NAD 302

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A friend of mine brought 2 NAD 302 amplifiers and asked me to rectify the problem with the second amp. I connected both amp to the same kind of speakers, set both amps bass and treble at the center. Then connected a CD player to the first amp, set the volume to half way and listened to the loudness. Then I connected the same signal to the second amp, set the volume half way and listened. I noticed that there is a significant difference in loudness. The second amp sounds much softer than the first one. All the settings on both amps are the same. I have not checked any component on any amp, as I do not have any idea where to start. Can someone please assist and advice me what can cause this problem or where to start from.


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It's basically yet another update of the original NAD 3020. I'd expect the same customers and their typical ways of abusing amplifiers would apply. It seems someone is trying to bi-amp with another dodgy Ebay offer. NAD 302 Manual - Stereo Integrated Amplifier - HiFi Engine

1.Someone has already attempted to replace a noisy pot with a different value or type of resistance law etc.
2. Coupling caps in the preamp (electrolytic) may be failing.
3. Muting FET control circuit is damaged.
4. Lots of other possibilities

You need to be systematic and separate the pre. and main amps to isolate where the gain is lost. Use a basic signal tracing procedure:
You can use a standard tone like 440Hz from a signal generator or off the internet, tone generator software etc. and measure the output voltage of the relevant section because ears are just not accurate or at a constant relationship to speakers. Use an appropriate fixed input level at the input, then proceed along the signal path to the section's output with the meter, comparing the signal voltage of both amplifiers and channels as you go. An insulated, shielded probe like an oscilloscope probe is best but you can struggle with short leads, twisted pair wire and attach a film cap, about 0.01uF, to the +lead and use the other lead as a contact, then attach a crocodile clip to the -lead for a ground. Make sure by listening occasionally too, that when you measure, the signal isn't just background noise or hum.

Assuming both channels have the same low volume or perhaps tone setting from your description, then you are looking for a common fault such as a dual pot. or its wiring, muting control circuit etc, common ground connectors, sockets etc. and anywhere that looks tampered with - this is where I'd look first.

Eventually, you'll find a discrepancy which could be almost anywhere really, if it comes down to the unlikely situation of the same parts failing in both channels.

It's not rocket science or expensive work, just tedious. Post links to service manuals to encourage others to take an interest in your repair threads and read up on some standard repair techniques for yourself.
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