Active DI Schematic Capture

flux

Reviewed the schematic block-by-block and fixed what was actionable: replaced the rail test-point placeholders with Keystone 1040 parts, added missing/provisional MPN metadata, corrected several generic passive package choices, classified key nets, and re-ran checks clean. I updated Schematic Capture Notes with the review findings and remaining placeholders.
Open items:
  • Exact Flux parts are still needed for THAT1646W16-U, Hammond 1140-DB-A, FFSAS02BFR, RACM30-15DK/277/OF, and some mechanical connectors/switches.
  • Direct schematic symbol rearrangement was not available through the exposed tools; I documented this and avoided making PCB layout changes.
Suggested next steps:
  1. Manually tidy schematic symbol positions in the editor, or rerun when a schematic-arrange tool is available.
  2. Create/import exact custom parts for the remaining placeholders.
  3. Move to mechanical/package verification before PCB layout.
jakey0101

jakey0101

Can you delete the components specifically part of the GLB network?

flux

I’ll find the components tied specifically to the GLB option, remove only those, then verify the A0/chassis domains remain otherwise intact.

flux

Removed the GLB-specific component JP_GLB from the schematic. I verified the checked ERC/DRC categories are still clean, and updated Schematic Capture Notes to record the GLB removal.
Open items:
  • A0 and CHASSIS remain separate elsewhere; connector shield/chassis terminations were not changed.
  • No other GLB-specific schematic components were found.
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Schematic

Layout

Preview

Project Specification
Project Overview
  • Project: Active DI
  • Status: Draft schematic capture in progress
  • Device: High-quality active direct injection / instrument interface for passive and active instruments.
  • Core architecture: Passive thru, selectable input load, OPA1656 input buffer, OPA1612 Sallen-Key HPF and output driver, transformer-isolated XLR output, optional THAT1646 balanced TRS line output.
Intended Use
  • Recording passive guitar, passive/active bass, synths, drum machines, and similar sources into an audio interface or mic preamp.
  • Installed in an aluminium enclosure with internal mains-derived ±15 V rails.
  • First build is prototype/validation oriented; production-final schematic and PCB release are blocked by several measurement and mechanical gates.
What the Device Should Do
  • Provide a hardwired passive thru output that remains available when unpowered.
  • Offer selectable DI input loading: 10 MΩ, 2.126 MΩ, 999 kΩ, and 221 kΩ nominal.
  • Buffer the high-impedance input with TI OPA1656ID at unity gain.
  • Provide selectable 2nd-order high-pass filtering at approximately 20.54 Hz, 42.37 Hz, and 82.68 Hz.
  • Use a master level control to attenuate both output branches.
  • Drive an isolated mic-level XLR output through a Hammond 1140-DB-A transformer.
  • Support a DNP/populate-later THAT1646W16-U balanced TRS line-output branch.
Main Features
  • Passive thru jack directly tied to input tip before the DI input resistor.
  • RF/protection input network with 1 kΩ series resistor, 100 pF RF return option, and optional BAV199 rail clamp.
  • Four-position input impedance selector using a permanent 10 MΩ reference plus selectable grounded shunts.
  • Unity-gain OPA1656 input buffer; unused second channel parked as follower at audio 0 V.
  • Unity-gain OPA1612 channel A HPF and channel B post-level driver.
  • Parallel-ganged dual 50 kΩ Alps RK27 master control.
  • Transformer XLR polarity reversal via DPDT switch; connector pin 1 to chassis only.
  • Optional balanced TRS output using THAT1646W16-U with rail clamps, ferrites, RFI capacitors, and sense-loop capacitors.
  • Internal XP Power FFSAS mains entry and RECOM RACM30-15DK/277/OF ±15 V PSU.
System Architecture
Diagram



  
    
  
  
    
  





























Instrument Input Jack
Hardwired Passive Thru
1 kΩ RF / Protection Resistor
Selectable Input Load
OPA1656 Unity Buffer
OPA1612A 3-Position Unity HPF
3.3 µF Coupling + Parallel RK27 Master
OPA1612B Unity Driver
Source Option + 3.3 µF + Hammond 1140-DB-A
Polarity Switch + XLR Mic Output
Optional THAT1646W16-U Line Driver
Balanced TRS Line Output
FFSAS + RACM30 ±15 V PSU
Hardware Subsystems
Input / Thru / Protection
  • Neutrik NMJ4HCD2 input and thru jacks.
  • Tips hardwired together; sleeves to A0_INPUT.
  • IN_PROTECTED node created after 1 kΩ series resistor.
  • 100 pF C0G RF capacitor returned via jumper-selectable chassis/A0_INPUT path.
  • Optional BAV199 clamp to ±15 V rails, DNP-capable.
Input Loading
  • Permanent 10 MΩ from IN_PROTECTED to A0_INPUT.
  • Selectable shunts: 2.7 MΩ, 1.11 MΩ, 226 kΩ.
  • Grayhill 56A30-01-1-04N switches only grounded ends of shunts; IN_PROTECTED should not be routed to the panel.
Buffer and HPF
  • OPA1656ID SOIC-8: channel A input buffer; channel B parked follower.
  • OPA1612AID SOIC-8: channel A unity Sallen-Key HPF, channel B unity driver.
  • HPF selector Grayhill 56D30-01-2-AJN stopped at 3 positions.
  • Capacitor pairs: 3.3 µF, 1.6 µF, 0.82 µF PP film, with 10 MΩ bleed across each capacitor.
  • HPF resistors: 1.65 kΩ feedback and 3.32 kΩ bias.
Output Stages
  • Master: 3.3 µF coupling cap, parallel-ganged dual 50 kΩ audio taper pot, 1 kΩ wiper stopper, 1 MΩ fail-safe to A0.
  • Transformer branch: mutually exclusive 0 Ω / 1 kΩ / 6.8 kΩ source options, 3.3 µF DC block, Hammond primary to A0_OUTPUT, floating secondary to DPDT polarity switch and XLR pins 2/3.
  • THAT branch: THAT1646W16-U SO16W as capture identity, DNP-capable until IC/support parts are obtained.
Power / Ground / Chassis
  • FFSAS02BFR filtered IEC inlet, switch, and fuse module.
  • RACM30-15DK/277/OF ±15 V open-frame PSU.
  • Rails: +15V_RAW, -15V_RAW, +15V_A, -15V_A, A0_PSU, A0.
  • PE bonds directly to chassis; GLB is only between A0 and already-bonded chassis.
  • XLR pin 1, TRS sleeve, and transformer can/shield options terminate at chassis.
Interfaces and Connections

