NAD 3120 Amplifier Hum
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- Опубліковано 8 жов 2021
- Attempting to resolve startup hum on my Vintage NAD 3120.
Spoiler alert - I ended up bypassing the muting circuit.
The 3120 is a variant of the 3020B, with tone controls and power meter not fitted.
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Thank you so much. I have had almost the exact same journey struggling with this, minus the oscilloscope. I already installed speaker protection so shorting JFET will be a perfect fix!
Shorting the JFET or taking out the JEFT and bypassing it, will definitely sort out the problem, but if you want to retain the muting circuit, a simple fix would be to replace C533, 47uf 50 volts with a higher value (and with higher voltage rating). I have found 1000uf 100volts to work just perfectly. If you want remove the thump, not a bad idea to build a small speaker protection circuit (or delay circuit) with a couple of transistors and a relay (per channel). You can remove the 1uH speaker inductor from the power amp section and introduce the delay circuit in its place (the 1uH inductor can be included with the delay circuit instead like this video: NAD 3020 project loving some 4 ohms - UA-cam). Some tips from my experience with loads of these 3020s.
Thanks - I watched your excellent video before attempting to fix my hum problem, and I am pretty sure I tried the hi value 1000u cap, which for whatever reason did not work. Can certainly look at it again. I also worked on a 3020i in another video, and I noticed that the muting circuit had been completely redesigned. I think you know that the 3020 doesn't have any protection circuit. I have a kit coming in the post which I will put externally, this will provide protection and start up delay. So this will make the muting circuit redundant anyway.
I have a continuous loud buzz on the right channel. It starts right away when I turn on the amp. It's not in the speakers or cables. The left channel sounds amazing with my dynaudio speakers.
I can't visually spot defective capacitors, although I can spot some glue which they have been attached with. I have checked the ground connections and loose screws and all seems fine. Do you maybe have suggestions what this could be? Thanks kindly for your ideas.
Continuous loud buzz to me sounds like one of the output transistors has failed short circuit. This would mean you have DC on your speakers, which is really really bad and can kill your speakers. This isn't a DIY repair, I'm afraid - you really need to have some electronics knowledge. I recommend you take it to a tech near where you live. Good luck!
Thanks for the video. Please turn up the gain in your microphone, volume is on the low side.
Thanks - I'll look at it!
I fixed two 3020A versions of the amp by increasing R548 to 180k.
My 3120 has had the start up hum ever since I bought it new in 1985. The hum dissipates very quickly, so I'm unsure why it's a concern at all?
Not a concern - it just isn't supposed to do it.