Follow up: my FR-12 is
incredibly quiet after this mod, and sounds just as good as it did before...
sans hiss. Really, really happy with how this turned out. It's remarkable how a simple opamp change can cut noise by 5dBA, which is going from
"that's annoying" all the way to
"goddamn, is this thing on?".
I also
think switching input/output opamps has slightly improved tonality as well, but don't quote me on it; this can be just my brain playing tricks on me. If there's an improvement, it's marginal at best, but i could swear the amp feels and reacts slightly better than it did before.
But yeah, very very very silent. Last night i played some video on my computer, which was hooked to Helix via USB, and it blasted audio through the amp at an ungodly volume: the FR-12 sits 60cm (2ft) away from me, and it's so quiet now that i accidentally left it on for an hour without even noticing
I'd say it's now about as quiet as the Powercab 112 it recently replaced.
Looking forward for reports from those modding with through-hole ICs (
OPA4227 et al). I expect similar results, without the hassle of dealing with surface mount parts - that alone will be probably worth the price difference for most.