Two Prong vs Three Prong???

Two prong or Three Prong? I've been using my ST120 VTA for a number of weeks now. No issues. Wonderful amplifier....One thing bugs me....The power cord has only two prongs, no ground and the instructions do not specify what wire goes where. Since it is a polarized AC plug it needs to be wired with the common wired to chassis and the hot to the fuse. My question is, would I be better off wiring with a three prong plug just for safety sake?

Yes - change it.
The neutral is a balance conductor - not a true ground.

I just rebuilt my VTA120 and started with the 2-wire but had the same thought of caution. So I put in a 3-wire cord. But then when I hooked it up to my preamp I got a little hum. Just to check I used a cheater plug to disconnect the amp from ground and it became dead quiet. Sounds great so I think 2-wire is fine.

corndog71 wrote:I just rebuilt my VTA120 and started with the 2-wire but had the same thought of caution.  So I put in a 3-wire cord.  But then when I hooked it up to my preamp I got a little hum.  Just to check I used a cheater plug to disconnect the amp from ground and it became dead quiet.  Sounds great so I think 2-wire is fine.

You might have a Ground Loop problem creating the hum when you ground the amp ? It's a very common problem with audio equipment.

I used this one : https://www.parts-express.com/6-ft.-AC-Power-Cord-Black-14-3-110-230. Put the ground wire on one of the transformer mounting screws. If I did it again, would go with either a smaller gauge or a cord with an On - Off switch in-line on the cord. No Hum, but my speakers are low efficiency in a room with an aquarium, so I may just not be able to hear it. Sounds good, and I feel more protected in case of a mishap. I have a portable GFI in-line also. In general, I think power cords , interconnects, and speaker wires that are well made , appropriately shielded .and of the proper gage are good to have. However, it is amazing how much less hocus pocus appeals to you when you have wired your own amp and you know the gages and types of wires used inside your amp. I am sure building your own speakers has the same effect, but I haven't done that.

3-prong and never a hum.

DavidR wrote:

3-prong and never a hum.

DavidR.... you have done a very good job in wiring your amplifier including the use of one single grounding point.

The blue arrow shows the single grounding point that you have used and where you have run all of your grounds to one single point.

The red arrow also shows where you have grounded the aluminum capacitor can back to the same single grounding point. This practice using a single grounding point eliminates ground loops and keeps your amplifier quiet and silent.

The only other thing that I will point out here is where you grounded you're green grounding wire on the chassis. That should also be moved over to the single ground source to also eliminate any small ground loop that might be created between the two grounding points.

Other than that you've done a very good job and that is the reason why your amp is silent.

On another note for others.....

Another problem is it may not be the amplifier itself some ground loops can be present in the wiring of the home that is creating the hum or noise that is being induced into the power line itself.

Anyway you have a good day and stay safe.

Tom Pickett wrote:

The only other thing that I will point out here is where you grounded you're green grounding wire on the chassis. That should also be moved over to the single ground source to also eliminate any small ground loop that might be created between the two grounding points.

That's a great point, Tom. I didn't think to do that. Just when I thought I had this stuff down. I'm still learning.

I wanted to continue on this topic so that everyone understands what a ground loop is so that many of you can solve this problem.

I went online and did some research and found a guy on YouTube that really explains what is exactly happening when you create a ground loop that creates a hum or a hiss in your speakers.

This guy is great and he nails it.

Have a good day everyone and stay safe.


Ground Loops: Grounding Series (Part 6) YouTube Link: https://youtu.be/PACur_GcTJ0

What are Ground Loops?
- Ground loops occur when two different points in an electrical circuit are intended to be at the same potential, but in fact, are at different electrical potential. Current will flow between whatever conductors are available in the circuit.
- Unintended current flow is what causes issues in electrical systems, like electrical “noise”, or interference in many different signals that are being transmitted.

How to Avoid Ground Loops:
- Make sure that all circuits, especially signal circuits, are referenced to one point as ground
- Remove grounding connections at the various pieces of equipment through the system
- If you remove these types of connections, make sure to implement proper grounding techniques to make sure they still have a path to ground through a single point.

