Debugging Fender amp sim from LXH2 PCB design

[jrc4558]

Hi all!
Threw a PCB toghether for a fender amp simulator found at http://home3.netcarrier.com/~lxh2/fenpre4.gif

Populated it and fired up. Works, but...
output volume knob (output level on the schematic) works for the first third only. the last two thirds are a gradual fade into hiss. (like an uneven panoramic control between slightly crackling white noise and audio signal from the effect)
I used one dual and two quadruple op-amps for the circuit with the dual opamp carrying the first and last stages (1 and 10 in schematic). i power it with batteries, so the impedance of the gound is low. what could be the problem? is it the cross-talk between the opamps within the package? or could it be the fact that i plug in directly into souncard without buffer?
NB this behavior was only obseved with the soundcard, since i didn't test it with anything else...

i will post the layout once i get home tonight...

[petemoore]

or could it be the fact that i plug in directly into souncard without buffer?
  I don't figure the impedance of O>I, but a volume control problem as you described...there's a reason for it, but I'd try a different 'amp' and some more *poking around the Circuit near the volume control...that shouldn't happen.
  Test the resistances of the volume pot, each side from wiper to see if it sweeps resistance with no dropouts.

[jrc4558]

Here's an overlay of the layout.
http://aronnelson.com/gallery/album111/FenderSIMOverlay
quad opamps are in backwards though... I wasn't sure of the pinout at the moment of PCBing...

[jrc4558]

The problem has been narrowed down to op-amps hissing.
I had the same problem with http://www.runoffgroove.com/tonemender.html
I didn't find a solution to it, but it seems to me that what is wrong is the same thing as with the tonemeder.
I unsoldered the wire that feeds the output volume control and went looking for a quiet stage in the circuit. When I connected it to the wiper of the treble potentiometer (250kA), the effect became dead quiet, but much quieter than stock. I proceeded taking output signal from concequent stages. Basically, the effect starts gaining some hiss right after the post-tonestack buffer and becomes progressively noisier as the gainstages add up. It increases throughout the amp simulator, and stays relatively the same through the cab simulator. Thus, by the time the signal hits the output volume control, hiss is at significant level, and somehow feeds back (how I don;t know yet, no traces are shorted)
With gain at maximum I hear enormous amount of really buzzy op-amp clipping (I mean, blackface Fenders do clip hard and fuzzy, but this thing's distortion is as far from Fender's as I am from Blackmore)
I compared the noise levels to those of Condor Cab Sim. The noise coming from my PCB layout of LXH2 is ~500-600mV higher than that of the Condor (which is practically dead quiet, I dont read anything above 2-3mV on the software oscilloscope)
I checkes wires, I checked common grounds. I checked voltage supply. I checked all potentiometers. I checked ICs (as far as I could)
The thing hisses none the less.
What would be further steps to eliminate hiss?
Also - please feel free to comment on the layout.
Hope to hear from you guys soon.

[MartyMart]

What does the bipolar supply consist of ... how much filtering?
When I built the LXH2 from the same site, I had 220uf filtering caps on both rails
together with 100n poly's and 10n ceramics.
It's quiet as  a mouse !
2 X 9v battery powered

MM.

[jrc4558]

actually - no filtering at all. i thought, since i power it with batteries, there should be no noise. you say 220uF/.1uF and .01 uf?
i will give it a try tomorrow.
Marty, how did you find the distortion sound of the circuit? Does it sound smooth? Did you modify your circuit in any way?

Thanks!

[MartyMart]

The only two "mods" I did were, using three X 4148's for assym clipping and
I put a dual RC network at the back end, of 12k's and 2n2's.
This was because I didn't build the spkr sim section - just preamp !
It's pretty much noiseless unless the gains are wound right up.
I'm not saying that all your noise is PWR filtering ... you'll need to keep
the lead dress "neat" and not cross over any pot wiring etc ....

MM.

[jrc4558]

Ok, I tried adding large filtering capacitors to the power rails - no effect. I think the problem is in the layout and wiring. Will try to redo it somehow, at least with dual opamps all the way, to avoid hanging four opamps on the same power line... But will this help? No idea.

Is there any kind of rule of thumb in routing the power rails in the PCB? Should I perhaps use jumper wires for them and avoid putting them on the same plain as the signal path?