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In development · Rev 2027.4 draft

HUSH M4 schematics

A block diagram and a circuit for every stage of the four-channel analogue mixer, with component values worked out from standard published formulas. Each value is shown on the drawing, repeated in a table below it, and recomputed by a script in the source.

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. Every figure marked design target is a goal for the finished product. Every figure marked calculated comes from the formulas shown, with nominal component values and ideal op-amps. No value on this page was measured on a built unit; measured results will be published separately, with their method. The drawings are original; no manufacturer schematic or branding was copied. Interviews are cited as the design brief, not as an endorsement.

BD Block diagram

HUSH M4 block diagramSignal flow from four identical input channels, through the master section and FX loop, to the outputs and cue/headphone path, powered by a linear supply.BLOCK DIAGRAM — signal flowCHANNEL STRIP × 4 (identical)PHONOS1 RIAALINES2 trimSELECTPH / LNEQ 4-bandS13 + S3aFILTERS4 HPF / LPFFADERS5 VCACUE TAP / SUMpre-fader cue tapchannel outputs to the master busMASTER SUMS7MASTER ISOS3bFX LOOPS6 switchableMASTER FADERS7BALANCED OUTS7MASTER XLRBOOTH XLRREC RCAcue taps from each strip (pre-fader) feed the cue bus:CUE BUS + BLENDS7HEADPHONE AMPS8PHONES 6.35 / 3.5 mmPOWER SUPPLY S9±18 V regulated (LM317 / LM337), 5 V reference, star ground, double-insulated mains transformerIn development. Analogue path throughout: no converters in the path. The optional USB-C interface (S10) taps it; noscreens.
Block diagram: four identical channel strips, master section, FX loop, outputs, cue and headphones, power supply Swipe sideways to pan on a phone.

S1 RIAA phono preamp (moving magnet)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
StandardPlayback curve with time constants 3180, 318 and 75 µsR1, R2
U1Non-inverting, G = 1 + Rf1/Rg1 = 1 + 2.21k/220 = 11 (+20.9 dB); Cg1 = 470 µF puts the stage high-pass corner at 1/(2π·220·470 µF) = 1.5 Hz
U2Shelf: pole at Rf2·Cf2 = 20k × 160 nF = 3.2 ms (3180 µs), zero at Rf2·Cf2/(1 + Rf2/Rg2) = 318 µs. Rf2/Rg2 = 20k/2.21k ≈ 9
R3·C3Passive low-pass, 5.76k × 13 nF = 74.9 µs (75 µs)
U3Flat gain 1 + 6.19k/1k = 7.19 (+17.1 dB): total at 1 kHz = 38.1 dB, a design target of 38 dB for a 5 mV moving-magnet cartridge
NoiseU1 should be the lowest-noise op-amp in the design (NE5534-class, 3.5 nV/√Hz typ)R7, R6; the 220 Ω gain resistor contributes about 1.9 nV/√Hz (√(4kTR))
ResultCalculated RIAA error with these E96/E24 values and ideal op-amps: ±0.017 dB, 20 Hz to 20 kHz. A real build adds op-amp bandwidth error, part tolerance and cartridge loading; the 20 Hz IEC rumble corner is not included

S2 Line input and trim

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
TrimInverting stage, G = −Rf/Rin, Rin = 10k, Rf = 1.8k (fixed) + P1 50k: -14.9 dB to +14.3 dB
PolarityU5 is a unity inverter so the line path has the same absolute polarity as the phono path
FiltersRF low-pass 1/(2π·1k·220 pF) = 723 kHz; input coupling 2.2 µF into 47k = 1.5 Hz high-pass

S3a Isolator building block

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
SectionSecond-order Butterworth Sallen-Key, equal resistors and equal capacitors, f0 = 1/(2πRC), K = 1 + Rf/Rg = 3 − 1/QR3
LR4Two cascaded sections give a Linkwitz-Riley 24 dB/oct filter, each stage Q = 0.71R4
ValuesC = 10 nF. 300 Hz: R = 53.6k (f0 = 296.9 Hz). 3 kHz: R = 5.36k (f0 = 2969 Hz). Rg = 10k, Rf = 5.9k gives K = 1.59, Q = 0.709

