Acoustiphonic wiring

[DC Ross]

Hey all, 

I've wired up an acoustiphonic system with a push/push to select between the humbucker and the piezo, but getting some crazy hum/buzz.

Attached is the diagram. 

Any suggestions / input / advice?

Thanks

[curtisa]

Quick thoughts:

• You haven't drawn a bridge ground wire - is one installed in the guitar?
• The 100uF cap isn't suggested in the Acoustiphonic documentation - is it necessary?
• Double-check the jack connections - make sure you haven't mixed up the tip/ring/sleve soldering
• The shared volume pot for the mag/piezo isn't a suggested option in the Acoustiphonic manual. I guess it should work as you have it drawn, but without knowing the finer details of the circuitry on the PCB I'm not sure what effect toggling the volume connections between the two functions will have to the output. The suggested toggling option in the manual is to use the QSW terminals with a switch to select the two functions, with a separate permanently-connected volume pot for each function. I guess you could use a dual gang volume pot to emulate this wiring, but perhaps finding a dual gang volume pot with a built-in push-pull switch may be troublesome?

[DC Ross]

Thanks for the reply, Curtis

The bridge is grounded, I omitted it for the sake of brevity

The cap is from them, with an update to the documentation

The jack is definitely wired correctly (good to check though)

 

Everything does function correctly, but occasionally there is the weird hum. It's especially noticeable if I grab the yellow wire in the diagram (not even the contacts, just grabbing the sheathing will do it).

[curtisa]

All the literature I can find mentions that individual volume pots are used, rather than shared. There may be some particulars of the two volume circuits in the Acoustiphonic PCB that prevent you sharing grounds between the volume pots as you have done. The manual shows individual ground wires run to each pot, rather than one ground wire daisy-chained to each. This may be for convenience on Graphtech's part, but could also be a mandatory requirement of the circuitry.

If you have a spare volume pot try wiring it as per the manufacturer's diagram (without the push/pull switch) and see if that cures the hum issue.

Maybe you have insufficient shielding in the cavity?

When touching the yellow wire does it hum only when either the piezo or mag is selected?

Might also be worthwhile posing the question to Graphtech support and see if they have any ideas.

Rewiring it with a dual gang push/pull pot and using the 'QSW' terminals for the push/pull switch is probably the only thing I can think of that would minimise any problems with adapting the existing circuit to suit the application.

[DC Ross]

Thanks again,

The thing is, everything works the way it was wired -- and any hum there is, isn't consistent. Usually it's dead-quiet.

Wiring the push/push to the quickswitch wasn't really feasible, since a) I haven't seen any dual concentric push/push pots anywhere, (and it would be moot anyway since he wanted the single master vol.) and b ) I have no idea how I'd wire the rest of the circuit.

I wrote to their support, but haven't found a solution through them.

[curtisa]

14 minutes ago, DC Ross said:

Wiring the push/push to the quickswitch wasn't really feasible, since a) I haven't seen any dual concentric push/push pots anywhere, (and it would be moot anyway since he wanted the single master vol.) and b ) I have no idea how I'd wire the rest of the circuit.

Not dual concentric. Dual gang. Dual concentric is two stacked pots controlled with individual knobs. Dual gang is two stacked pots controlled by a common knob.

Bourns make them:

http://www.wdmusic.com/bourns_push_pull_pot_dpdt_a_taper_6mm_solid_shaft_250k_bourns_250pp_ss.html

You'd treat each element of the dual gang pot as if it were the two individual pots shown in the Graphtech documentation and wire it as such. The push-pull switch would be wired to the 'QSW' terminals. Grpahtech have a 'one of the other' mod for the quickswitch on their FAQ pages for people who only want to select between piezo and mag without the 'both' option.

Anyway, if the circut normally works with intermittent hum I'd be looking at faulty wiring - maybe your bridge ground isn't solidly bonded, or the cavity shielding isn't properly earthed. Also consider environmental issues, such as doing your testing next to a electric motor or computer.

[DC Ross]

Ah, of course... Thanks for the link!