Transformer isolation unit with buffered output prevents hum when running two amps
By Phil O'Keefe
There are times when all you need is one amp, a cable and a good guitar, but there are also times when you may want to combine the sound of two amps to produce a "composite" tone. This technique has often been used live and in the studio by artists such as Stevie Ray Vaughn, Billy Gibbons of ZZ Top and Mike Campbell of Tom Petty & The Heartbreakers, and it allows you to obtain tones and textures that wouldn't be possible with a single amplifier. Suppose you decide to take it a step further and add an effects pedal or three to the equation, including a stereo analog chorus or delay pedal for some nice stereo effects. Once you "go stereo", you may have a difficult time going back to mono -- stereo is simply a great way to add extra fullness and dimension to your sound. However, even a fairly simple two amp setup is likely to run into hum, noise and even phase problems that can have serious negative consequences for your overall guitar sound, especially once you start adding pedals to the rig. TheGigRig HumDinger (Figure 1) was designed to overcome these obstacles to good tone when using two amps simultaneously, and can also be used in other situations where you need to eliminate a ground loop.
Figure 1: TheGigRig HumDinger is quite compact at only 4.33" long, 2.24" wide and 1.26" high, but packs a buffer, a phase reverse and transformer isolation in its small case
The biggest issue you're likely to run into is a significant increase in the amount of "hum" you hear from the two amplifiers. This isn't just because two amps worth of hum is louder than just one (it is); but it is most likely also due to ground loop issues. Without getting too far into the basics of electricity and grounding, a ground loop is caused when there are multiple paths to ground, and a difference in potential between the two paths. Once you split the signal with an AB box (or inside one of the effects pedals) and send each to a separate amp, the possibility of ground loops arises via the two separate amplifiers and their separate electrical plugs; each of which offers a path to ground for the audio signal. Plugging both amps into the same electrical circuit can sometimes help reduce the hum, but odds are you'll experience some increased hum and noise, as well as an increased susceptibility to EMI (electro-mechanical interference) and RFI (radio frequency interference) in one or both amps.
Some players will use a three pin to two pin "ground lift adapter" to attempt to reduce the noise, and while this can sometimes be effective, it creates a potentially deadly situation where a fault in the amp or PA can lead to electrocution of the guitarist, and for this reason it's NEVER considered a good idea to defeat the AC power cord grounding pin of any amplifier, or to use an amp that has improper AC wiring or modifications.
SOME PROBLEMS - AND SAFE PATCHING SOLUTIONS
While there are several different methods that can be used to reduce or eliminate this noise, such as running chokes and filters, using balanced cabling and power or using fiber-optic cables to transmit the audio, for most guitarists, the most practical, effective and cost efficient solution is to use an inline isolation transformer on one of the signal paths while leaving the other one alone. If you're using two amplifiers, the signal runs from your guitar and through your cables and pedals and straight to one amp as normal. That amp (Amp 1) is grounded via its three prong AC plug, and provides the ground for the entire system. The second amp (Amp 2) is also plugged into its own grounded AC outlet for safety, but the HumDinger "breaks" the audio pathway to ground by decoupling the direct connection of that ground pathway. The coils of wire inside the isolation transformer do this by allowing the audio signal to be transferred from one coil to the other via inductive coupling, while eliminating any direct wire connection between them, thus breaking the audio ground path to the second amp -- which is still grounded to the wall outlet, but no longer serving as a second audio ground path.
If all of your effects are mono, you would normally patch the HumDinger after the last pedal, and then patch from the HumDinger's buffered output into one amp, and run the transformer isolated output into the second amp. The HumDinger takes care of splitting the signal and provides the second output.
Try the phase switch in both positions and see if one or the other sounds thicker and fuller. The "thinner" sound is generally caused by phase cancellation. When the switch is in the "up" position, both outputs are in phase relative to the input. When the switch is in the down position, the buffered output remains in phase with the input, but the transformer isolated output's phase is inverted relative to the input. Whenever you are working in stereo or with multiple amplifiers, speakers, or microphones, it's a good idea to check the phase once you have everything hooked up. Since there are so many ways it can get inverted in a complex stereo rig, the HumDinger's phase switch is more than just a useful bonus on a product of this type; it's a necessity. Fortunately, it works exactly as advertised, and if one of your amps happens to be out of phase with the other one (which is not at all uncommon), the solution is just a switch click away.
