As a happy builder of a few different BYOC kits, I was initially a little disappointed with the ParaEQ sound, but fixed a few things and now very happy. To me the sound was muddy and undefined, something I don't prefer in an EQ. Also, I noticed some odd DC offsets in the circuit and at the output that changed from time to time. To make a long story of investigation, trial and error, and resulting fixes short, here is a summary: (1) Funny DC offsets are were caused by using electrolytic coupling capacitors in bipolar circuits. The caps were rectifying some AC into DC offsets. Replaced all 10uf coupling electrolytics with non-polarized types, in this case Panasonic UES1E100MDM1TD. (2) Wooly, murky sound was caused by original opamps. Replaced quad opamps with 1664A JFET opamps on SOIC to DIP adapters. Replaced dual opamp with OPA2132. All should be JFET input types as needed for active filters and hi-z guitar input. The new opamps are faster and draw more power supply current. (3) could not get clean power with MAX1044 or the improved drop-in replacement LT1054 switched capacitor power inverters. Saw considerable power supply noise on audio output jack. So, (4) removed MAX1044 and replaced it with and external +- 15 power supply -- in this case an XP Power ECL30UD02-S, which is very clean, and shows no evidence of power supply noise on the audio signal output even at low levels. (5) Added some 10uf tantalum power supply bypass caps where the external power comes into the unit. (6) Added .1uf ceramic bypass caps to ground on the power pins at each opamp. (7) with the faster opamps noticed some high frequency ringing apparently due to PCB layout issues. So, added 10pf NPO ceramic cap across every 5.1k opamp feedback resistor. These are: R6, R19, R31, R13, R25, R39, and R47. (8) Also added 240 pf NPO ceramic (actually 2x 120 pf) from pin-3 of LEVEL pot (junction of C-17) to ground. (9) Observed HF behavior with scope and found no remaining parasitic ringing or frequency peaking above 100kHz. (10) Observed LF square wave response and found some LF droop, so replace C-1 input coupling cap with high quality .47uf film type, which substantially corrected droop. (11) then calibrated BOOST/CUT pots for best flat-est possible square wave response and labeled knobs at the zero boost/cut positions. (12) Set Q pots full CCW for maximum Q and used signal generator and scope to calibrate frequency pots (adjust pots for maximum peak at signal generator frequency). --- Ok! Now I am very, very happy with the ParaEQ and use it a lot, and for many purposes, not just guitar. Sounds great on guitar too, at least to my tastes for an EQ. Also, I would mention that there is an error on the ParaEQ schematic which shows C20 and C21 as 10uf. Actually, the caps supplied with the kit are 220uf.
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