Circuit Atlas · HUSH M4 · Rev 2027.4 draft
7 Calibration and alignment
A proposed procedure for a prototype. Nothing here has been carried out.
IN DEVELOPMENT · REV 2027.4 (DRAFT) · DESIGN TARGETS, NOT MEASUREMENTS
HUSH M4 is in development. This draft is published on zakkhutoretsky.com; it has no price and no ordering. Figures marked design target are goals for the finished product. Figures marked calculated come from the published formulas. No value on these pages was measured on a built unit. Interviews are cited as the design brief, not as an endorsement.
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7 Calibration and alignment
A proposed procedure for a prototype. Nothing here has been carried out. Use an isolation transformer on the bench.
| Step | What to do | Target |
|---|---|---|
| 1 Supply | Power up through a series lamp or variac. Measure ± rails unloaded and loaded; check ripple with a scope. | +18.3 / −18.3 V ± 0.4 V; ripple below the section 5 figure |
| 2 Offsets | With inputs shorted, measure DC at every stage output. | Within a few millivolts; trim or servo as needed |
| 3 VCA | Set the fader to minimum and maximum, adjust the VCA offset and symmetry trims per the chosen VCA’s datasheet, then check 0 dB at full travel. | 0 dB at full travel (±0.2 dB) |
| 4 RIAA | Inject through an inverse-RIAA network (or an instrument that produces one) and measure the response at 20, 50, 100, 500 Hz, 1, 2.1, 5, 10, 20 kHz, referenced to 1 kHz. | Within ±0.2 dB, tighter if the parts allow |
| 5 Trim and line | Check the trim range at both ends and the polarity through S2. | About −15 dB to +15 dB; polarity unchanged vs. phono |
| 6 Master isolator | Sweep the generator; confirm −6 dB at 300 Hz and 3 kHz on each band, and full kill at the pot minimum. | Crossovers within 5 % of design; kill beyond 60 dB |
| 6b Channel EQ | On each strip, sweep the generator with every knob centred, then one knob at a time to each end. Check the bell centres, the shelf corners at 180 Hz and 3 kHz, the ranges (+6 / kill, +10 / −27 dB), and the flat response with all knobs at 12:00. | Centres and corners within 5 % of design; ranges within 1 dB; kill beyond 60 dB; flat within 0.5 dB at 12:00 (targets for a prototype) |
| 7 Filter | Check the dead-zone relay bypass and the sweep ends of P5 and P6. | End frequencies within 15 % of the calculated range |
| 8 Output stage | Set the headphone output stage idle current at the prototype (Vbe-multiplier trim); check for thermal runaway. | Idle current chosen at the bench; stable at 50 °C case |
| 9 Balanced out | Check level balance between pins 2 and 3 and the common-mode rejection into a balanced load. | Legs within 0.1 dB of equal |
| 10 Soak | Run for four hours at the maximum level into a dummy load. | No drift beyond the offsets tolerance; heatsinks within limits |