Mutron III

[motterpaul]

I have been working on this for awhile. I get tone on all settings, but it is very distorted and overdriven. It seems very susceptible to pickup output and even RF.  For example, of I hold one of the toggle switches or just add a jumper wire (like an antenna) I get all kinds of new tones and volume increase.

To try to tame it I changed the trimpot from a 10k to a 100k (this controls the LED brightness) - and I pulled the LDRs back (like in the layout picture). I also used to 33uF electros in parallel to replace the 15uf on-polarized. Neither idea worked. Funny, these are two things that Javi also said he did, but he says his is working.

I have looked at it thoroughly and see nothing wrong. This is one of those circuits where a LOT of people have the same problems I am having.

If anyone has any ideas I would appreciate the help.

[Travis]

What IC are you using? Have you tried any different ones?

[motterpaul]

The recommended TL072s - funny, I thought about trying something lower gain, like an AD175, but those are what it calls for...

http://tagboardeffects.blogspot.com/2014/06/mutron-iii.html

[knight_yyz]

Could it have something to do with the led?  The stats on a cheap radio shack red LED compared to a top of the line of the same size, and depending on the mcd's ranging from 700mcd up to 35kmcd, how will this affect the circuit?  Most components have a tighter "tolerance" if you will (eg:100uf cap) but the LED is almost a random element to me.  

[motterpaul]

I really don't know anything about the LED - it seems REALLY bright (which is why I put in a 100k trimpot). I just happened to have it.

I have some 3mm I could try.

[rocket88]

You want yellow LEDs.

[knight_yyz]

I'm not sure what the led is doing in the circuit.  Which feature of the led is important here?  Forward voltage? The forward voltage range on a red led is between 1.7 and 2.8 volts depending on manufacturer and intended brightness.  Current is 20-25mA.  Reverse voltage is only 5v.  



Whenever I buy led's I make sure I know the stats on them so I can limit the current and make them last forever.  

[motterpaul]

Thanks Rocket.  I sure didn't see that anyplace but I will give it a try. I know I have the same LDRs as other people who got it working

Knight - in this case it is just about brightness - so the LDRs can get a varying reading that changes as the LEDs fluctuate.

[rocket88]

opp, for some reason i read that it was the phasor not the envelope filter. have phasers on my mind latey.

anyways, it does say in the notes to use 3mm diffused red. the LED should be really bright, also, did you encase the LED and LDRs? if not that cold be part of the issue. you need to make sure that they are seals do that ALL the light from the LED goes to the LDR, if not then it wont work properly and could make the effect sound distorted and messed up. you want the face of the LDR to completely face the LED. like they're kissing.

also, if you read all the discussion you'll find that this is very sensitive to high gain effects and pickups. so if you're using higher gain pickups that could be your issue causing the distortion, which is normal for this effect.

you may also want to just go over the board again and knife between the tracks, and check for poor solder joints and bad grounding.

good luck

[knight_yyz]

How do you "shine light" on 2 ldr's with only one led?  Does the light have to be direct or ambient?  The "beam" of an led is so narrow normally.  

[rocket88]

You need to bend the head of the ldr, which is the part that detects light, towards the led. In a lot of the circuits we have that use them you have 1 led and 1 ldr, so you basically do this then shrink wrap them to prevent ambient light from affecting them.



Once you shrink wrap them you essentially made a vactrol.

Now with this circuit you can't have the two ldr's face the led like that, so you want the face of the ldr to touch the side of the led.

[knight_yyz]

but there are 2 LDR's and only 1 LED

[motterpaul]

Thanks for your helps guys!  I have already built three other circuits that work fine with LDRs so I m familiar with them. For testing I have these covered in heavy gaffers tape which takes all light away. I started with them right up against the LED, just like I started with a 10k trimpot, but the LED is unbelievably bright, so a bigger trimpot (100k) did help.

Moving the LDRs backs did not help so much, I got pretty much the same sound, but just a little quieter. Bottom line, I think the LDRs are under control, a 3mm LED might help, but it also seems to be something related to the early gain stages - because there just shouldn't be so much overdrive.

[nocentelli]

Does the gain control help?

[rocket88]

Knight. Reread my comment above with the image.

In it I saw that since you have 2 ldr's and one led, you want the faces of the ldr's touching the sides of the led. Look at the layout again with my comment and I think you'll see what I mean.

[motterpaul]

nocentelli wrote

Does the gain control help?

Well, I tried dropping that to a c500k and actually I did get some improvement - less distortion, and better control over the signal.

But the problem with the circuit is that it has a lower gate threshold where it just farts off if it doesn't have enough input. So, while that helped I don't really see any more room for improvement there.