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Open Source Hardware
Blog The Audio Cable Buddy - Boosting Audio via Impedance Conversion
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Engagement
  • Author Author: shabaz
  • Date Created: 21 Nov 2018 1:21 AM Date Created
  • Views 1977 views
  • Likes 12 likes
  • Comments 7 comments
  • car audio
  • gym
  • impedance transformation
  • audio amplification
Related
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The Audio Cable Buddy - Boosting Audio via Impedance Conversion

shabaz
shabaz
21 Nov 2018

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.

 

image

 

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 : )

 

image

 

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!

 

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Top Comments

  • michaelkellett
    michaelkellett over 4 years ago in reply to shabaz +6
    1824906 from Farnell is their cheapest (£4.60 each) that might do - should give 8.2dB boost. It doesn't have a very good frequency response spec. 1172420 at £6.38 gives you more options of ratios and a…
  • shabaz
    shabaz over 4 years ago in reply to luislabmo +3
    Hi Luis! I actually didn't consider any specs : ) I randomly ordered a few transformers to try them out, because the cheap transformers have very few specs anyway. These ones I used are very far off spec…
  • shabaz
    shabaz over 4 years ago in reply to michaelkellett +3
    Hi Michael, Thanks for the information, the NTE4 is lower-cost for a real audio transformer than I thought. For this application an amplifier would be more suitable since high quality isn't needed, but…
  • shabaz
    shabaz over 4 years ago in reply to davedarko

    Hi Dave,

     

    No problem at all, here's the diagram:

    image

    The transformer windings are likely unmarked, but it is easy to identify by using a multimeter set to ohms; the fewer turns side will have lower resistance (there is no harm getting the tranformer windings swapped by mistake, the volume will be lower so you'll know to swap it around).

    Incidentally I also tried this transformer, it gives much louder volume but is physically bigger too, and more expensive. In one of the comments above, michaelkellett has some suggestions too.

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  • davedarko
    davedarko over 4 years ago

    Awesome project! Could you add a schematic for this, please? Might sound stupid as the pins are all visible, but I don't really know what pin goes to what coil. Thanks

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  • shabaz
    shabaz over 4 years ago in reply to michaelkellett

    Hi Michael,

     

    Thanks for the information, the NTE4 is lower-cost for a real audio transformer than I thought. For this application an amplifier would be more suitable since high quality isn't needed, but for general isolation I may pick a couple of them up... my sig-gen isn't isolated, and a few times I've needed that at audio frequencies.

     

    Out of curiosity I took apart one of the transformers used in this project.. the number of turns are approx 660 and 125 (to within 5 turns or so), and measured about 235mH and 4.4mH with the very poor quality meter. Based on rough calculations, u is somewhere in the ballpark of 400-800, so I'm guessing it's some steel core (the laminations do bend, but snap easily too).

     

    That's very interesting that you wound audio transformers : )

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  • michaelkellett
    michaelkellett over 4 years ago in reply to shabaz

    1824906 from Farnell is their cheapest (£4.60 each)  that might do - should give 8.2dB boost.

    It doesn't have a very good frequency response spec.

     

    1172420 at £6.38 gives you more options of ratios and a much better frequency response - but probably still a bit bass light which might bother you.

     

    You have to move up to the Neutrik NTE4 (Farnell 724609 at £11.50) to get a real audio transformer - 200ohm/3.2k matching, 20Hz - 20kHz response, specified distortion etc.

     

    But electronics would be cheaper.

     

    A (very) long time ago I used to make little transformers like this using mu metal laminations (iron or ferrite can't give the frequency response)

     

    MK

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  • shabaz
    shabaz over 4 years ago in reply to luislabmo

    Hi Luis!

     

    I actually didn't consider any specs : ) I randomly ordered a few transformers to try them out, because the cheap transformers have very few specs anyway. These ones I used are very far off spec. They supposedly should be 1300:8 impedance ratio, which should mean 12:1 voltage ratio, but I'm seeing 5:1 voltage ratio at various frequencies, so the transformer they supplied is incorrect. It's actually a non-ideal transformer because the impedance is really low on the primary side, and it is loading the phone output. I tried to measure inductance, but I don't have the correct tools for this : ( I have a cheap $20 LCR tool that picks some high frequency for the measurement whereas I tested audio at 1kHz and 100Hz.

    As rusgray says, bandwidth and distortion are also factors. I have no control over this with a cheap transformer : ( but even a single mid-range real audio transformer will easily cost $20+, putting it out of reach for such a basic project. However, with the cheap transformer the distortion will be very minor compared to the audio quality from gym machines, and even ok for basic music in the car - the speaker-to-line converters from car audio stores use the same transformers I'm sure - they won't use high quality transformers either!

    I've ordered some more transformers, so if I find a better one I'll update in the comments here, but the ultra-low cost one is usable. I'd like to find one that doesn't load the iPod/phone too much (this one has too low inductance at audio frequencies : (

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