Audio Amplifier Repair, Do I Need An Isolation Transformer? - Page 1
Forum section: electronics / testgear

author: Timmox
author_rank: Contributor
timestamp: April 23, 2025, 08:37:35 am
content: Hi All and apologies if this post is in the wrong place.I'm getting in to audio amplifier repair and I'm researching what I need. I will be working on the power supply side as well as the audio side as that seems to be a common point of failure.Is it necessary (or a good idea) to use an isolation transformer? If yes can anyone give recommendations. I live in the UKThanks for your time.Tim.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: April 23, 2025, 11:53:40 am
content: Hi Timmox,an Isolation-Transfo is not a must, but can't hurt (if used propperly).Beware of bridged-outputs (common with Digital-Power-amps),so better invest in a Differential-probe.Good luck

author: KungFuJosh
author_rank: Super Contributor
timestamp: April 23, 2025, 12:22:16 pm
content: Don't get an isolation transformer, especially if you got the idea from a certain "Uncle" on youtube who gives dangerously lethal advice.Get a differential probe if you need to probe in that manner.Thanks,Josh

author: madires
author_rank: Super Contributor
timestamp: April 23, 2025, 12:34:26 pm
content: I agree, no need for an isolation transformer in that case. But it might be a good idea to build a dim-bulb tester.

author: Zenith
author_rank: Frequent Contributor
timestamp: April 23, 2025, 12:37:11 pm
content: They are mainly used with things with transformerless power supplies, such as vintage AC/DC radios and TVs. Incorrect wiring or a fault can have the chassis live at mains voltage relative to earth (and you). If it's powered through an isolation transformer, it will be floating, rather like connecting one terminal of a battery to earth and leaving the other unconnected - you wouldn't expect any current to flow. Neutral is not necessarily at earth potential anyway.I'm sure I don't need to tell you to be extra careful with valve HT lines, which can be lethal, and faults can leave capacitors charged for a long time. Even solid state supplies at + and - 50V should be treated with respect.You may be tempted to get a variac, which is a variable transformer. A lot of people assume these are isolating transformers, but they are not. They are autotransformers.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2025, 01:04:46 pm
content: No, you don't need one.Quote from: Zenith on April 23, 2025, 12:37:11 pmThis just means you should ground the chassis with an earth wire while you work on it.Before you start:Test the chassis with a low-Z multimeter. If it's live then you know your first job - fix the input wiring. After that you can ground the chassis.

author: EvgenyG
author_rank: Regular Contributor
timestamp: April 23, 2025, 01:26:20 pm
content: It is best to troubleshoot power supply and audio amplifier separately. Audio amp is best to power/test with a lab bench power supply and limit current. You will need a polar power supply, may be more than one. Quite often, audio equipment needs 3 different voltages or even more.

author: norbert.kiszka
author_rank: Frequent Contributor
timestamp: April 23, 2025, 02:11:56 pm
content: It depends if PSU is SELV or PELV type. In this second one, You can power whole amplifier with separation transformer, instead doing this on the scope. Anyway, differential (active) probe, will be the best and safest solution.