Step-Up Transformers 120/230V AC for EU Amplifiers Used in the USA
European musicians who travel to the USA with their own amplifiers often ask us what specifications a step-up autotransformer should have when used with European 230V amplifiers. This situation also applies to Jaaz Acoustics Gig and Compact amplifiers, which use 230V AC electronic power supplies. Their compact dimensions and low weight make them ideal for air travel, allowing musicians to pack them into their luggage and then operate them in the USA, where the mains voltage is 120V AC (see the picture of the JA Gig amplifier packed in a suitcase).
There are many suppliers of 120/230V AC step-up autotransformers on the European market. A good selection of transformers is offered by www.toroidy.pl.
The transformer should be selected according to:
Input voltage – for the USA this is 120V AC
Output voltage – for EU amplifiers, in our case Jaaz Acoustics Gig and Compact, this is 230V AC
Transformer power rating (VA) – this is selected according to the amplifier type and the Crest Factor of the signal being amplified
Crest Factor is the ratio between RMS power and peak power (Pmax). Musical signals vary significantly in Crest Factor. An acoustic guitar signal at the power amplifier output often has a Crest Factor considerably greater than 7 dB (peak-to-average power ratio approximately 5:1). An electric guitar signal with certain effects typically exceeds 5 dB (peak-to-average power ratio approximately 3:1). Keyboard instruments reproducing sine-wave signals have a Crest Factor of 3 dB at the amplifier output (peak-to-average power ratio 2:1). Synthesizers generating square-wave signals have a Crest Factor of 0 dB (the ratio between peak power and RMS power equals 1).
In practical terms, a signal with a high Crest Factor is relatively “sparse” from the perspective of the relationship between peak level and average power. It contains high peaks but relatively little average energy. A typical example is an acoustic guitar. This also affects the amount of electrical power drawn by the amplifier from the mains supply.
Example 1
If you are playing an acoustic guitar expressively and the Powerclip LED on a JA Gig 4i amplifier indicates that the peak output power of 250 W has been reached, then with a Crest Factor of 7 dB (peak value to RMS value ratio = 5:1), the amplifier will draw from the mains:
250 W / 5 × 1.1 (efficiency correction factor)
which equals approximately 60 W.
This means that when using an acoustic guitar, an autotransformer rated at approximately 55 VA plus an appropriate safety margin would be sufficient.
In any case, the 100 VA autotransformer available as a JA accessory at https://www.jaazacoustics.com/transformer-120-230v-us will be adequate.
Example 2
If you are playing an electric guitar expressively and the Powerclip LED on a JA Gig 4i amplifier indicates that the peak output power of 250 W has been reached, then with a Crest Factor of 5 dB (peak value to RMS value ratio = 3:1), the amplifier will draw from the mains:
250 W / 3 × 1.1
which equals approximately 87 W.
This means that when using an electric guitar, an autotransformer rated at approximately 87 VA plus an appropriate safety margin would be sufficient.
In any case, the 100 VA autotransformer available as a JA accessory at www.jaazacoustics.com will be adequate.
Example 3
If you are playing a keyboard instrument reproducing a sine-wave signal and the Powerclip LED on a JA Gig 4i amplifier indicates that the peak output power of 250 W has been reached, then with a Crest Factor of 3 dB (peak value to RMS value ratio = 2:1), the amplifier will draw from the mains:
250 W / 2 × 1.1
which equals approximately 138 W.
This means that when using keyboards producing sine-wave signals, an autotransformer rated at approximately 140 VA plus an appropriate safety margin would be sufficient.
In this case, the 100 VA autotransformer available as a JA accessory at www.jaazacoustics.com will not be sufficient.
A higher-rated autotransformer of at least 170 VA should be selected (for example, the nearest available rating from the Toroidy.pl range is the 200 VA model).
In such cases, it is only necessary to ensure that the transformer provides a protective earth conductor connection so that the Class I amplifier can be properly connected to the protective earth of the 120V AC supply system.
Example 4
If you are playing a synthesizer generating a square-wave signal and the Powerclip LED on a JA Gig 4i amplifier indicates that the peak output power of 250 W has been reached, then with a Crest Factor of 0 dB (peak value to RMS value ratio = 1:1), the amplifier will draw from the mains:
250 W / 1 × 1.1
which equals approximately 275 W.
This means that when using keyboards producing square-wave signals, an autotransformer rated at approximately 280 VA plus an appropriate safety margin would be sufficient.
In this case, the 100 VA autotransformer available as a JA accessory at www.jaazacoustics.com will not be sufficient.
A higher-rated autotransformer of at least 300 VA should be selected (for example, the nearest available rating from the Toroidy.pl range is the 300 VA model).
In such cases, it is only necessary to ensure that the transformer provides a protective earth conductor connection so that the Class I amplifier can be properly connected to the protective earth of the 120V AC supply system.
Conclusion
The above considerations show that if you do not wish to analyse the nature of your instrument signal and its Crest Factor, the step-up autotransformer must be sized according to the maximum power drawn by the amplifier from the mains supply.
This will generally correspond to the power rating of the amplifier power supply in a solid-state amplifier, or to the power rating of the mains transformer in a valve amplifier, including all applicable design safety margins.
However, if the Crest Factor of the signal is known (for example, in the case of an acoustic guitar), a smaller autotransformer can be used advantageously. Such a transformer will also be significantly lighter and easier to transport.
A 300 VA transformer typically weighs approximately 2 kg, while a 100 VA transformer weighs approximately 0.7 kg.
An incorrectly selected autotransformer with insufficient power capability for the Crest Factor of the signal will exhibit excessive temperature rise.
The temperature of such a transformer is best measured using an infrared thermometer or a thermal imaging camera.
As a simple practical check, if you can comfortably hold the hot transformer in your hand, its temperature is probably below 50°C, which is still an acceptable operating temperature.
If the transformer becomes too hot to hold comfortably, its temperature is higher, indicating that the transformer is undersized for the Crest Factor of your signal.
Taking ambient temperature into account, the transformer temperature should ideally not exceed 40°C.
