Noisy noise generator

[Michael Buchstaller]

Hello list members,

again i am buggering you with one of my silly questions.

I wanted to make a very simple noise source for my modular. So i took two zeners
in series; a resistor to V+ and one to V-;  the junction between them should go via a
small decoupling cap into an opamp.

OK, it sorta works, but the output is very weak and rides on approx. 100 mv of a distorted
50 Hz sine wave.  Now my first thought was that this would be mains cycles bleeding through
the power supply (a simple 7815/7915 combo on my bench), so i tried to smooth it with lots
of caps, but it did not change anything. When i probe the +, - and GND lines with the scope
probe, i cannot see any noise, hum or the like. I even shielded the zeners with a grounded
ring, but also no help.

The (unwanted 50 HZ) noise is approx. 20 times the usable real noise signal.

I have used a LM324 in the 1st instance, then replaced it by a TL084, but no change. (i did have
only quad opamps here now, but i bet a 741 would not help, too)

Here is the schemo and two photos of the experimental board:

http://www.geocities.com/m100de/noise.html

What am i doing wrong ???


-Michael Buchstaller