MoodRing - Weird oscillation noises

[culturejam]

Got a MoodRing board worked up today. Sounds pretty good, but there are two issues:

1) Sometimes, and for not apparent reason, there is high-pitched oscillation that creeps into the signal and won't go away until I kill the power.

2) If I hit the strings hard, there is a weird sound in the echoes. It has the same pitch as the aforementioned oscillation.


My initial thought is that oscillation in this type of circuit often originates at the voltage regulator. So I may replace it and see if that helps. I'm also wondering if the limiting LEDs are somehow making that odd noise on loud input signals. I hear that noise after the LEDs light up with a hot signal.

Anybody experienced anything similar? I did a search but didn't find a whole lot on this circuit.

[culturejam]

Did some poking around.

I definitely think it's regulator-related. I noticed that after the schottky protection, the supply voltage is where it should be (~8.8V), but on the supply pins of the op amps and the regulator, it was down to about 5.8V (and the regulator's output was about 4.2V).

So I put a different regulator in (one that I got from Mouser), and it improved slightly to about 6V on the supply pins and regulator output of about 4.5V.

When I remove the regulator completely, supply voltage goes back to where it should be. So I'm thinking the regulator is not able to handle the current draw of both the brick and the other PT2399.

Anyway, with the better regulator, the circuit works a lot better. But if I hit the strings hard enough to make the LEDs turn on, that weird oscillation comes back and won't go away until I power down the circuit.

[madbean]

Well that is just strange because I have not had a problem at all with the L05, and definitely no unwanted oscillation. The LEDs should only light up when the dwell knob is quite high and normal feedback oscillation occurs (to serve as a volume clamp). Any chance your brick or PT2399 is hosed? What if you remove the two LEDs altogether?

[culturejam]

I swapped the 2399 already, but no change.

My next test will be to remove and/or replace the LEDs. Then I'll try the brick. Might record a sound sample as well just for reference.

[culturejam]

Just did a little datasheet snooping.

Typical current draw for the brick is 60mA, and for the 2399 it is 15mA. Each op amp draws about 1.4mA x 4 op amps = 5.6mA. So the grand total for "typical" operation (whatever the hell that is) would be ~ 80.6mA.

LM7805 outputs a max of 100mA, so under typical conditions, it should be sufficient.

But brick and swing as high as 100mA by itself (at max). So maybe in my case, the brick is running somewhat high (say, ~80mA) and the PT2399 is its typical 15mA and the op amps are doing their normal thing, and that would bring the total draw right to limit of the regulator.

All theory of course, but interesting.

[culturejam]

Pulled the diodes, but the noise is the same. Not sure what is going on with this build. 

[Morgan]

Have you signal tested the circuit? It seems odd that you can get the LEDs to light up without runaway oscillation already occurring. Triple checked resistor values? Maybe a gain resistor is an order of magnitude high in one of the PT op amp stages or something.

[culturejam]

I'm thinking the same thing, Morgan. I probably screwed up a resistor value somewhere.

But I would be curious to see what others with properly working builds are getting for voltages on the op amp power pins and on the regulator's output.