Recently, I built a tube-based overdrive as an effect for guitar, which is basically a tube preamp with an EQ. The power supply for the tubes is around 340V dc.
I housed it in a metallic box, which is properly earthed. Lets name the effect TubeDrive.
The problem is this: the device will be used along with a guitar amp. The signal ground in the guitar amp is at all times connected to earth, for safety reasons and minimum noise. Pretty obvious so far. But this means that should I connect the TubeDrive's signal ground to earth, I would create a ground loop and induce noise.
So, the plan was to properly earth the chassis of the TubeDrive, and leave the signal ground floating.
By experimentation, this (allegedly) resulted in destroying the power transistor and mosfet of the power supply 2 times, I think due to overvoltage issues caused by the fact that the signal ground was not equal to earth potential, when I removed the input/output cables, thus cancelling the connection to earth via the main amp.
Measuring the voltage difference between the audio ground of the TubeDrive and its chassis earth with no cables attached, resulted in about 150V or so, which is obviously unsafe.
I considered permanently connecting the audio ground to the chassis earth via a 10R(5W)//10nF(1kV) combo, to ensure that there is no voltage difference but not making a ground loop. Of course, I fear that signals picked up by the 10R resistor would induce noise, but maybe this will not be an issue.
Another option is to add an earth to ground switch to the TubeDrive, but this is not completely safe, since if it is left accidentally off without cables connected to the TubeDrive that would earth the ground, the same problem would rise.
Anyway, these are my initial thoughts on the subject. What are your thoughts on this?
I housed it in a metallic box, which is properly earthed. Lets name the effect TubeDrive.
The problem is this: the device will be used along with a guitar amp. The signal ground in the guitar amp is at all times connected to earth, for safety reasons and minimum noise. Pretty obvious so far. But this means that should I connect the TubeDrive's signal ground to earth, I would create a ground loop and induce noise.
So, the plan was to properly earth the chassis of the TubeDrive, and leave the signal ground floating.
By experimentation, this (allegedly) resulted in destroying the power transistor and mosfet of the power supply 2 times, I think due to overvoltage issues caused by the fact that the signal ground was not equal to earth potential, when I removed the input/output cables, thus cancelling the connection to earth via the main amp.
Measuring the voltage difference between the audio ground of the TubeDrive and its chassis earth with no cables attached, resulted in about 150V or so, which is obviously unsafe.
I considered permanently connecting the audio ground to the chassis earth via a 10R(5W)//10nF(1kV) combo, to ensure that there is no voltage difference but not making a ground loop. Of course, I fear that signals picked up by the 10R resistor would induce noise, but maybe this will not be an issue.
Another option is to add an earth to ground switch to the TubeDrive, but this is not completely safe, since if it is left accidentally off without cables connected to the TubeDrive that would earth the ground, the same problem would rise.
Anyway, these are my initial thoughts on the subject. What are your thoughts on this?
Not necessarily so.
That is often a problem in complex multi unit rack setups, or in a Recording Studio environment, where absolute minimum hum/noise is expected, specially because "once recorded it´s there forever" , but not that important in a live playing setup.
Not even in a Home Hi Fi setting ... you don´t see many ground lift switches in DACs, preamps and power amps, even if they are connected all together do you?
And I am talking High end setups.
Besides that, SAFETY is a big concern, doubly so for Guitar players who are all the time connected to ground and metallic chassis through guitar strings and hardware, compared to a Hi Fi setup where you are usually not.
Please don´t.
I suggest both the amplifier and the mains powered preamp have their proper safety ground, and to minimize or kill "possible" ground loops take the precaution of plugging both in the same mains outlet ground.
Which means plugging both in the same multi outlet extension cord.
Don´t do anything dangerous trying to correct a "perceived" problem which may not even be there ... or just at a tiny level, safety first.
Just avoid ground loops by using same ground for all.
To boot, each one has its own power supply so buzz causing charging pulses run internal to each unit, in principle no supply currents of any kind should run through interconnecting ground, and if all arte referred to a same common point, you don´t have to deal with voltage differentials there.
Now if you are plugging them onto different outlets ... yes, "ground to ground" difference might exist ... so avoid it 🙂
Many thanks for your reply!
Actually, I was considering this solution too, since I tried it at home and it worked ok, I just wanted a universal solution that would allow me to have the same results everywhere.
Coming to think of it, wouldn't an audio isolation transformer do it better too? In order to isolate the output of the TubeDrive. Do you think that such a solution would be ok? Of course, provided that the circuit's ground is earthed.
EDIT: for example, I found this cheap transformer that should work, provided I terminate it correctly.
Last edited:
- Status
- Not open for further replies.