Earth loop reduction amp

A simple circuit designed for unbalanced audio circuits that suffer from earth loop problems.

Earth loops

Earth loops occur because although in the ideal world, all earth connections are at an equal potential, real world conductors have both resistance and inductance, and when current flows, an there is an inevitable voltage drop across the earth leads. This can result in that annoying mains hum when various pieces of audio equipment are linked together, each with their own earth lead.

Solutions

The professional solution is to use balanced audio systems. These systems are expensive and do not relay on an audio ground but rather, a positive and negative drive signal.

Another solution is to use a high grade audio isolation transformer. These devices can be very expensive and cheap devices color the audio signal by their inductance and the varying characteristics of the ferrite material over varying frequencies.

Yet another solution is a balanced audio op amp, of the instrumentation amp variety. Companies like Analogue devices and Bur Brown produce a number of these devices. One again, expensive and not something you are likely to find in your junk drawer.

Then there is this circuit. Most of the parts can be found in your average enthusiasts junk box, and the transistors can be any general purpose BJT.

circuit of earth loop reduction / elimination amp

Not a true balanced amp, but this circuit performs more than adequately in eliminating those pesky earth loops noises.

Basic operation

The basic operation of the circuit is as follows

The input earth is connected to the main earth with a 4k7 resistor, this allows it to "kind of" float, allowing Q1 to respond to the variances between the input and the input earth.
The relatively high resistance of R4 means very little current flows from the main earth to the input earth when there are a earth voltage differences.
Any AC voltage on the input is also produced across R1, irrespective of the voltage across R4.
This current  produces a voltage of equal magnitude, but inverted, across R2.
As R2 has one leg connected to the supply, and the emitter follower circuit Q2 and R5 is also connected to the positive supply, the AC voltage on R2 is converted to an AC current through R6. And as R6 is connected to earth, the output signal is with respect to earth, and Q2 once again inverts the signal back to the original phase.

Biasing

R3,R7 and C1 provides a stable bias voltage for Q1 via R8.
The voltage at Q1's base is set to about 4,3V.
R1 drops about 3V over it, and this is mirrored in R2.
R5 and R6 have the same current and resistance and thus the roughly 2,5V over R5 translates to roughly 2.5V over R6.
The voltage gain of the circuit is 1, but this can easily be changed by changing the various resistor values.



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