S3b Three-band isolator (master section)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
SplitLP4 at 300 Hz, HP4 at 300 Hz then LP4 at 3 kHz, HP4 at 3 kHzR4
Band gain×2 band amp and a 10k linear pot: minimum = full kill, centre = unity (the pot divides by 2), maximum = +6 dB
SumInverting sum, 10k per band. This is the master isolator. The channel EQ (S13) reuses the same split with the low crossover at 180 Hz and a fixed middle leg
Open taskAllpass delay trim between the three paths so the bands recombine flat in phase; to be done on a prototype

S4 Channel filter

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
SectionsButterworth Sallen-Key (12 dB/oct), one low-pass and one high-pass, Q = 0.71R3
Low-passC = 4.7 nF, R = 1.87k + P5 500k dual-gang log: 18,108 Hz down to 67 Hz
High-passC = 10 nF, R = 2.67k + P6 250k dual-gang log: 63 Hz up to 5,961 Hz
ControlOne knob: low-pass counter-clockwise, high-pass clockwise, with a centre dead zone
True bypassA window comparator on a third wiper track drives relay K1 across the dead zone: IN goes straight to OUT and neither section is in the path. At the dead-zone edge both sections sit at their open ends (LP 18,108 Hz, HP 63 Hz)
ResonanceButterworth, Q = 0.71: low resonance, no peakR3
ClickThe relay switching click and the step at the dead-zone edge on the real circuit are prototype measurement items; nothing is claimed here

S5 Channel fader (control-voltage VCA)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
LawVCA control constant 6.1 mV/dBR8: 100 dB range needs 0.61 V
Control pathP7 10k linear fader on a 5 V reference → buffer → 47k / 6.49k divider → 0.607 V maximum
Audio pathRin = Rf = 10k sets 0 dB at full travel; the audio never passes through the fader
CrossfaderNot fitted, as stated on the /mixer page (left out on purpose); a second control bus is a footprint option only

S6 FX send / return

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
Send10k pot, follower, 10 µF and 100 Ω to the jack; nominal 0 dBV, unbalanced
Return2.2 µF coupling, 47k bias, follower, 10k return pot, SW2 into the master mix
BypassSW2 removes the loop from the circuit entirely: no effect sits in the signal path when it is off

S7 Master, booth and cue

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
SumInverting summer: gain per input = −Rf/Rin = −10k/10k = −1
OutputImpedance-balanced driver: follower plus unity inverter, 47.5 Ω in each leg, Rout = 2 × 47.5 = 95 Ω
Level+28 dBu = 19.46 Vrms balanced, 13.76 V peak per leg. On ±18 V rails with 16.5 V usable swing per leg the balanced limit is about +29.6 dBu (1.6 dB above the +28 dBu design target). On ±15 V rails (13.5 V swing) it would be only +27.8 dBu: this is why the design uses ±18 V
CueChannel pre-fader taps into an inverting sum, then a cue / master blend pot into the headphone stage

S8 Headphone amplifier

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
DriveP = V²/R. 250 mW into 32 Ω needs 2.83 Vrms (4.00 V pk, 88 mA rms); 250 mW into 64 Ω: 4.00 Vrms; 100 mW into 600 Ω: 7.75 Vrms (10.95 V pk)
TopologyOp-amp with a complementary emitter-follower stage inside the loop, 0.47 Ω emitter resistors, 10 Ω series output resistorR12
Damping32 Ω / 10 Ω = 3.2: a design choice for short-circuit safety, not a high-damping design

S9 Power supply

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 (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
Rails±18 V, LM317 / LM337: Vout = 1.25 × (1 + R2/R1) = 1.25 × (1 + 3240/240) = 18.29 VR5
Rectifier2 × 22 Vac, 50 VA toroid, bridge, 29.7 V DC nominal; 4700 µF reservoirs, ripple = I/(f·C) = 1.06 V pp at 0.5 A, 100 Hz
Headroom10.4 V above the regulated output at the ripple trough, 7.3 V with mains −10% (LM317 asks for 3 V)R5
HeatEach regulator dissipates about 5.7 W at 0.5 A: needs a heatsink
BudgetEstimated 124 op-amp sections (62 dual packages) × about 8 mA = 0.53 A per rail against the 0.5 A design current. An estimate, not a measurement

