Preamp Stage#
The preamp stage processes the input signal before it reaches the power amplifier. It provides input selection, isolation, gain control, equalization, and crossover filtering.
Signal Flow#
Input Section#
Input Types#
The amplifier accepts two input types:
| Input | Connector | Sensitivity Range |
|---|---|---|
| RCA (Line Level) | PJ501 | 0.2V - 4.0V |
| Speaker Level | CN501 | 0.4V - 8.0V |
Isolation Amplifiers (IC501-IC504)#
Each channel has an isolation amplifier (NJM4565M dual op-amp) that:
- Provides input buffering
- Allows mixing of RCA and speaker-level inputs
- Isolates the input from downstream stages
Normal Voltages (all NJM4565 op-amps):
| Pin | Function | Voltage |
|---|---|---|
| 1-3 | Inputs/Outputs | 0V (signal dependent) |
| 4 | V- | -14V |
| 8 | V+ | +14V |
Gain Control#
Variable Gain (VR501-VR504)#
Four 9-turn precision potentiometers control input sensitivity:
| Control | Channels | Range |
|---|---|---|
| VR501 | CH1/CH2 | 10K dual pot |
| VR502 | CH1/CH2 | 10K dual pot |
| VR503 | CH3/CH4 | 50K quad pot |
| VR504 | CH3/CH4 | 50K quad pot |
Bass EQ#
Bass Boost (SW501, SW503)#
Channels 3 and 4 feature selectable bass EQ:
| Switch | Position | Effect |
|---|---|---|
| SW501 | FLAT | No boost |
| SW501 | BOOST | Bass enhancement |
| SW503 | FLAT | No boost |
| SW503 | BOOST | Bass enhancement |
The bass EQ circuit uses additional op-amp stages (IC513, IC515) to provide low-frequency boost when enabled.
Variable Crossover Filters#
Filter Types#
Each channel pair has independent crossover filters controlled by VR503/VR504 and SW502:
| Setting | Filter Type | Frequency Range |
|---|---|---|
| HPF | High Pass | Variable |
| LPF | Low Pass | Variable |
| FLAT | Full Range | No filtering |
Subsonic Filter#
A subsonic filter removes ultra-low frequencies (<20Hz) that waste amplifier power and can damage speakers.
Op-Amp Inventory#
The preamp stage uses 18 NJM4565M dual op-amps:
| IC | Function |
|---|---|
| IC501-504 | Input isolation amplifiers |
| IC505-506 | Gain stages |
| IC507-508 | Buffer/Line out |
| IC509-510 | Filter stages |
| IC511-512 | Filter stages |
| IC513-515 | Bass EQ / Filter |
| IC516-518 | Output buffer to power amp |
All op-amps are powered from the ±14V regulated rails.
Line Output#
Pre-Out (RCA)#
The amplifier provides buffered line outputs for daisy-chaining to additional amplifiers:
| Output | Channels | Level |
|---|---|---|
| PRE OUT CH1+3 | Mixed 1&3 | 0.5V @ 0.5V input |
| PRE OUT CH2+4 | Mixed 2&4 | 0.5V @ 0.5V input |
Common Preamp Issues#
No Output (One or More Channels)#
- Check op-amp supply voltages (±14V rails)
- Verify input connections
- Test gain potentiometer continuity
- Check for failed op-amps (rare)
Distorted Output#
- Check ±14V rail voltages (should be stable)
- Look for damaged filter capacitors
- Verify gain settings aren’t too high
Noise/Hum#
- Check ground connections
- Verify input isolation
- Look for damaged input protection diodes (D101-D502)