Can ya hear me - pumpin on yer stereo?

[Steve Conner]

As long as you don't allow it to form a complete shorted turn around your windings, there should be no eddy current losses, but it should still shield against RF. This is the idea behind the Faraday shields in better quality transformers.

Sticky-backed copper tape would do. I've used it for shielding all sorts of things, but it's quite expensive. Maybe the kitchen foil idea is the most cost effective after all.

If you can get it working quietly next door to a big broadcast transmitter it will probably work anywhere else.

[peskywinnets]

Over the next day or so, I'm going to try both these tests ....

1. Completely encase the pickup in kitchen foil (thereby a closed eddy loop will be formed!)

2. Encase with foil arrangement but in a broken incomplete loop.

I'll use my ears, then a bode plot.

Pound to a pinch of sh** , I'll not be able to discern any difference...& personally, as much as I love a good theory, I don't care two hoots about losses attributed to Eddy, just so long as the sound isn't degraded.

Watch this space!

[Mike Sulzer]

Speaking of over-theorizing, I don't think you have proven that it was rf pickup. Your preamp could have been oscillating as a result of the poor layout. The extra capacitance/shielding from the foil could killed killed it. Was the hiss pure, or did it carry some hint of signal on the strong station?

[peskywinnets]

Speaking of over-theorizing, I don't think you have proven that it was rf pickup. Your preamp could have been oscillating as a result of the poor layout. The extra capacitance/shielding from the foil could killed killed it. Was the hiss pure, or did it carry some hint of signal on the strong station?

I actually knew the RF was there all along (I'd previously heard radio stations outta my circuit - but was kicking that particular can down the road pending sorting the excessive hiss issues first!)...previously, towards trying to kill off the radio reception, I tried some caps across the differential (+ve/-ve) inputs - & many different values....but it made little difference.

the difference from shielding of those wires (& it was a rough arrangement - not totally encased all the way along their length) was nothing short of astonishing (ie through the aforementioned VST Amp sim).....radio recption gone along with a whole HEAP of hiss.....yet the pickup signal stayed as bright a broken glass.

I guess I'm not fully understanding why the RF picked up by the +ve sig & -ve sig pickup wires are not cancelling by the three opamps arranged into an instrumentation amp? (just to reiterate....I only wrapped the pickup to breadboard wires in foil - not the pickup itself) ....does common mode rejection only work well at lower frequencies?

[Steve Conner]

Yes, common mode rejection can only be relied on at audio frequencies, for two reasons:

Stray capacitances are never quite balanced, so the circuit will be unbalanced at RF.

If enough RF gets into a preamp it will drive the front end into non-linearity and so be detected, just like in an AM radio. Not only are the RF signals unbalanced as shown above, but even if they were balanced, the odds of the two front end transistors having equal detection sensitivities are very small. So in general the detected RF in a balanced preamp won't cancel out. Mackie use a common-mode RF choke in their mic preamps to keep it out.

[Mike Sulzer]

Yes, it is very unlikely that equal amplitude and phase signals at rf will get through the two first stages so that the cancellation could have a chance of occurring in the diff amp, not that it likely would in any case. I still wonder if an oscillation might have been occurring. Sometimes an oscillation can make an amp into a better detector of rf by driving it into the non-linear range.

[Joe Gwinn]

Sticky-backed copper tape would do. I've used it for shielding all sorts of things, but it's quite expensive.

What is cheap and useful is very thin brass shim stock: McMaster-Carr.

This is 0.001" thick, but there are other thicknesses, and one can get an assortment to figure out which thickness works best. Electrically, there is no difference; the issue is mechanical.

One can cut brass shimstock with ordinary scissors, form by hand, and it is solderable.

Insulate between layers with mylar or paper tape.