squealing in a Sparkling Yellow OD

[HamishR]

(EDIT)  Now fixed - see below


Bit odd - I have built this pedal before with no trouble but this time drew up my own layout so I could fit it into 20 columns, therefore squeeze it into a 1590B.  I copied a fair bit of the layout from one I had done for a Honey Bee previously (which works perfectly!)

Can anyone see why this layout might cause the M pot to squeal when turned fully CCW, ie to full bass?  It's sort of not a problem because I never use it with the M control fully CCW but I think it might be useful to know why it is doing this.  The pedal works perfectly otherwise and I really like it.  :-)

[MAO]

The layout looks solid to me, although the schematic I found on FSB I think shows a 4k7 before the volume.

Your layout and Rockett's have 47k.

But i dont see how that would cause squealing.

That M control is new to me and a bit puzzling, but it looks like a dual purpose variable high pass filter, 1 side before the distortion, and the other after the output buffer.

I cant reason out how that control can cause squealing as the frequency ranges on both sides seem somewhat low.

With the M control maxed, the control is grounding out the 4k7 on lug 3. I would probably check and see if any thing else grounds out when that control is maxed.  

 But looking at your layout, the only things likely would be the 3130 pin 6 or the 9v supply, neither of which would cause sqealing.

Or try this, disconnect lug 3 of the M control and see what happens when maxed. Then reconnect it and disconnect lug 1 and see what happens with it maxed. The symptom should most definitely change with one of those lugs disconnected. That should tell us something.

[HamishR]

Thanks MAO!  I hadn't seen the schematic before - I based my layout on Rockett's.  I will definitely change the resistor to output just because it should be right.

The M control is a Bjorn specialty, very much like a Nature control on a Honey Bee.  I guess the idea is to increase bass one way, increase treble or mids the other.  In the past I have split the control to two pots to create a more conventionally operating treble and bass.  But it seems to work ok in this pedal.

Coming from an amp-building background I was wondering if I have somehow created a parasitic oscillation or something.  I will try disconnecting pins 1 and 3 and see what happens.  Disconnecting pin 3 might not even do much!

Good ideas, thank you.  :-)

[MAO]

I woke up this morning with these exact thoughts.

 "... I have split the control to two pots to create a more conventionally operating treble and bass"

Coincidentally, I just got parts in to build the bearfoot Model G, looks like it has the same type of control. Will be interesting.

I'm curious your findings with lug 1 or 3 dissconnected.

Do you have an audio probe to spot exactly where and when the oscilation enters the signal chain?

[HamishR]

After posting last night I went back and read through the posts about the Sparkling Yellow OD and it appears that I had the same exact problem when I first built it from a different layout!  I had forgotten - I build so many of these things...  So it's likely not the layout unless by some freakish coincidence the same thing happens in two different layouts.  

And when I turn everything down to listen to the noise it makes when Nature is turned to the bassy extreme it is not quite a conventional squeal (!) but is more like a high-pitched purring sound.  Almost like high-pitched motor-boating.  Weird.  I'm almost inclined to leave it alone because under normal use it's all fine and sounds damn good in fact.

I don't have an audio probe.  Up until now I've never found the need for one.

[lasermonkey]

Have you tried a buffer in front of it? I've made a few dirt pedals, including the EQD Talons, Boss HM-2 and Chunk Systems Black Dog which squealed on certain settings. A Klon-type buffer cured the Talons and HM-2, though the only thing that would stop the Black Dog from howling was hooking up a another, buffered pedal in front of it!
Maybe worth a try.

[HamishR]

That's probably a very good idea, Lasermonkey.  Thing is, under normal use it's not an issue so I don't think I can be bothered!  Terrible I know.

I did change the resistor to Volume 3 to 4K7 from the 47K I had and I think it has probably made the pedal a tiny bit louder - it sounds great! So the only real mod I have done is to increase the value of the input cap from 22uF to 47uF, and i really like that change.  It's not night-and-day different, just a little fatter.

So weird that you are making a Model G, MAO.  I just made one today having got all the bits together last week.  I wanted to build it again because I remember liking it last time but couldn't work out what the C control was supposed to do.  So I've made it again and if the trace is right it does next to bugger all, just like when i made it before.  It seems to shift the frequency of the mid peak a little but it's very subtle.  I think it could be handier to measure the pot where I like it and stick a resistor on the board to do that job and use the available spot for a treble control.  It's just a hair too bright for me.

[MAO]

Equally funny is I've had a 3130EZ for a good while now, forgot what I bought it for. And was torn on which to build, Yellow Sparkling or the Model G.

After listening to all the demos, the Model G sounded different enough than the supro-ish and fender-ish pedals I already have, figured I'd try that one 1st.

Probably end up building both :0) if I do, I'll get back to you on my results with the sparkling yellow)

And I see what you mean regarding the Model G's "C" control, looks like another strange one.




 

[HamishR]

If I compare the Sparkling Yellow with the Model G I think the SY is probably more useful to me but then it does sound more like all my other pedals!  The Model G was probably a good choice.

Made a Sweet Honey today.  Tomorrow I have something to look forward to - I can play it loud enough to hear what it sounds like.

[HamishR]

So I built another Sparkling Yellow OD today and it got me thinking about this squeal again - because the new one squeals exactly the same.  When i look at the Model G the 4u7 going to Nature doesn't connect directly to pin 2 of the chip.  It connects to pin 2 (and Gain) via a 1K resistor which also goes to a 220n to Vref .  There is a 1K in the Sparkling Yellow which also goes to a 220n from pin 2 but this time the 220n goes to ground.  Still, I suspect that if I connect the 2u2 nature cap to pin 2 via that 1K I might avoid the squealing.

Could it be that a slight error crept into the trace?  It would only be a tiny mistake but it might be the answer.  I will test and report.  

[HamishR]

Fixed!  Yup, moving where the Nature cap connects fixed the problem.  So here is a completely working layout.  If you want completely stock use a 22n input cap.  I have changed it to 47n because it works better with a Strat like that.  :-)

[MAO]

That's great news!
Any change in tone?

[HamishR]

Not really.  It might reduce the effectiveness of the Nature control a tiny bit but not enough to worry about.  I have a feeling that this is really how the circuit should be.  It sounds a lot how I remember my original sounding, as far as memory of sound is reliable!

[Travis]

That’s great HamishR, thanks for posting

I wonder if there’s a similar issue with the Pink Purple Fuzz that I and several others had oscillation from