Computer Audio Asylum: CCS output stage by John Swenson

Here is my output stage. This is based on the self bias CCS from Gary Pimm.

As you can see its made out of either one or two boards per channel. The first is setup as a current mirror, the actual IV conversion occurs across R20. The output impedance is low enough that you can probably use it as is in most applications. The second board is a buffer, it isolates the IV conversion from the load and provides a very low impedance driver to whatever you hang on the output. Its actually a very good headphone amp!

Note there are a couple resistor value changes from Gary's design, R10 and R15.

The values for R1 and R20 were chosen for use with a 1543 DAC that is run in the 7-8.5 V VCC range with an output voltage of 3.5. R11 is adjusted to give the appropriate voltage on the DAC output, note that is NOT the voltage across R11, measure the voltage at the MU pin.

The DAC output pin gets connected to the MU pin of the CCS, I know that seems like a weird place to use as an input, but that IS the correct place.

R1 needs to be adjusted so that without the DAC connected there is 30mA flowing through the circuit, the 120 ohm resistor is just about right for a 3.5 V at the MU pin.

R20 is critical, this is where the IV conversion takes place, but its also where a large amount of power is dissapated, with these values there is about a 40V drop across R20 (which is why it needs a 70V supply) that comes out to 1.2 watts, so it had better be a pretty hefty resistor. I personally would probably use a 5 watt Kiwame, but thats one of the "tweaks" you can do. The higher its value the greater the audio output voltage, but also the greater voltage drop, which means a higher supply voltage and more power used by R1. If you drop the value, you have less drop and dissapation but also lower audio output. The 1.3K gives a good compromise.

The R1 on the second board also uses a fair amount of power (about half a watt) so you want something appropriate there as well.

If you use the follower you don't need the C10 on the first board, just the one on the second.

If you are using this with a DAC that has a zero volt output such as a PCM1704 or PCM56 the bottom of the first board needs to be at -5V instead of ground, the second board can stay referenced to ground.

The effective input impedance of this design is about 10 ohms which is much lower than the 270 of the direct IV resistor version, that makes a HUGE difference in sound on my system.

Have fun and let me know if you have any questions or want to share how it sounds.

John S.