S10 USB-C interface (optional tap)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
PurposeRecord the four channels and the master over one USB-C cable, and play host audio back to rear line outputs. A tap: it listens to the analogue path and is not part of it. The mixer works with the board removed or switched off
ConvertersTwo ES9842PRO 4-channel ADCsR14: eight record channels (CH1–4, master L and R, two spare). Datasheet figures: dynamic range +122 dB in 4-channel mode (+128 dB mono), THD+N -116 dB, PCM / TDM words up to 32 bit. The ES9843PRO is an alternativeR15
USB deviceXMOS XU316 running USB Audio Class 2.0, asynchronous, as in the XK-AUDIO-316-MC reference (8 in / 8 out, up to 384 kHz, 32-bit)R16. The playback DAC part is not chosen in this draft
32-bit floatA file and transport format. The converters output integer words; the host converts to 32-bit float when it writes the file. Float has a 24-bit significand (about 144 dB), which holds the 122 dB converter range (about 20.3 effective bits) without rounding. The real dynamic range is whatever the datasheet and the analogue noise floor give; float does not add range to the analogue path
Data rateEight channels of 32-bit words: 24.6 Mbit/s at 96 kHz, 49.2 Mbit/s at 192 kHz (calculated); USB 2.0 high speed carries 480 Mbit/s
GroundingA digital isolator between the USB side and the audio ground (design target). Converters run from the board’s own audio clock
Not claimedNo measured figure for the assembled interface; the datasheet figures are the converter alone

S11 Texture (optional)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
PlacementAfter S4, before the fader, switched by relay K2. Off is the analogue strip with no converter in it
Option A, analogueFour-pole ladder or OTA filter with a drive stage in front, resonance by feedback, 24 dB per octave. The Moog ladder is analysed by Stilson and SmithR17 and modelled non-linearly by HuovilainenR18. Component values are not set in this draft
Option B, DSP cardADC, DSP and DAC per strip or shared and assignable. Modes: Clean SVF, Ladder (zero-delay-feedback form, after ZavalishinR19), Comb, Bitcrush; 4× oversampled (2× in the efficient mode); drive before the filter; resonance; LFO or envelope-follower modulation of the cutoff. Latency in the browser model: 51 samples
ControlsTEXTURE knob (amount; zero is off), MODE button with LED. The same signal flow runs in the browser model on /mixer
Not part of this revisionFormant and Noise / air modes

S12 Spatial FX send / return (optional)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
StatusDesign target. Nothing here is built or measured. The browser model on /mixer runs the same four modes in floating point so the idea can be heard; it is not the analogue design
TopologyFour post-fader taps through 10k resistors into an inverting sum (gain −Rf/Rin = −1), a send level pot, the FX engine, a return level pot and a hard-bypass switch SW3 into the master sum of S7. With SW3 out the engine is out of circuit, as for S6
Option A, BBD ensembleMN3207, 1024 stagesR22: delay td = 1024 / (2 × fclock); the datasheet gives 2.56 ms at 200 kHz to 51.2 ms at 10 kHz and a +4 to +10 V supply. A 6.4 ms voice needs 1024 / (2 × 0.0064) = 80 kHz (calculated). Typical THD 0.4 % and S/N 73 dB under the datasheet’s own test conditionsR22. Three voices with clocks 120 degrees apart give the ensemble; a clock generator and anti-alias / reconstruction filters are needed and are not drawn. Companding is a design choice for noise, with no part chosen
Option B, digital delayPT2399R21: 44 kbit RAM, delay set by the VCO frequency through one external resistor (10 kΩ recommended, up to 50 kΩ allowed in the datasheet note), THD 0.5 % and SNR better than 90 dB typical, +5 V. Echo feedback would be an op-amp mix of the output back to the input with a pot; wow and flutter from modulating the VCO resistor current is a circuit idea of the designer, not a datasheet claim, and is untested. The datasheet’s delay-time table is on a page not included in the copy read, so no delay range is claimed
Option C, spring and plateReal springs and plates are not modelled by a short BBD. A DSP card (ADC, DSP, DAC) or a dedicated reverb IC would be needed; none is chosen. The plate algorithm in the browser model follows DattorroR20: input diffusers 142, 107, 379, 277 samples, tanks 672 + excursion / 4453 / 1800 / 3720 and 908 + excursion / 4217 / 2656 / 3163 samples at 29,761 Hz, decay-diffusion 0.70 and 0.50, 0.6 output taps from the paper’s Table 2. The spring is the designer’s own: 30 first-order all-pass stages per line for dispersion plus a delay and damping loop; it is not taken from a source
Controls (model)Per-channel send; time / size, feedback / decay, tone, return; mode; tap tempo or tempo sync for the echo. Feedback is capped inside the model: echo 0.85, spring 0.95, plate decay 0.93, BBD regeneration 0.5. Return at 0 adds exact zeros
Browser model limitsNo hiss, clock noise or aliasing of a real BBD is modelled; tape echo has no head or tape model beyond a saturating soft clip and low-pass filters. CPU: measured in Node on the test runner only (see tests/dsp_test.js); no phone was measured

