Building McIntosh C28
Started implementing a plan
#Goal
- Rebuild the left-channel low-level stage of the McIntosh C28 preamplifier by removing the modern NE5532-based front end and replacing it with the discrete, transistor-based topology derived from the uploaded low-level-stage PDF and BOM spreadsheet.
#Implementation plan
1. Analyze the Current Schematic
1. Locate the existing left-channel NE5532 op-amp based circuitry in the current project.
2. Identify and mark the components that conflict with the desired C28 discrete stage.
2. Remove or Isolate Conflicting Circuitry
1. Remove or isolate the modern left-channel structure to prevent interference with the new design.
3. Integrate C28-Derived Left-Channel Stage
1. Refer to the uploaded low-level-stage PDF and BOM spreadsheet to identify all specific components, specifically the discrete transistors (e.g., Q1, Q3, Q5, Q7) and their supporting resistors and capacitors.
2. Add the identified discrete components into the project schematics for the left-channel low-level stage.
3. Wire the added components according to the connectivity and topology shown in the reference documents.
4. Verification and Documentation
1. Review the updated left-channel schematic for correctness against the C28 reference, ensuring that all connections and part values match the BOM.
2. Document the changes made, explaining the transition from the modern NE5532 circuitry to the discrete C28-based left-channel stage.
#Success criteria
- All conflicting modern op-amp components in the left-channel are removed or isolated.
- The discrete transistor-based left-channel low-level stage is accurately built according to the provided PDF and BOM, with correct parts and wiring.
- The schematic reflects a faithful C28-derived left low-level discrete stage ready for production.
Task completed.
0:19
Execution stopped due to insufficient credits.
6:52
Execution stopped due to insufficient credits.
5:31
No changes applied.
0:04
Applied changes.
nets {
net unnamed = C24:P2;
net unnamed = VR2:CW_2;
net unnamed = J1:3, J2:3, C4:P2;
net unnamed = C24:P1;
net unnamed = IC2:2OUT;
net unnamed = R10:P1, IC2:1OUT;
net unnamed = R6:P2;
net unnamed = C16:P1, IC3:VCC-, IC4:VCC-;
net unnamed = C27:P1;
net unnamed = C7:P2;
net unnamed = IC3:VCC+, IC4:VCC+;
net unnamed = IC2:1IN-;
net unnamed = VR1:CCW_1, GND:GND, VR3 CCW_2 - R16 P2:GND, VR2:CCW_1, R4 P2 - J2 1:GND, J2:1, C16:P2;
net unnamed = R10:P2;
net unnamed = VR1:CW_1;
net unnamed = IC2:VCC+;
net unnamed = R4:P2;
net unnamed = R4:P1;
net unnamed = C4:P1;
net unnamed = R14:P1;
net unnamed = J2:2;
net unnamed = VR2:WIPER_1;
net unnamed = IC2:1IN+;
net unnamed = VR1:CW_2;
net unnamed = C27:P2;
net IN_L = J1:2;
net unnamed = R11:P2;
net unnamed = VR1:WIPER_2;
net unnamed = C22:P2;
net unnamed = IC2:2IN+;
net unnamed = C11:P1;
net unnamed = IC2:VCC-;
net unnamed = R3:P1;
net unnamed = IC2:2IN-;
net unnamed = C17:P1;
net unnamed = IC3:2IN-;
net unnamed = VR2:WIPER_2;
net unnamed = C11:P2;
net IN_R = J1:1;
net unnamed = VR1:CCW_2;
net unnamed = VR2:CW_1;
net unnamed = C7:P1;
net unnamed = IC3:2OUT;
net unnamed = R11:P1;
net unnamed = R6:P1;
net unnamed = IC3:2IN+;
net unnamed = R3:P2, IC3:1IN-;
net unnamed = C22:P1;
net unnamed = IC3:1OUT;
net unnamed = R14:P2;
net unnamed = VR1:WIPER_1;
net unnamed = VR2:CCW_2;
net unnamed = R13:P1;
net unnamed = IC3:1IN+;
net unnamed = R13:P2;
net unnamed = C17:P2;
}
Applied changes.
Applied changes.
Applied changes.
Added 28 components
0:07
Added 1x BC550CBU as Q1 (Pins: C, B, E)
Added 1x BC546BTA as Q3 (Pins: B, E, C)
Added 1x BC639 as Q5 (Pins: B, E, C)
Added 1x MJE340G as Q7 (Pins: E, B, C)
Added 17x Generic Resistor as R19, R21, R23, R25, R27, R29, R31, R33, R35, R37, R39, R41, R43, R45, R47, R49, R85 (Pins: P2, P1)
Added 7x Generic Capacitor as C7, C9, C11, C13, C15, C17, C18 (Pins: P1, P2)
Diagram
- Preserve the essential McIntosh C28 style signal path: input stage, gain control, tone shaping, and output buffering.
- Use modern low-noise dual audio op-amps for practicality and serviceability.
- Use dual analog supply rails for headroom and low distortion.
- Leave room for later additions such as phono equalization, filters, loudness contour, balance, and tape monitor functions.
- Input connectors and source selection entry points
- AC-coupled high impedance input buffer
- Gain and volume stage
- Active Baxandall bass and treble control section
- Low impedance output buffer
- Split-rail supply filtering and local decoupling
- Line-level operation
- Dual supply rails based on +/-15V analog operation
- Film capacitors in the audio path where practical
- Star-ground friendly grounding strategy for later PCB work
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