When looking at schematics and builds of vacuum tube amplifiers employing output transformers some people notice a curious inconsistency. On some amplifiers, the output (i.e. secondary) side of the transformer is grounded, on others, it is not. This can lead to a dilemma for amplifier builders: To ground, or not to ground? Luckily, there are some simple guidelines which will almost always yield the correct answer.

This is one of those topics that doesn’t get discussed very often. Or when it does, lots of absolutes are thrown around without many specifics. When perusing this website, the casual reader will find numerous examples of amplifiers with no secondary grounding of the output transformer whatsoever. Yet other designs on the website will have a specific ground reference connection made on the output side of the output transformer. So the real question concerns how to determine which is the best approach and when is it required.

There are three main considerations involved in answering this grounding question. I would like to discuss each in turn. I will remind the reader that I am only addressing topologies which employ an output transformer. Output transformers not only transform impedance levels to allow the driving of low impedance loads, they also provide a significant safety isolation layer between the amplifier outputs and potentially lethal internal amplifier voltages. It is important to understand this function and what it means to the users of the amplifier.

Output Grounding when Employing Feedback

The first, and only absolute, case to be discussed is when the amplifier topology employs feedback and the feedback loop includes the output transformer. In this instance, the output side of the transformer must be referenced to the same reference as the input side of the feedback architecture. Without this reference, the feedback factor β is undefined and the amplifier will not be stable. This is usually achieved by tying one side of the output winding to the signal ground. Which side of the output winding to tie to the ground reference (and which side to the feedback loop) depends on the specifics of the feedback architecture.

Output Grounding of Speaker Terminals

Concerning amplifiers without transformer encompassing feedback, the factors driving the grounding decision are more subtle. As I stated above, a primary service provided by the output transformers is to provide a high level of isolation between the internals of the amplifier and the speaker terminals. This renders the exposed terminals of the amplifier safe to inadvertent contact in operation. This is an important consideration with speaker cables run as they often are. For example, around baseboards, under furniture, under carpets, or anywhere they might be caught, pulled, disconnected, or shorted.

There is no inherent advantage to grounding the secondary of the output transformer when driving speakers. It does not improve amplifier performance, it does not increase noise immunity, and it may complicate matters if any active speaker components are part of your sound system downstream of the amplifier.

However, if someone really wanted to ground one side of the secondary windings on the output transformers, they can certainly do that. In the same way that it doesn’t improve performance, it also doesn’t really harm performance either. And it does not reduce the safety isolation by any great margin, unless the amplifier signal ground is also left floating. In this case it might be more beneficial from a safety standpoint to ground the secondaries to the chassis ground so that they are bonded to neutral Earth.

For these reasons, I prefer to leave the output transformers floating in my amplifier designs intended to drive speakers. My professional experience leads me to believe that this is the best option in these cases.

Output Grounding of Headphone Jacks

The grounding of headphone jacks is one area where more caution is required. This is not because of signal level. Most headphone outputs are very low level; on the order of milliwatts. This has more to do with human physiology and how headphones are used.

When worn, headphones become capacitively coupled to the body. They are are constantly being subtly moved and shifted. A consequence of this movement, is that a static charge can build up on the transformer-headphone circuit over a few minutes. It is not generally a high charge, but it still can build. It also means that, in an ungrounded output circuit, the charges built up on the body and the circuit are likely larger than they would be it the small charges built up on the body were being continuously AC coupled to Earth.

Now the physiology part. As it turns out, the human outer ear is about 50 times more sensitive to static discharges than the hands or feet. Throughout a typical day your hands and feet are exposed to hundreds of static discharges. The vast majority of these discharges are far too small to feel; on the hands and feet. However, these same small charges can not only be felt on the ear, they can be painful. When putting on or removing headphones, it is very easy for stored static charges to attempt to equalize across the ear. If you have a headphone amplifier that is giving subtle (or not so subtle) zaps to your ears it is likely the fault of the grounding scheme. Many headphones are thrown away as defective when this happens and it’s usually not the fault of the headphones at all.

The best way to combat this tendency is to make sure that the secondary of any transformer driving a headphone has a good ground reference. Especially an Earth ground reference. This not only prevents static charges to be built up on the output circuit, it provides a capacitively coupled path for static charges on the body to disperse. This greatly reduces the risk of getting a shock when standing up or shifting around while wearing headphones.

So these are the considerations I take into account when deciding to ground the secondary of an output transformer. I was asked the question recently and I thought that a longer post was required to fully discuss the whole issue.

As always, questions and comments are welcome.