Stratoblaster radio interference

[bdevlin]

I built two of these things and both have radio interference.  One was built on perf point to point and the other was the PCB from GGG.  One is 9V battery powered and the other is powered by a wall wart.  Curiously the wall wart one also has a hum that is equal in frequency to a low B on a guitar.  Any ideas to fix the radio interference??  I have tried a .01 cap across the power jack for the DC hum to no avail.  I have also tried a 220pF cap from input to ground for the radio interference also to no avail.

[rockgardenlove]

The hum is probably just a wall wart with bad filtering.  Use a large value electrolytic cap (>100) to try and smooth it out.  That might help.

Try  putting a resistor in series with the input.

[bdevlin]

Won't a resistor in series with the input just cut the signal level all-together?  Also, the board has a 100uF cap in parallel with the power line.

[brett]

Hi
For the hum, use a smalll resistor (100 ohms) is series with the 9V input, followed by a 100uF or larger cap (470uF 16V are quite small) from the 9V rail to ground.  This will filter out everything except the lowest of low frequencies.  (This trick works on everything except some digital power-hog pedals).  The resistor adds a LOT extra filtering power to the usual cap.  (Except for semi-flat 9V batteries that hhave plenty of internal resistance)

Unfortunately, that JFET works right up into the UHF range.  Use a small cap from the gate to ground to bypass the highs brought in by your radio antenna guitar and human body.  In older circuits you often see 0.001uF caps used for this, usually placed right at the input.
cheers

[tcobretti]

I've been having the same problem with my V847 Vox wah.  It seems weird to me that this would happen and I wouldn't have heard about it here.  I understand the fix, but has anyone had this happen with their wah?

[bdevlin]

Thanks for the tips.  I'll give them a try.  I'm hopeful with the DC hum.  The radio interference.....not so sure.  These high gain transistor effects seem very susceptable.

[bdevlin]

Hi
For the hum, use a smalll resistor (100 ohms) is series with the 9V input, followed by a 100uF or larger cap (470uF 16V are quite small) from the 9V rail to ground.  This will filter out everything except the lowest of low frequencies.  (This trick works on everything except some digital power-hog pedals).  The resistor adds a LOT extra filtering power to the usual cap.  (Except for semi-flat 9V batteries that hhave plenty of internal resistance)

Unfortunately, that JFET works right up into the UHF range.  Use a small cap from the gate to ground to bypass the highs brought in by your radio antenna guitar and human body.  In older circuits you often see 0.001uF caps used for this, usually placed right at the input.
cheers

This worked.  Thanks a million!  I ended up going with a 470pF cap.  I think I heard too much high freg. cut with the .001 and a 220 pF didn't quite cut all the radio interference.

[brett]

Hi
glad it worked.  Thinking about it a bit more, I've realised that a more elegant solution is a cap to bypass the drain resistor.  For a 5 to 10k resistor, a 0.001uF cap should do the trick.
cheers

[bdevlin]

Hi
glad it worked.  Thinking about it a bit more, I've realised that a more elegant solution is a cap to bypass the drain resistor.  For a 5 to 10k resistor, a 0.001uF cap should do the trick.
cheers

Hmm.  Not sure what you mean.  In the schematic there is a 22k resistor that goes from the Drain to +9V.  Do you mean the cap would be in parallel to that?

[brett]

Yeah.  In most JFET boosters/buffers the drain resistor is smaller: 4.7k to 10k.  For the 22k in the Startoblaster, 0.001uF would be too high.  330pF would be better.