Trem Lune noise hiss

[captntasty]

I put the tremulous lune together yesterday and was happy with it except for a noisy hiss.  With depth up it follows the LFO - with depth dialed back it is constant.  I searched the forum and found nothing.  I've tried different opamps with no success.  Any ideas?  I've read such great feedback about this effect - can't believe that if others had this problem it wouldn't be considered unusable. 

[ayayay!]

Can you post the schem?

[captntasty]

Sorry, I forgot... it's the tonepad layout.
http://www.tonepad.com/getFile.asp?id=84

[oskar]

With depth up it follows the LFO - with depth dialed back it is constant. 

Just guessing:
The hiss is related to the amplification (current through the VTL5C2 LED. Which is either
LFO driven (depth up) or constant (depth down)).

If you get noise on the input it would off course behave this way. As you have allready tried different
OP-amps it's either the input cap (turned the wrong way. Measure the input cap/1M resistor junction)
or the vactrol. ( But I don't know how optocouplers are supposed to behave when they don't behave...     )

Are the 1k/ 330R resistors correct values?
How does it behave apart from the hiss?
Voltages!

[Paul Perry (Frostwave)]

If the noise is being modulated, then the noise is before the LDR.

If the noise is a steady hiss - like the white noise between stations on the old FM radios before they had automatic muting - then I think it must just be normal for the effect, with those particular components & chips.
If the noise is more crackly & intermittent, think leaky capacitor somewhere.

Alternatively, some resistor or cap may be the wrong value, lowering the gain of the system, so that you are having to crank up the gain after the input, and thereby amplifying the noise that is always there but not normally noticed.
Looking at the circuit, I can't see that it would be very noisy.

[captntasty]

If the noise is being modulated, then the noise is before the LDR.

Yes, the hiss is modulated - when I dial the depth all the way back to zero it becomes constant.

There is some hiss/crackle on the depth pot - like there would be in a ZVEX S.H.O. - DC voltage on that pot, right?

Are the 1k/ 330R resistors correct values?

Yes - I did use the following substitutions and mods which are posted on the Tonepad site:

In the Audio path:
Change the two 47k resistors to 220k.
Change trimpot to a 10k, you may want to use a panel mounted pot for easier adjustment instead.
Add a 330pF capacitor across opamp pins 6 and 7.

In the LFO:
Change 100ohm resistor to 2k7.
Change the 100uF cap to a 10uF.

Symmetry mod parts (two diodes and 500k pot)

edit:  I almost forgot - I used a home-rolled LED/LDR, 3mm red LED and small LDR from RatShack.

Could these changes have had some effect?
With all the good reviews this thing gets I can't imagine nobody noted this before.

I will take measurements and post later tonight.

[Zben3129]

If the noise is being modulated, then the noise is before the LDR.

Yes, the hiss is modulated - when I dial the depth all the way back to zero it becomes constant.

There is some hiss/crackle on the depth pot - like there would be in a ZVEX S.H.O. - DC voltage on that pot, right?

Are the 1k/ 330R resistors correct values?

Yes - I did use the following substitutions and mods which are posted on the Tonepad site:

In the Audio path:
Change the two 47k resistors to 220k.

Could these changes have had some effect?
With all the good reviews this thing gets I can't imagine nobody noted this before.

I will take measurements and post later tonight.

These two resistors set the gain for the stage, with gain being R_in / R_fb  (or is it the other way around?). Either way you would be looking at a current gain of 1, but I think this can change the voltage gain. Something like that, I think, I'm no EE I just try to accurately repeat what others teach me 

I guess long story short, take a look at these.

The 100 ohm to 1.7k is also a drastic change, but I don't think you are having LFO problems.

Zach

[oskar]

These two resistors set the gain for the stage, with gain being R_in / R_fb  (or is it the other way around?).

It actually is the other way around: ...gain being  R_fb /  R_in

[Unbeliever]

With all the good reviews this thing gets I can't imagine nobody noted this before.

I've never noted any noise problems with the 'lunes I've built - the design isn't inherently noisy, it's much more likely it's something you've done. Build it over again from scratch, it's a simple circuit and can easily be done on perf or similar.

If you still have trouble, then contact the designer - Dann Green @ 4ms pedals.

[Zben3129]

These two resistors set the gain for the stage, with gain being R_in / R_fb  (or is it the other way around?).

It actually is the other way around: ...gain being  R_fb /  R_in

Of course, I just had to guess wrong   


Zach

[captntasty]

So I replaced the 220K resistors with the original value - 47K and no more hiss.  I did lose some overall volume so I may try 100K.
Thanks for all your help....

[Zben3129]

Glad its fixed 

Can I ask why you subbed 47k/47k for 220k/220k in the first place? Was it just a matter of what you had on hand?



Zach

[captntasty]

The substitutions were updates that were given on the Tonepad site.