Small Stone True Bypass Pop (i swear i've tried all the normal advice!)

[drummer4gc]

Hopefully the final thread in my Small Stone Woes collection...

Here's the schematic:



I've hardwired the feedback side of the color switch to be always on, and I replaced the 27k resistor in the feedback loop with a 250k pot. I cut the resistive element carefully so that when the pot is fully counterclockwise, the feedback circuit is open - thus no feedback, just as if the color switch was turned off.

I also have the thing wired up true bypass. When the feedback pot is at 0, the unit switches quietly. With any feedback dialed in, there is a loud pop when turning the effect on.

There is already a 470k pulldown resistor at the output. I added a 1M pulldown at the input with no luck. Replaced input and output caps with new film boxes - no luck. Tried a new switch - no luck.

I've been staring at this schematic trying to figure out why this pop is getting through all these defenses, and I can't figure it out. There are some other mods on the unit (rate LED, indicator LED, volume adjustment, and wet/dry mix), but I've taken all of them out for testing and the pop persists.

Here's the last clue - the unit consistently pops the loudest when turning the effect on when the rate LED is in the "off" part of it's cycle. I believe it's at this point when the LFO is putting out the highest voltage. That's got to be related somehow, right?

Anyone have an idea why adding feedback is causing pop or some other suggestions for things I haven't tried yet?

Thanks so much!

[R.G.]

Get out your meter, and measure the DC level across the 470K resistor on the output of the effect. This should be 0.0000000V. If it's not, in either switch position, you will get pops.

[drummer4gc]

Yep, it dances between .000 and .002 to the rate of the lfo. Interesting, though, is that it continues to do this even when feedback is cut off and I'm not experiencing pops.

Is there a solution for this?

[drummer4gc]

Just for reference:

http://www.diystompboxes.com/smfforum/index.php?topic=69693.20
http://www.ehx.com/forums/viewthread/663/
http://www.madbeanpedals.com/forum/index.php?topic=12518.0

It's a well-established issue, apparently. I'm about ready to ditch the TB idea and just throw a nice buffer in it instead, but I hate letting solvable issues go unsolved.

Any thoughts that haven't been offered up are very much appreciated!!

[R.G.]

Yep, it dances between .000 and .002 to the rate of the lfo. Interesting, though, is that it continues to do this even when feedback is cut off and I'm not experiencing pops.
Is there a solution for this?

You can try reducing that 0.1uF cap feeding the 470K pulldown resistor from 0.1uF to 0.01uF. Right now it's letting through audio down to a nominal cutoff of 3.4Hz if I punched the right buttons on the calculator. You don't need that much. Try cutting things back to 34Hz by cutting the cap by a factor of ten. This reduces LFO feed through if it's an LFO problem.

It's not clear exactly what is going on, as there is not enough information to tell. This is a raw guess. Could be a lot of things, and might require oscilloscopes to really tell.

[Mark Hammer]

What comes immediately *after* the Small Stone?

[drummer4gc]

The amp . I've tried my tube amp and a small practice amp - both pop.

.01 output cap might have changed the tone of the pop a bit, but she's still there.

I appreciate your replies!

[R.G.]

With the changed cap, does your amp still show the same dancing range?

Is there a DC offset on the input?

I take it that the thing didn't do this before the true bypass conversion; correct me if that's wrong. So try putting a 1M from that 100K at the input to ground. There are some true bypass conversions that will leave that open and generate pop from the input cap.

We're rapidly running out of wild guesses that might help.

[drummer4gc]

RG - does the 1M go on the input jack side of the 100k or the cap side?

I've made a little headway - I have the 4.7k biasing resistor in the input gain/buffer stage changed to 3.3k in series with a 5k pot for volume trimming, and I notice that the pop occurs if the volume and feedback settings are up too high together. If I dial one up and the other down accordingly, I can still hit unity volume with a much lesser pop, unless the feedback is way up and then the volume has to come way down. Still a minor pop though.

[R.G.]

RG - does the 1M go on the input jack side of the 100k or the cap side?

It doesn't matter, either will do. You're establishing a DC path to ground for the cap.