Author Options:

I have built the "Sweet Portable Guitar Amp" but there is a very load hiss.? Answered

I have built this and its working fine, except there is a very load hiss along with the audio. I have only made 1 modification: I have connected the negative terminal of the battery to the 2nd signal of my stereo 1/4 inch socket, and the ground to the ground circuit ground. This causes the circuit to turn on when a mono jack is plugged in.

Any ideas why I am getting this hiss? It sounds a bit like static on a TV but I have used shielded cable.



The forums are retiring in 2021 and are now closed for new topics and comments.
Quercus austrina
Quercus austrina

Best Answer 10 years ago

You have a "load" (loud) hiss along with the audio. This usually signals very high gain.

If you turn the gain knob (between pins 1 and 8) toward maximum resistance (the unconnected end of the variable resistor), does the hiss go away or become reduced? If you short the VR out (have just the 10uF cap in the path), does the hiss go away?

Are the capacitors inserted in your circuit with the correct polarization? + goes toward the incoming signal, - goes toward the output.

You say you have used shielded cable. Are the rest of your wires twisted? This helps cancel out noise.

Have you shielded your enclosure? While this may not stop hiss, it helps keep other noise out of your circuit.

Your power on scheme should not introduce any hiss unless you chose the wrong connection. Verify that you used the tip for signal, ring for -V, and sleeve for GND.

Lastly, are any of the components old? I mean over 10 yrs old. If so, they may be deteriorating and introduce noise. Newer components should be ok.

Do one thing at a time and check the operatin of the amp. Once the hiss stops, you will have found the problem. Troubleshooting at its most basic.



Answer 10 years ago

Thanks for quick replies
The gain was already on the minimum so i tried shorting out the VR and I heard some of the strangest noises I have ever heard.

I checked all the caps those are fine.

When you say 'Are the rest of your wires twisted?' do you mean the output wires? I twisted them and there is no difference.

By the way I recently built the Little Gem Guitar amp which works well, with thesame wires, and has no interference, while this one picks up cell phone interference.

I don't have a shield for this.

Based on what you said I just realized what could be wrong. My only other forum post on this site is about exactly the same problem, just a slightly different cause! I can't believe I made the same mistake twice in a row! The problem is that I am converting a stereo source to a mono but by mistake transmitting the 2nd signal along the GND. Really silly mistake! I will try to find away around.

Thanks for the help!


10 years ago

I'm not sure exactly how you have the battery hooked up. You should wire it exactly as drawn and see if that fixes it, if not then you either have a mistake in the wiring somewhere or a bad part.

Does it hiss even when no instrument is plugged in?

Did you try changing to a different cord?