GEOFEX Humfree splitter build - help needed

[Sonicboom]

Hi Guys,

I'm attempting to build a 2 chl. humfree splitter and have run into some noise problems.  Hoping someone can help by giving me a sanity check on the design and required grounding.

First, I'm using a modified version of R.G's basic schematic with a charge pump that will be connected to a PP2+ isolated power supply.  The design requirements are 2 separate channels that can be switched true bypass independently.  When not bypassed, each channel can be switched independently from buffered to non-buffered (for cases where the input is already buffered).

The transformers are Edcor 10K:10K that are enclosed in a DIY mag. shield because the enclosure is aluminum - the shields aren't grounded.   I've also lined the enclosure with copper tape to form a Faraday cage that will be tied to a common pedal board ground via a separate wire.   The pdl. bd. ground is tied back to the amplifiers through a common power strip.  The input and output jacks are isolated from the enclosure, meaning that the circuit is floating from earth ground.  The only connection btw. channels is via the op amp's power - the charge pump.

I've read in other posts that people have had noise problems with a Humfree until they connected amps to both the isolated and non-isolated outputs.  Since this is a two channel box, there will not be any non-isolated outputs as the orig. design calls for.

Operating Conditions:

• It works fine in Bypassed mode - quiet, clean signal.
• When engaged in Buffered mode using either a battery or PP2+, there is a slight vol. drop from bypassed mode and an increase in hum.
• When engaged in non-buffered mode using either a battery or PP2+, there is a fair amount of hum.
• In both cases, the hum drops to near zero if I connect the input and output grounds - similar to what others experienced in previous posts.

Questions:
1. Am I over-thinking this?
2. Will the common power source cause noise problems?  The DC input is isolated and its ground wire is not connected to the enclosure.
3. Will I have internal ground loop issues if I use only one OPA2134 op amp for both channels?

Modified R.G. Humfree


Circuit as built - shielded input wires with magnetic shielding

[Sonicboom]

Anyone??

The main question is how should I ground this so that each channel is independent of the other and does not cause ground loops. 

Currently, it is wired such that the power supply, which is isolated coming in, has its own ground separate from signal ground.  Each signal channel also has its own ground - they are not shared with power or each other. 

I'd love to get this working.

Thanks.

[Sonicboom]

Well, I got it working and boxed up.  Now, there is no diff. btw. the buffered and bypassed signals in terms of signal levels, noise, or tone.  The xfmrs. are a little overkill, handling up to 20v RMS with a flat freq. response, but at ~$8 ea., who cares.  Between the xfmers. and the +/- 9v supply, I shouldn't have any headroom issues.

The enclosure was an experiment with a marbling technique.  It actually looks pretty good; the photos don't really do it much justice.