Book

Circuit Atlas · HUSH M4 · Rev 2027.4 draft

B Schematics

Original drawings. Each stage also has its own schematics page.

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.

See this control Open the mixer

B Schematics

Original drawings. Each is followed by its value table in section 5.

RIAA moving-magnet phono preamp, one channelThree op-amp stages: flat input stage U1, 3180/318 microsecond shelf U2, passive 75 microsecond low-pass and flat gain stage U3.S1 RIAA PHONO PREAMP (MM) — one of four identical channelsMM INRin47kCLsel. pF−+U1x11 +20.9 dBRf12.21kRg1220+Cg1470 µF−+U23180/318 µs shelfRf220kCf2160 nFRg22.21kR35.76kC313 nF75 µs−+U3+17.1 dBRf36.19kRg31k+Co10 µFRb100kRo100TO INPUT SELECTIdeal-op-amp calc: RIAA error max 0.017 dB, 20 Hz to 20 kHz (E96 / E24 nominal values). 1 kHz gain 38.1 dB. U1: low-noise op-amp; U2, U3: dual low-noise op-amp. All op-amps on ±18 V.
S1 RIAA phono preamp (moving magnet) Swipe sideways to pan on a phone.
Line input and trim, one channelInput coupling, RF filter, an inverting trim stage with a rheostat for plus and minus 15 dB, and a unity inverter that restores polarity.S2 LINE INPUT AND TRIM — one of four identical channelsLINE IN+Ci2.2 µFRb47kRrf1kCrf220 pFRin10k−+U4G = −Rf/RinRt1.8kP1 50k trimRin210k−+U5unity inverterRf210kTO INPUT SELECTTrim range -14.9 dB to +14.3 dB (formula G = −Rf/Rin, Rf = 1.8k to 51.8k, Rin = 10k). RF filter corner 723,432 Hz(1/2πRC), input high-pass 1.5 Hz.
S2 Line input and trim Swipe sideways to pan on a phone.
Sallen-Key building block for the isolatorA second-order Butterworth low-pass and high-pass Sallen-Key section with equal resistors and equal capacitors; two in cascade make one Linkwitz-Riley 24 dB per octave filter.S3a ISOLATOR BUILDING BLOCK — 2nd-order Butterworth Sallen-Key (two in cascade = LR4)Low-pass sectionHigh-pass sectionINR153.6k*R253.6k*C210 nFC110 nF−+OAK = 1.59Rf5.9kRg10kOUTINC110 nFC210 nFR253.6k*R153.6k*−+OAK = 1.59Rf5.9kRg10kOUT*R = 53.6k for the 300 Hz crossover (f0 = 296.9 Hz) and 5.36k for 3 kHz (f0 = 2969 Hz); C = 10 nF; K = 1 + Rf/Rg =1.590, Q = 1/(3 − K) = 0.709 (Butterworth: 0.707).
S3a Isolator building block Swipe sideways to pan on a phone.
Three-band isolator topologyInput splits into low, mid and high bands with Linkwitz-Riley 24 dB per octave crossovers at 300 hertz and 3 kilohertz. Each band has a times-two amplifier and a 10 kilohm linear pot, then all three sum.S3b THREE-BAND ISOLATOR — master section (the channel EQ reuses it at 180 Hz, see S13)INLP4 300 Hz2 × SK ButterworthHP4 300 Hz2 × SKLP4 3 kHz2 × SKHP4 3 kHz2 × SK×2band ampP210k linLOW×2band ampP310k linMID×2band ampP410k linHIGHSUMinverting 10kLinkwitz-Riley 24 dB/oct = two cascaded 2nd-order Butterworth (Q 0.707). Pot: min = kill, centre = unity (the x2 bandamp), max = +6 dB. Allpass delay trim between bands is a prototype task.
S3b Three-band isolator (master section) Swipe sideways to pan on a phone.
Per-channel one-knob split filterA Butterworth Sallen-Key low-pass tuned by a dual-gang log pot and a Butterworth Sallen-Key high-pass tuned by a second dual-gang pot. A window comparator on the knob wiper drives a relay that takes both sections out of the signal path across a dead zone at the centre.S4 CHANNEL FILTER — one-knob split filter, 12 dB per octave, relay true bypassLow-pass, CCW of centre: 18.108k down to 67 HzHigh-pass, CW of centre: 63 up to 5.961k HzINR11.87k + P5aR21.87k + P5bC24.7 nFC14.7 nF−+OAK=1.59Rf5.9kRg10kLP OUTINC110 nFC210 nFR22.67k + P6bR12.67k + P6a−+OAK=1.59Rf5.9kRg10kHP OUTCentre dead zone: true bypassWindow comparatorwiper in dead zone?Relay K1DPDT, audio contactsdriveK1 energised in the dead zone: IN goes straight to