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Noise from speakers 2 hours after switch off????

Pine Marten

pfm Member
I have 2 monoblock Avondale NCC220+Minicap6 power amps in Naim 110 boxes, original transformer, mains switch etc. I switched off the amps and as usual, the LEDs gradually whent out and there was a tiny "plop" from each speaker in quick succession about 20 seconds later. This is normal. I then switched off the HiCap. I dont leave the gear running as I know I wont have time to listen properly to music again till next weekend. About 2hours later I heard a distinct click or crack, like the sound I sometimes get when the fridge starts up, it was certainly from the rh speaker! Now, I removed the speaker leads and with my DMM measured 40mV dc, which slowly fell by degrees to 10mV over about 15 mins. BTW the rh speaker has the longest straight length of NAC5, 4m ish, the lh run is taped up in a folded bow tie shape bundle to cancel out induced currents etc. Equipment is powered from the standard ring main, there being only one, as the hovel only has three rooms+ bathroom. Supply to the house is overhead and at least 8 other properties share the same phase. Should I be worried? It never happened with the standard monoblock 110's! Comments please?
 
Does it happen if you don't switch off the Hicap?

I am surprised you switch it off Naim pre amps seem to need two days to warm up, and it won't use much power.

Pete
 
If all the gear is turned on, I do get the odd "mains click" type noise, both channels. I know I dont have the cleanest or most robust mains supply, with so many properties fed from the same lines and post mount transformer. Power cuts happen 3 or 4 times a year. Voltage varies too, between 238 and 242. Lamps dim momentarily if neighbours are using a tumble dryer or big microwave. My mains instalation was new about 35 years ago, when the derelict cottage was rebuilt. It had no supply previously. Many of the neighbours is much much older, cast iron termination boxes and two rubber insulated thumb thick cables! I dont like leaving stuff turned on, I'm of the opinion that this only speeds up ageing of big electrolytics and may be why my S/H HiCap was a bag of poo before I serviced it! Gear is switched on a good few hours before I need it on Friday evening as I come in from work and then stays on untill Sunday bedtime.
 
I wouldn't worry about it.. or your mains supply.

Your stance over switching gear off is to be encouraged! I always advise my customers to switch gear off, no matter what the manufacturer may say.
 
I've had to trawl through my memory banks for this and must say, it's very unusual

Going back in time, my pal had a 42/110 setup driving Lowthers. He used NACA5
cables and one day, I noticed the sound of a radio prog coming from the kit. The
amps by then, had been powered down for some time and I could only surmise a
situation caused by the length of his cables being resonant at the frequency of a
local transmitter less than a mile away.

Some ferrites snapped around his cables at the amp end put paid to that problem.

Hope that helps,

Les
 
^ Another reason for the Naim 3.5m speaker cable limit perhaps? It's just outside the FM band.

NACA5 makes a great dipole. If only they'd fit that inductor on the output.

RE: the original enquiry, it is my experience on our own amps with CAP6 and a mono NCC200 board that there's not enough current draw after the constant-current stages of the amp have shut down to fully discharge the power supply. Two boards sharing the power supply were fine but a single board took hours to discharge. In the end I fitted a couple of 2W 3k3 bleeder resistors across the outputs of the CAP6 (the same ones we use on the NAP250 power supply boards). This fixed the issue completely.
 
I suspected that the generous reserve in the Minicaps might be the reason. Even though I am very close to a powerful FM regional transmitter, I was doubtful it was to blame. I am a radio amateur and know a bit about RFI. Will give the bleed resistor idea a try if I notice the problem is a regular occurence. I was just a bit concerned that odd clicks from an otherwise switched off system could cause expensive damage to my amps or speakers! Thanks all!
 
I'm taking the Intermediate exam next week. It really does seem that inductors fix everything! Most of the Naim-based RFI I've seen has been due to odd speaker cable placement and geography, most notably the bi-wired NAP250's in line of sight to Wrotham and Crystal Palace. Les's NCC200 inductor stopped that in its tracks.
 
Good luck with the exam! I have no radio at present, long story, persevered for years in the current QTH, which just happens to be at the bottom of a steep sided river valley in the foothills of the Pennines, just couldnt get a decent signal out! Doesnt help when your main interest is VHF/UHF either! Rain and wind lashed nights and weekends on local hills with portable and homebrew antennas with rigs and linears keeping the inside of the Landrover warm was fun though. Working stations in Norway and Sweeden on 2m aurora, deeep into eastern and southern Europe on 2m and 70cm tropo etc. G6BCC
 


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