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Old 18th September 2008, 10:50 AM   #1
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Default Hiss From An Ampli Channel

Today I switched on my Nad 3020 and I heard a permanent, loud hiss from one speaker
The amp has been recapped few months ago and never caused problems.
Thank you in advance for any help to understand (and possibly to fix it!!!)
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Old 18th September 2008, 10:57 AM   #2
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Hiss ? Hum ? Does it vary with volume ? Is it "hiss" or is it unstable perhaps.
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Old 18th September 2008, 11:15 AM   #3
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It is a hiss and it doesn't vary with volume. But if I increase the volume a hum also appears.
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Old 18th September 2008, 11:26 AM   #4
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Hi,
If you can post a circuit it should be fixable. Do you have a 'scope ? Can you see what's actually coming out ? Does the hum appear just on the one channel ?
Really need to see circuit
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Old 18th September 2008, 12:17 PM   #5
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Unfortunately I don't have a 'scope. I tried to listen to a cd and the sound comes just from the speaker with the hiss and with an abnormal high volume. The other speaker seems to be dead
You can find the schematics at this ws:
http://web.295.ca/~peak//audio.html
Thanks so much
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Old 18th September 2008, 05:47 PM   #6
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Hi Calamaro,
Can't get the link to the schematic to work --someone must have one though Anyone ?
You say the sound comes out of the speaker at high volume and that the other is dead. That sounds very strange, almost liked cracked print somewhere.
Does the volume control not turn the sound down at all ? That points to something very basic ( it's not been dropped or anything has it ) like an open circuit ground somewhere.
A circuit will reveal all I'm sure
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Old 18th September 2008, 05:54 PM   #7
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Found this http://www.tcaas.btinternet.co.uk/index-4.htm

Can you confirm, does the volume control NOT work. Do you get sound even with it on minimum.
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Old 18th September 2008, 06:13 PM   #8
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I wonder if you can have gotten oscillations on the output.
This comes from some speakers have an inductive impedance.
The impedance rise from 8 ohms to much more at higher frequencies.
The amplifier can get Un-Stable.

To prevent this we use Zobel filter across output of amplifier.
This will flatten the impedance seen by the amplifier.
So it does not rise (as much) at higher frequencies.

Schematic NAD 3020 power.
http://www.tcaas.btinternet.co.uk/nad3020pwrhr.gif

The amplifier will see
the resistor R657 (10 ohm) in parallel with speaker, at higher freq.
This parallel impedance seen, will always be lower than 10 ohm!

Check that ZOBEL filter is at work!
C635, C637 and R657.
Across the output nodes.

100nF - 100nF - 10 Ohm
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Old 18th September 2008, 07:53 PM   #9
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Quote:
Can you confirm, does the volume control NOT work. Do you get sound even with it on minimum.
Yes Mooly, volume control don't works

Lineup, I had no problem at all 'til this morning, I definitely don't understand what happened
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Old 19th September 2008, 07:12 AM   #10
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Oh dear !! Somethings happened to it me thinks Check very carefully for damage and for cracks in and around the volume control area. Have a measure on ohms with it all unplugged and see if the volume pot "works".
There is something very fundamental wrong here.
Do you know how to check it on ohms and what to measure ?
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