One point about the input stage not mentioned specifically is whether the soft clipping switch, shown in the schematic as a stereo slider, received the contact cleaner treatment.
There could be a dirty contact where the soft clipping circuitry is shorted across in bypass mode but the connection is not clean.
Another point not covered is whether the fault is in the right or left channel.
There could be a dirty contact where the soft clipping circuitry is shorted across in bypass mode but the connection is not clean.
Another point not covered is whether the fault is in the right or left channel.
Try changing C415 and C416 (220uf 10V) I had a similar problem with a Linn Intek and changing the capacitor solved the problem.
Thanks again, all, for the suggestions.
If I was not clear, I did do the freezing test on all the components, especially the transistors. (I just didn't do the Vbe tests that were suggested by Mooly as I don't have a scope.)
I had already tried cleaning the soft clipping switch (and switching it back and forward).
I tried changing C415 - it's the left channel that has the problem - as I had a 220uf/10v to hand. No change. (Curiously, C415 & C416 were in fact 220/65v on the actual board - maybe they knew that they could cause problems.)
I checked the Vbe's with a DVM as suggested by Mooly, they seem OK but that's not surprising as the crackle is just intermittent.
I did the freezing test on the early stage transistors again, but more strongly, in fact it does change the noise a bit, but it get worse when cold, rather than better.
I do get a few seconds of silence when first switched on after being off for a long while. Maybe 10 seconds or so. Is that's a clue? And there is a continuous hiss as well as the crackle. It's quiet, but there is none on the other channel.
Anyhow, I've ordered some c2240/a970 pairs for the early stage as it looks like they are the most likely candidates.
Thanks again.
If I was not clear, I did do the freezing test on all the components, especially the transistors. (I just didn't do the Vbe tests that were suggested by Mooly as I don't have a scope.)
I had already tried cleaning the soft clipping switch (and switching it back and forward).
I tried changing C415 - it's the left channel that has the problem - as I had a 220uf/10v to hand. No change. (Curiously, C415 & C416 were in fact 220/65v on the actual board - maybe they knew that they could cause problems.)
I checked the Vbe's with a DVM as suggested by Mooly, they seem OK but that's not surprising as the crackle is just intermittent.
I did the freezing test on the early stage transistors again, but more strongly, in fact it does change the noise a bit, but it get worse when cold, rather than better.
I do get a few seconds of silence when first switched on after being off for a long while. Maybe 10 seconds or so. Is that's a clue? And there is a continuous hiss as well as the crackle. It's quiet, but there is none on the other channel.
Anyhow, I've ordered some c2240/a970 pairs for the early stage as it looks like they are the most likely candidates.
Thanks again.
These are not easy faults as I mentioned before. You checked some resistors you say, did you check the specific ones I mentioned ?
Anatech mentioned ceramic caps... and he is correct, they can occasionally give trouble, the ones to look out for are the 'compressed disc' type which look like two saucers stuck together.
Anatech mentioned ceramic caps... and he is correct, they can occasionally give trouble, the ones to look out for are the 'compressed disc' type which look like two saucers stuck together.
I had intermittent crackling in one channel of my NAD 2700. It was my iPhone transmitting something occasionally causing the static.
If temperature change of the transistors changes the crackle then they are the likely culprit, unless it's something nearby being indadvertedly being cooled by overspray. Usually transistor stop crackling when cold, but it can be the other way around. Crackly transistors will likely still measure correct Vbe and even measure OK in a simple out of circuit tester.
I don't know if it's already been mentioned, but have you checked the power supply rails? Do it when the crackling occurs and make sure the voltages are steady, not jumping around. The input stage to the power amplifier has its own supply fed by R465 (+) and R467 (-) and filtered by C429 and C431 respectively for the left channel, R466, R468, C430, C432 for the right. With no signal the voltage across the smoothing caps should be ~56volts (will depend on the mains voltage being exactly as rated) and rock solid during the crackles. If it varies with the crackles something is might be wrong with one of those components. Also the the biasing zener diodes D407, D409 (L) and D408, D410 (R) could crackle. Being semiconductors these should also be tested with freezer. With small components I try to let get just a single drop of freezer to fall on the component I am testing.
I don't know if it's already been mentioned, but have you checked the power supply rails? Do it when the crackling occurs and make sure the voltages are steady, not jumping around. The input stage to the power amplifier has its own supply fed by R465 (+) and R467 (-) and filtered by C429 and C431 respectively for the left channel, R466, R468, C430, C432 for the right. With no signal the voltage across the smoothing caps should be ~56volts (will depend on the mains voltage being exactly as rated) and rock solid during the crackles. If it varies with the crackles something is might be wrong with one of those components. Also the the biasing zener diodes D407, D409 (L) and D408, D410 (R) could crackle. Being semiconductors these should also be tested with freezer. With small components I try to let get just a single drop of freezer to fall on the component I am testing.
