More info on MIDI In phantom power please!
- Thread starter Me
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duesentrieb
Active member
Sure, that would be AC driven by your 9V AC in. If it is built into the voodoo pedal thing, just bypass it.
I just posted it to show you that it is easy to double volts
I just posted it to show you that it is easy to double volts
Me
Member
Always good to learn more things about electronics.duesentrieb":3d0lg5k9 said:I just posted it to show you that it is easy to double volts
Unfortunately too often I spend my days testing other peoples stuff, not making my own though. 
Apparently Zachary Vex uses a "negistor" circuit to make high voltage in his tiny Z Vex nano head. Trying to work that one out just makes my brain hurt though.
Me
Member
One more question before I go have a look tonight: Is the midi phantom power from the Herbie totally floating or is the -ve connected to ground?
If so, and the ground of the audio is connected to the "digital" ground in the Pedal Switcher then I'll have made a massive ground loop, or tried to effectively short out half the rectifier in the Pedal Switcher. Obviously Peter may be expecting all MIDI pedals to be floating. At least the pedal switcher may be creating a ground loop (even if a negative is connected to ground via a cap).
If so, and the ground of the audio is connected to the "digital" ground in the Pedal Switcher then I'll have made a massive ground loop, or tried to effectively short out half the rectifier in the Pedal Switcher. Obviously Peter may be expecting all MIDI pedals to be floating. At least the pedal switcher may be creating a ground loop (even if a negative is connected to ground via a cap).
Me
Member
I've had a long evening of much experimentation with a multimeter and oscilloscope...
I don't believe it's an under voltage problem as I could power the VL Pedal switcher from a DC linear PSU with it plugged in to the guitar in jack and brought it down to about 7V input and it still didn't get noisy. The Pedal switcher runs it's smoothed 12V to the relays and to the buffer op amp which I don't use. I removed a resistor to the buffers power and it still hummed, so it's definitely not that. The MIDI/opto isolators for midi ports in it runs off a 5V regulator which seemed to be working fine with the AC PSU (15V pk/10.7 RMS -> 13.65 VDC from the rectifier), the phantom power from the Herbie (12VDC -> 10.7V from rectifier), or at least down to the 7V from the DC PSU (don't know what it was past the rectifier but that was still ok out the regulator and noticeably less than the Herbie output). So I think I've happily ruled under voltage out.
I then looked at grounding issues. It seems there is only about 2 Ohms from Pedal switcher chassis ground to the audio out and power ground. So it could well be a ground loop issue. It appears (at least when off!) that the Herbie phantom power is floating as it's open circuit to motherboard ground and chassis. When on though the -ve looks like 0 V and the +ve like 12 V relative to the chassis. It seemed to be able to power the pedal switcher with only the +ve side of the phantom power connected, so it definitely looks like there's a 2nd ground somewhere. Oddly it still buzzes with only the + phantom power connected though (using the pedal switcher to Herbert audio lead as ground). Maybe thas to be expected though I guess if there's current flowing down the audio screen. It has the same 11-ish volts at the main smoothing cap in the pedal switcher when only phantom powered with the +V.
I tried unsoldering the Herbert phantom power and connecting the voodoo labs PSU to a pair of flying leads coming from the pins. It's lovely and quiet, so it's not a MIDI cable interference issue. It means if I do decide to cut a hole in the back of it and put in a DC phantom power jack it should at least work without any nasty surprises!
So overall, it looks like it's some kinda ground loop and doesn't happen with a totally floating/isolated MIDI phantom power. But the Herbert ones look like the -ve is connected to 0V somewhere. Never a problem with a floating audio-free MIDI patch change pedal only, and I'm guessing all the MIDI capable pedals switchers aren't phantom powerable. Trust me to be difficult and try to do something no-one else has tried! Unless there's a fully floating mod I don't think I'll be able to use the Herbie phantom power to run anything that audio passes through with a common audio/power ground. This may be the reason Voodoo Labs never gave that option in the first place.
