half-solved... help! Crackle & hum with Fender MA10 clone

[Mngnt]

Hi All,

I'm building a Fender MA-10 clone.  I've got it all sorted out and built.  Problem is, the circuit crackles and buzzes at anything above around 30% volume.

I've checked connections, tested for solder bridges, etc.  Everything looks OK.

I've narrowed down the problem.  I'm using this IC (I've tried a second one to make sure it's not bad):
http://pdf.datasheetcatalog.com/datasheet/sanyo/dis_pdf_e/LA4140.pdf

When I make a connection between me and either pin 1 or 4, the problem completely disappears.  (To make the connection, holding a small screwdriver bit and touching the pin works best).

I've replaced the cap bridging pins 1&4 to no avail.  I'm using a 470pF cap there as per the MA10 design.

I'm too newb to understand exactly what me touching the pins does circuit wise, and thus, what a potential solution may be.  Can anyone help me?

Thanks,

Alex

[PRR]

Touching those pins cripples the chip's gain.

That's not the answer.

It does suggest that lay-out is critical!! The chip has gain to hypersonic frequencies. Small stray signal leaking from output to input will make it howl. Super-sonic howl can't be heard, but it upsets the chip. Hisses crackles and buzzes are popular symptoms.

Have you copied Fender's layout as close as possible?

Have you kept the output wires as far as possible away from the chip and the input wires?

Do you have a hefty capacitor pin 7 to pin 5?

Do you have the cap from pin 1 to pin 4? 100pfd is suggested for general use. For guitar, 1,000pFd won't cut the highs and might make it a lot less critical about layout.