Ground loop issues can also arise when you use a single amp along with an AC powered effects unit; since they are also plugged directly into the wall outlet, they can also provide a second pathway to ground. The HumDinger can also help in these situations; just patch in the HumDinger immediately after the last AC powered pedal in the chain. The pedal will supply the path to ground while the path from the pedals to the amp is broken by the inline transformer of the HumDinger. It's important to again point out that the amp is still safely "grounded"; the transformer inside the HumDinger merely breaks the direct connection that creates the secondary path to ground, and thus eliminates the cause of the hum problem.
For studio use, the HumDinger allows you to connect the buffered output to your guitar amp while sending the isolated output to your audio interface or mixer's High Impedance input for direct recording - no direct box needed, and without any ground loop issues from being connected to the amp and to the AC powered interface or mixing board simultaneously.
One potentially confusing patching scenario is when using stereo effects pedals and two amplifiers with the HumDinger. There is only a single input on the HumDinger, so how should you connect things if the last pedal in your effects chain has stereo outputs? I recommend connecting Output 1 from the stereo pedal directly to Amp 1, and connect Output 2 from the stereo pedal to the Humdinger's input, and use only the Isolated output on the HumDinger; connecting it to Amp 2.
One very important point to be aware of for safety's sake is that you must maintain one pathway to ground at all times. The buffered output on the HumDinger is designed to be the "main" output of the unit, and is grounded, while the transformer isolated output is specifically designed not to be. If you are not using the main output on the HumDinger, then it is crucial that you have a solid path to ground somewhere else in the setup. When running a stereo effects chain as I described earlier, the signal running through the pedals and directly to the amp provides that direct ground pathway. The other channel, with the HumDinger inserted but with only the isolated output connected, is the isolated signal and is ungrounded.
THE SOUND IS IN THE IRON... AND BUFFERS
As the old saying suggests, the sound quality will be directly affected in a device of this type by the quality of the transformer, and fortunately, the HumDinger sounds quite good, with minimal coloration; mainly a slight thickening to the sound that is hardly worth mentioning due to how subtle it is. Unlike passive isolation transformers, the HumDinger has a buffer in the signal path in front of the transformer. (Figure 2) This helps to offset the coloration and signal loss of the transformer itself, and contributes significantly to the HumDinger's relatively uncolored sound. The main output is also buffered. The buffers sound good and will drive long cable runs without the high frequency and level losses you would otherwise expect to encounter. A buffer is definitely something that is generally recommended anyway for those who are running large pedalboards with true bypass pedals, and again, when splitting the signal to drive two amps, such a buffer is helpful to insure proper impedance and levels are maintained, and it is a welcome addition to the HumDinger's feature list.
Figure 2: The HumDinger block diagram from the manual, showing the dual buffers - one on the main buffered output and one immediately in front of the isolation transformer that feeds the isolated output
There's not a whole lot more to say about the HumDinger. The only real complaint that I can come up with is the placement of the 9VDC (100mA) power supply jack at the bottom of the unit, directly beneath the blue Power LED - a rather unusual location. However, since it's not really a "stomp box" and your feet don't ever need to go near it, the HumDinger can be mounted practically anywhere on your pedalboard, the location of the jacks and switch and power plug shouldn't really be a major issue.
I've spent much more time trying to describe what the HumDinger does and how it can benefit players than I have on the sound of it because, well, it really doesn't have much of a sound to speak of. Your rig may sound better with it in line, but that's because of what it's correcting. It does exactly what it's designed to do with no muss and no no fuss, and once you have it connected, you can pretty much forget about it - along with any worries about hum. It eliminates the second path to ground and kills the ground loop hum. It keeps your signal levels, impedance and tone intact, even when driving long cables and using a large pedalboard filled with true bypass pedals, lets you flip the phase if you need to compensate for one amp being out of phase relative to the other, and it does it all at a reasonable price. If you're running a stereo or dual amp setup, is definitely a must-have. It will kill what ails your tone.
Dimensions: 4.33" L x 2.24" W x 1.26" H.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.