It has an additional auxiliary TRS input per module, debalanced by a THAT1240 and bypassing the Jensen mic input transformer. It's a modified TAC 511 Advance from Total Audio Control (member Sahib, whose advice was invaluable). The rack itself is also worth mentioning. Definitely a job for a clean magnifying glass, sharp tweezers, and a steady hand! That said, once I got the rhythm down I actually found it a lot quicker than my standard through-hole resistors and a Lorlin, and it's the only way I found to get close to replacing a 9mm pot with a rotary switch! Now the low and mid bands boost in 1db steps, very tightly matched (mid band cuts are much shallower, due to the way the cut / boost works in the Helios, but that works for me as I tend to cut in software for the greater control). I matched all the EQ capacitors and inductors as tightly as possible, and then replaced the EQ gain pots with Grayhill series 56 rotary switches (solder tag version), to which I soldered resistor ladders using 0.1% thin film 0603 SMD resistors. Two of the modules (9 + 10, with white caps on the +48V switch rather than red) are matched for stereo work. They're the standard Helios "0011 upper module" design (2128 amp as mic pre > passive EQ > 2128 for makeup gain), though with Jensen input transformers and a discrete output buffer, and of course they sound fabulous. The modules are based on board layouts by Matt from Iron Age Audioworks (member Dogears), and I'm indebted to him for his help with this project and his extraordinary generosity in sharing his work. This is quick build report on a special project I completed yesterday, which has been many months in the planning and execution: 10 channels of Helios-style preamp + EQ in a custom 500 series rack!
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