S13 Channel EQ (four-band, per channel)

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) (schematic) Swipe sideways to pan on a phone.
ItemValue / formula
StatusDesign target. Nothing here is built or measured. The browser model on /mixer is floating-point DSP that follows the same control law; it is not a simulation of this circuit
BandsHI shelf 3 kHz: +6 dB to full kill. HI MID bell 1.1 kHz: +10 dB to −27 dB. LO MID bell 350 Hz: +10 dB to −27 dB. LO shelf 180 Hz: +6 dB to full kill. Every knob is exactly 0 dB at 12:00 (centre detent). Where these figures come from is stated in the manual, section 5, S13
Strip orderTrim, LO MID bell, HI MID bell, three-way LR4 split with LO and HI band pots, then the filter (S4) and the fader (S5). The cue tap stays pre-fader
Bells (two stages)Each bell is an inverting op-amp stage with a 10k linear pot and a band-pass filter tuned to the bell frequency, the virtual-earth boost / cut arrangement described by ESPR23. ESP lists multiple-feedback, twin-T and state-variable band-passes as options; this design picks multiple-feedback for part count (a design choice). At the pot centre the band-pass has no effect and the stage is a unity inverter, so it is flat. Two inverting stages in series keep absolute polarity. Bell Q about 0.9 at full boost (design choice). Component values are not calculated in this draft
Shelves (kill)A plain Baxandall shelf cannot reach full kill, so LO and HI reuse the S3a LR4 split insteadR3, R4: crossovers at 180 Hz (R = 88.7k, f0 = 179.4 Hz) and 3 kHz (R = 5.36k, f0 = 2969 Hz) with C = 10 nF. LO and HI bands each get a ×2 band amp and a 10k linear pot (minimum = kill, centre = unity, maximum = +6 dB); the middle leg gets ×2 then a fixed divide-by-2; an inverting sum recombines
Kill depthDesign target: beyond 60 dB in the stop band of the LR4 split, as for S3b. Not measured
Model versus circuitThe browser model uses second-order shelf and peaking sections (the textbook biquad forms, not cited to a page in this draft) with the same corner and centre frequencies and the same plateau levels, and a −80 dB floor for kill. The LR4 split has steeper transitions near 180 Hz and 3 kHz, so the two differ in the transition regions
HeadroomBoosts stack: with every band up the strip can add well over 10 dB at some frequencies. Headroom for that has not been analysed in this draft. The model on /mixer keeps its limiter on the master output only
Op-amp sectionsBells: 2 sections each, 4 per channel. Split: 12 sections per channel (see the budget in S9)

References

Formulas and datasheet figures are cited by number in the tables. Standards are listed by title; the IEC pages are catalogue entries, the standards themselves are paid documents and were not read in full.

