Hunting down noise sources in my current design...
I was disappointed with a noise output figure of about .6mV on my most recent project and went looking for the source. I'd worried about this one because I'd really crammed the componentry close together-- I was especially worried about the input transformer picking up stray noise from the power toroid, or the fact that I was also running the V+/G/V- as a twisted bundle right under the input transformer...
Nope and nope. The source of my noise-- the bloody input coupling cap. Apparently when the film/foil cap is half the size of your fist, it becomes a dish-freakin'-antenna for stray signals. Simply putting it inside a 1/4" thick metal case is not enough. Apparently one also needs to shield it from everything inside the case too (the things I do to completely kill DC offset).
The test that identified the input coupling Orange Drop as the interference source was to wrap it in aluminum tape and ground the tape to the chassis. That alone killed off all last traces of hum/hash (measured by sticking my ear right up onto the cone, in the dead quiet basement workshop with everything else electric turned of at 3am) although there's still stray measurable white and RF noise on the scope...
Now I'm wondering-- do I add a sheet-metal 'clamshell' shield over and under the entire input stage PCB? Do I ground that to chassis or signal ground? Maybe it's also time to go to shielded cable for internal connections...
Something 'new' every day
I was disappointed with a noise output figure of about .6mV on my most recent project and went looking for the source. I'd worried about this one because I'd really crammed the componentry close together-- I was especially worried about the input transformer picking up stray noise from the power toroid, or the fact that I was also running the V+/G/V- as a twisted bundle right under the input transformer...
Nope and nope. The source of my noise-- the bloody input coupling cap. Apparently when the film/foil cap is half the size of your fist, it becomes a dish-freakin'-antenna for stray signals. Simply putting it inside a 1/4" thick metal case is not enough. Apparently one also needs to shield it from everything inside the case too (the things I do to completely kill DC offset).
The test that identified the input coupling Orange Drop as the interference source was to wrap it in aluminum tape and ground the tape to the chassis. That alone killed off all last traces of hum/hash (measured by sticking my ear right up onto the cone, in the dead quiet basement workshop with everything else electric turned of at 3am) although there's still stray measurable white and RF noise on the scope...
Now I'm wondering-- do I add a sheet-metal 'clamshell' shield over and under the entire input stage PCB? Do I ground that to chassis or signal ground? Maybe it's also time to go to shielded cable for internal connections...
Something 'new' every day
