bblender pops loudly when bypassing the first few dozen times..

[cloudscapes]

Hi all, I've been trying to get this blender to work:

http://seanm.ca/stomp/bblender.html

..and unfortunatelly it pops quite loudly when I hit the bypass switch. after doing for a minute it stops, though. it doesn't matter which effect I have in the loop. for the sake of testing, I've shorted the send and return.

I'm using a tl074. pin voltages are as follows:
1-4.63
2-4.63
3-4.20
4-9.20
5-4.21
6-4.69
7-4.69
8-4.62
9-4.62
10-4.20
11-0
12-4.19
13-4.63
14-4.63

I've added 1m pulldowns on both the input and output, as well as on the send and retur. I've swapped between several opamps. I've swapped out all capacitors, different brands even. the amp is a peavey classic 30 and isn't known to have problems with effects. I'm powering it with a diago power station, the same one I use to power 12 pedals when I gig without issue. I've also tried a battery but reverted to the PSU. I've got an extra 10uf cap across power rails. I've checked all caps, all of them are oriented as the schematic. I've redone the layout thrice. twice on breadboard and once on PCB. each time the same problem.

ideas?

[cloudscapes]

when I power the thing on, DC voltage offset between in and out goes from 1.5ish volts and gradually down to nearly 0 over the same time that it takes for it to stop popping. when I cut the power it goes a couple volts into the negatives and also takes a bit of time to go up to 0.

I know it's a cap, but I don't know enough about caps or opamps to know why.

[John Lyons]

If you are getting voltage at the in or out (outside of the cap) then either the pulldown resistor isn't working
or miswired (maybe not a good connection to ground) or the cap is leaky (not block DC as is should, nearly completely.
Make sure the pulldown resistors are on the outer of the caps (farthest from the circuit).

[slotbot]

hey man

what might be more revealing is to measure the voltage across the input and output caps individually instead of from input to output. (from power up as you did)

[cloudscapes]

thanks guys.

pulldowns are indeed well connected, and read as they should.
3.84v across the input cap. stable
on power up the output cap goes from about 1v to 4.2v gradually over around 30 seconds.
I've replaced the caps again, same readings.
none of the component leads are touching, and I had redone it in a different palce o nthe breadboard the second time. I had used fresh resistors as well. since last week, I've tried three different switches (one of them salvaged from a pop-free effect).

[slotbot]

try to lower the output cap to something like .1 uf. i would suspect this is where teh pop is coming from.

you will have the same problem in theory but the cap should charge 100 times faster (and so the problem should disapear 100 times faster?).

10 uf is a bit crazy. basically that cap will make highpass filter with the input impedance of whatever stage its being fed too. usually input impedance is pretty high. you can find the cut off frequency by

1/2piRC

So if the input impedance was say 100k and you used .1 uf output your cut off would be around 16 Hz so thats ok with 10uf in the same situation it would be i think .16 Hz which is practically DC.

I would replace all teh 10uf actaully because it is way overkill especially as coupling between 2 op amp stages. but experiment and see if you get any better results.

good luck!

[cloudscapes]

thanks again!

yeah, replacing most of the caps to 0.1uf did the trick. I dont know a whole lot about capacitors, outside of using them as low-pass/high-pass filters or for a charge. when it comes to decoupling or needing one to let audio pass but not affect its sound, it makes about as much sense as string theory to me 

and I didn't want to change those cap values initially because I know this particular circuit is used a lot on these forums, "so it must be without flaw. must of been something I done.."

thanks for 'splanation!

[earthtonesaudio]

Glad you got it fixed!

Just want to praise slotbot for an excellent post.  You accurately identified the problem, proposed a solution, and explained a little of the theory all in one post.  That's what makes this forum great!

[differo]

Glad you got it fixed!

Just want to praise slotbot for an excellent post.  You accurately identified the problem, proposed a solution, and explained a little of the theory all in one post.  That's what makes this forum great!

+1 still learning about circuit designs myself, good explanation that makes sense for beginners(or non-pro's