At a glance
- volume control circuit for up to 4 channels or 2 fully symmetrical channels
- buffered input with 47kOhm input impedance
- Burr Brown volume control chip PGA4311
- +/-5V symmetrical voltage
- digital control connection decoupled by optocouplers
- PCB available in our shop
This circuit is based on the Burr Browns PGA 4311. This chip provides volume control for 4 independent channels. Operating voltage is +/-5V. While this would be problematic for professional audio level, it is enough for home audio (2v peak voltage). The circuit should be located near the power stage, because it is not using additional buffer stages after the PGA4311. However, it is not a problem to drive a few meters if the power stage has a high input impedance. For the input, an additional buffer stage has been added. It is based on a quad opamp like the OPA1664 (which is a good choice, because it is also designed to work on +/- 5V power supply). To decouple the digital input from the analog part, optocouplers will be used.
The major disadvantages of this design are
- it needs 3 additional parts
- driver circuit must be able to source 10mA current
- depending on the optocouplers, speed of the SPI transmission must be low
However, none of these issues seems to be critical for this use. The additional parts are cheap and the final PCB is still very small. Microcontrollers like Atmel’s MegaAVR series can source up to 40mA per port and even the Raspberry Pi allows a port current of up to 16mA. Speed is also not an issue, because setting the volume needs only a few bytes of data to transmit.
Pin assignment of the control connector
The control connection uses 6 pins. However, 10 pin connectors are easier available. Therefore you can use a 6-pin or a 10-pin connector. Because the connector uses a common anode connection you can even use a simple 4- or 5 pin connector.
| Pin | Signal |
|---|---|
| 1 | COMMON ANODE (+) |
| 2 | COMMON ANODE (+) |
| 3 | CLOCK – |
| 4 | COMMON ANODE (+) |
| 5 | CS – |
| 6 | COMMON ANODE (+) |
| 7 | DATA – |
| 8 | COMMON ANODE (+) |
| 8 | COMMON ANODE (+) |
| 8 | COMMON ANODE (+) |
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Hi,
What was the reason SDO has not been made available on the header for daisy chaining, to obtain more channels?
Is it a 4-layer board?
What is the reason to have included a front opamp?
Thanks
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Is it possible to have several PGA4311 boards controlled by Raspberry Pi at once for 8 or 12 channels?
Very interesting piece. You say that you can control via PC? Is that with RS232 commands, and subsequently could you use a controlsystem (Crestron, AMX/RTI etc)
What volume commands are availible +,- set vol? Is there feedback?