Hey all!
I'll preface this thread by saying that i'm not a complete noob to electronics, but i'm damn close to it so please bear with my apparent lack of technical knowledge
Last year I built a simple passive footswitch out of 2 3PDT switches, some LEDs and the appropriate resistor (which i can't remember the value of) for a 9vdc connection. I'm using it to switch between piezo and magnetic pickups on my guitar. I've attached a simple wiring diagram for you all to ridicule haha.
I'm having ground hum issues between the two outputs, so I have two questions for y'all:
First, I'm wondering if there's anything i can do to to improve the wiring of the box (Ground loops?).
Second, I've thought of adding a 1:1 isolation transformer to the 2nd output of the switch, but have no idea how to pick one based on the output impedance of the piezo on my guitar (Again, not quite sure as my multimeter is at work... The suggested trim pot for passive installation is 5Mohm if that's any indication :/).
Any suggestions are greatly appreciated. Thanks!
I'll preface this thread by saying that i'm not a complete noob to electronics, but i'm damn close to it so please bear with my apparent lack of technical knowledge
Last year I built a simple passive footswitch out of 2 3PDT switches, some LEDs and the appropriate resistor (which i can't remember the value of) for a 9vdc connection. I'm using it to switch between piezo and magnetic pickups on my guitar. I've attached a simple wiring diagram for you all to ridicule haha.
I'm having ground hum issues between the two outputs, so I have two questions for y'all:
First, I'm wondering if there's anything i can do to to improve the wiring of the box (Ground loops?).
Second, I've thought of adding a 1:1 isolation transformer to the 2nd output of the switch, but have no idea how to pick one based on the output impedance of the piezo on my guitar (Again, not quite sure as my multimeter is at work... The suggested trim pot for passive installation is 5Mohm if that's any indication :/).
Any suggestions are greatly appreciated. Thanks!
Attachments
I'm not good at reading physical layouts, only schematics, so may I ask, what exactly does the box you built do?
You only need one footswitch to switch between two inputs and one output, so what does the second footswitch do? And what do the three LEDs indicate? And what are the outputs from the guitar's TRS jack - ground, piezo, and magnetic?
1) Are you using two different amplifiers, perhaps one acoustic and one electric? If so, are they both plugged into the same AC outlet? (If not, you can get ground loops that cause hum.)
2) Does the hum level drop dramatically when you put your hand on the strings of the guitar? If so, you may be dealing with a 9V power supply that has a floating output (neither + nor - line is grounded). If so, you might try grounding your switcher pedal through the cable to the amp - so make sure the output jacks are not insulated from the case.
Piezo sensors have such a high impedance at low frequencies that, as far as I know, an isolation transformer isn't going to work. It isn't practical to wind a transformer with a high enough self-inductance to work with a passive piezo guitar pickup.
On the other hand, an active buffer to work with such a piezo pickup is almost child's play. You can use any typical guitar "clean boost" or buffer pedal that has the normal 1 Meg input resistance. Simply wire a 2.2 nF (same as 2200 pF) capacitor across the input of the buffer (i.e., straight across the output from the passive piezo pickup), and you're all set.
The added capacitor will drop the voltage from the piezo pickup a bit, but will make it compatible with the 1Meg input resistance of the buffer or clean boost pedal. My go-to pedal for this and many similar situations is a Danelectro Fish-n-Chips graphic EQ/clean boost, which is inexpensive, works very well, and gives you additional control over your sound.
Since you already have 9V power inside your footswitch pedal, you also have the DIY option - to simply build a tiny JFET buffer into the box. It will only need four or five small and inexpensive components, and since you already have the skills to solder switches and LEDs, I think this will be well within your ability, even if you're new to electronics.
The JFET buffer will give your piezo pickup the very high input resistance it needs, and simultaneously present a low output impedance to your guitar amp, so that it is much less likely to hum. If you decide to go this route, I / we can give you more information so you can proceed.
-Gnobuddy
You only need one footswitch to switch between two inputs and one output, so what does the second footswitch do? And what do the three LEDs indicate? And what are the outputs from the guitar's TRS jack - ground, piezo, and magnetic?
Two questions:
1) Are you using two different amplifiers, perhaps one acoustic and one electric? If so, are they both plugged into the same AC outlet? (If not, you can get ground loops that cause hum.)