OUT.Neither Sallen-Key section is in the signal path.At the dead-zone edge both sections are at their open ends:LP 18.108k Hz, HP 63 Hz.Relay click on the real circuit: a prototype item.One knob, one centre dead zone. CCW of the dead zone: P5 (500k log dual-gang) sweeps the low-pass. CW: P6 (250k logdual-gang) sweeps the high-pass. fc = 1/(2πRC) and Q = 0.71 (Butterworth, low resonance, no peak).The knob also drives the comparator through a third wiper track. The browser model on /mixer uses its own frequencyrange and a smoothed software bypass; it is not this circuit.
S4 Channel filter Swipe sideways to pan on a phone.
Channel fader as a control-voltage VCAA 10 kilohm linear fader pot between a 5 volt reference and ground sets a control voltage. A scaler brings it to 0.6 volts, which drives the control port of a generic exponential VCA. Audio never passes through the fader. The crossfader bus is not fitted.S5 CHANNEL FADER — control-voltage VCA (no crossfader fitted)+5 V REFP710k lin 60 mm−+U6bufferRt47kRbt6.49kEc+ 0 to 0.61 Vcontrol portAUDIO INRa10kVCAgeneric exponential−+U7I-to-VRf10kTO SUM BUS AND CUE TAPVCA control constant 6.1 mV/dB (THAT 2180-series datasheet): 100 dB range needs 0.61 V; scaler 47k / 6.49k gives 0.607V from the 5 V reference. Rin = Rf = 10k sets 0 dB at full travel. Pin-out and bias come from the chosen VCAdatasheet.
S5 Channel fader (control-voltage VCA) Swipe sideways to pan on a phone.
FX send and returnA send level pot feeds a follower and a 100 ohm build-out resistor to the send jack. The return jack has coupling, a bias resistor, a follower and a return level pot into the master mix, with a switch that hard-bypasses the loop.S6 FX SEND / RETURN — switchable, hard-bypassed when offSendFROM MASTER ISOP810k log−+U8follower+Cs10 µFRs100SEND JACKReturnRETURN JACK+Cr2.2 µFRr47k−+U9followerP910k logSW2 FX in / bypassRm10kTO MASTER MIX BUSSend and return are line-level, unbalanced, nominal 0 dBV. Followers: dual low-noise op-amp. The loop is out ofcircuit when SW2 is off (no effect in the signal path).
S6 FX send / return Swipe sideways to pan on a phone.
Master, booth and cueFour channel feeds sum in an inverting amplifier. The master isolator, FX loop and master fader follow, then balanced drivers for master and booth XLR outputs. A separate cue summer takes the channel cue taps.S7 MASTER / BOOTH / CUEMaster sum and chainCH1R110kCH2R210kCH3R310kCH4R410k−+U10inverting sumRf10kMaster isoS3bFX loopS6 switchableP1010k log masterTO DRIVERSBalanced driver (master XLR and booth XLR; booth has its own level pot)IN−+U11follower (+)Ro147.5XLR PIN 2 (HOT)Rn10k−+U12unity inverterRf10kRo247.5XLR PIN 3 (COLD)Rout = 2 × 47.5 Ω = 95 Ω (impedance-balanced)XLR pin 1: chassis via 10 Ω || 10 nFCue: channel pre-fader taps into an inverting sum(10k each), then cue / master blend pot 10k, then S8All pots 10k, all sums unity per input. The sum node is a virtual earth, so crosstalk through the bus is low byconstruction; verify on a prototype.
S7 Master, booth and cue Swipe sideways to pan on a phone.
Headphone amplifierA low-noise op-amp drives a complementary emitter-follower output stage inside the feedback loop, followed by a 10 ohm series resistor to the headphone jacks.S8 HEADPHONE AMPLIFIERBLEND+Ci4.7 µFRb47kP1210k log level−+U13NE5532-classQ1 NPNBD139-classQ2 PNPBD140-class+18 V−18 VRe10.47Re20.47Ro10PHONES 6.35 / 3.5 mmCalculated drive (P = V²/R):250 mW into 32 Ω: 2.83 Vrms (4.00 Vpk), 88.4 mA rms250 mW into 64 Ω: 4.00 Vrms (5.66 Vpk), 62.5 mA rms100 mW into 600 Ω: 7.75 Vrms (10.95 Vpk), 12.9 mA rmsFeedback is taken before Ro. Output-stage idle current is set at theprototype (Vbe-multiplier bias), then stabilised by the emitter resistors.
