K6JRF's Page
formerly W6FZC
Hum Elimination Techniques Page
(Oct 12, 2013)

This section discusses the various devices to eliminate ground loops and AC hum from your audio equipment.
To find out more about hum and ground loops and how to avoid same, click on the link to the Rane Technical library to read Tech Note 110 entitled "Sound System Interconnection".

Balanced to UnBalanced Adapters

Here's two devices that I've used to get rid of hum and give some RFI immunity. Both work very nicely to isolate various audio equipments and keep everything hum free! Most times this occurs when going from balanced to unbalanced equipments.

The red box shows a modification to the Radio Shack Adapter (Unbal-Bal) that isolates the input and output windings of the internal transformer to give complete isolation. It's main drawback is the lack of low frequency response, due to the very small transformer.

Two Balanced to Unbalanced Isolators

The spectrum plot shows the response to wide band pink noise with and without the adapter. Since it has a small transformer, some low frequency loss is expected and is definitely evident.

Radio Spectrum Plot of Radio Shack Adapter

EbTech Hum Eliminator

In searching the InterNet, I found a company called EbTech that makes a two channel Hum Eliminator (HE). Check out the SpectraPlus picture showing the flat bandpass and voice response. Plus it's "SHIELDED" so no hum pickup!! Can be purchased from Sweetwater for about $70. Direct link here. Ebtech Hum Eliminator o Eliminates AC hum quick and easy
o Breaks groundloops safely while leaving all signal grounds intact
o Automatically translates signal voltages to match differences in ground potentials, avoiding clipping
o Converts between balanced and unbalanced lines
o 1/4" TRS 'smart' jacks for all inputs and outputs
o Completely passive design with audiophile quality components
o Frequency response 20Hz to 70kHz +/-0.5dB
o Distortion less than 0.005% THD @1kHz
o Crosstalk better than -97dB

This report details the results of testing the EbTech Hum Eliminator (HE). The static tests using pink noise tests were very almost identical and very flat frequency response. The case is 1/8" thick with torx head screws so you can't peek inside. It weighs about one lb and is very ruggedly constructed.

A second device that works very well is the EbTech Hum Eliminator (HE). The HE is basically a 1:1 ratio isolation transformer with great bandwidth.

After rewiring the HE unit into the audio system, I made a PN sweep to from 20hz to 20Khz. This device is supposed to have at least 70Khz bandwidth!

The picture shows a full bandwidth pink noise sweep reference level, GREEN and the response BLUE line. Note that it follows the input response very nicely.

JENSEN transformers are unique because they have typically a 200Khz bw. The true test of a transformer is to excite the device with a square wave and look at the transient response of the device. The second set of tests does just that. A 50hz square was applied as reference, PURPLE and compared to the response with the HE inline RED. This response did not use the PEAK HOLD function in SpectraPlus. Note that the HE unit follows the input very nicely indicating that it has adequate bandwidth and provides good fidelity to fast rise wavefronts.

Ebtech Hum Eliminator

After bench tests, I wired it into the station in the tape (minidisk) playback loop that drives the balanced modulator in the FT1000D. The slight bit of hum was removed. The hum rejection due to AC line pickup was ok also. I mounted the unit in the back of my audio rack and there was no trace of hum pickup.

The internal transformers are magnetically shielded and everything works as advertised! Long story short, two thumbs up!

Radio Shack Ground Loop Isolator

Another Radio Shack device used in an automobile for car stereo ground isolation is their GROUND LOOP ISOLATOR (GLI) device, PN 270-054.

Picture of RShack GLI (not very good!)

The static tests using pink noise and voice tests were very exciting leading yours truly to attempt to rewire my transmitter/audio equipment interface to use the device. Such was not to happen!!

First the static tests. Referring to the picture, the PN and voice tests were nothing sort of spectacular! The slight difference in the turns on the secondary added about 1db more gain. But the tracking was just terrific.

This is were the good part ends. The device is useless for any serious needs because any, and I mean any, sort of hum field induces hum and buzz and destroys it usefulness. Just bringing it near the front panel of my FT1000D induced so much hum that it was almost deafening. The unit is NOT magnetically shielded. Long story short, the RS unit may work ok in an automobile. It's definitely not intended for use around AC lines!

Radio Shack Ground Loop Isolator

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