Voltage Controlled Amplifier with Balanced Input

Rajkumar Sharma
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SKU: EL150155
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The project presented here is a Voltage Controlled Amplifier (VCA). A VCA board helps users control the audio level from a remote potentiometer. This board requires a balanced audio signal. Gain control voltage can be adjusted through potentiometer R13. The project works with a dual (±15V) power supply. Screw terminal connectors are provided for signal input and signal output. The project is built using THAT2181LA chip which is a VCA, designed for high performance in audio-frequency applications requiring exponential gain control, low distortion, wide dynamic range, and low control-voltage feedthrough. This chip controls the gain by converting an input current signal to a bipolar logged voltage, adding a DC control voltage, and re-converting the summed voltage back to a current through a bipolar antilog circuit. Stereo control of VCA with a single potentiometer is possible.

THAT2181–series trimmable Blackmer voltage-controlled amplifier (VCA) ICs are very high-performance current-in/current-out devices with two opposing-polarity, voltage-sensitive control ports. They offer wide-range exponential control of gain and attenuation with low signal distortion.

Trimming

The trim should be adjusted for minimum harmonic distortion using PR1. This is usually done by applying a middle-level, middle-frequency signal (e.g. 1 kHz at 1 V) to the audio input, setting the VCA to 0 dB gain, and adjusting the SYM trim while observing THD at the output. In 2181, this adjustment coincides closely with the setting which produces minimum control-voltage feedthrough, though the two settings are not always identical.

Features

Power Supply +/-15V DC @ 40mA
Total Harmonic Distortion 1V 0dB Gain 0.0025%
Screw Terminal for Balanced Audio Input
Header Connector for Power Input
Screw Terminal for Audio Output
On Board Power LED
5mm Mounting Holes
PCB Dimensions 53.34 x 25.72mm

Connections

CN1: Balance Audio Signal Input, Pin 1 = Audio-Hot, Pin 2 = GND, Pin 3 = Audio Cold
CN2: Pin 1 = Audio Output, Pin 2 = GND
CN3: Pin 1 = 15V DC, Pin 2 = GND, Pin 3 = -15V
D1: Power LED
R13: Gain Adjust Potentiometer
PR1: Symmetry Adjust

Schematic

Parts List

BOM	QNTY.	REF.	DESC.	MANUFACTURER	SUPPLIER	SUPPLIER PART NO
1	1	CN1	3 PIN SCREW TERMINAL PITCH 5.08MM	PHOENIX	DIGIKEY	277-1248-ND
2	1	CN2	2 PIN SCREW TERMINAL PITCH 5.08MM	PHOENIX	DIGIKEY	277-1247-ND
3	1	CN3	3 PIN MALE HEADER PITCH 2.54MM	WURTH	DIGIKEY	732-5316-ND
4	2	C1,C4	47PF/50V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
5	3	C2,C5,C10	47uF/35V ELEKTROLYTIC	WURTH	DIGIKEY	732-8508-1-ND
6	1	C3	22PF/50V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
7	4	C6,C8,C11,C12	100nF/50V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
8	2	C7,C9	10uF/25V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
9	1	C13	1uF/25V CERAMIC SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
10	1	D1	LED RED SMD SIZE 0805	OSRAM	DIGIKEY	475-1278-1-ND
11	1	PR1	50K TRIMMER POT	BOURNS	DIGIKEY	3296W-503LF-ND
12	5	R1,R3,R6,R9,R13	10K 5% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
13	1	R2	2.2K 5% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
14	1	R4	100E 5% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
15	2	R5,R8	20K 1% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
16	1	R7	51E 1% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
17	1	R10	5.1K 1% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
18	1	R11	4.12K 1% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
19	1	R12	130K 1% SMD SIZE 0805	YAGEO/MURATA	DIGIKEY
20	2	U1,U3	BA4560 SOIC8	ROHM	DIGIKEY	BA4560F-E2CT-ND
21	1	U2	THAT2181A SOIC8	THAT CORP	MOUSER	887-2181AS08-U