Table

InterfaceSignals / Nets
Input jackJ_IN_TIP, A0_INPUT
Thru jackJ_THRU_TIP hardwired to input tip, A0_INPUT sleeve
Input load switchLow side of R_Z_HIGH, R_Z_NORMAL, R_Z_LOW, common to A0_INPUT, one open position
HPF switchBUF_OUT and HPF_N1 commons; capacitor-bank throws to HPF_N1/HPF_N2
Master potLEVEL_TOP, LEVEL_WIPER, A0
XLR outputXLR_2, XLR_3, pin 1 to CHASSIS
TRS outputTRS_TIP, TRS_RING, sleeve to CHASSIS
Mains / PSUPE, CHASSIS, +15V_RAW, -15V_RAW, A0_PSU
Power and Runtime Expectations
  • Mains powered; no battery/runtime requirement.
  • RECOM RACM30-15DK/277/OF provides ±15 V rails at up to 1 A per rail.
  • Audio circuitry uses local 100 nF and 10 µF decoupling per IC rail region.
  • Rail-entry 470 µF/35 V hybrid capacitors are preferred layout defaults subject to footprint/startup review.
Power Tree and Power Budget
Diagram



  
    
  
  
    
  


























Mains IEC L/N/PE
FFSAS02BFR Filter / Switch / Fuse
RACM30-15DK/277/OF
+15V_RAW
A0_PSU
-15V_RAW
+15V_A
-15V_A
A0 Analog Return
OPA1656 / OPA1612 / optional THAT1646
Selectable GLB to CHASSIS
PE'
Dedicated Chassis Stud
  • Detailed current budget remains a validation item; installed PSU has large margin for the audio circuitry.
Manufacturing and Assembly Expectations
  • First implementation may use SMF ENIG ProtoBoard and daughter boards where sensible.
  • Final PCB/layout is not production-final until package, dimensions, enclosure, and measurement gates are closed.
  • Use correct SOIC-8 footprints for OPA1656ID and OPA1612AID.
  • Use SO16W for THAT1646W16-U; do not substitute SOIC-8.
  • Use schematic placeholders/generic footprints only where Flux library parts are unavailable, with actual names preserved in component properties.
Physical Design Expectations
  • Aluminium enclosure with safe mains/PE bonding.
  • Keep high-impedance input area away from PSU, mains wiring, transformer, and switch bundles.
  • Verify large PP film capacitor bodies, Grayhill switches, RK27 pot, transformer, IEC inlet, and RACM30 spacing before drilling or PCB release.
Important Design Decisions
  • Hardwired passive thru is retained; no switched/active thru defeat in baseline.
  • Input impedance is selected by grounding shunt lows, not by routing IN_PROTECTED to the panel.
  • HPF uses exact single-value PP film capacitor pairs and exact 1.65 kΩ/3.32 kΩ resistors; historical series/parallel workarounds are superseded.
  • Master level precedes the output split and attenuates both outputs.
  • Transformer output is first-build primary output; THAT1646 line output is capture-ready but DNP-capable.
  • PE/chassis safety bond is never lifted; GLB is only an audio/chassis reference option.
Assumptions
  • Library-missing parts may be represented with closest suitable symbols/footprints while preserving actual part names/MPNs in properties.
  • The first schematic capture is allowed to include explicit DNP/selection footprints for unresolved items.
  • Pending ordered parts are electrically selected but not physically reconciled.
Open Review Gates
  • Pending order receipt and dimensional inspection.
  • Hammond lead phase, shield destinations, mounting/lead geometry, and low-frequency measurements.
  • Final transformer source-resistor fit value.
  • THAT1646 sense capacitors, matched ferrites, and both-output polarity implementation.
  • Compact 100 nF local decoupler MPN/footprint.
  • Exact GLB population.
  • Rail-entry capacitor footprint and RACM30 inrush/fuse behavior.
  • Enclosure scale layout, mains safety clearances, and panel drilling.
Change Notes
  • 2026-07-21: Created from supplied schematic-ready design spec, architecture reconciliation, and project plan attachments. Current task is schematic capture in Flux.

  • Project Overview

  • Intended Use

  • What the Device Should Do

  • Main Features

  • System Architecture

  • Hardware Subsystems

  • Input / Thru / Protection

  • Input Loading

  • Buffer and HPF

  • Output Stages

  • Power / Ground / Chassis

  • Interfaces and Connections

  • Power and Runtime Expectations

  • Power Tree and Power Budget

  • Manufacturing and Assembly Expectations

  • Physical Design Expectations

  • Important Design Decisions

  • Assumptions

  • Open Review Gates

  • Change Notes