Another old article that hadn’t made the transfer from my old site. This tutorial shows you how to make a resistor adapter for your MP3 player, to cheaply and easily increase sound quality.

A resistor adapter is simply a small adapter between an MP3 player and headphones that has a resistor on each of the sound channels. The reason for this is to increase the resistance of the headphones, forcing the MP3 player to work harder to drive them. Many players (with the Cowon D2 being the most known player with this issue due to its powerful amp) can’t perform well with low impedance headphones such as lightweight 32 ohm headphones or earphones that can go as low as 16 ohm. Beside the issue with the volume control not being sensitive enough to work well at volume levels as low as you might get in such cases, the amplifier in the player simply can’t perform as it should under such low loads.  This issue, called roll-off, mostly affects bass and the effect can be seen very easily in this RMAA graph (courtesy of elektrokrishna.com).

As you can see, the bass frequencies are seriously compromised at 16 ohm load (left side of the graph – it should be flat). At 80 ohm it’s a lot better, but only at 300 ohm is it really as flat as it should be (technically lower than that but there’s nothing showing a load between 80 and 300 int hat graph). This issue is fixed by simply adding a resistor adapter in between the player and the headphones, upping the load on the player and giving you better sound quality (a lot more bass in this case) – granted your player has a roll-off problem.

Tutorial

First off you need to gather parts. You need two identical resistors, and by that I mean IDENTICAL resistors. Two resistors that are said to be 40 ohm, might as well be 38 and 42 ohm, so I urge you to use a multimeter and measure the resistors until you find two that match. As long as they match eachother, it will be fine. Depending on your headphones and their impedance, you need to balance the added impedance versus the loss in battery life (don’t panic; higher volume level caused by the harder-to-drive headphones means a slight loss in battery life, obviously) and the fact you still need to be able to turn the volume up high enough for you to hear (so no 1 kohm resistors, people). I usually recommend about 40-50 ohm for 32 ohm headphones, as 80 ohm is a nice level – even if it doesn’t make the curve above completely flat. If you have a D2 or another player with a powerful amp, you could easily do 100 ohm for the adapter. The parts used aren’t exactly that expensive (not if you buy in bulk, anyways, one by one they might be) so why not make several to experiment?

When you have your resistors, you also need a short wire, a female 3.5mm (1/8″) headphone jack, a male 3.5mm headphone jack, some wide heatshrink, a hot glue gun and a soldering iron (the picture shows two of everything since I made two adapters while taking the pics, a custom cable job for a buyer – see the “custom cables” link at the top!). Of course this is what I used in this particular tutorial, as with all tutorials I do I can’t show you every single design you can do for a resistor adapter, I can only show you one and then you have to take the methods I use and implement them on your own adapter. In other words, you don’t need to match the parts perfectly with what I use. For example, this tutorials shows how to make an angled adapter – you might want a straight one, or one with a short extension cord- etc.

For the 3.5mm male headphone jack, I do recommend the ones I use here – they are simply old earphone plugs split open and stripped to the bare plug. These are excellent low profile plugs and far better than the DIY ones you get at “The Shack” because they are small and cheap. You can also get these from cheap audio cables sold at DealExtreme.

First off, solder the wire to the ground channel of your 3.5mm female jack. On a male 3.5 mm jack, the ground channel is the root of the plug, the right channel the middle of the plug, and the left channel the tip of the plug. You have to extrapolate this to the female jack to see what solder point is what channel. The ground channel is normally in the logical place, but on some connectors – like the ones I use here, the middle solder point of the female jack is actually the left channel and vice versa, as inside the plug there are metal springboard-like connectors that go in opposite direction of the each other, making the channels switch place inside the connector. This can be seen on picture 7 below where you can see inside the connector as the top plate is clear plastic. Bottom line: make sure that what you think is the correct solder point for a specific channel actually is just that.

Using the hot glue gun, glue a resistor to each side of the female jack.

Cut the ground channel wire to length and solder it to the ground connector on the male 3.5mm jack like the picture shows.

Hot glue the male 3.5 mm jack to the female 3.5 mm jack, pointing as shown above. Notice the insides of the female connector and how the springboard-connectors make the channels switch place. Your 3.5 mm female jack might be like this, or it might not. You have to find that out or you risk mixing up the channels.

Solder the resistor legs to the male 3.5 mm jack. Again, make sure you match the channels properly – the tip of the male plug is the left channel, which means that the tip of the solder-end of the male jack is also the left channel. Right is in the middle on both as well. Match this with the correct resistor.

Test that it works. You can use this file to check that you got the channels right (or find out which is which on the female connector if you haven’t already). The first 5 seconds of the audio file plays using both channels, the next 5 seconds is left channel only, while the last 5 seconds is right channel only.

Encase the entire thing in hot glue. This insulates against short circuits and makes the whole thing a lot more sturdy. Please don’t skip this step because it looks ugly, your adapter will self-destruct after 5 minutes of use.

To make it a bit better looking, you can use heatshrink. I found that 20 mm (diameter) heatshrink worked well for the two adapters I made here. I recommend getting heatshrink that’s narrower than what you need and then use needle-nose pliers to extend it. That way you get better shrink ratio to better cover the less-than-symmetric adapter you just made.

That’s it, you now have a resistor adapter. If you’d like, you can take a look at the video below. It’s very old, from the first resistor adapter how-to I made. Still, might be useful in some way (though I doubt it).

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