[quote author="EZ81"]Thanks for all answers, looks like another opamp with adjustable gain (a nice extra) will do the gain. there will also be a offset adjust trimmer.
[quote author="bcarso"] This circuit is working hard with an LF347 section driving the 510 ohm attack tau resistor, although maybe the voltage required at the output is fairly small hence making it less of an issue.[/quote]
Do you have any suggestions what to look for? A very common quad opamp that can drive the output to 5v max? The load on C2 is very high impedance (ADC of a uC) All i know about opamps is that 5532 are ok and 741 are &%$§&%, so i just used what the SSL metering used...
[quote author="moamps"] T=(R2+R4)*C2[/quote]
Milan, in simulation (as i don't have a working scope) R6 and R3 also seem to influence the time constant.
[quote author="moamps"]R2 is 200k in the NS datasheet. (C2 is 3u3).[/quote]
Thanks, i mixed this up in the schematic.[/quote]
The offset trimming will typically only be an issue if you try to realize the extra gain as PRR, Milan, and I have been proposing, and should be applied to the first and possibly second existing stages.
The two input R's to the second stage shouldn't have much effect on tau because the second op amp should always have the loop closed with one or the other component of its feedback network---that is, the input is a "virtual ground". In your simulator, see if the voltage at the inverting input is moving much---it shouldn't be if all is well. So, after a burst of audio and subsequent charge up of C2, make the audio go away---the op amp should make its output go low to roughly one diode drop below ground, in order to keep the inverting input at about ground potential.
As far as higher output current garden-variety Bifet amps, I don't know. Probably it is not really a big deal, because you will just get a bit of current limiting at worst which will effectively add to the attack time.
It would be good to know what the input Z of your A/D actually is---some might not be that terribly high-Z. Maybe yours is.