Shoot the moon based tremolo debugging

[Les_Portes]

Hi everyone,

I am using the shoot the moon tremolo signal path (and only the part between the IN and the OUT, i.e. the TL72 and attached components) for an optical tremolo project.
I followed exactly this schematic : http://imgur.com/RgvdJem, but I only built the bottom part : the two rectangles and the Vr part.

The LED modulation is done physically.

The build is done, but when testing I get a lot of noise along the expected sound. The tremolo works, i.e. the output volume is modulated as expected, but I get my input mixed with a lot of noise.

If I replace the LDR with a 220 Ohm resistor, I get strong noise and signal, if I replace it with a 10k resistor, I get a low noise and signal (maybe with less noise). I used these values as they more or less match my LDR behavior.

I double checked everything, my TL072 voltages seems OK to me :
1 - 4,6
2 - 4,6
3 - 4,4
4 - 0
5 - 4,4
6 -4,6
7 -4,6
8 -9,1

Any idea what could be causing that noise ? Maybe this circuit is designed for very low lighting only ?

Thanks for any help, I am not an experienced builder and I don't know how to progress here...

Regards

[duck_arse]

Les_Portes - welcome to the forum.

when you say "done physically", what do you mean? can you show us photos of what you have built, please? and describe the sound of the noise.

[Les_Portes]

Here are some pics.

The whole thing

The LED is in the arm-thingie, it lights the LMD intermittently depending on the motor speed and the used disk.

I read through the forum for similar issues, informative, but so far I am still stuck.

Just the incriminated perfboard

[Kipper4]

Welcome
It's really hard to make out what's going with the pcb.
As a side note. You do realise that the led and ldr are linked as a vactrol?

Is the led flashing?

edit
http://www.diystompboxes.com/smfforum/index.php?topic=95269.msg824665#msg824665

[Kipper4]

" I used these values as they more or less match my LDR behavior."

This is unlikely.
A darkened ldr will be at least 500k and more than likely 10's of Meg.

[Les_Portes]

"As a side note. You do realise that the led and ldr are linked as a vactrol?"
I did not know the word (I'm not a native English speaker), thanks for the tip but yes I know it's a type of opto coupler

"Is the led flashing?"
No, but the disk rotates and create the intermittency, my inspiration comes from that box http://makezine.com/projects/make-33/optical-tremolo-box/
But I wanted to add a buffer to his circuit, as his solution behavior depended a lot on what was plugged in.

"A darkened ldr will be at least 500k and more than likely 10's of Meg."
You're right of course, in my configuration it goes from 250ohms to 350K ohms, I only matched the lowest value. I will check the behavior with higher values.

Thanks for your time everyone, and thanks for your welcomes

[Les_Portes]

I noticed that touching the gain pot was noisy, so I tried grounding it (the shaft & body of the pot) and it cut all sound, just a high hiss... There is no short circuit between the pot & its 3 inputs... I am a little lost here, any advice on what may be causing that ? Faulty pot maybe ?

[pinkjimiphoton]

either a crack in the phenolic plastic the pot terminals connect to, or a solder bridge would be my first thought

[Les_Portes]

I soldered a new pot, and... It works 

I added a resistor (10k) in series with the LDR to curb distortion, as I'm using an high power LED for optimal visual impact 

One question before I finalize the prototype : should I solder the pin 2 and 3 of the pot together ? I read somewhere that leaving an unsoldered input was bad practice, but it seems to work fine like that !

[pinkjimiphoton]

you can do that, but it's not necessary. the wisdom is that if the wiper connection fails in the pot, there will still be some resistance across the pot. you can do it... or not... as you choose.

in some cases it will make a difference in the way stuff sounds, but mostly if in the audio path. jme, ymmv

[PRR]

> led and ldr are linked as a vactrol?

Les_Portes has abandoned the electronic LFO (low frequency oscillator) and is instead using a motor and a wheel with a hole in it. Almost the same thing. Much bigger. Possible to have custom wave-shapes with different cut-out shapes. Possible to wow-wah the tremolo rate with a finger (or toe).

[smallbearelec]

when testing I get a lot of noise along the expected sound.

Does the noise go along with the running of the motor? When I experimented a long time ago with this kind of effect, I had to give a lot of attention to powering the motor separately and shielding it from the modulator circuit.

[duck_arse]

when you say "done physically", you really mean done physically. vr interesting. the fun you could have, with eggshaped wheels and multi holes and hole shapes ......

[Les_Portes]

I did more tests, everything works fine now, only issue is that I did not use a brushless motor on that prototype, so... Still some noise, but this time it's purely mechanical

Indeed disk possibilities are huge, and I hope that it will make it fun to use and to witness (it's designed for an instrumentalist who do not use effects usually).

you can do that, but it's not necessary. the wisdom is that if the wiper connection fails in the pot, there will still be some resistance across the pot. you can do it... or not... as you choose.

Makes sense ! Thanks for the insight, I will test and solder if it does not cause any change as I would prefer to keep the resistance if the pot fails.

Does the noise go along with the running of the motor? When I experimented a long time ago with this kind of effect, I had to give a lot of attention to powering the motor separately and shielding it from the modulator circuit.

Motor and effect alimentation are separated for now, I will have to keep an eye on that when I put everything together.

Thanks you all for your help !