2) Does the hum level drop dramatically when you put your hand on the strings of the guitar? If so, you may be dealing with a 9V power supply that has a floating output (neither + nor - line is grounded). If so, you might try grounding your switcher pedal through the cable to the amp - so make sure the output jacks are not insulated from the case.
I think we may have found the most likely cause of the hum problem. Passive piezo transducers have an extremely high output impedance, and as far as the guitar amp is concerned, it's like having no guitar plugged into the other end of the cable. Most amps will hum in this situation.
Piezo sensors have such a high impedance at low frequencies that, as far as I know, an isolation transformer isn't going to work. It isn't practical to wind a transformer with a high enough self-inductance to work with a passive piezo guitar pickup.
On the other hand, an active buffer to work with such a piezo pickup is almost child's play. You can use any typical guitar "clean boost" or buffer pedal that has the normal 1 Meg input resistance. Simply wire a 2.2 nF (same as 2200 pF) capacitor across the input of the buffer (i.e., straight across the output from the passive piezo pickup), and you're all set.
The added capacitor will drop the voltage from the piezo pickup a bit, but will make it compatible with the 1Meg input resistance of the buffer or clean boost pedal. My go-to pedal for this and many similar situations is a Danelectro Fish-n-Chips graphic EQ/clean boost, which is inexpensive, works very well, and gives you additional control over your sound.
Since you already have 9V power inside your footswitch pedal, you also have the DIY option - to simply build a tiny JFET buffer into the box. It will only need four or five small and inexpensive components, and since you already have the skills to solder switches and LEDs, I think this will be well within your ability, even if you're new to electronics.
The JFET buffer will give your piezo pickup the very high input resistance it needs, and simultaneously present a low output impedance to your guitar amp, so that it is much less likely to hum. If you decide to go this route, I / we can give you more information so you can proceed.
-Gnobuddy
Hi,
I am not sure how is your guitar designed and whether it has a built in piezo preamp.
I also understand the problem here is some kind of a ground loop, which leads to hum. Additionally, the guitar has two output jacks with their ground connected and you use two signal cords to connect it elsewhere.
If the piezo pickup is NOT connected to ground on one (guitar) end, it will surely simplify the solution.
Here's how I would try to do it for both pickups connected to the same ground:
For the piezo input, make at least a simple piezo preamplifier, which you would have to supply with some power, like in guitar effects.
The same could apply for the magnetic pickup input, with a buffer of your choice, but there might be no need for that.
Isolate the input jacks from the enclosure, so you can connect the ground(s) as you need.
Use only one input jack to get the ground and leave the other one floating. At the end of the day, it's still the same ground.
Connect input signals (sources) to switching circuitry (that is, a switch) and off to output jack. If you need any indication, a 3PDT switch should be enough for this purpose.
In the case you have the piezo not sharing the signal ground, you might be able to do a bit more, even with transformers if you like - some kind of preamp + signal isolation buffer. Basically the issue is, that any input signals should have grounds connected on the device you plug them into, and not necessarily on their source, as it might be in your case. I might be wrong though.
I also noticed you seem to need some kind of A/B switch? In such case there are at least a few designs out there to guide you through.
Hope this helps a bit!
Cheers,
Jakub
I am not sure how is your guitar designed and whether it has a built in piezo preamp.
I also understand the problem here is some kind of a ground loop, which leads to hum. Additionally, the guitar has two output jacks with their ground connected and you use two signal cords to connect it elsewhere.
If the piezo pickup is NOT connected to ground on one (guitar) end, it will surely simplify the solution.
Here's how I would try to do it for both pickups connected to the same ground:
For the piezo input, make at least a simple piezo preamplifier, which you would have to supply with some power, like in guitar effects.
The same could apply for the magnetic pickup input, with a buffer of your choice, but there might be no need for that.
Isolate the input jacks from the enclosure, so you can connect the ground(s) as you need.
Use only one input jack to get the ground and leave the other one floating. At the end of the day, it's still the same ground.
Connect input signals (sources) to switching circuitry (that is, a switch) and off to output jack. If you need any indication, a 3PDT switch should be enough for this purpose.
In the case you have the piezo not sharing the signal ground, you might be able to do a bit more, even with transformers if you like - some kind of preamp + signal isolation buffer. Basically the issue is, that any input signals should have grounds connected on the device you plug them into, and not necessarily on their source, as it might be in your case. I might be wrong though.
I also noticed you seem to need some kind of A/B switch? In such case there are at least a few designs out there to guide you through.
Hope this helps a bit!
Cheers,
Jakub
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