  • [R1] RIAA equalization: time constants 3180, 318 and 75 µs (IEC 60098 / RIAA) — Wikipedia, RIAA equalization
  • [R2] S. P. Lipshitz, “On RIAA Equalization Networks”, Journal of the Audio Engineering Society 27(6), pp. 458–481, June 1979 — AES E-Library record
  • [R3] J. Karki, “Analysis of the Sallen-Key Architecture”, Texas Instruments application report SLOA024B — TI SLOA024B
  • [R4] S. Linkwitz, “Active Filters”: LR2 and LR4 crossovers as cascaded Sallen-Key sections, Q0 = 0.71 per LR4 stage — Linkwitz Lab, Active Filters
  • [R5] Texas Instruments, LM317 3-Pin Adjustable Regulator datasheet (Rev. Z): Vₒ = 1.25 V × (1 + R2/R1) + Iₐdⱼ·R2 — TI LM317 datasheet
  • [R6] Texas Instruments, NE5532 dual low-noise operational amplifier datasheet (5 nV/√Hz typical at 1 kHz) — TI NE5532 datasheet
  • [R7] Texas Instruments, NE5534 low-noise operational amplifier datasheet (3.5 nV/√Hz typical at 1 kHz) — TI NE5534 datasheet
  • [R8] THAT Corporation, 2180 series voltage-controlled amplifier datasheet (control constant 6.1 mV/dB) — THAT Corporation datasheets
  • [R9] IEC 60268-3:2018, Sound system equipment — Part 3: Amplifiers (specification and measurement methods) — IEC webstore
  • [R10] IEC 62368-1:2023, Audio/video, information and communication technology equipment — Part 1: Safety requirements — IEC webstore
  • [R11] W3C, Web Audio API (the browser demonstration on /mixer uses it; it is not the analogue design) — W3C Web Audio API
  • [R14] ESS Technology, ES9842PRO 4-channel ADC datasheet v0.5.1 (dynamic range, THD+N, 32-bit PCM / TDM words) — ES9842PRO datasheet
  • [R15] ESS Technology, ES9843PRO 4-channel ADC datasheet v0.4.1 (alternative part) — ES9843PRO datasheet
  • [R16] XMOS, XK-AUDIO-316-MC USB audio multichannel reference: USB Audio Class 2.0, asynchronous, XU316 — XMOS sw_usb_audio, app_316_mc
  • [R17] T. Stilson and J. Smith, “Analyzing the Moog VCF with Considerations for Digital Implementation”, ICMC 1996 — CCRMA, Stilson and Smith
  • [R18] A. Huovilainen, “Non-linear digital implementation of the Moog ladder filter”, DAFx-04 — DAFx-04 paper archive
  • [R20] J. Dattorro, “Effect Design, Part 1: Reverberator and Other Filters”, Journal of the Audio Engineering Society 45(9), pp. 660–684, September 1997 (plate topology, delay lengths and output taps, Fig. 1 and Table 2) — CCRMA, Dattorro
  • [R21] Princeton Technology Corp., PT2399 Echo Processor IC datasheet V1.9, March 2022 (44 kbit RAM, VCO delay set by an external resistor, THD 0.5 % typ, SNR better than 90 dB typ, +5 V application circuits) — PT2399 datasheet
  • [R22] Panasonic MN3207, 1024-stage low-voltage low-noise BBD datasheet (delay 2.56 to 51.2 ms for a 200 to 10 kHz clock, supply +4 to +10 V, THD 0.4 % typ, S/N 73 dB typ). Read from a copy hosted by Synthrotek. — MN3207 datasheet (hosted copy)
  • [R23] R. Elliott (ESP), “Equalisers, The Various Types And How They Work”, Elliott Sound Products, March 2015, updated February 2021 (Baxandall and James tone controls; boost / cut stages with a band-pass filter between virtual-earth op-amp inputs; multiple-feedback, twin-T and state-variable band-pass options; gyrators) — ESP, Equalisers
  • [R19] V. Zavalishin, The Art of VA Filter Design, Native Instruments. Cited by title and author; no link was checked for this draft.
  • [R12] D. Self, Small Signal Audio Design, 3rd edition, Focal Press / Routledge (2020). Book, no link.
  • [R13] P. Horowitz and W. Hill, The Art of Electronics, 3rd edition, Cambridge University Press (2015). Book, no link.

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