Seventh Circle Audio

The output of voltage regulators U5 and U6 is set by the ratio of R31/R30 and R34/R33,
respectively. Substituting 1K8 resistors for R31 and R34 raises the supply voltage to +/-20V, giving a few dBs of additional headroom. The 1K8 resistors are optional and not
included with the kit.

Mine are currently 2k for 18V. Wouldn't I need 2.2k?

I went ahead and installed 2.2k to my 8 channels. Bumped them up to an average of 19.65 V. Gave me some more headroom before clipping. What was the reason for the 18V change? I see pics in the assembly "how to" with 20V pcb labels.

Tim, do you have any feedback on rhthminmind's observations?

I too was curious about seeing 20V on the PCB in the assembly instructions.

How much of a difference in headroom does this mod make? How does this compare with the headroom of some of your other modules?

I did this mod (using 2.2k resistors) about 3 years ago, and recently have had several of my T15 modules fail. The only symptom I had at first was a blinking power LED light upon startup, and intermittent problems with a few of the modules. Then, with one of the modules I experienced catastrophic failure (capacitors exploding, smoke, etc). The problem appears to be overheating due to the higher voltage. After Tim repaired that module, he recommended replacing the resistors in my other modules with the original values to lower the voltage to the original 18v spec.

I did have my SCA at the top of my rack unit, where ventilation was poor and may have contributed to overheating. But my (older) C84 modules are still going strong -- leading me to believe the problem is with the T15 mod.

I would proceed with extreme caution of you want to try this mod, make sure you have good ventilation and maybe use IC sockets to make it easier to replace blown ICs. As @rhythminmind points out, the assembly instructions are quite confusing, and pictures show an older rev board. If I could do it again, I would not have done the mod. There is still plenty of headroom without it, imo.