Lumitron (MuTron III) low volume noise issue

[rhaas]

Something to try would be to add a 100pF cap from the output of IC3B (pin 7) to the inverting input of IC3B (pin 6).  If IC3B is oscillating that should stop it.

I recently built a Lumitron and had the same issue as the OP. Swapping IC3 to a TL072 (or even a 4580DD) helped a bit, but I can happily say that the 100pF cap completely eliminated the noise. Thanks for the suggestion, Rob Strand!

[LukeFRC]

Something to try would be to add a 100pF cap from the output of IC3B (pin 7) to the inverting input of IC3B (pin 6).  If IC3B is oscillating that should stop it.

I recently built a Lumitron and had the same issue as the OP. Swapping IC3 to a TL072 (or even a 4580DD) helped a bit, but I can happily say that the 100pF cap completely eliminated the noise. Thanks for the suggestion, Rob Strand!

likewise - I've had the same problem too - also a Musikding kit.

No signal, no noise
Small amount of signal - a crackle type distortion
Signal turns on the vactrols no distortion noise.

If I turn the volume up full I don't get the distortion.


Very glad to have found this thread!

[LukeFRC]

P.S. Here is how it looks btw, happily dwelling next to the amazing L4 Bass Preamp by aion.

I have also built the L4 and the Lumnitron! I love the L4, it's the closest thing to the kind of full amp sims you get on the hx stomp in analogue, I've found!

[LukeFRC]

I thought this would be an easy and valid fix, shifting/adding one more 330R to proper current share. but I'm not sure about that feedback connection on R22A - the original 330R stopped the feedback going as far as 0V. Rob?

Should help quite a bit.  R22A and R22B should be 2x330=660 ohm, so 680 ohm standard E12 value.  In the original both LED currents pass current through the 330R and are sensed.  Now R22A is only sensed and it current from one LED passes through it.   As a bonus 680R helps tracking even more.

Can you explain more about the advantages of this method vs. putting the LEDs in series? Here's my wild swing at what's happening -

• Straight parallel: senses both LEDs, but poor tracking if the LEDs are mismatched
• Series: senses both LEDs more accurately, and reacts accordingly with better tracking than straight parallel
• Modified parallel (Duck's diagram): uses only one LED as the reference point and applies the same current to both

Taking it as a given that all three methods would perform well if the LEDs were perfectly matched, what's the difference between series and modified-parallel if they are not matched?

I'm going to do a PCB revision, but it'll be tricky to fit in an extra resistor so I'm wondering if it's just a nice-to-have, or if it's a better-in-all-cases sort of thing and it should be the way everyone builds the Mu-tron circuit when using two optos.

Can report back the 100pf cap mod works perfect