Audio isolation transformers

Allen Avionics Audio Isolation Transformers page describes the typical ground loop problem situation with the following drawing:


If you want to do the ground loop elimination in audio path, you have to cut the galvanic connection but pass the whole audio range. The simplest and most common way to do the isolation is use audio transformer. High-quality audio transformers cover whole audio band with good response.

Usually when you install an audio galvanic isolation to almost any place in the audio cable connection is solves the ground loop caused humming problem nicely.

Xitel Ground Loop Isolator page gives a good example how to use a ground loop isolator (audio isolation transformer) to solve ground a typical ground loop problem:


To make an audio isolator you have to get two audio transformers which have 1:1 transformation ratio and greater than 1 kohm impedance (prefererably many kilo-ohms for consumer line level applications). There are high quality audio transformers in the markes that meet those specs.

There are several ways how to wire an audio isolation transformer. Here is a basic wirings for isolation transformers for unbalanced and balanced lines:



There are variations how ground are wired on different audio isolation transformers. There are various ways with their advantages and disadvantages. The followinf solution circuit diagram from Allen Avionics Audio Isolation Transformers page conbines all different variations to one device, so you can always easily select (with a switch) the one that works best on you specific application:


The 3-position switch will eliminate the need to cut pin “1″ to lift the audio ground and the connection to the shield. Switch Positions:

1. Chassis shield connected. Audio ground not connected. Acts as a balanced line transformer to break ground loops. Eliminates hum and buzz.

2. Chassis shield and audio ground connected. Corrects balance on audio lines.

3. Chassis shield and audio ground not connected. Breaks ground loops. Provides audio isolation.

When selecting a transformer for your project keep in mind the quality. Cheapest transformers for audio use have some problems on the sound quality, typically distorted bass response and attenuating in high-frequency response. Basically a poor transformer slows down high frequencies more than low frequencies (because of transformer leakage inductance). This allows the low frequencies to pass first, creating what we perceive as a “fat/warm” tone. Inadequate frequency response on the low end (roll off at like 20Hz), causes low frequencies to be “slowed”, allowing the upper frequencies to be heard first, this is perceived as “barky/ brittle”.

When installing audio isolation transformer to an audio system HIFI people usually ask does that transformer affect the sound in some way and if it does something how much effect does it have. The truth is that the effect of adding an audio audio isolation transformer to a system that does not have ground loop problem could be anything from unnoticeable to normal listener to well noticeable. How much the transformer affects depends on the transformer quality and properties of the equipment between which the transformer is connected. So if there is no humming on the system, the transformer usually has some slightly negative effect on sound quality.

In case an audio transformer is installed to a system with ground loop humming, the effect of it will for sure be possible. The annoying ground loop humming will be heard by everybody. When the isolation transformer get rid of that, the would quality is very much improved at that moment (typically the negative effects of the transformer to sound are much smaller than the annoying humming is). It is a good idea to keep an audio isolation transformer on your tool box when you work with audio/video systems, and use it in you get ground loop problems.


  1. Tomi Engdahl says:

    Lightning Boy Audio 2020i Amplifier Isolation Transformer

    The 2020i is specifically an audio isolation transformer, unlike the 2020 Instrument transformer. Its a different product is not interchangeable with the 2020 Instrument Transformer. Aside from a different application, it does have the same transformer in it as the Instrument Transformer. The big difference with the 2020i is there is no ground connection between the input and output jacks. Due to this, there is only one practical application for the 2020i and that is to eliminate ground loop noise in multi-amplifier guitar rigs, ie. a stereo rig. Whenever you have more than one path to ground you have a ground loop. The transformer is an LBA-2020, which is wound in-house by Lightning Boy Audio. The 2020i is available direct from Lightning Boy Audio

  2. Tomi Engdahl says:

    Lightning Boy Audio 2020S Steel-Core Instrument Transformer

  3. Tomi Engdahl says:

    How to make the audio transformer

    Winding the output transformer for vaccum tube audio.

  4. Tomi Engdahl says:

    Audio Transformer Measurements and Modeling

    This page presents measured data for selected audio transformers and shows how well the measurements permit the transformers to be modeled using LTSpice. The transformers selected are ones that might be considered for ground loop isolation when used with Softrock receivers, or when computer-based data transmission/reception programs are used with receivers or transceivers.

