Sunn Beta preamp PCB - Help!

[cylens]

Hey all! Long time no see!

After successfully building the Sunn Beta (Bass) preamp thanks to the excellent layout by Alex, I decided to have PCBs manufactured.

I'm quite happy with how it came out BUT I have a very weird noise problem:

When the gain is quite high, I have a very loud squealing sound at the output (and not only at the output, as I've used a probe all along the circuit). I have noticed that when the body of the drive pot is not connected to ground (when the circuit is not boxed and I'm not touching the pot), there is no noise.

As soon as I touch the pot or when the circuit is boxed with the case grounded, the noise is there.

It's very frustrating because, I've checked the PCB layout along the circuit times and times again and didn't find anything... I've swapped all the ICs between the vero-based build and 2 of the PCB-based builds with no change...

the only thing I have not tested is replacing the PCB-mounted drive pot (dual, 6 lugs) with a non PCB-mounted one (as it is in my vero-based build)

apart from a mistake when drawing the PCB layout (which I do not exclude, though can't find after at least 5 full checks...), I do not have a clue... so if anyone has an idea or come across such a problem... I am all hears!

best

[Sensei Tim]

What are you using to make the pcb layout? eagle? kicad? something else?

Can you post the schematic and picture of the layout?

sometimes noise like this (squealing, oscillation, etc) is caused by routing traces too close together.  are you using a ground pour on your top and bottom layers?

[cylens]

hey, thanks for your answer!

i'm assuming, it's something like this...

i'm using a ground plane on my bottom layer only, used Eagle for this.

here's a print file for top (red) and bottom (blue) layer (without the ground plane, though).



and here's the schematic I've used (using a 4069 instead of a 4049, so the pinout is different).



I can't be 100% sure layout vs schematic is ok, but honestly, I've checked it so many times...

If we're talking about traces too close, the traces between the 4069 pads on the bottom layer are quite suspicious :D

If you have any advice/resource on "how to route a pedal PCB properly", I would be quite grateful!!!

thanks in advance!

[cylens]

did I miss it completely and should have used a -Vcc (-15V) plane instead of 0V/ground plane????...

[Sensei Tim]

where are you getting your +15V and -15V from?  I don't see a power section in your schematic or on your board.

When I am using a bipolar power supply i usually have it on my PCB so I feed 9V into it and it will generate a +15V and a -15V output.

[cylens]

I have a small daughterboard with a 1054, basically this one: http://tagboardeffects.blogspot.com/2014/01/bipolar-voltage-converter.html

[Sensei Tim]

you have 5 ICs running off the LT1054.  I'm not sure if the 1054 can supply enough current for all of those ICs.

I have a circuit that uses 4 dual opamps on ±15V rails from a 1054 and it just barely has enough juice to power it.  I think a 1054 can only supply 10-20mA @ ±15V

If you run a DRC and ERC on your schematic and layout do any warnings or errors pop up?  I assume you're using Eagle?

[cylens]

thing is, I have the same schematic-based circuit running perfectly on veroboard... so I have to stick to a design flaw or error on my side...

I must admit I have never used ERC or DRC (or even schematics) in Eagle.

I think I'll redo it from scratch avoiding anything to pass between the 4069 pins...

[Sensei Tim]

you're using Eagle, right?

just click on the ERC button and it will give you any errors or warnings about your schematic like nets that look like they might be connected but actually aren't, etc.

if you run ERC in the board window, it will let you know if any of your traces are overlapping others or too close or other design rule violations.

If you want to send me the sch and brd eagle files i can have a look when i have some free time.

T

[Sensei Tim]

also, where did you have the boards made?