Why would active vs passive guitar cure my buzzing ??

[pedro]

Dear all,
I thought I'd cured my buzzing 18w - but sadly not quite yet...

after I sorted the filament winding erreor I used my Godin to check the amp and all seemed great.
But I now discover that I'd used the active socket ( the guiatr has 2 jacks and synth access ) .

When I use the passive socket I get a bad buzz behind the sound. Ive checked with another good guitar ( Swamp Ash ) and buzzing is also there, also the squeal is there again at high vol knob settings. Bugger...

So now I need help again - could it be that the active circuit presents a much different loading to the amp input and therefore cures the problem ?
Could it be a problem with the grid resistors for the input stage ? I cant see how as its the same on all 4 sockets on both channels.

As soon as I use the active on the Godin its gone - perfect amp ! Passive, its rendering the amp unuseable at present. I MUST find this - the tone of the amp is fantastic otherwise. Incidentally the active is not just louder - so its not just noise floor - its a definite BUZZ. Like playing next to flourescent tubes.

Is there anything I can check / try to fix it ?
It must be input stage issue - with the jacks unpluged and grounded the amp is one of the quietest I have.......
TIA
Pedro

[zaphod_phil]

I'm not sure about the active/passive thing, but buzz is usually RF coming into the amp. Here is what you can do about it.

1 - Connect the ground side of your input jacks to chassis via 0.01uF caps with very short leads. This is to get rid of buzz coming in ob the guotar cable.

2 - Ensure the chassis is shielded on all sides when the cab is closed.

3 - Use an EMI-filtered IEC mains connector for where the power cable plugs into your chassis.

I usually find buzz problems when a light dimmer is operating somewhere in the vicinity.

[pedro]

Thanks Phil,
I'm going to try everything...

Ive taken a set of digi photos of the innards last night so I'll post them onto the troubleshooting section this week some time when I have chance.

I did wonder of there is a methodical way I could use my second hand scope to track down the culprit - looking for ripple etc ?? I am a scope novice though so would need some guidance here.

Any thoughts anyone ?
Rgds
Pedro