SpiritBox noise/hiss

[DougieFresh]

I just finished a SpiritBox and the reverb effect is working (with the pots behaving as expected), but I'm getting a very noticeable hissing noise when the Depth knob is turned up. With Depth all the way down, I get a clean (no reverb) and loud guitar tone with no hiss at all. Also no hiss at all when bypassed.

I've tried by best to figure out the problem, but I've reached the end of my (limited) abilities. Here's what I've done so far:

• Carefully inspected all components and reflowed all solder joints. Swapped out the two TL072s and PT2399, with no change.
• With the Depth turned all the way up, I poked around with a signal probe to see if I could identify where the noise starts, focusing on the inputs and outputs of the ICs. No noise coming off any signal outputs of the PT2399. For the first TL072 (labelled IC1 on the schematic), I hear the noise on OUTPUT B and no noise at all on the inverting and non-inverting INPUT B. For the channel A of IC1, and for both channels on the other TL072 (IC4 on the schematic), I get no noise on any inputs or outputs. I also get no noise on the input or two outputs of the Belton BTDR-2H.
• I verified that the problem is NOT with the TL072 chip itself… When I swapped the chip used as IC1 with the one used as IC4 (which had no noise on any inputs or outputs), I still still get the same noise only on IC1 OUTPUT B.

What would you recommend as the next step? I'm attaching the schematic here for reference with the noisy probe point circled. Thanks very much in advance!

(Edit: fixed image formatting...)

 

[darwin999]

I would take an old toothbrush and some isopropyl alcohol and gently remove all that solder flux (which can conduct at a low level), and then carefully re-inspect your solder joints - looking for any bad contacts or solder bridges.

 

[DougieFresh]

I would take an old toothbrush and some isopropyl alcohol and gently remove all that solder flux (which can conduct at a low level), and then carefully re-inspect your solder joints - looking for any bad contacts or solder bridges.

Nice tip about removing the remaining solder flux (I'll remember that for future builds). Unfortunately, after carefully removing the excess flux and inspecting/verifying all the solder joints, there's no change.

 

[BuddytheReow]

Try swapping out the PT2399 chip if you've got an extra.

 

[darwin999]

Unfortunately, after carefully removing the excess flux and inspecting/verifying all the solder joints, there's no change.

Rats, sorry about that. But flux can conduct at the ~megaohm level, and thus cause crosstalk and other unwanted noise. Good luck with this.

 

[Bio77]

Try swapping out the PT2399 chip if you've got an extra.

This is the likely culprit. You could also try probing both sides of C3 and C4. Lastly, I have a 9V battery rigged with a snap and a barrel jack. Sometimes some pedals get weird noise from a particular power source.

 

[DougieFresh]

Thanks everyone. Just following up for anyone having the same issue...

I tried replacing the PT2399 and the power adapter with no change. After carefully testing various parts of the circuit again with the signal probe, I'm pretty confident in the end that it's the Belton brick generating the hissing noise that's then being amplified further by the TL072. When I crank up the speaker while listening with the probe, I could definitely hear a hiss on the output of the brick that's not on the input, even when testing the unused output channel (pin 6) that's not connected to anything else. I thought (hoped) that some noise might be getting into the brick via its dedicated L78L05 voltage regulator, which is a component that I believe can be easily damaged from heat while soldering. I removed that and replaced it with a new socketed one but again no change.

In the end, I just bit the bullet and added in a passive RC low-pass filter between the Belton output and the next part of the signal chain (pin 3 on the Depth pot). After experimenting with a few cut-off frequencies, I settled on 1.3 kHz (R=1k, C=120n) which seems a bit low but it removed almost all of the hiss and had only a minimal impact on the sound of the reverb signal. The pedal sounds much better now!

 

[juice886]

Yes, this circuit definitely has some amount of white noise/hiss. PT2399 and Belton Brick are not hi-fi devices by any means and involve some level of hiss that becomes noticable in this configuration. Changing TL072 to something with better noise characteristics (OPA2134) helped me a bit with that as I recall but only to some extent as the main culprits are PT2399 and Belton Brick. I think this circuit could have been optimized for better signal to noise ratio.

 

[JTEX]

I think this circuit could have been optimized for better signal to noise ratio.

Pet peeve of mine. I think nearly all audio circuits could be optimized for better signal to noise.

 

[juice886]

nearly all audio circuits could be optimized for better signal to noise.

I'd say nothing is perfect. Especially guitar effects