High Gain+buffer=no squeal. Why?

[idy]

Several times I've made high gain FET pedals that squeal when you 'max' the controls (black '65, this time DLS mkIII). Shielded wire, better lead dress did not help (although it turned out the circulated schematic of the '65 had incorrect values for some of the caps that limit high end).

The thing that does kill the squeal is putting a buffer before it! Why would that change the tendency of these circuits to oscillate?

[dschwartz]

Most squeal issues is parasitic currents going back to the input. Fets are very high impedance so are very sensitive to small currents..the proportion of signal current vs parasitic current is vastly improved with an input buffer, which boosts the signal current, squashing the parasitic currents.

Getting rid of squeals, noise and other undesirable artifacts is an art itself, mostly related to layout design, rf filtering and coupling/ decoupling techniques.

Even my control layouts have to be a compromise between avoiding this issues and user friendly positions..

[Quackzed]

my limited understanding suggests that high impedance lines ' like the ones from a passive guitar pickup' are better at  picking up 'noise' and electromagnetic interference. with a buffer, you take that high impedance 'weak' singal and make it a low impedance 'strong' signal. well strong as in better at driving long cables without losing highs and without being as affected by noise...
buffers basically take a wimpy passive 'loose jump rope' signal and turn it into a tough active 'steel pipe' signal. the noise 'wind' can blow the jump rope around easily and create oscillation and other crud, but the wind cant blow the steel pipe around and so you get less crud.
thats my twisted understanding of it for ya...  but i believe that its basically the high impedance signal vs. the low impedance one.  low impedance is more resistant to interference...

[J0K3RX]

I have battled with this many times...
Good example: read over the Black Forest posts  51 pages of jfet insanity... 
http://www.diystompboxes.com/smfforum/index.php?topic=98954.0

Somewhere in that topic I discovered that a buffer in front of the pedal eliminated the hum and squeal... I noticed that if I had a tube screamer in front of the Black Forest it eliminated the unwanted noise even when it was bypassed. The tube screamer I was using was a stock Ibanez TS7 which does not have "true bypass" so even in bypass mode the signal was being buffered..

In any case Daniel pretty much summed it up perfectly with what he posted!

[idy]

Thanks all.