Power Supply Problems

[Adji]

Hey guys. Haven't been on in a few weeks, hope your all doing okay?

I have a small problem which I am sure is an easy fix.

I built a small amp based on the Smokey Amps schematic taken from Dano (BeavisAudio's) website. It works great when powered from a 9 Volt battery, but when I plug in a 9 volt power supply I get this horrible screeching, buzzing sound. It sounds like something which has been 'circuit bent.'
Any ideas why?

BTW I tried a few different power supplies and got the same problem.

[GibsonGM]

I'm sure you have the polarity right!! Right?
Sounds like a filtering/decoupling issue.  I'm imagining that the wall warts aren't very well regulated, so you may need to add some filtering where the PS enters the board.  Perhaps a resistor followed by a cap to ground...

A 'try this first' trick may be to add some decoupling caps across IC power supply pins (not familiar with the circuit so I dunno what you've got there).   Try .1uF or so across the + and - terminals of any opamps....sometimes that will help to stabilize them.

[GibsonGM]

PS - a 2nd thought...you're not using a 9V battery at the same time as the power supply, right?  Like, you're sure the battery is disconnecting when you plug in the PSU?  Doubling the voltage could cause some issues, too!    Disregard if that's not the problem...

[Adji]

Hey man, still very much a noob so I only THINK I have the polarity right. I was just kind of assuming that it is centre negative?

Battery is definitely unplugged when wall socket is in. The 'amp' is powered on a 9Volt DC Jack and to power via a battery I had to use a Battery snap to (V barrel plug (thing) adapter.

[composition4]

As per LM386 datasheet

Try adding (to the output at pin 5) a 10 ohm resistor and a 0.047uF cap to ground to prevent oscillation

Like shown in the "Little Gem" schematic here:



Let us know how that goes

[Adji]

Hey man thanks for that. It did improve the situation but it is sitll pretty bad. It sounds like a very Lo-Fi fuzz type sound and the notes are hardly distinguishable from noise, but there is improvement.

Any more ideas matey?