U4B performs similar a similar scaling, but
R18 offset the control signal so that U2 is essentially off at a control voltage of zero, but
increases to a gain of zero dB when the control voltage is 5 V.
U5 is the breakpoint generator. U5B scales and offsets the 0-5 V control to approximately ±0.6 V, as well as inverting the polarity. It is easiest to understand the breakpoint
circuitry by looking at one half of the circuit at a time. While the 0-5V control signal is below
2.5 V, the output of U5A is negative, and D4 is off. Since pin 2 of U5A and pin 2 of U4A are
both at zero volts, there is no current through R6 and R8, and the output of U4A is purely a
function of the 0-5 V control signal via R9 and R13. As this signal goes above 2.5 V, the
output of U5A goes positive, D4 conducts, and the breakpoint generator begins to affect
the output of U4A. While the output of U5B is significantly more positive than -0.5 V, the
gain of U5A is two, and its affect on the EC- of the VCA is to approximately double the slope
of the control signal. As the output of U5B continues to approach -0.6 V, D1 begins to turn
on, and the gain of U5A begins to increase rapidly. This results in the second breakpoint.
The operation of the other half of the circuit is basically the same, but all of the breakpoints occur below the 2.5 V at the input. The results, given in figure 2, show the gain of
right and left channels, which correspond to U1 and U2 as a function of control signal and
temperature. As the second breakpoint kicks in, there is a distinct gain error as a function
of temperature. This turns out to be relatively insignificant.
Copyright © 2000 - 2008
by THAT Corporation
All rights reserved
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; [email protected]
Design Note 120
Page 2 of 4
Doc 600130 Rev 02
VCAs in a Pan
Potentiometer Application
THAT Corporation Design Note 120
Gain Control
2
SW1A
R22
R1
R2
D1
22k
5k1
1N4148
R28
6
133k
5
V-
R6
43k
C1
1n
R5
2
10k
3
82k
1
1
D3
1N4148
Lin C7
47u
R23
300R
6
DP3T2
Inv.
Gain Control
V-
100n
U4A
5532
5
43k
R20 R17
133k 267k
C2
3
U5A
R14
20k R15 9k1
43k
SW1B
R16
V-
2
R7
1N4148
R19
300R
D4
1N4148
5532
D2
R13
R8
9k1
20k
7
U5B
5532
47u
43k R9 DP3T2
Zero to 5V control signal
R18
133k
R26 20k0
EC- 4 7
SYM
1
V+
8
IN
OUT
V20k0 GND
5
EC+
6
R3
3
C4
22p
C6 R24
Rin
82k
R10
Control
V+
C3
100n
U1
2180B
2
1
3
Rout
U3A
5532
5k1
VR27
20k0
C5
U2
2180B
V+
2
EC- 4 7
R25 1 SYM V+ 8
OUT
IN
20k0 GND V5
EC+
6
3
R4
5k1
22p
6
7
5
Lout
U3B
5532
V-
7
U4B
5532
VFigure 1. Pan Pot Circuit
Figure 3 shows the deviation from ideal constant power summing. Note that the vertical
axis is in dB. This result is calculated with the equation:
error = 20%log e
Ec − gain
VT
+e
Ec + gain
VT
, since the gain for either channel is
Ec
e VT
As the graph shows there is less than 1 dB deviation from ideal, and the temperature
errors so prominent in Figure 2, have minimal impact on the gain error, because they occur
when the gain of a particular channel is significantly below that of the more prominent
channel. SW1 provides a means for shaping the gain at the midpoint. Putting equal value
resistors in parallel with R9 and R15 will lower the gains at the midpoint to -6 dB, while
putting double their value in parallel will lower the gain at the midpoint to -4.5 dB.
All of the results shown are based on ideal values of components and exact values of the
supplies and reference. For a given "real world" application, the designer may need tighter
tolerances on components, and to generate an accurate, stable negative reference for generating offsets.
Copyright © 2000 - 2008
by THAT Corporation
All rights reserved
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; [email protected]
Design Note 120
Page 3 of 4
Doc 600130 Rev 02
THAT Corporation Design Note 120
VCAs in a Pan
Potentiometer Application
20
0
L@20degC
L@25degC
L@30degC
L@35degC
R@20degC
R@25degC
R@30degC
R@35degC
-20
dB
-40
-60
-80
-100
0
0.5
1
1.5
2
2.5
3
Control Signal
3.5
4
4.5
5
Figure 2. VCA gain as a function of control signal
10
7.5
5
2.5
Err@20degC
Err@25degC
Err@30degC
Err@35degC
dB 0
-2.5
-5
-7.5
-10
0
0.5
1
1.5
2
2.5
3
Control Signal
3.5
4
4.5
5
Figure 3. Gain error with respect to constant power summing
Copyright © 2000 - 2008
by THAT Corporation
All rights reserved
45 Sumner St; Milford, MA 01757 USA; www.thatcorp.com; [email protected]
Design Note 120
Page 4 of 4
Doc 600130 Rev 02