Minor problem with Karate Shop

[bcalla]

Hi - This is my first Madbean pedal, but I have ~ 20 successful builds.

My Karate Shop worked first time (great board & documentation, BTW!).  The problem is hiss.  The hiss is pretty loud, and gets worse as I turn the frequency knob clockwise.  I just verified all component values and orientation.  All stock values, with C1 using the recommended substitute of 2.2n.

Has anyone else had this issue?  Does anyone have any suggestions?

Thanks, Bob

[madbean]

What kind of power supply are you using? That is a possible culprit. How much hiss are experiencing versus signal level (approximately, I mean).

[bcalla]

I use a Gator G Bus to run my board.  I just tried a dedicated 9v supply with the same result.

To clarify, I get hiss at every position of the frequency knob, but the pitch changes low to high as I turn it clockwise.  It may not get louder, but it seems louder with the higher pitch.

If I turn the volume down on my guitar the hiss seems overwhelming.  If I turn the guitar up, the hiss is still there but only noticeable when I stop playing - it kind of resembles single coil hiss in that regard.  However, it is loud enough when not playing to make me not want to keep the pedal engaged.  BTW I have been doing most of the testing on my strat, but I also tested it on an Ibanez with 2 HBs with the same result.

[bcalla]

I did some testing with my audio probe.  The hiss is introduced at pin 7 of the TL074; none of the other pins have it.  I swapped the chip, no difference.

With a fair amount of experimenting I figured out that the pedal is useable if I keep the gain at ~ 10 o'clock or lower (10 o'clock is ~ unity gain).  There is still hiss at all frequencies, but at this gain setting it is barely noticeable and it actually sounds pretty good.

However, I would really love to conquer the hiss, so if anyone has any suggestions please let me know.

Thanks!

[madbean]

Some suggestions:

1) Try putting a cap in parallel with the gain pot (lugs 1 and 3). Try 100pF - 470pF.
or, 2) If you have unity at 10 ock, then you could swap the pot for a lower value, say 10k.

1) is probably the easiest (and better) solution.

[bcalla]

I will try option 1 and report back.  Thanks!

[bcalla]

It took a while for me to get a chance to do this, but it made a difference.

The hiss is still there, but with a 470p cap I can turn the gain up to ~ 2 o'clock before it becomes a real problem.  I could leave it like this and be happy, but I wonder:
- How high can/should I go?
- What are the trade-offs?
Thanks.

[stecykmi]

noise like this shouldn't happen has you describe... can you confirm the pot is good?

the circuit is noise-free at pin5?

[bcalla]

I will go back in with my audio probe & check pin 5 again.

I used board mounted pots, so swapping it out is going to require some surgery.

I have built a couple dozen pedals, but I don't really understand all I should about electronics.  Can you help me understand what could be wrong with the pot?  It performs its function - increases / decreases gain as it rotates.  I understand how a dirty pot could introduce scratching as it is turned.  But what could fail or be defective in a pot that would introduce a hiss that varies in pitch with the setting of the frequency pot?

Thanks

[stecykmi]

i only suggest checking out the pot as it's directly connected to pin 7 and sets the gain for that opamp stage. if you used a brand new pot, it's unlikely that the pot is bad.

another suggestion is to make sure the circuit is grounded properly, it will need to be grounded to the enclosure.

[bcalla]

There is no hiss at pin 5, only pin 7.  The spare ground pad on the board is connected to the input jack (in addition to connecting the +/- from the power supply jack to the 9v/G pads on the board).  So the circuit is grounded to the enclosure.