Screeching noise when powering On and Off a NAD C326BEE stereo amp

[cincin]

I have a NAD C326BEE stereo amp that screeches in both channels (though not exactly the same screech in both channel) when I power it on, and then again after I power it off, using the front panel power switch. It sounds like a needle scratching across an LP, and is independent of the volume pot level.

The service manual (with schematics) for this amp is here.

The amp also has an occasional pop in the left channel (independent of volume, source, or whether there's a signal), but that symptom is very intermittent and difficult to troubleshoot. The pop will sometimes activate the protection circuit of the amp, which makes it go offline (red light on front panel)

The amp has a protection relay that turns off the speaker outputs until a second after the amp powers on (and immediately when I power it off), so the screech doesn't actually come out of the speakers, but it does come out of the headphone jack.

With this easily reproduceable screech, and a bit of help, I'm hoping to be able to finally fix this

The screeches are present on both output channels before the headphone/speakers split, so it's not just on the headphone circuit.

The power amp area of the PCB is a bit discolored from heat. The amp was on continuously for many years, but used rarely.

I have disconnected the pre-amp and power amp sections by removing the RCA jumpers at the back, and the noise is still present on the output channels, which I believe rules out the pre-amp.

I have:
- Replaced a seized trim pot
- Resoldered every joints that looked suspicious
- Tested the large capacitors of the power supply board
- Carefully inspected the catalytic capacitors
- Tapped around with a chopstick (no noise)
- Scanned around with an infrared thermometer and ended up plucking out C207 because it was hotter than C408 (equivalent in the other channel), but it tested fine.
- Sniffed around with a Carlson Super Probe type signal tracer, which sniffed the screeches coming out of most of the power amp section, both sides of the PCB, but otherwise no randomly noisy parts that I could find.

I wish the signal tracer probe was giving me a hot or cold indication when sniffing around, but the screech seems to permeate the power-amp circuit when it happens, so no areas stand out as particularly suspicious.

I'm hoping the turn on/off symptom can help narrow down which parts to pluck and test. Insights greatly appreciated.

Thanks!

[CaptDon]

I am not sure I follow what you are saying? The screech not present in the speakers because the relay disconnects them at power up/down. O.K., that is often why the relay is there, to eliminate the pop and the squeal that most poorly designed amps seem to always have. As for the squeal in the headphones, they may be tapped to the output stage prior to the relay and don't disconnect during power cycle. You state there is a screech coming from the power supply at power down??? Is this a switch mode power supply, if so many of them have that symptom as they go out of regulation while the input energy capacitors run dry. The power supply screech (if it is a switched mode supply) may be magnetically coupling into the amplifier circuitry and coming from the huge irregular pulses of a power supply trying to regulate when there is not sufficient energy remaining to stay in regulation. The only cure may be replacing every capacitor in the entire unit. Some SMPS supplies have an undervoltage lockout UVLO that prevents the supply from trying to regulate when its input supply is out of range. They tend to not squeal or hiss at power down. Almost every power amplifier I have ever tested or worked on had severe power up/down characteristics that were mostly cured by a 'speaker relay'. At some point during power down voltage decay the amplifier's very own negative feedback loop circuitry is the very thing that causes the squeal due to the amplifier entering a non-linear range due to undervoltage on its supply rails. Many folks 'just live with it'. Carver solid state amps are known for 'speaker thump' mainly at turn off. There were several 'factory' mod suggestions and in the end they still thump!!

[cincin]

No screech coming from the power supply, the screeches are all in the output channels, both at power On and power Off.
I see where the power supply screech confusion came from, I typed "power supply" when I meant "power amp" at on e spot in my original post. I've corrected that now. Sorry about that.

Are you saying that being able to hear the power On and Off screeches in the headphones may be normal? From what you describe, it sounds like stereo amps tend to have power On/Off screeches (I had no idea), and for some reason NAD may have decided not to protect the headphones from that noise? If that's the case, then those screeches may be "normal" and not a symptom of my pop problem, as I was hoping...

I appreciate the insight! imagine how long it would've taken me to find and fix a noise that is "by design"!

[JustMeHere]

Sounds like the characteristics of a class A amp. 

[SeanB]

Looks like they use that output relay to disconnect the speakers with both power on and off, as well as when you plug in headphones, so likely the noise is there always, just muted when you have speakers alone. Easiest fix for the noise with headphones in would be to add in another relay, low power type, that will simply act to short the headphone socket pins when not energised, so that the noise is not heard. 12V relay, fed from Q353 collector and 12V, with DPCO contacts, arranged to short pin 3 and 5 of J101 when not energised. Does not need high ratings, any small low power 12V DPDT relay rated over 100mA contact current will do. Should work well, seeing as there already is input muting when power is transitioning on and off, and also for overload.

[u666sa]

You gotta track that pop. Look for it with oscilloscope at the output. See what it looks like. Then go down the amp, maybe half way down and then if you see it there as well, further down, if not, then go up. It should be a cap. So once found where the pop coming from, you gotta inspect all caps with LCR meter.

[CaptDon]

We know the screech/thump is in the power amp part. Doubt the problem can be found with a scope since with the negative feedback of most amplifier circuits the anomaly will be present everywhere in the power amp chain. Usually in an amp with dual supplies (for instance + and - 40vdc) the weird anomalies come from one supply running down quicker than the other and the amp tries to balance the output at zero thereby going non-linear and attempting to become an oscillator instead. As stated earlier, with the age of the amplifier the O.P. may want to shotgun the amplifier stage with all new capacitors and just be done with it.

[cincin]

Thanks, gang.

Easiest fix for the noise with headphones in would be to add in another relay, low power type, that will simply act to short the headphone socket pins when not energised, so that the noise is not heard.

I'm not actually worried about the headphones being protected from the screech. I was just hoping that the reproducible screech was a symptom, and might help me find the cause of the pop. If the screech is normal then I will live with it.

You gotta track that pop. Look for it with oscilloscope at the output.

As CaptDon pointed out, I'm pretty sure the pop can be found all over the power amp section. I'm suspecting a cap, but others have pointed out that it could also be a diode or transistor. There are so many parts in this amp, I'm hoping to limit it down to the biggest suspects before shotgunning it.

Usually in an amp with dual supplies (for instance + and - 40vdc) the weird anomalies come from one supply running down quicker than the other and the amp tries to balance the output at zero thereby going non-linear and attempting to become an oscillator instead. As stated earlier, with the age of the amplifier the O.P. may want to shotgun the amplifier stage with all new capacitors and just be done with it.

I could shotgun all the electro caps, but others have suggested I should check the transistors first. All the caps plus all the transistors is quite a few parts, but my attempts at narrowing it down have so far failed... so I may just have to start replacing everything.

I appreciate the guidance. Without it I'd still be chasing the screech.

So, cheers!