Restoration Tips & Notes Working with audio media (mostly tape) restoration

2012-12-12

The Phone Plug—Uses and Misuses

Filed under: audio,audio-video systems design,video — Richard L. Hess @ 18:06

The ubiquitous phone plug, especially in the 1/4 inch (6.3 mm) size, is extremely confusing to the uninitiated. I was at a church today where I struggled to get a video to play properly in advance of an event in a few days.

The church had a PC and two 60-inch diagonal video monitors (another story). The audio was fed to the good-quality 16-input mixer (Allen and Heath, I think) from the PC’s headphone output.

In the video, there are two places where there is speech and the music is faded into the background. When played in the church, the voice disappeared! This created some angst coupled with erroneous assumptions. I hope this post will perhaps help solve this problem for others.In more time than I want to admit, I pulled the 3.5 mm stereo plug from the computer and the built-in speakers played the voice fine. I then realized what had happened. The people who added the computer to the mixer had taken a 3.5 mm to 3.5 mm stereo cable and a 3.5 mm to 6.3 mm stereo adapter and plugged the output of the computer audio stereo headphone jack into a balanced mono line input of the mixer. Tip-Ring-Sleeve to Tip-Ring-Sleeve, what could be easier, AND MORE WRONG?

This is where different circuit topologies on the same connector can cause a great problem. We have known this for a century, which is why there are so many different mains (hydro/power/line cord) plugs. Each has a separate voltage and current rating. We also see this with USB vs. FireWire vs. HDMI and a whole host of computer standards that use dedicated connectors. Not so, for the over-century-old phone plug.

Wikipedia has a great article on the phone plug and my intent here is to alert the reader to the issue and not describe all of the options. This section addresses the problem in more detail that I will go into. There are three common usages of the 1/4-inch (6.3 mm) phone plug on most audio mixers.

(1) Balanced Line Connection – This is most likely the format which has the highest jack count on common mixers. Line-level (as opposed to mic-level)  inputs and outputs frequently appear on this connector. If the circuit is balanced, tip is the in-phase signal, ring is the out-of-phase (or reference) signal, and sleeve is chassis ground. If it is unbalanced, usually tip is the signal and ring and sleeve are chassis ground. In some instances (especially on outputs) the ring may be connected to ground via a resistor. This is an impedance balanced circuit and provides many of the benefits of a truly balanced connection and eliminates some of the negative aspects.

(2) Unbalanced Insert Point – This is most likely the second highest jack count on common mixers. Some or all of the input channels allow for the insertion of processing equipment in the channel. If you do not have special adapter cables or a very specific need, it is best to stay away from this series of jack, usually labelled “insert.” See note at end of article.

(3) Stereo headphone connection – There is usually only one jack wired like this and that is for the stereo headphones. Here left is on the tip and right is on the ring and the sleeve is common.

So, what happens when one connects a headphone connection (left tip, right ring) to a balanced tip-ring-sleeve input? The left channel is connected to the input with proper polarity. The right channel is connected to the input with reverse polarity. Anything that is equal on both channels will be removed from the program. What one hears is the DIFFERENCE between the left and right channels (right is subtracted from left). This explains why some of the videos in the past were very low (they were essentially mono) and why I had to drastically turn down the level control when playing my stereo video (they were amplifying the slight imbalance between left and right for mono signals). Also, when listening to the difference signal, the sound just isn’t right.

We solved the problem today as there were free inputs on the mixer and I was willing to donate a cable with a 3.5 mm to two RCA plugs and two RCA to 6.3 mm tip-sleeve (mono) adapters. Now, all works well and they can use stereo (if the rest of the system were wired in stereo which it appeared not to be). The sound was amazingly superior (even in truly summed mono). It does take up two faders.

My preferred solution if I am running a 3.5 mm output to mono is to purchase a 3.5 mm cable of sufficient length. It does not matter what is at the other end, but if you purchase a cable of adequate length with 3.5 mm plugs on both ends, you can make two adapters. It is simple. Along with the cable buy two 100 ohm resistors and a tip-sleeve mono plug for each adapter. Cut the cable in the middle. Solder the shield of the cable to the sleeve of the plug. Solder the two wires inside the shield to one end of each of the two 100 ohm resistors. Solder the free ends of both 100 ohm resistors to the tip terminal of the plug. Insulate well. Assemble the plug. Test it. Now you will have a mono sum signal for your balanced input (but you are defeating the balance, but that is fine if the cable run is short (i.e. on the same work surface, typically).

If you wish to place the computer at the end of a long mic run, you need to isolate the device from phantom power, lower the level, and balance it to avoid hum. The easiest way to do this is to use the above cable with a direct-inject transformer isolator. While not a true high-fidelity part, I have found the $20 Radio Shack/The Source transformer to be adequate as long as the signal level is not too high. Run the volume control of the PC low enough to not cause any bass distortion. These also work adequately with many guitars, especially those with built-in preamps. They measure surprisingly well and sound good when used as intended. For higher quality applications, a Radial Engineering Direct Box using a Jensen Transformer is about as good as it gets at about 11 times the price!

More detail on the insert point — There is an inter-stage “insert point” for each input channel (and sometimes output channels as well) that connects to a single jack. In normal operation, the internal switch contacts on the jack allow the signal to flow through uninterrupted. When a plug is inserted, the switch contacts open, the signal flows out the tip and is returned to the ring (usually, though please check your mixer’s instructions to confirm that it is not wired in the reverse). The insert jacks also make a handy place to derive a direct output, usually pre-fader. To do that reliably, I make up TRS-to-TRS cables where the tip and ring on the mixer insert-jack end are tied to the tip on the recorder end and the ring and sleeve on the recorder end are tied to the sleeve on the mixer insert-jack end. In this way, we are reliably connecting the tip and ring together on the mixer insert-jack while connecting that signal only to the tip for recording.

 

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