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Background:

Chances are you've heard of the vintage 312 preamplifier card. And if you've actually used an original (circa 1970), properly configured, then you know what an awesome and unique coloration it imparts: bold, tough, compressed, and rich. This kind of tone can only come from such a primitively designed circuit: classic American transformers directly coupled to a single discrete op amp gain stage. The Eisen Audio DIY500 mkII universal template was designed to exploit this style of circuit: a single op amp gain stage with transformer I/O, but allowing for a great variety of different parts and flexibility in how they are connected. In fact, using our EAC service several customers had asked us to transfer the contents of a 312 card, or to build from modern parts something which sounds like a vintage 312. The transfers were always easy, but it's hard to find those old cards, and we'd had little success finding modern equivalent parts... until now!

Due to production variation and common unavailability, some third-parties have taken to building their own all-important 2520 discrete op amp. In our 5+ years of servicing unsupported equipment we have heard several of these substitute parts and found that none of them sounded like the real thing (circa 1970)... until now. We feel that the Scott Liebers SL-2520 red dot discrete op amp is, in context, virtually indistinguishable, so we're using it.

Then there are the transformers. The originial 312 card that we like so much used an inimitable 2622 1:8 ratio input transformer, which was later replaced with a rather different 1:10 ratio part. This 2622 was unobtainium until our friend, Ed Anderson, spent nearly two years creating a completely authentic reproduction! As for the 2503 output transformer, it would at first seem like your run-of-the-mill steel quadfilar device. However, we've found that the version supplied circa 1970 had a particular signature to it not found in contemporary off-the-shelf equivalents. Again, Ed to the rescue. His is the only recreation we know to carefully reproduce the proper composition and treatment of the vintage steel laminations. Add these to the list.

Lastly are the finer details of the support circuitry. This included sourcing capacitors of the same (inferior) values and composition as those employed 40 years ago, which is a challenge of both willpower and resourcefulness when vastly superior components dominate today's markets. Also, forcing ourselves to include the input filter capacitors, which compromise the specs in a few ways, leaving a noticeable tonal change; and bravely omitting input and output coupling capacitors the way original 312 cards did – these are not popular decisions for the modern designer, but we've made them, and the results are worth it!

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