I'm starting this thread to be a reference list of suitable chips. This is supposed to be a fairly high quality volume control, useful for hifi setups or PA systems.
Basically, for controlling volume, you reduce the volume with the help of a potentiometer, and then you buffer the high impedance output of the pot wiper with a opamp or a transistor.
A digital potentiometer is a chip with a built in resistance that can be varied via a digital interface. There are lots of them around. These could be useful, but need additional opamps. I wanted chips with the opamp integrated, in order to simplify the design. (adding any opamp is not difficult, but choosing the right digital pot and opamp and getting rid of noise, THD, etc. might be not so easy)
So far I found 5 chips:
| chip | USD | supply V | interface | audio performance | comment | | - | - | - | - | - | - | | max5486 | 2.50 mouser or farnell | +2.7V to +5.5V | up/down pushbutton | SNR 90dB THD 0.01% | only SMD | | pt2257 | 2 ebay | 3 to 9V | I2C | SNR 90dB THD 0,07% | | | pt2259 | 3 at ebay | 4 to 10 V | I2C | SNR 95dB THD 0,07% | | | pga2311 | 10 at mouser or farnell | 10V (+/-5V) | serial | SNR 120db THD 0.0004% | | | m62429 | 5 at ebay | 5V | serial | SNR 85db THD 0.01% | creates its own ground ref V |
The chip "that2181" from http://www.profusionplc.com/index.php has superior audio performance, but no digital interface.
Power supply: This is not so trivial, because you can't just use the power supply of the arduino. The switching processes inside the atmega chip introduce small spikes in the Vdd rail, which will be audible.
Also, the volume chip needs a dual voltage power supply. The dual voltage requirement can be cirumvented in a fashion: because there is actually not much current flowing in and out of the ground, you can create a ground point with a voltage divider and an opamp used as buffer. Or you omit the ground alltogether and connect the inputs and outputs with capacitors. However the capacitors will affect frequency response, if not carefully selected.
A 5V power supply will allow a maximum output amplitude of roughly +/- 2V or 10dBu or 8dBV, which is enough for PA use or for hifi use. However there is no safety headroom left, since 10dBu is PA standard.
My plan for the power supply so far: Use one 9V or 10V power supply (depending on the chip) and filter this power supply with a low pass RC filter and possiby an inductance for the analog audio chip. This filter will take away some voltage, depending on the current drawn, but there will still be enough headroom left, if the resistor is not too big. The arduino will step the 9V down to 5V with its internal voltage requlator.
Any thoughts or suggestions for other chips?
BTW, there is a fully assembled board with a PT2259 or m62429 chip available, including voltage regulator, at ebay or other online stores, for around 10 - 20 USD.
This thread http://forum.arduino.cc/index.php?topic=244152.0 shows that it's possible to operate the board with an arduino.
P.S. My actual need is this (for guitar): http://www.pfeifferelectronics.com/products/leveler_volume_pedal.aspx However I need more than 8 steps, and I'm not willing to shell out USD180 + shipping + tax to Germany, and there are additional things supposed to go into the casing (LED metronome, effects bank switching)
Thanks for reading Thomas