Rbertalotto wrote:Two prong or Three Prong? I've been using my ST120 VTA for a number of weeks now. No issues. Wonderful amplifier....One thing bugs me....The power cord has only two prongs, no ground and the instructions do not specify what wire goes where. Since it is a polarized AC plug it needs to be wired with the common wired to chassis and the hot to the fuse. My question is, would I be better off wiring with a three prong plug just for safety sake?

Yes. Use a grounded 3-prong cord. It is just plain stupid to have an ungrounded exposed metal chassis.

I went back in and rerouted my ground wire and now it's dead quiet with no cheater plug needed.

Thanks again, Tom, for your advise.

corndog71 wrote:I went back in and rerouted my ground wire and now it's dead quiet with no cheater plug needed.

Thanks again, Tom, for your advise.  

Aman Brother ! We Love Our Tube Gear !

People with home electrical awareness assume "3 prong good. 2 prong bad" and blindly do a 3 prong conversion on their audio equipment. Sometimes this is fine and results in a "safer" piece of equipment. Other times it introduces a ground loop and drives them crazy trying to figure out why they have a 60 Hz hum. Most modern audio set ups have a wide variety of equipment hooked together - some 3 prong, some 2 prong some coax. Both the white, or "neutral" wire and the green "ground" wire ultimately end up at the same place in the circuit breaker box and are both wired to earth ground, but they carry microscopically different potentials on the way and "signals" other than 60 Hz. These can introduce ground loops. Sometimes, internal to the equipment, using chassis ground rather than "star" ground in designing or building an amplifier with either 2 or 3 prong AC connection can be sufficient to create noise on its own. Sometimes using improperly grounded shielded cable can create rather than solve ground loops. Sometimes even using too small a wire gauge can create loops.

Ground loops can drive you nuts.

stsk wrote:People with home electrical awareness assume "3 prong good. 2 prong bad" and blindly do a 3 prong conversion on their audio equipment. Sometimes this is fine and results in a "safer" piece of equipment. Other times it introduces a ground loop and drives them crazy trying to figure out why they have a 60 Hz hum. Most modern audio set ups have a wide variety of equipment hooked together - some 3 prong, some 2 prong some coax. Both the white, or "neutral" wire and the green "ground" wire ultimately end up at the same place in the circuit breaker box and are both wired to earth ground, but they carry microscopically different potentials on the way and "signals" other than 60 Hz. These can introduce ground loops. Sometimes, internal to the equipment, using chassis ground rather than "star" ground in designing or building an amplifier with either 2 or 3 prong AC connection can be sufficient to create noise on its own. Sometimes using improperly grounded shielded cable can create rather than solve ground loops. Sometimes even using too small a wire gauge can create loops.

Ground loops can drive you nuts.

I feel fine and has no hum with my 3-prong equip.

stsk wrote:People with home electrical awareness assume "3 prong good. 2 prong bad" and blindly do a 3 prong conversion on their audio equipment. Sometimes this is fine and results in a "safer" piece of equipment. Other times it introduces a ground loop and drives them crazy trying to figure out why they have a 60 Hz hum. Most modern audio set ups have a wide variety of equipment hooked together - some 3 prong, some 2 prong some coax. Both the white, or "neutral" wire and the green "ground" wire ultimately end up at the same place in the circuit breaker box and are both wired to earth ground, but they carry microscopically different potentials on the way and "signals" other than 60 Hz. These can introduce ground loops. Sometimes, internal to the equipment, using chassis ground rather than "star" ground in designing or building an amplifier with either 2 or 3 prong AC connection can be sufficient to create noise on its own. Sometimes using improperly grounded shielded cable can create rather than solve ground loops. Sometimes even using too small a wire gauge can create loops.

Ground loops can drive you nuts.

"Sometimes using improperly grounded shielded cable can create rather than solve ground loops. Sometimes even using too small a wire gauge can create loops."

Very well said and 100% accurate... Sometimes a small gauge ground wire can act like an in circuit resistor creating a ground-loop.