Strange problem with pedal

[cornelius]

I made The Roseyray pedal and I've got strange noise when I turn the Tone pot to max position. In this position pedal sounds very powerful but this noise (some called it 'motorboat noise') makes it impossible to use pedal now. I checked out on this forum to see if someone explained how it's possible to avoid this problem (which is obviously common for this pedal) and I found out that i should put 2 1uF caps there... I did it, but this noise is still here... Does anyone suceed to solve this problem, folks???

This way I can just imitate some engine-sound :-(

[Paul Perry (Frostwave)]

Are you using a battery or a plug pack? Does it make any difference?

[Mark Hammer]

I am convinced that it is a biasing issue.

If you have been on the forum long enough, you may remember that I original called it the "Voodoo Plus" because I had based it around the Voodoo Labs Overdrive pedal (or at least the schematic that was posted of it).  The VLO "re-uses" the biasing of the first op-amp for the second one, so I thought I could do the same.  That was my mistake.  There are two big differences between the Roseyray and the VLO.  One is that the VLO does not provide any feedback path from the output of stage 2 back to the input.  The other is that the VLO only provides a small amount of gain in stage 2 because it is a recovery stage after the clipping diodes (i.e., it adds back a bit of the volume that is subtracted because of the clipping diodes), whereas the Roseyray adds much more gain to get additional clipping from the LEDs.  The small amount of DC offset that might exist in the op-amp gets magnified at the output of stage 2, then fed back to the input without any DC blocking.   

I need to redraw it with proper DC blocking and re-bias of stage 2, so that those small errors don't turn into big ones.  I'll let everyone know when that's done.

[cornelius]

Thank you, Mark! I can't wait to see the new schematics...

I use just 9v battery, I don't like ac-dc adapters, PSUs...

[biggy boy]

I am convinced that it is a biasing issue.


I need to redraw it with proper DC blocking and re-bias of stage 2, so that those small errors don't turn into big ones.  I'll let everyone know when that's done.

The schematic at
http://hammer.ampage.org/files/Roseyray.gif
Has it been updated to include the DC blocking Caps, to eliminate the motorboating?

I would like to build this circiut!

Thanks
Glen

[Mark Hammer]

I believe I posted an update to my old Photobucket site....if only I could access it   I registered with Photobucket before my workplace instituted filters.  I went so long without putting anything new on the account that I forgot the password.  When I click to request that they send me a new password or some means of getting into my account, the e-mail from Photobucket gets sent to the e-mail address where I originally registered from, which is, you guessed it, my work e-mail.  The e-mail filters block me from ever receiving what Photobucket has sent.

In any event, it is a simple change.  Just make sure that there is a cap (.47uf is good) before the 6k8 resistor shown before the tone control, and a 1uf cap between the output of the second stage and the scoop filter after it (just before where you hit the 22k/.001uf junction), and you should be alright.  Those will block any DC voltage from finding its way back to the input of the second op-amp.

[biggy boy]

Ok
Thanks Mark
I think I'll redraw the schematic in Eagle, to make a board.
I'm going to add the second leg to gound with a resistor and cap, filter deal on the second stage