Headphone Jack + Stereo Receiver = Fail

A headphone jack and stereo walk into a bar. Stop me if you've heard this one...

Have you noticed that when you hook your ipod or similar portable media device to your receiver, the result is, well....problematic?

First off, you have to turn the volume all the way up on your portable device, and then turn your stereo up too loud also. And then the resulting sound is a bit anemic, nowhere as good as other sources you have plugged into your stereo like a CD. In fact, it is not nearly as good as what you hear through good quality headphones.

What gives?

I've seen a great deal of both speculation and misinformation around the interwebs regarding this problem, so I'll try and lay out the facts in understandable way, and show you the best way to hook up your ipod/portable media device to your stereo.

Synopsis

Connecting your MP3 player/ipod to your stereo via the headphone jack is a bucket of fail
Impedance and Signal Level mismatches are the problem
How to fix it: Line Level Output
Various Ipod-compatible devices for Line Level connecting

 

Wait, What do you mean by Stereo?

First off, let's define what I mean by "stereo." I mean a device that includes both a preamplifier and a power amplifier that is somehow connected to speakers. This could be that bitchin' old Marantz you have hooked up to equally bitchin' vintage Bose 901 speakers. Or it could be one of those sweet "mini-component" systems with Line Input Jacks. Or it could be a luscious Adcom preamp hooked into some type of compact MOSFET amp into bookshelf speakers. For our discussions, that's what I mean by "stereo", something with a Line In or Aux In capabiltiy.

Conversely, what I do not mean is a "Designed for Ipod" clock radio, or a Logitech sound-dock with 2" speakers. If it has a built-in Ipod or Zune or whatever dock, it is not what we are talking about here. Why? Because such devices are typically designed to take the characteristics of a portable sound device's output circuitry into account. You just drop your media player in the anointed slot, and you are good to go.

Problem 1: Impedance Mismatch

If you all you give someone is a screwdriver, then all jobs will have to be resolved with that screwdriver. It drives screws well, but it sucks as a hammer. Same thing with a portable media device. Most only have one output jack, and it is labeled "Headphones." Since that is all we get, that's what we use to connect the device to the stereo.

The headphone jack is designed, funnily enough, to drive headphones. And a stereo receiver is designed to expect line level signals, something altogether different. The resulting mismatch is a killer for both convenience and sound. This is because devices have impedance, and they are mismatched.

Impedance is a somewhat complicated subject if you really want to get into the details. In fact, even measuring it accurately requires math, equipment and the blood of a Peruvian Wombat Toad. But for our purposes, it really is quite simple: impedance is the opposition to the flow of current. Literally, impedance tries to impede your electrons. The unit of measure for impedance is ohms, signified by the ever-hip symbol: Ω

In working with audio devices for the reproduction of music, the goal is to have impedance matching: the output impedance of the device making the sounds should closely match the input impedance of the device amplifying the sound. This impedance matching works to ensure the most efficient transfer of power (the audio signal in our case) from device to device.

So our headphone jack is designed to power headphones so it has an output impedance in the range of 30Ω to 100Ω. Line level expects a much higher impedance, typically on the order of 200Ω to 10,000Ω. And therein is the first problem: this mismatch results in an inefficient transfer of power from the ipod to the amplifier. And that is one of the reasons you have to turn your ipod all the way up.

Problem 2: Signal Level

Second, signal level: This is the voltage that a device puts out. Unlike impedance, output signal level is very easy to measure. The unit of measure is volts. V

The exact definition of line level signals for consumer audio is hard to pin down--different manufacturers adhere to slightly different standards, sometimes even within their own product line. But the Line Level voltage for consumer audio is pretty much defined as a maximum signal voltage of around 5-6 volts. Measure the output of your typical CD player or Tape Player component and you will see maximum voltages in that range.

Now perform the same measurements on your Ipod. You'll see a maximum voltage of between 2.1 and 2.9 volts. That's about half what a Line Level device should do!

Wait! you say... Are you sure? That sure sounds low..Let's look at some numbers...

Quick Percy! Trot out Some Numbers!

To become better acquainted with this dilemma, I cleared off one of my workbenches and assembled a motley crew of various impressive looking devices. To test the actual line level voltages, I hooked up my trusty oscilloscope and set it to measure peak-to-peak voltage. I wanted to see the peak output level using various connections to the iPod Touch.

For this test, I chose a recent recording so as to get the maximum Hip Hollywood Mastering signal. In other words, I choose The Pot from Tool's 10,000 days release. 

Source

Highest
Voltage
Output

Ipod headphone jack

2.84 V

Ipod line out jack  2.94 V
Apple Universal Dock "Line Out" 2.01 V
Typical component CD player Line Out 5.76 V

As you can see, the headphone output is a measly 2.84 volts max. Whereas the line level output of component CD player (designed to be a 'real' line out) is close to a whopping 6 volts, double the headphone jack output. I also tested the output of the Line Out port on the Apple Universal dock, and we'll talk more about that in a bit... I've posted voltages here, but you can use handy on-line calculators to convert to dbU/dBV if you want. (For a great article on levels, dbU/dbV etc, see the excellent bottlehead page.)

