Here is an idea I had for those of you who want to try something a little different in your DAC/CD output stage. Take a look at the schematic which is attached. Basically it is a common base amplifier used as an I/V stage suitable for PCM1702/1704/63 or similar current output style DACs. This is STILL VERY MUCH A SKETCH at the moment but I thought some of you may wish to have a play and tweak it.
T1 forms the actual gain transistor and I suppose it can be considered the only transistor in the signal path. T2 creates a voltage reference for T1s base. T3 and T4 form constant current sources the current through these should be nearly identical.
The emitter of T1 will provide a low impedance point for the DAC chip. The DC voltage at this point will probably need trimming to zero by a pot arrangement around Rib enabling you to vary the bias current through T1 either side of the current level set by fixed current source T3.
I am not sure what the bias current should be but I would say that 10mA would be a good starting point.
Ri is where the actual I/V conversion takes place and the value chosen for this resistor is a direct compromise between output level and output impedance. Its fairly clear to see that its value is the output impedance. The maximum output level (RMS) of the circuit is equal to the maximum RMS current from the DAC chip multiplied by the value of Ri. For example if it is being used with say a PCM1704 (1.2mA) peak and Ri=1K then the maximum drive level is 0.8485v RMS with an impedance of 1K. Clearly this is not your usual CD Player drive but should be suitable for amplification by a good pre-amp, a ZEN for example.
It will be important to watch the voltage drop across Ri to ensure that T1 has an adequate Vce drop, you may need to raise the supply voltage. Speaking of supply it will have to be very well filtered as any noise on the positive supply will appear at the output.
Lastly this design has a very wide bandwidth and digital noise will be passed through to the output. The bandwidth can be decreased by adding a capacitor across Ri. There is lots of room for experimentation here and results will probably vary for different amplifiers and systems.
I would really like some opinions on this so let me know what you think.
T1 forms the actual gain transistor and I suppose it can be considered the only transistor in the signal path. T2 creates a voltage reference for T1s base. T3 and T4 form constant current sources the current through these should be nearly identical.
The emitter of T1 will provide a low impedance point for the DAC chip. The DC voltage at this point will probably need trimming to zero by a pot arrangement around Rib enabling you to vary the bias current through T1 either side of the current level set by fixed current source T3.
I am not sure what the bias current should be but I would say that 10mA would be a good starting point.
Ri is where the actual I/V conversion takes place and the value chosen for this resistor is a direct compromise between output level and output impedance. Its fairly clear to see that its value is the output impedance. The maximum output level (RMS) of the circuit is equal to the maximum RMS current from the DAC chip multiplied by the value of Ri. For example if it is being used with say a PCM1704 (1.2mA) peak and Ri=1K then the maximum drive level is 0.8485v RMS with an impedance of 1K. Clearly this is not your usual CD Player drive but should be suitable for amplification by a good pre-amp, a ZEN for example.
It will be important to watch the voltage drop across Ri to ensure that T1 has an adequate Vce drop, you may need to raise the supply voltage. Speaking of supply it will have to be very well filtered as any noise on the positive supply will appear at the output.
Lastly this design has a very wide bandwidth and digital noise will be passed through to the output. The bandwidth can be decreased by adding a capacitor across Ri. There is lots of room for experimentation here and results will probably vary for different amplifiers and systems.
I would really like some opinions on this so let me know what you think.