Hi -- my homemade guitar combo amp is powered by a Quilter solid state amp which is powered by a centre positive 25 volt power supply. It is positioned next to a guitar effects pedal which is powered by a centre negative supply. Generally, everything is working fine, but I noticed the other day that if the metal jack going into the effects pedal comes into contact with the metal jack plugged the amp’s speaker output, it produces a very loud buzz through the speaker and you see little sparks where the two metal jacks are in contact! I’ve never come across this kind of thing before, and I’m wondering if anyone can shed any light on this for me?

 

Power adapters are usually isolated.

Does the power adapter have a 2-pin or 3-pin plug?

Get a test meter and measure voltages between the connector outside barrel and electricity supply earth with the meter set to AC and again set to DC. If you get no appreciable voltages, test for continuity between the connector and earth. Ideally you should see an open circuit.

If the power jack on either adapter is connected to earth then you might have a problem.

 

Hi -- my homemade guitar combo amp is powered by a Quilter solid state amp which is powered by a centre positive 25 volt power supply. It is positioned next to a guitar effects pedal which is powered by a centre negative supply. Generally, everything is working fine, but I noticed the other day that if the metal jack going into the effects pedal comes into contact with the metal jack plugged the amp’s speaker output, it produces a very loud buzz through the speaker and you see little sparks where the two metal jacks are in contact! I’ve never come across this kind of thing before, and I’m wondering if anyone can shed any light on this for me?

I've experienced buzzing sound when I touch the input to an amp in the past. It's a 60 cycle hum which could be described as a buzzing sound. In MY case it's just the amplifier amplifying the presence of AC present in all homes. As a general rule you always plug all things into an amp you're going to use BEFORE you turn the power on.

In your case I doubt it's anything you need to be concerned about. Do this test: Touch the tip of the jack when it's plugged into the amp. It will likely hum.

 

Power adapters are usually isolated.

Does the power adapter have a 2-pin or 3-pin plug?

Get a test meter and measure voltages between the connector outside barrel and electricity supply earth with the meter set to AC and again set to DC. If you get no appreciable voltages, test for continuity between the connector and earth. Ideally you should see an open circuit.

If the power jack on either adapter is connected to earth then you might have a problem.

There are two power adaptors. One delivers 9V for the effects pedals, and the other delivers 24V for the amplifier. Both adaptors plug into the mains with three pin plugs. To test with the supply earth, I take it I would put one of the test meter terminals into the earth terminal of the mains supply?

 

I've experienced buzzing sound when I touch the input to an amp in the past. It's a 60 cycle hum which could be described as a buzzing sound. In MY case it's just the amplifier amplifying the presence of AC present in all homes. As a general rule you always plug all things into an amp you're going to use BEFORE you turn the power on.

In your case I doubt it's anything you need to be concerned about. Do this test: Touch the tip of the jack when it's plugged into the amp. It will likely hum.

I know the kind of thing you mean when you touch the tip of the jack. It's very different from that. And this happens after everything is plugged in prior to switching the amp on. It's a very loud buzzing sound, and then there are the sparks, which I've never come across before in years of using effects pedals. I was wondering if it had anything to do with the differing polarity of the power supplies. As I say, when the jack casings are not in contact, everything seems to be working fine.

 

There are two power adaptors. One delivers 9V for the effects pedals, and the other delivers 24V for the amplifier. Both adaptors plug into the mains with three pin plugs. To test with the supply earth, I take it I would put one of the test meter terminals into the earth terminal of the mains supply?

Yes. Unplug the adapter from mains and check if there is continuity between the outside of the barrel plug and the earth pin of the AC plug.

 

RETURN the effects pedal with the center negative power jack to the seller. Center negative is unforgivably poor thinking.. The loud buzz is because of the polarity reversal short circuit.

 

RETURN the effects pedal with the center negative power jack to the seller. Center negative is unforgivably poor thinking.. The loud buzz is because of the polarity reversal short circuit.

Unfortunately, centre-negative effects pedals are pretty universal in the field of electric and amplified acoustic guitar. And although there might very well be some poor electronic thinking in their centre-negative design, they don't actually present any obvious practical problems to the millions of guitarists who use them. And this is the first time in decades of practice that I've had any such problem, which might, as I have speculated, have to do with, as you say, 'polarity reversal'. My solution might have to be to use a plastic casing jack plug for the speaker out connection on my amp. As I say, the problem does seem confined to that particular localised metal-to-metal contact.

 

Yes. Unplug the adapter from mains and check if there is continuity between the outside of the barrel plug and the earth pin of the AC plug.

Just checked on this. The 3-pin mains plug for the 9V effects pedal adaptor has a plastic earth terminal, so this adaptor is not connected to earth. But 3-pin mains plug for the centre-positive amplifier adaptor has a metal earth terminal and there is continuity between that and the outside of the barrel plug carrying the 24 V output to the amp.

 

I've now tried disconnecting the earth, but the same thing is happening.

 

What happens is that the outside barrel of the power connection becomes connected to the metal housing that it is in, as does the sleeve portion of the output jack that the amplifier input connects to. The lazy reason for center negative is often that the sleeve also operates a switch to disconnect the internal pedal battery positive and connect the external supply.

 

Just noticed you live in Royal Oak. I lived there for a while myself when I was studying PhD Philosophy at Wayne State University.

 

It is quite a university indeed!! With a broad choice of studies. And a serious parking challenge. I did not attend the school, but did a project for them once. They are still happy with it, although they have broken it a couple of times. I designed the controls for the crash sled in some lab at Wayne State. It certainly is an interesting place to do work on a project. Vastly different than most places that I have done projects.
I had a friend who got a PhD in audio engineering there many years ago. I did not know that was even possible.

 

Where's @Audioguru again ? He'd probably have a very authoritative answer.

 

So far, I cannot make any sense of the observations.
Positive or negative centre pin should not make any difference. The signal paths in and out should have been AC coupled.
If any metal chassis is at a DC potential other than earth ground, you should be able to measure this voltage with a DC voltmeter.

 

So far, I cannot make any sense of the observations.
Positive or negative centre pin should not make any difference. The signal paths in and out should have been AC coupled.
If any metal chassis is at a DC potential other than earth ground, you should be able to measure this voltage with a DC voltmeter.

Please consider that AC coupling is almost always limited to the signal conductor and very seldom addresses the shield/common return, also known as "ground". The only exception that I am familiar with is balanced lines, such as microphone XLR connections.
Shields are very seldom AC coupled only. Besides that, it was the shield part of the connectors that sparked and buzzed.

 

Please consider that AC coupling is almost always limited to the signal conductor and very seldom addresses the shield/common return, also known as "ground". The only exception that I am familiar with is balanced lines, such as microphone XLR connections.
Shields are very seldom AC coupled only.

Who said anything about the shields being AC coupled?
Let's assume that the shields are galvanically coupled.
One unit is powered on positive supply voltage. The other unit is powered on negative supply voltage.
I don't see how sparks can fly here. There is something else going on of which we have not been told.