I’ve always thought of ground loop isolators, the type that they sell at RadioShack, to be crude and temporary solutions to problems that should be fixed with balanced audio circuits. This is why I was so intrigued to read through Bill Whitlock’s Seminar on Ground Loops.
Bill Whitlock is a name that you can’t avoid if you spend much time in the audiovisual industry. Originally I thought Bill had something to do with Whitlock, an AV integration company, but that is a different and equally well-known Whitlock. Bill is a brilliant engineer. He holds two patents related to balanced audio circuits and AC voltage regulation. However he takes educating very seriously. He could have easily made it by in life without educating, so I have to assume that it’s a labor of love to bring neophytes up to speed. It probably doesn’t hurt his company, Jensen Transformers, brand either.
I’ve never considered a ground loop isolator the optimum solution for a problem. I’ve even worked on designs where a senior consultant spec’d unbalanced passthrough audio plates with input transformers (ground loop isolators) at the rack. The solution at the time seemed ungainly, a value-engineered approach to a problem that should be solved with balanced audio circuits.
So I was shocked to read his paragraph about why powered balancing circuits aren’t as great as I thought they were.
A wide variety of commercial interface devices are “active” (i.e., powered) devices. Although they incorporate many useful features, they invariably use differential amplifier circuits to “isolate” their unbalanced inputs. As explained later, the ground noise rejection of ordinary differential amplifiers is extremely sensitive to impedance imbalances in the driving source. With unbalanced sources, their entire output impedance becomes “imbalanced” and typically ranges from 200 Ohms to 1 kOhms or more. Under these conditions, the noise rejection of differential amplifiers is quite poor.
Some other highlights from his seminar:
- Ground lifts should never be used to solve ground loop problems. Lifting a ground can be a serious safety concern. It’s against code and can make you liable for any injuries that result. The reason is that if there is a fault in the neutral cable the ground provides a safe route for the current to return to the earth. This also trips the circuit breaker, so the short isn’t prolonged any more than necessary. Ground lifts exist to provide a separate safety ground where a grounded power outlet isn’t available.
- When a ground cannot be provided you should use a ground-fault circuit interrupter or GFCI. “A GFCI works by sensing the difference in current between the line and neutral conductors. This difference represents current in the hot conductor that is not returning in the neutral – the assumption is that the missing current is flowing through a person. If the difference reaches about 5 mA, an internal circuit breaker is tripped.”
- Ground loop isolators, aka transformers, come in 2 flavors: output and input transformers. Input transformers use Faraday-shielding which eliminates capacitive couplings between windings. Magnetic fields may still pass otherwise current wouldn’t be induced on the isolated wires. Output transformers attenuate low frequencies in the audio signal more than input transformers.
- There is something called parasitic capacitance between power lines and the ground within equipment. Basically the insulators that separate the power lines in equipment from the chasis and signal ground function as capacitors. Capacitors in series with an alternating current function as a high pass. These parasitic capacitances can contribute to the overall ground loop problem.
- Protective ground connections for Cable TV should be made to the same earth rod used for utility power, but it may not be. This is a problem because the earth is not a perfect conductor. Not even close. Soil has resistance which allows voltages to develop across different grounding rods.
This image in particular caught my attention.
For weeks I’ve been living with a problem with my TV sound system. The problem started when I decided to upgrade my speakers. I had been using a pair of Mackie 824 studio monitors from my audio engineering days as my computer speakers. I decided they would be put to better use if I used them as my television speakers, especially after purchasing an Apple TV. Now I regularly listen to music on the couch rather than in front of my computer. So I moved the speakers over and connected them to the direct output from the television which is just two unbalanced RCA jacks. The input on the Mackie speakers is balanced, so this irked me a little bit. On top of that the remote control would not control the volume of this output.
I discovered a headphone output on the side of the TV, and although the output is still unbalanced it worked perfectly with the TV volume control. Aside: no consumer TV has a balanced output, and I’d even be surprised to find a professional one that does. Unfortunately when I powered on the speakers and adjusted the volume to a comfortable level there was a terrible hum. What made it so terrible is that it sat at this perfect level where it would interfere with quiet music. It wasn’t loud enough to abandon the approach completely, but it wasn’t quiet enough to ignore. I tried to optimize the gain structure by turning the volume of the TV to 100% and turning down the speaker input trim. This actually improved the hum to the point of being quiet enough to ignore in most circumstances. However the program audio would distort in loud portions of songs, movies and TV shows. This distortion fell into the same goldilocks range of annoying. There was no sweet spot to be found. I tired the TV volume at 75% and the speaker input trim a little higher. Distortion and/or hum could not be avoided. I actually found that the hum was less noticable on the direct out of the TV, so I left them plugged into that output. The problem with this approach was that I had no volume control unless I initiated Apple’s “AirPlay” from the computer. Then I could control the volume from the computer, but otherwise the volume was uncomfortably loud until I played classical or other equally quiet, dynamically rich, music. To make matters worse the speaker input trim is an inaccessible little knob that looks like a screwhead on the rear of the speakers.
So I left it that way for weeks. If I wanted to listen to those speakers I would have to mute the integrated TV speakers. Turn on my Mackie speakers. Then initiate AirPlay from my computer in order to have the necessary volume control.
Now refer back to that ground loop illustration from above. When I saw this something clicked, and I realized that a ground loop isolator could fix my problem. I’m in a multi-tenant apartment building, so I have no idea how they have the utility power ground and the Cable TV ground configured. Even if they were on the same earth rod the impedance difference in the CATV ground path and the utility power path would probably be enough to give rise to a ground loop with the help of parasitic capacitance issues that are inherent to all electronic devices. I stopped by a RadioShack an hour later, picked up their eponymously branded ground loop isolator, and plugged it in at home. At first I inserted it into the direct out of the TV. With my head inches from the woofer I could immediately tell that the low level hum was completely gone. Excitedly I added the RCA to 1/8″ stereo trs adapter to the isolator and jammed it into the headphone jack. I was astonished. The audio was perfect. I wasn’t driving the headphone amp to 11, so there was no distortion. The ground loop was gone, so there was no hum or noise at all. I could control the volume with the TV remote. How can this be possible? Such a simple fix. No balanced audio required.
The funny thing is that if I were less informed about audio I may have just gone to RadioShack and bought this ground loop isolator as a first step. “What’s that? A hum exterminator? Do you have an extreme hum exterminator? Cause my hum is pretty bad. There’s some distortion and noise too. Do you have a distortion isolater?”
So I’m here to say, you’re not too good for a ground loop isolator. Recording studios use them for direct inputs from instruments. I knew that, but I didn’t know the details of why, which is why I ignored them when my TV sound system gave me problems. Even Bill Whitlock, the innovator behind modern balanced audio circuits, calls ground isolators a silver bullet. “Use ground isolators at problem interfaces. Isolators are a ‘silver bullet’ solution for common impedance coupling, which is the major weakness of unbalanced interfaces.”
Silver bullet. I didn’t even bother with getting a high quality input transformer. That may peev Bill as a representative of Jensen, but for my purposes it was all I needed.