Introduction

I've often heard the issue that the audio isn't loud enough when people plug in their iPods/phones and so on into music systems, even if they turn the volume up to maximum on their device or the amplifier.

Often the problem is down to the fact that the 3.5mm plug is being used for two different roles. On the iPod/phone the plug is being used to carry a headphone output signal. But on the music system, the socket is used to accept a line level signal.

Headphones have a low impedance (perhaps tens of ohms) whereas the line level input could be something like 1kohm or 10kohm.

If the voltage level is too low for the line input, it can still be high enough for lots of volume through headphones. My mobile phone manages 766mV RMS output with no load attached. That's sufficient for some line inputs, but not others.

As soon as the load increases, that output may drop; it won't go higher than the unloaded 766mV RMS.

In the drive for a healthier lifestyle : ) you may have noticed that gym running machines with audio inputs can sound quiet too, when you plug your phone in. When I'm running, I cannot hear the music clearly : ( I wanted a little louder music. Not a massive boost, but enough to hear it over the sound of the motor and my gasps for breath while running. This ultra-simple, 30-minute project addresses that problem.

It could also apply to car audio inputs, and elsewhere too.

How does it Work?

A pre-amplifier could be one solution, but it isn't always necessary. Besides, then you need to find power from somewhere, such as a battery. Sometimes just a transformer is needed. It can be used to perform an impedance conversion that results in a higher voltage at the output, provided that the load impedance isn't too low. It sounds too good to be true, and it is to an extent - although a perfect transformer will achieve this, in practice the transformer will have an impact on the audio quality, introducing some distortion. You can buy high-end audio transformers, but they are really expensive. Instead, I decided to use a cheap transformer. This is a similar approach to in-car 'speaker-to-line-level converters' that you can buy to adapt legacy car hi-fi equipment (with only speaker outputs) to a new car amplifier, although they usually have other circuitry in there too, to present a suitable load to the legacy car hi-fi. The quality is adequate for in-car applications, and so I planned to do the same thing for gym applications.

Building It

This is really easy. There isn't a circuit because it is so trivial. A 3.5mm cable3.5mm cable is cut in half, and the transformers (one each for left and right audio) are inserted in-between. The end that will plug into the iPod/phone needs to have fewer turns. Any transformer intended for old-school portable radio speaker applications is fine. I used some cheap transformers from aliexpress. If the transformer is unmarked, then a multimetermultimeter set to ohms can be used to establish this. For an enclosure, I just used what I had to hand, which was a small potting boxsmall potting box. I cut a piece of plastic to act as a lid, and used epoxy glue inside, to secure the transformers, cable and lid. Job done : )

Using It

Just plug and go : ) You may wish to label the side intended for the iPod/phone or the amplifier. The sound should be noticeably louder. As a very quick test, I applied a 1kHz sine wave from my mobile phone, and the output using the Audio Cable Buddy was transformed from 766mV RMS to 1.38V RMS unloaded. With a 1kohm load resistor, it dropped to 1.1V RMS - still louder than without the transformer.

I also tested at 100Hz, and the output was 1.27V RMS with no load, dropping to 1.03V RMS with the 1kohm load. This is good enough as far as I'm concerned!

Summary

With little effort and cost, it's possible to boost the audio for increased gym happiness : )

This solution is easy, costs less than $5, and requires no batteries.

Thanks for reading!