I've been working on 312 clone that was built by someone else and the history is unknown (it was sold as an OSA pre-production 312 clone, but it was obviously built by an amateur, it has a vintage 2503 and a potted, unmarked 2520). I ordered EA2622 to replace the Jensen that was in there. In the process of adding the new trafo I noticed several components were missing. I replaced the 5.1k R with a 10k R in the Zobel network, then added the 1000pf caps (C6, C7 in original 312, C5,C6 in the Classic API VP312 circuit).
I've now hit on a point that doesn't seem to relate to the original 312 schemo or the VP312 schemo. On the VP312 there is a 200 R and a 330uf between opamp in (-) and the gain pot. In my project there is no R and a 470uf, but the cap is in the reverse orientation from the VP312 (ie, the + is on the gain pot side). It seems to me that this was mistake by the original builder, but with my limited understanding of theory, I;m just not sure what to make of it.
I intend to change the diodes, caps, add a few more R's, and volume pot, hoping to make it more of a VP312 with vintage output trafo. I also just completed the gar2520 build so I have two to try in this pair.
So I'm just messing around trying to do some good. As it is now the preamp sounds pretty smooth with limited testing... it sounds a tad crisp in the high end and seems to be missing a little low or low mid. It sounds very hi-fi though and WAY better, smoother than it did before I started. In comparison, the former design sounds clunky, though present and the new design sounds smooth, crisp, almost compressed and just a hair more distant. So far, so good I'd say.
I just joined here, but have been half-assing my way around circuits for about seven or eight years, though I'd be lost without people like you.
I'd also like to hear the story on these early OSA? pres. There seem to be a few different configurations floating around out there, but all based on the 312 circuit. They look like maybe they came as DIY partial kits and people gathered their own parts.
Thanks for the input!
I've now hit on a point that doesn't seem to relate to the original 312 schemo or the VP312 schemo. On the VP312 there is a 200 R and a 330uf between opamp in (-) and the gain pot. In my project there is no R and a 470uf, but the cap is in the reverse orientation from the VP312 (ie, the + is on the gain pot side). It seems to me that this was mistake by the original builder, but with my limited understanding of theory, I;m just not sure what to make of it.
I intend to change the diodes, caps, add a few more R's, and volume pot, hoping to make it more of a VP312 with vintage output trafo. I also just completed the gar2520 build so I have two to try in this pair.
So I'm just messing around trying to do some good. As it is now the preamp sounds pretty smooth with limited testing... it sounds a tad crisp in the high end and seems to be missing a little low or low mid. It sounds very hi-fi though and WAY better, smoother than it did before I started. In comparison, the former design sounds clunky, though present and the new design sounds smooth, crisp, almost compressed and just a hair more distant. So far, so good I'd say.
I just joined here, but have been half-assing my way around circuits for about seven or eight years, though I'd be lost without people like you.
I'd also like to hear the story on these early OSA? pres. There seem to be a few different configurations floating around out there, but all based on the 312 circuit. They look like maybe they came as DIY partial kits and people gathered their own parts.
Thanks for the input!
