Part 2 of a series showing repairs and modifications to an inexpensive, used preamp, the Gemini PA-7000. This preamp retailed for about $100 USD when new. The makers apparently kept the price down by 1) employing op-amp-based circuits (with no discrete transistors) and 2) using "jellybean" op-amps (4558's) in those circuits. The 4558 isn't bad, and of course it's cheap, but there are better options that are still relatively cheap. The 5532 would be a typical choice for a replacement, but the 2068 has measurably better specs and still has a low price tag, less than $1 USD. For most of the sub-circuits employing 4558's, noise is already undetectable, but the phono preamp sub-circuit provides a lot of amplification, produces measurable noise at the output, and is therefore the obvious place to upgrade the op-amp.
TLDR: I measured a drop in full-volume hiss of 2 dB after replacing the 4558 in the phono preamp with a 2068.