Hey guys, long time, hope you are all doing OK... I just rewired my OBEL and noticed when my guitar's loop switch is ON that with the stereo cable unplugged from the guitar the audio signal is still coming out the mono cable. Anyone see this behavior?
Normally the obvious solution is to either turn the loop switch OFF, or else plug in the stereo cable and use it as designed. But I am investigating some squealing I sometimes get in my ampless pedalboard which suggests the signal is going through some funky pathways unless there are buffers (or buffered pedals) in the right places to prevent the squeal.
The TL;DR might be this: would an output coupling cap at the end of the buffer help to block audio from running backwards into the output?
OK, here are the details (hopefully there are some sharp electronics gearheads still on this board). .. I've wired the guitar according to the Tiger schematic where the pickup selector always goes to the buffer input, then the buffer output goes to the DPDT loop switch like this:
Pickup SW -> BUFFER IN -> BUFFER OUT -> Switch middle left -> off: top left hardwired to right / on: bottom left to TRS TIP (fx send), bottom right from fx loop TRS Ring (fx return) -> 25k volume potentiometer (log) left terminal -> wiper to mono send TS tip (actually TRS with ring for buffer battery negative/buffer ground) / right pot terminal to ground.
Volume sweeps nicely over full range. There are 2 single coil pups and one HB pup, they each have their own tone cap and pot (SC's each have a 250k pot (actually ~242k), HB has a 500k pot (actually ~490k), all before the pickup selector output.
Plugging in a stereo cable into the fx loop TRS jack (with other cable end disconnected) with the fx loop switch ON gives me ~10k between tip and ground (makes sense since buffer has a 10k resistor to ground at end of the circuit), and ~25k between ring and ground (makes sense since the ring is connected to the 25k volume pot. Now I get ~35k between the tip and ring - makes sense since they have a common ground, the resistance is 10k+25k. Note I tested with just a TRS plug with no cable to eliminate possibility the cable was to blame).
[Note: Turning the fx loop switch OFF disconnects the fx loop TRS jack from the buffer out and the volume pot. In this case the tip and ring do not go to ground and the resistance between tip and ring is at least 390k (probably more - not sure why it wasn't immediately higher or why the meter kept reading higher and higher until I got tired and stopped measuring, possibly running backward from ground through tone caps?).]
So how can it be that the audio signal is getting from the buffer out to the volume pot when the fx loop switch is ON but the stereo fx loop cable is disconnected - could it be running out the buffers 10k resistor to ground, then from ground up right terminal of the volume pot and then out the volume pot wiper? Or is it something more basic like crosstalk within the TRS jack (it has been re-soldered for different installations; I've ordered a replacement).
Before rewiring the guitar I first installed the OBEL without a buffer, got a squeal in my pedalboard loop, then installed a non-Tiger buffer and the squeal went away pretty much completely. But I had numerous mods to make and so I routed space for batteries, fully shielded the cavity, replaced the volume and tone pots, and installed a TIger-based buffer I built, and now I get the squeal a lot when I flip the fx loop switch OFF.
Clearly I could take a look at potential ground loops and misaligned wire strands, but wanted to check if anyone has been down this road before where the buffer did not eliminate squeals elsewhere in their signal chain. I used a star arrangement to connect all grounds to a common spot (pickup selector switch ground screw). The cavity is fully conductive (used aluminum tape) and there is an aluminum shield under the pickguard (I know, peeps like copper and conductive paint, I used what I had on hand). Maybe with the pickup switch and pots already pressed on the shield I didn't need some of the ground wires.
Any other thoughts or suggestions? As I said above, I'm wondering if adding a polarized coupling cap at the end of the buffer help to block audio from running backwards into the output.
Thanks,
Ned
Normally the obvious solution is to either turn the loop switch OFF, or else plug in the stereo cable and use it as designed. But I am investigating some squealing I sometimes get in my ampless pedalboard which suggests the signal is going through some funky pathways unless there are buffers (or buffered pedals) in the right places to prevent the squeal.
The TL;DR might be this: would an output coupling cap at the end of the buffer help to block audio from running backwards into the output?
OK, here are the details (hopefully there are some sharp electronics gearheads still on this board). .. I've wired the guitar according to the Tiger schematic where the pickup selector always goes to the buffer input, then the buffer output goes to the DPDT loop switch like this:
Pickup SW -> BUFFER IN -> BUFFER OUT -> Switch middle left -> off: top left hardwired to right / on: bottom left to TRS TIP (fx send), bottom right from fx loop TRS Ring (fx return) -> 25k volume potentiometer (log) left terminal -> wiper to mono send TS tip (actually TRS with ring for buffer battery negative/buffer ground) / right pot terminal to ground.
Volume sweeps nicely over full range. There are 2 single coil pups and one HB pup, they each have their own tone cap and pot (SC's each have a 250k pot (actually ~242k), HB has a 500k pot (actually ~490k), all before the pickup selector output.
Plugging in a stereo cable into the fx loop TRS jack (with other cable end disconnected) with the fx loop switch ON gives me ~10k between tip and ground (makes sense since buffer has a 10k resistor to ground at end of the circuit), and ~25k between ring and ground (makes sense since the ring is connected to the 25k volume pot. Now I get ~35k between the tip and ring - makes sense since they have a common ground, the resistance is 10k+25k. Note I tested with just a TRS plug with no cable to eliminate possibility the cable was to blame).
[Note: Turning the fx loop switch OFF disconnects the fx loop TRS jack from the buffer out and the volume pot. In this case the tip and ring do not go to ground and the resistance between tip and ring is at least 390k (probably more - not sure why it wasn't immediately higher or why the meter kept reading higher and higher until I got tired and stopped measuring, possibly running backward from ground through tone caps?).]
So how can it be that the audio signal is getting from the buffer out to the volume pot when the fx loop switch is ON but the stereo fx loop cable is disconnected - could it be running out the buffers 10k resistor to ground, then from ground up right terminal of the volume pot and then out the volume pot wiper? Or is it something more basic like crosstalk within the TRS jack (it has been re-soldered for different installations; I've ordered a replacement).
Before rewiring the guitar I first installed the OBEL without a buffer, got a squeal in my pedalboard loop, then installed a non-Tiger buffer and the squeal went away pretty much completely. But I had numerous mods to make and so I routed space for batteries, fully shielded the cavity, replaced the volume and tone pots, and installed a TIger-based buffer I built, and now I get the squeal a lot when I flip the fx loop switch OFF.
Clearly I could take a look at potential ground loops and misaligned wire strands, but wanted to check if anyone has been down this road before where the buffer did not eliminate squeals elsewhere in their signal chain. I used a star arrangement to connect all grounds to a common spot (pickup selector switch ground screw). The cavity is fully conductive (used aluminum tape) and there is an aluminum shield under the pickguard (I know, peeps like copper and conductive paint, I used what I had on hand). Maybe with the pickup switch and pots already pressed on the shield I didn't need some of the ground wires.
Any other thoughts or suggestions? As I said above, I'm wondering if adding a polarized coupling cap at the end of the buffer help to block audio from running backwards into the output.
Thanks,
Ned