  5. Tomi Engdahl says:

    Step Up Transformers SUT for Moving Coil Cartridges Explained – SUT Series Part 1

    BG133 – An explanation of how Step-Up Transformers for Moving Coil Phono Cartridges work.

    Building a Step Up Transformer for Moving Coil Cartridges MC – SUT Series Part 3

  6. Tomi Engdahl says:

    DIY ground loop isolator audio noise filter or hum eliminator

  7. Tomi Engdahl says:

    1:1 600ohms between a dac and a tube preamp idea
    WE111c belongs on the mix bus for a vibe-y country record!

    WE111C are a wrong and expensive choice for this. Despite much internet folklore they are all but invisible in the audio path, …

  8. Tomi Engdahl says:

    Yep. 30-15K +_ 0.5dB – sounds like a real plan to increase fidelity. A repeating coil with a ratio of 600/xxx ohm. Who knows what phase shift or other nasties come in addition to the reduction in frequency response. I wonder who builds stereo equipment to this spec?
    This belongs in a museum.

  9. Tomi Engdahl says:

    Discussion from

    Yep. 30-15K +_ 0.5dB – sounds like a real plan to increase fidelity. A repeating coil with a ratio of 600/xxx ohm. Who knows what phase shift or other nasties come in addition to the reduction in frequency response. I wonder who builds stereo equipment to this spec?
    This belongs in a museum.

    I would hazard a guess the net effect is to roll off the HF a bit. We used to call that a treble control. ;)

    Indeed, the harmonic distortion characteristic for transformers provides perceived “smoothness” (aka. “added even-order harmonics”) – not like turntables, but like tube amplifiers. In fact, what is generally recognized as “tube sound” is in fact “transformer sound”, as OTL (Output Transformer Less) tube amps sound just as clinical and “correct” as solid state.

    First be sure your line driver from equipment is capable for driving 600 Ohm load. Transformers are used in studio gear which have topology designed to drive them. For example, Manley use WCF with 6414 for driving output transformers and it do sound great. Most of customer audio gear have 4558 or TL071 so it can turn to be disaster :D

  10. Tomi Engdahl says:

    Test – Do Audio Transformers Mess Up Sound? – Part 1

    Lets see and hear the impact on the sound of having an isolation transformer connected to an Audix TR40 measurement mic and a Shure Beta 58

    00:00 Introduction
    00:54 Transformer Specs Radial JS3, Twin ISO Whirlwind ISO2
    02:14 Crimson, Cinemag and Lundahl
    02:48 Do transformers hurt the sound?
    04:16 The test setup
    06:35 Baseline curve with Audix TR40 mic and hardwire split
    08:41 Compare Audix TR40 into 1 vs 2 console channels hardwire
    09:48 Impact of Whirlwind ISO2 connected to TR40 mic
    11:20 Transformer alters sound of mic everywhere
    12:35 impact of Radial JS3 connected to TR40 mic
    13:53 Impact of Whirlwind ISO2 connected to Beta58
    15:52 impact of Radial JS3 connected to TR40 mic
    16:38 Outro

  11. Tomi Engdahl says:

    Do Audio Transformers Change the Sound? – Part 2

    OK, here is part 2 where I test a Beta58, Royer 122 ribbon mic with various transformers as well as we listen to the transformers themselves. Also I do a null test of various transformers with direct signal.

    00:00 Intro and recap part 1
    00:39 Test setup
    02:40 Why do transformers change mic sound?
    03:27 Beta58 with and without ISO2 connected
    04:24 Beta58 with and without JS3 connected
    06:01 Beta58 with and without Cinemag CMMS2 connected
    06:43 Beta58 with and without Crimson connected
    07:59 Listen to Radial Twin ISO vs direct
    09:42 Listen to Whirlwind ISO2 vs direct
    10:29 Listen to Radial JS3 vs direct
    11:22 Listen to Cinemag CMMS2 vs direct
    11:50 Null transformer with direct and see differences
    13:18 Royer 122 ribbon mic with JS3, ISO and Twin ISO vs direct
    16:04 Summary of impacts on sound
    17:18 The reason I avoid transformer splits

  12. Tomi Engdahl says:

    Transformers vs Capacitors DC Blocking & Overload

    Lets look at audio transformers on Smaart and compare to capacitors at differing levels

  13. Tomi Engdahl says:

    Understand DI Boxes, Impedance and Transformers Part #1

    DI Boxes, Impedance and Transformers Part #2


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