Discrete transistor based CFB I/V stage

Since I figured out the reason for needing all those caps in my earlier DAC designs was all brought on by using passive I/V, I'm now a total convert of active I/V in order to do away with the sheer bulk.

Having tried single transistor I/V and loved it, I found there was still some improvement to be gained by biassing the common-base transistor with additional current sources to reduce its input impedance. Since getting down to the region of 1ohm would require some 25mA of bias which isn't well suited to portable applications I decided to have a go at using feedback to obtain the impedance I'm seeking.

I'm not using an off-the-peg CFB amp because they still turn out to be fairly power supply quality susceptible (subjectively speaking) so here's a design I hope that greatly reduces the supply impedance requirements so that it can be used in a portable player.

The picture shows the second prototype I/V stage, coupled to a 6th order Chebyshev anti-imaging filter with NOS droop correction. Since its using cooking-grade discrete transistors and darlingtons with no boutique components the BOM cost of this board is under $2. Schematic to follow.

I've pasted up the schematic from LTSpice - the DAC is the controlled CS at the bottom to the left, the output comes from the darlington emitter at the right. The voltage sources are LEDs in practice - two in series for the input transistor, one for the cascode bias. I ran a quick sim on this followed by the the filter stage it showed about -60dB 2nd and 3rd harmonic when fed with a high level squarewave from the pulse generator. The 3rd is most likely from the squarewave (around 12kHz) so that's a decent amount of rejection at 36kHz. Agreed 0.1% nothing special but then again probably inaudible in practice.

Just fired this up and apart from getting the initial DC biassing wrong (hence the two paralleled resistors in the CCS bottom left) it works first time. And the sound is satisfying to put it mildly....

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