AionFX Andromeda - Loud hum + Pop, goes away when drive pedal engaged after

[clement]

Hi everyone,

After many builds without issues (or at least issues I could solve by myself), the day has come that I need further help!Hopefully with your support this can be solved before I get grey hair
I'll be as complete as possible.

The build is a kit of AionFX's Andromeda (kit from Musikding in Germany). It's based on a Nobels ODR-1. I'm getting a loud white-noise type sound, and the pedal pops when engaged/disengaged. More details below.
Thinking I made a mistake building it, I ordered and built a second identical kit. Result: exact same problem. I couldn't find mention of anyone having a similar problem with this pedal, and Klaus at Musikding confirmed he didn't hear about this before.

Here are detailed characteristics of the problem:

• Constant white noise-type sound when the pedal is engaged; no issue when not engaged- Loud pop when engaging/disengaging
• Volume of the noise is fixed: independent from the pedal's Level control, independent from the guitar's volume pot; and it is there even with no pedals plugged in
• Noise is about as loud as the guitar playing - i.e. much louder than typical hum/hiss- The louder the amp, the louder the noise
• Noise goes away when a drive/boost pedal placed after in the signal chain is turned on
• Noise doesn't go away when a pedal placed before in the signal chain is on
• Noise doesn't go away when a delay/reverb pedal placed later in the signal chain is turned on
• When the Andromeda is engaged, I am getting a small pop when I turn on my delay pedal (pop doesn't happen with Andromeda off)
• Beside the noise, the pedal is working fine, all controls work as expected. It sounds nice, except for the noise

What I've tried/done:

• Testing Andromeda alone, outside of pedalboard: same problem
• Testing with good 9V battery: same problem
• Testing with different jack cables and amps: same problem
• Testing within enclosure, and with a pedal tester (kit from FuzzDogs in the UK): same problem
• Measure DC voltage between ground and input/output jack tip, with pedal engaged: 0.0v
• Swapped ICs and transistors for others: same problem
• Measured components when building (especially for 2nd build): all within tolerances
• Some voltage readings in the circuit: see picture enclosed -- nothing jumped at me as being an issue, but my understanding is limited. GND in green = continuity with main ground.

https://aionfx.com/app/files/docs/andromeda_documentation.pdf


Hopefully this gives a good starting point to narrow down the problem...Thanks in advance!
Clement

[antonis]

Hi & Welcome..

All ICs voltages are fine..
Same for Q2..

Q1 is out-of-order..!! 
(check for R3 proper connection..)

P.S.
Q1 Gate seems to get about 1V DC from input..
(place a 10nF - 100nF cap right before R1 and take measurement again..)

[clement]

Hi Antonis,
Thanks a lot for the reply & the welcome!

I double checked the readings on Q1 after your message, and realized I noted them wrong: I mixed Source and Gate readings - sorry for this!
These are the actual readings on Q1 (with new battery):

• D: 9.4v
• S: 0.9v
• G: 0.0v

Similar readings for the 2nd build (Source: ~ 0.8v).
I also checked R3: 2k96 on both builds, and properly connected to GND and Q1's Source.

I can try adding the cap before R1 this weekend, but not sure if needed considering correcting readings -- there's actually no voltage at the Gate. Also no voltage at input, R1 nor R2.

I'll also try swapping Q1 with others, but I'd be surprised if that's the issue: I already replaced it before with no improvement, and the problem is the same on the 2 separate builds...

Any other idea? Any further voltage readings I can do to diagnose better?
Otherwise I'll try going through the circuit with the audio probe this weekend.

Thanks again!

[antonis]

Q1 Source voltage is low..
(it should lie between 3.5 - 5.5V, give or take..)

Maybe particular 2N5457 transconductance is far apart from (wide spead, anyway..) average one..