Oscillation problem

[PBE6]

Worked like a charm!!! Thanks karbomusic!!! Looks like I have a new design habit. =D

On a side note, I ended up lowering the amount of gain in the make-up gain stage from 12 dB to 4 dB as the original values were causing the opamp to clip. The lower gain also makes the saturation control much more useful.

After almost a year of getting my keister kicked by this build, I think I've finally killed it - with kindness -  and an enormous amount of help from this forum. Thanks all!!

[karbomusic]

Certainly glad you got it worked out.  

The lower gain also makes the saturation control much more useful.

That may also kill the whine, not sure if you checked before adding the ground mod but reducing the gain often helps the whine.

[PBE6]

I fixed the whine first, the gain change was an afterthought. Your suggestion about grounding the disengaged input was the magic bullet in this case!

[PBE6]

I was playing around with this box again last night, and noticed something else odd. It actually works great now when engaged and when bypassed, but if I unplug my guitar while it's on it squeals like a pig! This makes sense based on previous discussions, when the input is hanging this thing tends to oscillate something fierce. But why is that? I don't think I really understand what's going on here. I thought the feedback caps would be enough to kill any high frequency gain mishaps.

Can anyone provide some insight on this particular circuit? The version where the diodes are dumping signal directly to ground?

[karbomusic]

I was playing around with this box again last night, and noticed something else odd. It actually works great now when engaged and when bypassed, but if I unplug my guitar while it's on it squeals like a pig! This makes sense based on previous discussions, when the input is hanging this thing tends to oscillate something fierce.

That is normally cured if the input jack has a switch for the 9V battery which has the additional advantage of powering down the circuit before it can squeal much. If you don't have that, you can use a jack that simply grounds the input when unplugging the cable. You know this already I'm sure.

I have a completely different circuit with the same whine issue and the only thing I think I haven't addressed is simply trying different op amps. I think I have addressed almost everything else. I'm sure it is due to my adding a few dB of additional gain at the last stage so I'm gonna swap some stuff and see what happens.

[PBE6]

I've connected the negative terminal of the battery to the ring of the stereo input jack so the power is off when there is no input plug inserted. The input is also grounded when the circuit is bypassed as per your suggestion, which has cured the whine. The noise I'm talking about happens when the circuit is engaged and the cable is plugged into the input but not the guitar. I'm used to buzzing in this situation, but not squealing! On the bright side, it's easily avoidable.

Just looking at the circuit, the first stage can provide up to 227x gain (47 dB), although the TL072 is definitely going to max out long before it gets that high. After that the diodes cut the signal down to a max of about 0.8 V (unless the "Sat" control is dialed to mix in some of the original signal). Would that kill any oscillation from the first stage? Or is it here to stay once it's created?

I dialed the second stage gain down a bit to 1.7x (5 dB), which is pretty modest. I don't think that would be contributing.

But I don't really see any loops now that the diodes are dumping signal to ground. Am I missing something obvious?

Maybe the most likely cause would be an opamp limitation. Is there a particular opamp that works well for Dist+/OCD style pedals?

[karbomusic]

The noise I'm talking about happens when the circuit is engaged and the cable is plugged into the input but not the guitar. I'm used to buzzing in this situation, but not squealing! On the bright side, it's easily avoidable.

Ah.... I missed that, yea, you're just moving the "free hanging" part to yet another location. It's an endless circle if not careful and the reason it's best to cure the whine at the source, which as evidenced by my troublesome circuit, not my area of expertise. 

[PBE6]

Mine either!!

[juan_felt]

5k6 is "stiffer" than usual, so I doubt it's the problem. Some capacitance would help, but I don't think it's causing instability.

I think it might do that - as that's happened to me before when I accidentally left a bypass cap off.

@ OP: Hang 22uF or bigger cap from the Vref to ground. If the oscillation is still there, that wasn't it, but this certainly won't make it worse.

Hi R.G.,

I'm trying to solve a very similar issue to the one described on this thread. I put together in one PCB a BSIAB2 (with an active Baxandall instead of the tone control) and a LPB1. Both effects are great separated, but when I engage both at the same time with relatively high gain on the BSIAB and high output on the LPB1, I get a really loud, high pitch feedback, that changes when I move the controls (eq, volume..). I also built an exact same pedal, and when I turn on the BSIAB on the first one, and the LPB1 on the second one (both max level), I don't get the noise.

Both PCB have ground planes. I have 47uF cap to ground on the Vref, and a 100uF on the 9v supply. Both effects have their own polarity protection Diode.

Do you have any idea of what could be causing the problem?

Thanks!

[juan_felt]

BSIAB2 schematic



LPB schematic is similar to this one, plus the 100uF cap from 9v to ground

[manocharli]

Hi, I would like to make a contribution to the case of the oscillation in the Boneyard clone circuit. In my case I made the single version. the oscillation beep made it impossible to use the pedal. buy several 7660s to try and at the first exchange for another 7660s of another brand goodbye oscillation. I hope I have helped something. regards