Preamp and Headamp 2 in 1?

[dirkwright]

I think most CD players will put out 2volts max. At the least, you'll have higher distortion with higher output from the opamp. Historically, most designs put the gain stage after the volume pot and fancy designs put a buffer ahead of the volume pot also, as well as a buffer after the gain stage. But, it's a free world, go ahead and experiment.

[GLooP]

Originally Posted by dirkwright

I think most CD players will put out 2volts max. At the least, you'll have higher distortion with higher output from the opamp. Historically, most designs put the gain stage after the volume pot and fancy designs put a buffer ahead of the volume pot also, as well as a buffer after the gain stage. But, it's a free world, go ahead and experiment.

Actually, I have only my USB DAC right now and the Dac IC is not buffered. So this topology will blend greatly since most DACs are buffered then there is the volume pot and the amp. What I do is adding a gain stage which will buffer the DAC and then implement a amplifier, whill will be then responsible to drive cables' capacitance safetly. I have not reinvented the wheel, even O2 is constituted this way - gain-pot-I/V.

BTW: I really appreciate your input, dirkwright, thank you!!!

[sgrossklass]

Better don't omit R4 as suggested, it limits pot scratching. Do include a coupling cap in between the pot and R4, else you'll have input bias current flowing through it, which may turn it into a scratch fest.

If in doubt, I'd rather swap the values of R6 and R12 (or make both 100R), power transistors can have a good bit of input capacitance. Any reason you're using a SE output stage with CS instead of a push-pull affair?

I'd distribute gain in between the two stages. Having gain only in the first one gives phenominally low noise but is a little fussy to work with in the real world. Remember that the input may have to accept up to 2.5 Vrms + 3 dB to account for intersample-overs (about 3.25 Vrms), so I wouldn't give it a lot more gain than 6 dB when running on +/- 12V.

As far as using a super low Vnoise OP in the input is concerned, it's probably a waste. Ever bothered calculating source noise? Besides, this is a line-level stage that has to handle levels in the Vrms range, so it will offer plenty of instantaneous dynamic range in any case. This is not the case for the second stage, where even the output may only be in the mV range.

[dirkwright]

Hmmm, never heard of "pot scratching" but I guess it's possible. It would depend on the type of pot used I guess.

[ethanolson]

I understood it to be the cap that overcame the scratchy pot.