If there's a mod to totally isolate the Herbert phantom power let me know, but it doesn't look simple. Otherwise I might just buy a DC jack socket and get the drill out. Easier to have a 9V PSU in the rack than out on a long mains lead near the front of the stage and it trying to make my wah pedal hum with the magnetic emissions!
(Unfortunately I wasted quite a while to begin with thinking the Herbie phantom power was noisy by powering the oscilloscope from the opposite side of the room off a multi plug with lots of computers and digital stuff on. Got suspicious, moved it to the 4-way the Herbie was coming off and it suddenly went fine!
)
I don't believe it's an under voltage problem as I could power the VL Pedal switcher from a DC linear PSU with it plugged in to the guitar in jack and brought it down to about 7V input and it still didn't get noisy. The Pedal switcher runs it's smoothed 12V to the relays and to the buffer op amp which I don't use. I removed a resistor to the buffers power and it still hummed, so it's definitely not that. The MIDI/opto isolators for midi ports in it runs off a 5V regulator which seemed to be working fine with the AC PSU (15V pk/10.7 RMS -> 13.65 VDC from the rectifier), the phantom power from the Herbie (12VDC -> 10.7V from rectifier), or at least down to the 7V from the DC PSU (don't know what it was past the rectifier but that was still ok out the regulator and noticeably less than the Herbie output). So I think I've happily ruled under voltage out.
I then looked at grounding issues. It seems there is only about 2 Ohms from Pedal switcher chassis ground to the audio out and power ground. So it could well be a ground loop issue. It appears (at least when off!) that the Herbie phantom power is floating as it's open circuit to motherboard ground and chassis. When on though the -ve looks like 0 V and the +ve like 12 V relative to the chassis. It seemed to be able to power the pedal switcher with only the +ve side of the phantom power connected, so it definitely looks like there's a 2nd ground somewhere. Oddly it still buzzes with only the + phantom power connected though (using the pedal switcher to Herbert audio lead as ground). Maybe thas to be expected though I guess if there's current flowing down the audio screen. It has the same 11-ish volts at the main smoothing cap in the pedal switcher when only phantom powered with the +V.
I tried unsoldering the Herbert phantom power and connecting the voodoo labs PSU to a pair of flying leads coming from the pins. It's lovely and quiet, so it's not a MIDI cable interference issue. It means if I do decide to cut a hole in the back of it and put in a DC phantom power jack it should at least work without any nasty surprises!
So overall, it looks like it's some kinda ground loop and doesn't happen with a totally floating/isolated MIDI phantom power. But the Herbert ones look like the -ve is connected to 0V somewhere. Never a problem with a floating audio-free MIDI patch change pedal only, and I'm guessing all the MIDI capable pedals switchers aren't phantom powerable. Trust me to be difficult and try to do something no-one else has tried! Unless there's a fully floating mod I don't think I'll be able to use the Herbie phantom power to run anything that audio passes through with a common audio/power ground. This may be the reason Voodoo Labs never gave that option in the first place.
If there's a mod to totally isolate the Herbert phantom power let me know, but it doesn't look simple. Otherwise I might just buy a DC jack socket and get the drill out. Easier to have a 9V PSU in the rack than out on a long mains lead near the front of the stage and it trying to make my wah pedal hum with the magnetic emissions!
(Unfortunately I wasted quite a while to begin with thinking the Herbie phantom power was noisy by powering the oscilloscope from the opposite side of the room off a multi plug with lots of computers and digital stuff on. Got suspicious, moved it to the 4-way the Herbie was coming off and it suddenly went fine!
)
Me
Member
AH-HAAAA!!!! Just noticed a very interesting phenomenon. Now with the oscilloscope connected closer to the amp, when the grounded crocodile clip of the oscilloscope is connected to the Herbert phantom power -ve (not using differential probes) the hum goes LOADS quieter. I repeated it with a screwdriver from chassis to pin 1 and it wasn't noticeable over the normal faint hum. Is there a reason I can't connect the midi port phantom power -ve to earth better inside with a good thick short low-impedance wire? It seems that rather than float the supply to remove the ground loop I could try to make the phantom power ground good enough that most of the current would far rather go down the midi lead, not the audio one, and the hum will pretty much be gone. Or is that bad because if I go somewhere with higher magnetic fields I might end up with surprise hum on a gig night that I can't get rid of that wouldn't occur the other way with the plug? I also don't want to make more ground loops inside the Herbert and obviously have no circuit diagram so don't know what other grounds the Phantom power has.
duesentrieb
Active member
Hmmm - easiest way is to isolate the signal from other grounds by using a line isolation transformer (Palmer offers these little helpers).
