# Transformer for high power audio amplifier - using old transformer from UPS?

#### RogueRose

Joined Oct 10, 2014
375
I was looking into making a high powered audio amplifier and it seems that one of the biggest components is the transformer. I've also seen amplifiers (commercial) run from 12v up to mid 30's, depending on the maker and size.

I've taken apart many large UPS battery backups of 2200-3000 watt rating and even higher. They have very large transformers, usually of 12v or 24V - IDK the KVA rating of the transformers but they are quite large, about 1.5-2x the size of the average MOT in physical size and weight, if that gives any reference. Each unit I took apart had 2 identical transformers.

I also have some 12v SMPS supplied from servers that are rated at 1200 or 1600 watts. IDK if these could be used in the place of a transformer, they are very efficient and were super expensive when they were new, on the price of near $300-400 each IIRC. IDK if these could also be used as the source of DC current. The reason I questioned if the transformers could be used is because I've often seen transformers for amplifiers advertised as "audio transformers" and they were often much higher priced than same rating normal transformers. I don't know if they are built with special components or something or with different requirements. As far as the type of audio amplifier, I'm not sure if it would be a A, B or AB class but I would also like to make a class D for a sub woofer amp. I do have some very well build line filters that were made for communications equipment. These are 120/240v rated and for 1660w/2,700w PSU so they should be able to handle the power for the transformers if the line needs filtered. I guess if these transformers can't be used for this, I could use them to make a nice sized isolation transformer with 2 of these? #### Dodgydave Joined Jun 22, 2012 9,459 Your problem is what voltage the transformer will give out on it's secondary side, it maybe no good for your design, chances are a ups transformer will give out 15V or 24V to charge the batteries, or be used in reverse in the inverter. I would search Ebay for ready made amplifier modules to see if they fit your design criteria, you might be able to buy one cheaper than making it . #### crutschow Joined Mar 14, 2008 25,434 I've often seen transformers for amplifiers advertised as "audio transformers" and they were often much higher priced than same rating normal transformers. Transformers labeled "audio" are usually for the audio output stage. They are more expensive because they have to operate over a wide frequency range. Here we are talking power transformers, which only have to operate at line frequency. As Dave noted, their output voltage can be a limiting factor. A ±24V peak signal will give a maximum power of 70W into 4Ω or 35W into 8Ω. If you went with a bridge amplifier, that would double the peak voltage with four times the output power. #### dendad Joined Feb 20, 2016 3,602 What is the spec of your amplifier you want to make? Are you going to use valves or transistors? And what is to power it? I have seen power transformers pressed into service as audio transformers. A lot will depend on the quality of the output you are after. Just recently, I made a PA amp for my son. It was inside an old HiFi graphic equalizer. The case was mostly empty so a toroidal power transformer was added, along wit a big bridge rectifier and large caps. The amps I used were 2 x stereo class D boards, so this box runs 4 speakers. I was pleasantly surprised at the way it works. Each board is a TPA3116D2 amp. Note that the the speaker is labeled as "Left Channel input" :0 These are available on Ebay for less that$10.
It may be worth a look at.

#### Ramussons

Joined May 3, 2013
871
The reason I questioned if the transformers could be used is because I've often seen transformers for amplifiers advertised as "audio transformers" and they were often much higher priced than same rating normal transformers. I don't know if they are built with special components or something or with different requirements.
No, you cannot use a "power" transformer as a "audio" transformer. A power transformer handles low frequency - 50 or 60 Hz. An audio transformer will need to handle frequencies from about 40 Hz to about 20 KHz. This means that the construction is sophisticated.

Special Core material with a low u to reduce Hysteresis.
The laminations of the core are made thinner to reduce the Eddy Current losses.
Each layer of the primary has a "spacer" to reduce Inter winding Capacitance.
Bifilar winding to balance DC resistance of each side of a Push Pull Primary.
An Air Gap in the core to avoid core saturation in case of a Single Primary.
The windings ratio is based on the Plate/Collector characteristics of the output device and the Speaker Impedence.

All these make an Audio Transformer more expensive than a power transformer.
The transformer in a SMPS is designed for High Frequency ( > 50 KHz) and cannot be used for Audio.

#### crutschow

Joined Mar 14, 2008
25,434
Perhaps we need some clarification from the TS about what the transformer is to be used for in the audio amp.

#### RogueRose

Joined Oct 10, 2014
375
I thought I was talking about a power transformer, the thing that makes home audio receivers very heavy. I've taken a number of these apart (and some musical amps) and they all have large power transformers, usually dropping voltage down from 12-35v IIRC. The amps I've seen are ones that have 100-120watts RMS x 5 or 7 channels @ 8 ohm, or a guitar amp (400-600w RMS rated at 1,000-1,200 peak wattage - maybe 4 ohm though..)

So from what I had looked at I thought that most of the amps had a large power transformer to drop the 120AC down to 15-35V that is useable by the Mosfets/IC. IDK, maybe it only drops the V to 50-70 or even up to 170-200V, IDK how the power supply works inside of high power audio amps.

I just didn't think the power transformers were anything special and didn't know they used transformers in other parts of the amp that deal with high frequency and are more sensitive.