In conclusion, our iPod is only providing roughly half the voltage expected by the Line Level input on our stereo. The result is: you have to turn the iPod all the way up, which means it is operating at it upper limits and therefore introducing more noise and distortion than is ideal. And then you have to turn the stereo up, which in turn amplifies the noise and distortion more.

So Why is My Portable Device Output So Low?

Is it just that the designers were dumb? Or it is some great conspiracy to deprive me of my right to great sound? Well, it's neither. Instead is part of the Great Engineering Compromise.

The key word in Portable Media Player is "portable"--it runs on batteries. To ensure maximum battery life, engineers strive to reduce power usage where ever possible. And one of the more power hungry parts of a portable media device are the amplifiers--the parts of the circuit that raise the fairly small voltage from the Digital-to-Analog (DAC) converts to a level that will power headphones. In order to keep power requirements low (and battery life high) the output of portable devices loosely standardizes on a lower power output, around the 2 volt range that we saw in our earlier tests. That is enough to nicely drive headphones, but now enough to drive Line Level inputs.

Line Out to the Rescue?

If you have done some research on your IPod or other portable device, you may have come across the concept of a Line Out jack. For example, Apple has provided such a feature on the IPod product line. There is no plug for it, rather, you access it through the dock connector. Some portable media players even expose the line out jack, making it easy.

The good news is that Line Out jacks to sound better than headphone jacks for connecting to a stereo. First off, the line out jack addresses the impedance mismatch. And depending on your specific unit, it usually bypasses the headphone amplifier completely, thereby taking the noise, impedance and tone coloration out of the equation.

The bad news is that the Line Out jack on most portable media devices does not solve the output signal level. For example, the above tests showed us that on the Ipod, both the line out jack on the Apple Universal Dock and the Line out cables still put out a miserly signal of around 2 volts. So we still have that problem.

To address the line level problem, we'll need to use a preamplifier.

Build a Preamplifier and Be Amazed!

Here are two great Beavis HiFi preamplifier projects that solve all of the heaphone jack/impedance/line level problems. Warm up your soldering iron and take a look:

 

Accessing the IPod's Line Out Connectors

You could use Apple's Universal Dock product, after all on the back of the unit it has a jack labeled "Line Out". This does provide better sound, but you still have to turn your IPod all the way up, and then turn your stereo up.

As with the battery saving design of the IPod itself, the circuitry in the Universal Dock is designed to be powered by the IPod itself if no charging cable is connected. That means it uses almost the same circuitry as the IPod. In fact, if you open up the Universal Dock, you will find much of the same circuitry as exists inside the IPod: DAC chips and opamps for amplification. So to conserve power and to keep the output circuitry design consistent, the Line Out jack on the Universal Dock still puts out a power level around half as what is expected from Line Level devices.

LODs: Line Out Dock cables

This consists of an ipod dock connector spliced into a 3.5mm stereo jack. You can find these all day long on ebay and they average about 30 bucks. A typical one looks something like this. Here are a couple of things to look out for when buying a Line Out Dock cable:

  • A LOD should not cost more than around 30-40 dollars. They are usually handmade and from most vendors, represent good quality. The ones that are in the 50-100 dollar range and claim attributes like Solid Gold or Pure Copper are pure bling, nothing more.
  • Check that the LOD being sold actually works with your model ipod. Some will work with the ipod classic devices, but not with the newer iphone or itouch models.

Also note that that LOD's plug into your ipod's dock connector, but do not provide a USB port, so you won't be able to charge or sync your ipod while using an LOD.

Commercial Dock Connector

A commercial dock connector is a device that is mass-produced and can be bought from brick and mortar stores like best buy or directly from a company's website. One of my favorites is the SendStation PocketDock. I like the PocketDock because it is very well made, very compact, and provides access to the dock connector through the USB port for charging and syncing. Highly recommended.

Other solutions are typically marketed as A/V dock connectors. This is a dock connector wired to an array of plugs for hooking up to video component out or composite/s-video out. They also include a set of plugs for Line Out. An example of this product would be the RocketFish A/V Cable which consists of a dock connector attached to a gaggle of wires which terminate in jacks for video and line out. These are very flexible units, but aren't terribly compact.

DIY Solutions

There are a variety of companies who sell an array of parts that make it pretty darn easy make your own customized Line Out solution. Stay tuned for an article on how to do that!

Conclusion

So know we know why the headphone jack is great for headphones but lousy for stereo/hifi reproduction. We also know how to gain access to the ipod's line out jack. Hopefully this information will help your quest for the best-bang-for-the-buck ipod-based stereo.

 


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