Re: Mains interference on power amp


"Kevin McMurtrie" <mcmurtri@xxxxxxxxxxxxxx> wrote in message
news:mcmurtri-FD49C7.15202701072008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <xtGdnZ7aopzXj_fVnZ2dnUVZ8qTinZ2d@xxxxxx>,
"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> wrote:

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.

It doesn't make much sense that a little soldering station would produce
so much electromagnetic noise. I suspect that your amp prone to
oscillating and that's what's showing up everywhere on your scope. Is
the HF hash still strong if the amp is off while you arc the contacts on
your soldering station?




No, the amp is not oscillating.

I think I know what the problem is now. The 3 relays that do the soft start
and the protect switching are on the same PCB as the protection circuitry.
I removed all 3 and the PCB underneath was not too clean. Since the relays
switch mains voltages, it is not beyond the realms of possibility that mains
voltage and any hash will make its way over to the circuitry via the dirty
PCB. The main protect line runs along all 3 relays, only 5mm from the relay
coils. I have also established beyond doubt that the protect circuitry is
actually being pulsed on briefly via this protect line.

I'm going to fit 3 new relays tomorrow and clean it all up, and hopefully
the nightmare will be over. I might even re-route that protect line.



Cheers,


Gareth.


.

Relevant Pages

  • Re: Mains interference on power amp
    ... I have a EV P3000 that is intermittently going into protect. ... Mains spikes are getting into the amp. ... It doesn't seem likely that a spike could get across the power supply -- ...
    (rec.audio.tech)
  • Re: Mains interference on power amp
    ... Mains spikes are getting into the amp. ... This is putting over 10 volts of HF hash onto the amp ... I have tried 0.1uF caps on the -26v rail on the protect switching ...
    (rec.audio.tech)
  • Mains interference on power amp
    ... Mains spikes are getting into the amp. ... This is putting over 10 volts of HF hash onto the amp ... I have tried 0.1uF caps on the -26v rail on the protect switching ...
    (rec.audio.tech)
  • Re: Mains interference on power amp
    ... Mains spikes are getting into the amp. ... This is putting over 10 volts of HF hash onto the amp ... I have tried 0.1uF caps on the -26v rail on the protect switching ...
    (rec.audio.tech)
  • Re: stereo amp cleaning
    ... Any protection circuit is there ... simply to protect the speakers from an amp failure, ... I was going to mention tubes but I'd already gone on too long... ...
    (sci.electronics.repair)