Bias servo in the AN-1651

[conorpeterson]

Hi all,

Please help me get some things straight on DC servos. I'm fairly new to audio electronics and my experience is mostly limited to the phono preamp described in AN-1651, which was an interesting first foray into this. I want to move onto a new project but I was hoping somebody could explain the design of the servo in that circuit before I get too far ahead of myself.

The schematic in that preamp (see attachment if you're unfamiliar) called for an LME49710 as the servo amp. Why did they use a relatively expensive op-amp for such a "dumb" function? Since the signal is leveled out to near-DC by that .2hz low-pass filter, couldn't I basically substitute a paperweight op-amp to do the job? I'm eyeing a tube of LM358s.

Related question: it would seem obvious that one wouldn't want the servo to have a noisy output, since any noise sent back would soil the "clean" output. So maybe that's one reason to avoid cheap op-amps in that scenario. But why not just run the servo output through a high-pass filter, one that mirrors the low-pass filter on the input?

In the schematic, that looks like what they're doing with C14+R16 (though I'm thrown off by R17, which I suppose might be there to ensure U3 "sees" the right output impedance).

So will any amp do, or should I be choosy about the devices I saddle with the burden of nulling out DC? Any comments most appreciated.

[sawreyrw]

U4 and U5 provide filtered supply voltages to the other amps. (Don't ask me why they need to do this, but I suppose it's for low noise.) U3 is the servo; a correction voltage is applied through R17 to the plus input of U2. This action keeps the average voltage at the output of U2 zero volts.