Re: Mains interference on power amp


"Earl Kiosterud" <someone@xxxxxxxxxxx> wrote in message
news:B6sak.197$713.167@xxxxxxxxxxx

"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> wrote in message
news:xtGdnZ7aopzXj_fVnZ2dnUVZ8qTinZ2d@xxxxxxxxx
Hi,

this really is driving me nuts now.

I have a EV P3000 that is intermittently going into protect. I think I
have established why.

Mains spikes are getting into the amp. A lot of them. I can make the
amp trip every time by switching off my soldering station slowly and
making its mains switch arc. This is putting over 10 volts of HF hash
onto the amp chassis, PCB earth traces and power supplies.

The trip circuit runs on the unregulated -26 volt supply that feeds
the -15 volt audio circuit, the +26 volt supplies the +15 audio. When I
do the soldering iron switch arc thing, the up to 10 volts of hash
appears on both the ground and the -26 volt supply, and even the amp
chassis. This immediately trips the circuit, muting the amp for 2
seconds before resetting and carrying on fine. Note this 26v supply
isn't very clean, it has around 2 volts of ripple to start with. (I have
compared this to another amp which has the same ripple).

I have tried 0.1uF caps on the -26v rail on the protect switching
transistor, and a pair of 0.1uF caps on the +and - 26 volt rails, no
help. I have also tried a 0.1 uF X cap accross the mains live and
neutral, still getting 10 volts of hash. All these are stilll in place,
except the one on the switching transistor.

The mains plug earth to chassis reads good. Switching ground lift on and
off has no effect. (just disconects signal input ground I guess). Both
existing X caps on the mains input read OK.




Not sure where to go from here.


Any ideas? Am I barking up the wrong tree thinking this is the problem?


Cheers,


Gareth.


If you're seeing a spike on the 26 Volt line AND on the ground of the amp,
then your scope is not seeing what the amp sees. You might need to float
the scope ground, grounding it only to the amp, or use it in differential
mode, putting one probe on the chassis. But...

It doesn't seem likely that a spike could get across the power supply --
not if it's a very short one. You should probably start at the protection
circuit, and try to see what it sees. Or...

Try disconnecting all cables, speaker, audio, antenna, ground wires --
absolutely everything except the power cord (no scope leads either -
nothing), then do your soldering switch thing and see if it still trips.
It may not, indicating that it's not sneaking into the power supply. Keep
everything else the same; don't even move any cables. Ground-induced
high-frequency noise can be the culprit, but rarely gets looked at.
--
Earl



Thanks, I have just gone back and noted that if I probe the ground of the
scope some hash is there, so this is indeed a misleading reading, the probe
must be an aeriel. With the probe removed from the scope there is still a
small amount of hash. Maybe this explains why putting caps on the power
supplies has no effect - there may not be any hash on the power supplies at
all.

I now have a second P3000 from the same guy with exactly the same symptoms.
It is here on my bench unopened. It has nothing connected to it, and when I
arc my soldering station this too goes into protect. He has brought me
these amps because they trip randomly at different venues, and I am assuming
now it is spikes on the mains that are the trigger. I am also assuming that
the amps should be able to tolerate an arcing mains switch without jumping
into protect.



Ah, while writing this I have just realised that it may not be going into
protect at all - it may be going into its power up cycle, which behaves
exactly the same as a momentary protect/reset. (The 2 protect LEDs light,
and the fans go full speed). There are 3 relays on the power supply input
side, 2 for soft start, a third for protect. These are situated within the
mains input/fusing part of the PCB. It could be that these mains spikes are
triggering a tired relay somehow and starting the power on cycle? I've been
looking at hash on the power supplies that probably isn't there, this is
starting to make a little more sense.



Cheers,


Gareth.






.

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