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I am experimenting with the frequency of a unique transformer and I want to vary the frequency of the current that goes into primary coil of the transformer.

I don't want to buy function generator because that outputs very low voltage and current, and I lack the expertise to make an inverter circuit using MOSFET or IGBT to amplify the low voltage and current that function generator or signal generator outputs.

During my research I came across an audio amplifier that can vary the frequency from 80Hz to 15kHz and its output can be connected to run a 1kW speaker.

I found one such amplifier here. it can run 260W speakers.

The specifications of the amplifier don't mention the voltage and current that goes into the speaker, it just shows the wattage, and not even the impedance of the speaker.

My requirement is to input 220 volts and up to 2-3amperes into my transformer with varing frequency. I am planning to transform the voltage and current that comes out of the audio amplifier output into 220 volts. What is the typical voltage and current that goes into the speaker of 1kW rating?

Can you suggest a good function or signal generator with an amplifier type of equipment that can amplify the function generator output to 220V?

Can you guide me to a circuit that I can give to a technician so that he can build the amplifier to amplify the output of signal generator to 220 volts?

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    \$\begingroup\$ About 16A and 63V for an amplifier intended to work with a 4 ohm speaker. About 11A and 90 V for an amplifier intended for an 8 ohm speaker. \$\endgroup\$
    – JRE
    Jun 14 at 11:59
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    \$\begingroup\$ What are you really trying to do? What kind of transformer are you trying to test? What is its input impedance? \$\endgroup\$
    – JRE
    Jun 14 at 12:00
  • \$\begingroup\$ @JRE that's different transformer nobody has built it, it's in research phase, i contacted the original inventor with my findings, he suggested to raise the frequency, now I need a way to raise the frequency hz by hz upto few KHz with output in 220 volts and 2-3 amps as per the load. \$\endgroup\$
    – Yogie
    Jun 14 at 12:03
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    \$\begingroup\$ Audio amplifier and speakers manufacturers frequently lie about output power if they do not provide detailed specs. Peak or Maximum power is actually double the real output power. Momentary power can also be much more than continuous power. Distorted power is also much more than undistorted power. \$\endgroup\$
    – Audioguru
    Jun 14 at 16:40
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    \$\begingroup\$ "During my research I came across an audio amplifier that can vary the frequency from 80Hz to 15kHz and its output can be connected to run a 1kW speaker." Amplifiers don't "vary the frequency". That specification is telling you that it can amplify signals containing frequencies between 80 Hz (which is high, so it has a poor bass response) to 15 kHz. \$\endgroup\$
    – Transistor
    Jun 14 at 18:21

2 Answers 2

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Consider a 1 kW resistive dummy load instead of a speaker.

To dissipate 1 kW at 220 V the resistance of the dummy load = (220 x 220) / 1000 = 48.4 Ω.

The load current = √ (1000/ 48.4) = 4.55 A.

A 220 V - 1 kW hot plate would make a good dummy load for an audio amplifier that is capable of delivering a continuous output of 1 kW RMS to a 48.4 Ω load.

The function generator is a must to input a signal to the amplifier at the required frequency.

An audio amplifier, capable of delivering a continuous output of 1 kW RMS to a 4 Ω load, would output 15.8 A at 63.25 V. For loading such an amplifier up to 1 kW using the 48.4 Ω load, the required transformer turns ratio would be 2 : 7 with the impedance ratio being 4 : 48.4 (transformer impedance ratio is equal to the square of the turns ratio).

enter image description here

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    \$\begingroup\$ Agreed, a 1kW speaker does not approximate well to a resistive load and can cause hearing if not structural damage at high output levels. \$\endgroup\$
    – Frog
    Jun 14 at 20:15
  • \$\begingroup\$ @vu2nan thanks for the concise and to the point answer especially the diagram, u answered exactly what I wanted to know \$\endgroup\$
    – Yogie
    Jun 15 at 14:11
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    \$\begingroup\$ Anytime, Yogie! I'm glad you found my answer useful. \$\endgroup\$
    – vu2nan
    Jun 15 at 15:57
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The amplifier you linked to is not a signal generator. It cannot itself generate a signal, variable frequency or otherwise. It is a regular power amplifier, able to deliver up to 260W of power into one or more loudspeakers, but you will still need a signal generator to produce the signal for this amplifier to amplify.

The 80Hz to 15kHz specification tells you what range of frequencies it can amplify without excessive signal attenuation. That's pretty poor for an audio amplifier, so I suspect this one is intended for public address. There's no mention that I can find of how many speakers it is designed to drive, or what impedance those speakers should have. The figure of 260W could be a total for multiple speakers, or a power rating per speaker. I suspect it's the former.


You don't say what impedance your speaker has. I'll assume 4Ω, though common values are 2Ω or 8Ω. You also don't tell us what shape of signal you intend to use, so I'll assume it's a sinusoid.

To deliver 1kW of power to a 4Ω load, if we were talking about DC, would require a certain voltage, which can be calculated using one of the variants of power law \$P=\frac{V^2}{R}\$, which gives us \$V_{DC}\$:

$$ \begin{aligned} V_{DC} &= \sqrt{PR} \\ \\ &= \sqrt{1kW \times 4\Omega} \\ \\ &= 63V \end{aligned} $$

This would correspond to a certain current, which is calculated using Ohm's law \$V=I\times R\$:

$$ \begin{aligned} I_{DC} &= \frac{V_{DC}}{R} \\ \\ &= \frac{63V}{4\Omega} \\ \\ &= 16A \end{aligned} $$

For AC, which is what you will be using to produce audio, we usually work in terms of RMS (root-mean-square), which is the equivalent DC that would deliver the same power to a resistive load by an AC source. For instance, 220V AC RMS from the mains will deliver the same power (to a resistive load) as a 220V DC battery would.

For sinusoids, their peak value is \$\sqrt{2}\$ times greater than a steady DC equivalent, so to calculate the peak values of a sinusoidal voltage source that would deliver the same power:

$$ \begin{aligned} V_{AC(PEAK)} &= V_{RMS} \times \sqrt{2} \\ \\ &= 63V \times 1.4 \\ \\ &= 88V \end{aligned} $$

The sinusoidal AC voltage you apply across your 4Ω speaker, to deliver 1kW of power, will rise to a maximum of +88V, then fall through zero continuing to −88V, and so on.

The corresponding current will be:

$$ \begin{aligned} I_{AC(PEAK)} &= I_{RMS} \times \sqrt{2} \\ \\ &= 16A \times 1.4 \\ \\ &= 22A \end{aligned} $$


There are a hundred ways to produce this kind of source, all of which are difficult. You are certainly better off buying off the shelf, and since home audio almost never calls for such power, you'll probably be looking for a public address system.

There are plenty of cheap audio waveform generators on EBay and Amazon and such, most of which will produce the signal you require, to be amplified. You could even use a PC or smartphone to get a good enough sinusoid (from the headphone or line-out socket), though you'll need to measure the signal amplitude yourself, using an RMS AC voltmeter.


1kW of audio power can deafen you, literally. Be cautious, and stand many many metres away from any audio source of that power before switching it on.

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