S8 Headphone amplifier Swipe sideways to pan on a phone.
Power supplyA 2 by 22 volt AC toroid transformer feeds a bridge rectifier and 4700 microfarad reservoirs. An LM317 regulates the positive rail and an LM337 the negative rail to plus and minus 18 volts. The mains side has a fuse, switch and protective earth.S9 POWER SUPPLY — ±18 V linearMAINS: HAZARDOUS VOLTAGE. Build only to a qualified engineer's design review and the local safety standard.LF1T 630 mASW3NPEchassis; bond T1 core and enclosureT1 toroid2 × 22 Vac 50 VAdouble insulated0 VBridgeD1–D4 3 A+C14700 µFLM317+18.3 V+Co110 µF+18 VR1240R23.24k+C24700 µFLM337−18.3 V+Co210 µF−18 VR1240R23.24kVout = 1.25 V × (1 + R2/R1) + Iadj·R2 = 18.29 V (LM317 datasheet, Rev Z); the LM337 mirrors it with reversed diodesand capacitors.Ripple = I/(f·C) = 1.06 V pp at 0.5 A, 100 Hz, 4700 µF. Input 29.7 V DC nominal, 28.6 V at the trough, 25.5 V at −10%mains: headroom over Vout 10.4 V (7.3 V low line); each regulator dissipates about 5.7 W: heatsink.Also: 5.0 V precision shunt reference for the VCA control scaler; every op-amp pair decoupled with 100 nF + 10 µF perrail; star ground at the supply.
S9 Power supply Swipe sideways to pan on a phone.
USB-C 32-bit float interface (optional tap)Four channel taps and the master pair pass through buffers into two four-channel ADCs, which feed a USB Audio Class 2.0 device controller on a USB-C connector. A separate DAC set returns host audio to rear line outputs. The tap is off the analogue signal path.S10 USB-C INTERFACE — optional record and playback tap, design targetCH1 tapBufferunity, 47ΩCH2 tapBufferunity, 47ΩCH3 tapBufferunity, 47ΩCH4 tapBufferunity, 47ΩMASTER L tapBufferunity, 47ΩMASTER R tapBufferunity, 47ΩADC AES9842PRO, CH1–4ADC BES9842PROmaster L / R + 2 spareTDMXU316USB Audio Class 2.0, asyncIsolatordigital, design targetUSB-Cto computerDAC sethost playbackRear line outsplayback onlyWhat this is, and is notThe analogue strips, isolators, filters and master run as in S1–S9 with the board removed or switched off: the interface only listens.ADC: 4-channel PCM, dynamic range +122 dB in 4-channel mode (+128 dB mono), THD+N -116 dB (datasheet figures, not measured here).122 dB ≈ 20.3 effective bits. 32-bit float is a file and transport format: its 24-bit significand covers that range without rounding.The converters deliver integer words; the host converts them to 32-bit float when it writes the file.Raw stream at 96 kHz, 8 channels, 32-bit words: 24.6 Mbit/s (49.2 at 192 kHz); USB 2.0 high speed carries 480 Mbit/s.Tap point per channel: before the fader (default) or after it, by a jumper. Own audio clock; USB is asynchronous.The real range is set by the analogue noise floor and the ADC, not by the float format.Design target. Parts named as reference choices from their datasheets; nothing here has been built or measured.
S10 USB-C interface (optional tap) Swipe sideways to pan on a phone.
Texture optionsTwo design-target ways to add texture after the channel filter: an analogue four-pole ladder or OTA filter with drive, and a digital texture card. A relay takes the texture out of the path when it is off.S11 TEXTURE — analogue ladder / OTA option and DSP card, design targetsS4 OUTRelay K2texture in / outA Analogue ladder / OTAdrive, 4-pole, resonance, cutoff CVB DSP texture cardADC, DSP, DACMode selectone of A / Bto faderWhen texture is off K2 routes S4 OUT directly to the fader: the path is the analogue strip of S1–S9 and no converteris in it.A. Four-pole ladder: Stilson and Smith analyse the Moog voltage-controlled filter (24 dB per octave, resonance byfeedback); Huovilainen adds the non-linear model.B. DSP card: modes Clean SVF, Ladder (zero-delay-feedback form, after Zavalishin), Comb and Bitcrush, 4× oversampled,with drive before the filter,resonance, and LFO or envelope-follower modulation of the cutoff. The browser model on /mixer runs this signal flow inan AudioWorklet.Controls: TEXTURE knob (amount; zero is off), MODE button with LED, and per-channel assign in the full design. Formantand Noise / air modes are not part of this revision.Design target. Nothing here has been built or measured.