If C415 has been changed it remains to see if the negative end has a good connection the earth at the chassis.
Another possible source of noise is the level of voltage across R447 and R449 in excess of 60 and what temperature these operate at.
The purpose of these is to feed 4v7 zener diodes protecting VR401 in the process generating a small amount of zener noise.
Since VR401 connects to the inverting input terminal any problem , small with noise, possibly worse with loose intermittent connection or bad solder joint would be amplified.
While R447 and R449 could be checked for accuracy of resistance - these have had a long service life so it might be best to replace them.
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The headphone outputs of the amplifier have 220 R series resistors.
You could use headphones for testing to save putting your speakers at risk.
That would make it possible to try moving the setting of VR401 quickly back and forth whilst monitoring output with a meter over say a dc output range of +/- 100 m.V. to see if that makes any difference to the noise.
You could use headphones for testing to save putting your speakers at risk.
That would make it possible to try moving the setting of VR401 quickly back and forth whilst monitoring output with a meter over say a dc output range of +/- 100 m.V. to see if that makes any difference to the noise.
This NAD uses breakers for protection.
Possibly the breaker in the one channel is faulty? Just a thought....
Possibly the breaker in the one channel is faulty? Just a thought....
The plugs on ribbon cables oxidizes and makes bad contact. Had that on a Nad214, I removed the plug and soldered the wires directly to the board. It caused intermittent DC offset of about 250mV and made scratching sounds as well.
Hi jhjove,
That's a very good thought. It wouldn't be the first time breaker contacts were defective. Couple that with a little DC offset and you have crackling.
That's a very good thought. It wouldn't be the first time breaker contacts were defective. Couple that with a little DC offset and you have crackling.
One could check that by listening on headphones with the speakers disconnected to see what if any change results. I had suggested listening on headphones in a different context with speakers disconnected - there has been no response in that regard.
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The only thing that might not work doing that is the magnitude of the load. The headphones have a few hundred ohms between the headphones and amplifier output. That may not be enough to create the noise current if it is breaker contacts.
Good idea though, worth a try.
Good idea though, worth a try.
Thanks, everyone, again for the suggestions.
I have tried listening through headphones, with the speakers on, and also with them off. The noise comes through the same in all cases.
I also tried temporarily shorting out the protection breaker switch. Still no change.
I wondered whether the problem could be narrowed down by grounding (or disconnecting) the signal somewhere. For example by disconnecting (or grounding) the base of Q417, or by disconnecting the bases of Q419 and Q421?
Maybe someone can say whether this is a good idea or suggest something else along these lines.
Anyhow, I've ordered new transistors for the stages before Q419 and will of course post the results when I have tried those.
Thanks all.
I have tried listening through headphones, with the speakers on, and also with them off. The noise comes through the same in all cases.
I also tried temporarily shorting out the protection breaker switch. Still no change.
I wondered whether the problem could be narrowed down by grounding (or disconnecting) the signal somewhere. For example by disconnecting (or grounding) the base of Q417, or by disconnecting the bases of Q419 and Q421?
Maybe someone can say whether this is a good idea or suggest something else along these lines.
Anyhow, I've ordered new transistors for the stages before Q419 and will of course post the results when I have tried those.
Thanks all.
An intermittent grounding problem could be an issue you could check for dc continuity with a meter with the amplifier turned off.
You still need ac continuity which you should look at series capacitors like C415 which you replaced, this having been replaced earlier by a higher voltage rated component.
If this component was open circuit then you would have close to silence since the gain of the channel would be the one so the output would be the same level as the input. This I doubt however if there is a dodgy connection anywhere between the negative end of C415 and earth you could easily have your present experience.
In post 24 you mentioned the problem starts about 10 seconds after power up. You could use that period to tap the pcb at points starting with the top of C415 broadly in line with traces/wire along the route to earth to see what that reveals.
You still need ac continuity which you should look at series capacitors like C415 which you replaced, this having been replaced earlier by a higher voltage rated component.
If this component was open circuit then you would have close to silence since the gain of the channel would be the one so the output would be the same level as the input. This I doubt however if there is a dodgy connection anywhere between the negative end of C415 and earth you could easily have your present experience.
In post 24 you mentioned the problem starts about 10 seconds after power up. You could use that period to tap the pcb at points starting with the top of C415 broadly in line with traces/wire along the route to earth to see what that reveals.
Hi Juanbic,
Grounding or opening connections in many circuits would really upset the DC conditions. You could, in an amplifier cause a major failure causing a lot of damage.
-Chris
Grounding or opening connections in many circuits would really upset the DC conditions. You could, in an amplifier cause a major failure causing a lot of damage.
-Chris