Helicopter sound in Nurse Quacky

[mark2]

I built this tagboard layout, and everything about the effect seems to work fine (with the exception of pulsing, described below).

However, I get a background helicopter sound (~380 bpm). It seems to be unaffected by any of the controls.  I tried switching to a battery with no apparent change.

Perhaps related: I get a pulsing of the effect (about 180bpm) that's more pronounced when the attack is turned up high. This results in a "wah wah wah" sound.

My diodes are matched, I used a lm1458 and 3904 (also tried a lower gain 3903). Only change I made was a 20K range instead of 25K.

Any pointers on where to look for this issue?

[Mark Hammer]

Question:  If you turn the sensitivity way down, such that the filter remains in the bass region, do you still get the "helicopter" noise?  If not, that suggests something is amiss with the components used to set the time-constants, such that you're hearing the impact of serious envelope ripple.

What cap value (going to ground) are you using in the envelope-follower section?

[mark2]

Hi Mark. I really appreciate you taking the time to give some input.

I have a 1nF going from 6 (-IN2) to GND.

If you turn the sensitivity way down, such that the filter remains in the bass region, do you still get the "helicopter" noise?

Yes, the sound remains and seems unaffected by the sensitivity.

By the way, do you recall ever noticing a rhythmic pulsing of the effect? (as if somebody is rocking a wah pedal) 

[Mark Hammer]

To answer your question, no.  But the general rule of thumb is that if something takes time to start happening, or occurs and reoccurs in repetitive/rhythmic fashion, one should suspect a capacitor.  Not always, but often, since rhythmic, oscillating things are typical of charge/discharge cycles.

Is the 47uf cap that smooths the reference voltage oriented correctly?

[mark2]

Thanks. Unfortunately that 47uF cap is correct. I'll keep looking for cap-related issues just in case though.

[Mark Hammer]

There are two major subcircuits here.  One is the filter itself, and the other is the side-chain that makes the filter move around.  If you unhook the one from the other, that should help identify roughly where the problem lies.  If you lift one end of the 22k resistor feeding the base of the transistor, it will separate those two subcircuits.  If you continue to hear the helicopter then there is a good chance the source is in the powering of the overall circuit.  If the helicopter goes away, however, then the likelihood is that the problem lies in the rectifier/envelope-follower section.

[mark2]

I've been waiting to post back since I want to do further investigation first, but it maybe a week or more before I get back to this so I just wanted to again thank you for more clues and insight. Looking forward to getting this working!

I'll split it up to help narrow down the problem.

[mark2]

I went back to this project today (after having done nothing to it since last post) and the sound is barely audible even at the highest amp settings. It's great now.

I'm wondering then if it must have been a power supply issue.

[11-90-an]

Probably just one of those I-leave-this-circuit-alone-and-it-suddenly-works/breaks 

[Mark Hammer]

I went back to this project today (after having done nothing to it since last post) and the sound is barely audible even at the highest amp settings. It's great now.

I'm wondering then if it must have been a power supply issue.

Been there, done that, had my passport stamped many times, and worn out the t-shirt, but still can't explain it. 

[PRR]

Ceiling fan?

(One summer, a guitar amp forum had several reports of uncontrolled tremolo, even on amps without tremolo. Once we hit on the ceiling fan theory, like 3 out of 4 reported that, yes, it was a fan bouncing tones.)

[mark2]

Ceiling fan is a good theory for me to keep in mind for later, but in this case it wasn't.