S11 Texture (optional) Swipe sideways to pan on a phone.
Spatial FX send and returnFour post-fader sends sum in an inverting stage to the FX send bus. Three engine options are drawn: a BBD with clock for ensemble, a PT2399 digital delay for echo, and a DSP card for spring and plate. A return level pot and a hard-bypass switch feed the master mix bus.S12 SPATIAL FX SEND / RETURN — optional; DESIGN TARGET, not built, not measuredSend bus: four post-fader tapsCH1 post-faderRs110kCH2 post-faderRs210kCH3 post-faderRs310kCH4 post-faderRs410kINV SUMU12, -Rf/Rin = -1SEND LEVELpot P10 10k logFX engine: one of three optionsA BBD ENSEMBLE / CHORUSinput LPF - BBD - clock - output LPFMN3207, 1024 stages: delay 2.56 to 51.2 ms (R22)B DIGITAL DELAY (ECHO)input LPF - PT2399 - output LPF, feedback potPT2399: 44 kbit RAM, delay set by a resistor (R21)No delay range is claimed here.C DSP CARD (SPRING, PLATE)ADC - DSP - DAC, or a dedicated reverb ICNo part chosen. Plate topology: Dattorro (R20)ReturnFROM ENGINERETURN LEVELpot P11 10k logfully down = no returnSW3 FX in / bypassRr10kTO MASTER MIX BUS (S7)How the browser model maps to this:Per-channel send, post-fader and post-pan, summed to a stereo bus; the return joins the mix bus before the limiter.Return at 0 adds exact zeros in the model. In hardware SW3 removes the engine from the circuit.The model runs four modes in floating point: tape echo, spring, plate (Dattorro) and BBD ensemble.Hardware options A and B cover only chorus / ensemble and echo. Spring and plate need option C.The model is not the analogue design.BBD parts run from +5 V (R22: +4 to +10 V) and the mixer from +/-18 V: a regulator and level interface are needed.Typical datasheet figures, each maker's own conditions: MN3207 THD 0.4 %, S/N 73 dB; PT2399 THD 0.5 %, SNR > 90 dB.Complete circuits are not drawn.
S12 Spatial FX send / return (optional) Swipe sideways to pan on a phone.
Channel EQ topology, design targetPer channel: two peaking stages, a low-mid bell at 350 hertz and a high-mid bell at 1.1 kilohertz, then a three-way Linkwitz-Riley split at 180 hertz and 3 kilohertz with a kill-to-plus-six-decibel pot on the low and high bands, a fixed mid leg, and an inverting sum.S13 CHANNEL EQ (4-band) — per channel; DESIGN TARGET, not built, not measuredINLO MID BELL 350 HzMFB band-pass + inv. op-amp, P2HI MID BELL 1.1 kHzMFB band-pass + inv. op-amp, P3+10 dB / -27 dB, centre = flat. Two inverting stages in series keep absolute polarity.LP4 180 Hz2 × SK, R = 88.7kHP4 180 Hz2 × SKLP4 3 kHz2 × SK, R = 5.36kHP4 3 kHz2 × SK×2band ampP410k linLO×2band amp÷2fixed×2band ampP510k linHISUMinverting 10kLO and HI: pot minimum = kill, centre = unity, maximum = +6 dB. The model on /mixer uses 2nd-order shelves and bellsthat match these corner frequencies and plateau levels;the LR4 split has steeper transitions near 180 Hz and 3 kHz, so the two differ there. Bell component values are notcalculated in this draft.R = 88.7k for 180 Hz (f0 = 179.4 Hz) and 5.36k for 3 kHz (f0 = 2969 Hz); C = 10 nF.
S13 Channel EQ (four-band, per channel) Swipe sideways to pan on a phone.

The mixerSchematicsManual (PDF, 685 KB)Glossary

For music supervisors: sync licensing for film, TV and ads →