How I Repaired Humming Sound in Ahuja 120 Watts Power Amplifier

 

Author: Basavaraj B M
Electronics Debug Engineer | 20+ Years Experience | Passion for Repair & Training

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Introduction

Power amplifiers are the heart of any sound system, and when they develop issues like humming or buzzing, it can ruin the audio experience. Recently, I received an Ahuja 120 Watts PA amplifier with a continuous low-frequency hum—even when no input was connected. This post shares how I diagnosed and repaired the issue step-by-step.

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Symptoms Noticed

• Constant 50Hz humming sound from speakers.

• Humming was present without any input signal.

• Volume control had no effect on the hum.

• The audio signal path otherwise seemed functional.

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Tools Used

• Digital Multimeter (Fluke 17B+)

• Soldering station

• Oscilloscope (for signal tracking)

• ESR meter

• Desoldering pump and wick

• Spare capacitors, grounding wire, star washers

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Step-by-Step Repair Process

1. Visual Inspection

The first step was to inspect the PCB and wiring:

• Found dust accumulation and slight corrosion around ground points.

• Noticed the power transformer was close to the input stage—a possible source of magnetic interference.

• The main filter capacitors looked aged but not bulged.

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2. Grounding Check

Using the multimeter, I verified:

• Chassis and signal ground were not bonded properly.

• Ahuja amps are sometimes susceptible to ground loop hum.

Fix:
I removed the ground loop by:

• Cleaning all chassis contact points.

• Adding star washers to the grounding screw for better contact.

• Tightly securing all ground wires to a single grounding point (star grounding method).

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3. Power Supply Filter Capacitors

The next check was the electrolytic filter capacitors (10,000µF/50V):

• Used an ESR meter and found ESR higher than normal (>1Ω).

• These caps were not filtering the 50Hz ripple effectively, leading to hum.

Fix:

• Replaced both filter capacitors with new low-ESR 10,000µF/63V caps.

• Soldered with care to maintain polarity and track integrity.

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4. Preamp Section Decoupling

Checked the decoupling capacitors (47µF to 220µF) in the preamp and tone control sections:

• A couple of caps had minor leakage and degraded ESR.

• Especially the one at the op-amp power rail.

Fix:

• Replaced aged decoupling caps with fresh Panasonic low-ESR types.

• This stabilized the DC rail feeding the preamp op-amp (4558).

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5. Signal Path Isolation

I disconnected the input and directly injected a clean sine wave at the power amp input.

• The hum disappeared, confirming the issue was before the power stage.

Also noticed that the RCA input jacks were loosely grounded, which can cause hum pickup.

Fix:

• Re-soldered input jack ground to the chassis.

• Used shielded wire for the input path from jack to board.

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Final Result

After all these fixes:

• Humming completely gone

• Clear and noise-free audio output

• Amplifier passed full-load test with 120W output at 4 ohms

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Conclusion

Humming in amplifiers is often due to bad grounding or faulty capacitors. In this Ahuja 120W PA amp, replacing old filter and decoupling caps, and ensuring proper chassis grounding, eliminated the hum. Always remember to isolate the stage causing the problem and approach logically—don’t just replace parts blindly.

If you're facing a similar issue in your amplifier, try these steps. Feel free to comment or reach out if you need help!