Sunking II Noise when bypassed

[thegreatwent]

Hello all!

Built a Sunking II as per the build instructions recently and all went without a hitch, though, when I last played it I noticed that there is some noise when the pedal is bypassed.
It only happens when playing and increases the harder you strum. Sounds like a harsh clipping hiss that's in the background.
It doesn't happen so much when playing very lightly, so it seems there is a threshold whereby it comes in with a certain input level.
I think it happens a bit when the pedal is engaged, though, it's hard to differentiate between the expected clipping and the noise in the background.

Anyone else experience this? Is this indicative of a common issue? Something to do with the buffer components?

Any help, as always would be appreciated.

Thanks!

[bordonbert]

I don't know the Sunking unit intimately but I have the build PDF.  There's nothing at all unusual about the schematic but I notice there are specific instructions as to how to "make the Sunking true bypass" and these are a bit unique.  At first sight it looked a bit strange with only a DPDT stomp switch, I would have expected a TPDT type here.  Then I looked a bit deeper and - yuk!

The input is passed through the first opamp which is intended to act as a clean buffer even when not in use.  Having a line buffer in the signal path when not on is not a nono for me, I know there are those who are retentive about this!  I've never found it a problem.  Tone suckout?  Hmm!  The second opamp input, the diode drive unit, is left connected to the output of this buffer and there is feedback from the distorted output applied back to it.  In the textbook world of perfect opamps this should make no difference, in the real world it will!  Does the background noise change character in OFF mode when you alter the gain pot?  The distorted signal is fed into the VB+ DC reference voltage circuit via the gain control in a very poor way.  Each of the opamps uses this as its V+/2 middle line!  Again in a real world situation the limitations of the "perfect" voltage supply setup will get in the way.

If you really like this circuit and want to stick with it I would really recommend you set this up for true bypass with a 3 pole switch instead of the 2 pole.  There are instructions in the build PDF to do this.  That way the input and output are completely disconnected from the circuitry at both ends and effects like this cannot happen, (you just have a different set of problems ).

[blearyeyes]

Swap out the chip to make sure you don't have a bad one.

[thegreatwent]

Thanks for the replies!

I will see if there's any noise when moving the pots in bypass.
Then I'll throw in a different chip.

I actually really like the Klon buffer. So much so that I've even built a standalone version that was on my board all the time. 

I wasn't patient enough to try and make the buffer footswitchable in this build.
This build was super tight as far putting in an enclosure. There is almost no wiggle room.

Thanks again and I'll report back after I get around to trouble shooting it.

[bordonbert]

I will have to start by saying that I owe you an apology.  I have been interested enough to take a second more serious look at this circuit and there are a couple of points I missed in my first quick scan which I should correct.  The PDF is too big to post so I have attached a small JPG of the original schematic taken from it.  If anyone wants the full document it is posted here:  http://www.madbeanpedals.com/projects/Sunking/docs/Sunking_ver.3.pdf

I criticised it for leaving the distortion generators in place when the pedal is deselected and connecting the output of that stage back to the input introducing distortion into the deeper works.  This does not happen quite like I said!  It isn't shown on the first schematic in the PDF but the selector switch shorts out the distortion diodes when not in use and that prevents any distortion from being generated in the second stage.  However it is still far from a good idea to leave that shaped feedback in place in the loop of the buffer amp which is used in all cases.

Something I did not mention but which I should, is that the output of both signal paths, clean and distorted, is actually blended at the output.  Each path terminates in a 68k resistor and the selector switch simply shorts out the resistor in the desired path bringing its signal level up.  The other path is then left in place but reduced by running through its 68k resistor before being summed at the output.

That might be fine for the on state where you can go from mild to hard distortion by blending the two but it's far from ideal in clean bypass mode.  It still leaves potential paths for unwanted signals to be fed into the output signal via IC2A and IC2B.  The test of turning each of the pedal Volume and Gain fully up and down in turn while playing in bypass mode should tell if this is the problem.  The Volume would solve the problem completely as it cuts out any contribution when fully down, (though it can't be used like that in practice), while the Gain may only change the character of the noise if the path through its circuitry and on through the IC2 opamps is a problem.  If the background fuzz is there when they are turned full up but gone when they are fully down then this is the culprit.

The real answer if the noise is a showstopper for you is to follow that true bypass procedure described in the documentation.  If this is an old pedal circuit, at the time that it came out TPDT switches may well have been expensive and rare so it needed economy measures to be able to use a DPDT switch.  Then the noise problem would have been noticed and diagnosed which could be why the true bypass mod has been added to the PDF.

Get it in true bypass mode asap!