Other than that I would install a 2.5mm jack to Herbie and run 9V AC as already suggested.
Other than that I would install a 2.5mm jack to Herbie and run 9V AC as already suggested.
Me
Member
I've looked into the audio transformer Olaf but it's probably too much work to mod the Pedal Switcher board, may not be possible to fit, and I'm not gonna risk lots of time and money on it since I want it working ASAP.
I nearly bought a 2.5mm jack at lunch time until I realised in the shop that the outer is grounded and it will effectively ground half the AC to chassis and force the internal pedal switcher voltages relative, so might hum, else even be trying to short half the a voltage to ground down the audio screen.
I'm gonna get an XLR plug, disconnect the Columbus footswitch XLR wires inside the Herbert and feed it in via that. So it's still totally floating relative to chassis, then back out the midi jack. This also means I can always put the Herbert back to stock without any holes in it in the very very unlikely case I'd ever sell it!
And the locking XLR means I can't accidentally disconnect it and power down all the pedals. It's just a standard microphone type XLR isn't it? I have a few of those at home going spare.
Thanks very much Olaf and Peter for all your help on this. Trust me and my crazy ideas to be awkward!
I nearly bought a 2.5mm jack at lunch time until I realised in the shop that the outer is grounded and it will effectively ground half the AC to chassis and force the internal pedal switcher voltages relative, so might hum, else even be trying to short half the a voltage to ground down the audio screen.
I'm gonna get an XLR plug, disconnect the Columbus footswitch XLR wires inside the Herbert and feed it in via that. So it's still totally floating relative to chassis, then back out the midi jack. This also means I can always put the Herbert back to stock without any holes in it in the very very unlikely case I'd ever sell it!
And the locking XLR means I can't accidentally disconnect it and power down all the pedals. It's just a standard microphone type XLR isn't it? I have a few of those at home going spare.Thanks very much Olaf and Peter for all your help on this. Trust me and my crazy ideas to be awkward!
duesentrieb
Active member
Yup, standard XLR.
Huh? Here's a standard jack for 9V stuff . . . insulated of course:
https://www.tube-town.net/ttstore/produ ... ntage.html
Huh? Here's a standard jack for 9V stuff . . . insulated of course:
https://www.tube-town.net/ttstore/produ ... ntage.html
Me
Member
Modded it via the XLR. Electrical taped the ends of the Columbus and existing phantom power wires and neatly tie-wrapped them back so they're completely safe but I can put it back exactly to factory spec if required.
All works! Doesn't hum. Hooray!
Well,it hums a bit, but that's even without any of the pedals or phantom power, and the clean channel's a bit distorted, hence why I think gonna change all my preamp tubes. That's a different story though. Spent half the evening swapping preamp tubes I already have around but still seems to randomly hum on all channels or make sounds of the sea on channel 1! I'm sure channel 3 sounds nicer with an RFT in v2 though. It got one of its old TADs in v1 again now. I think a couple went bad before so seems to have had a random mix in for a while, so probably due for an entirely new set and start afresh. I'm no longer sure which ones are balanced for v6 other than an ECC81.
All works! Doesn't hum. Hooray!
Well,it hums a bit, but that's even without any of the pedals or phantom power, and the clean channel's a bit distorted, hence why I think gonna change all my preamp tubes. That's a different story though. Spent half the evening swapping preamp tubes I already have around but still seems to randomly hum on all channels or make sounds of the sea on channel 1! I'm sure channel 3 sounds nicer with an RFT in v2 though. It got one of its old TADs in v1 again now. I think a couple went bad before so seems to have had a random mix in for a while, so probably due for an entirely new set and start afresh. I'm no longer sure which ones are balanced for v6 other than an ECC81.