The DB-25 Enigma

By: JG

AES-315YMany times, people look at the jacks on the back of a device to figure out what cables they will need. While this is not really the best way to figure things out, most of the time it will get results. If you see an RCA jack and ask for an “RCA cable”, you’ll probably find a suitable cable (though there are different types of cables that use the RCA connector). But if you don’t get a little more information, you’ll most likely run into trouble when you see a DB-25 jack on the back of your audio device.

DB-25, or D-sub, connectors were originally created for computer applications. The audio industry adopted the DB-25 as a way of getting multiple channels in and out of devices while taking up minimal space. In this regard, the DB-25 connector works great. However, the use of this connector is problematic because there is not one universally accepted way of wiring it. In fact, there are three widely used wiring conventions when it comes to DB-25 in the audio world.

Some devices use D-sub connectors as a way of inputting or outputting eight channels of balanced analog audio. Each balanced channel requires three pins—one for the positive signal, one for the negative, and one for the shield—and each channel is grouped in a triangle pattern by taking two pins from one row and one pin from the other. Eight channels require 24 pins and pin 13 is simply not used. The Hosa DTM-800 series balanced snake is wired this way. It connects to the DB-25 output and breaks out to eight XLR male connectors. The Precision 8 mic preamp by True Systems uses a DB-25 connector to output all eight channels. This makes it possible to run a single cable with eight channels to an audio interface or mixer.

Professional audio devices can also use one DB-25 connector for eight channels In & Out (I/O) using the AES3, or AES/EBU, format. This digital audio format enables devices to send two channels of audio along one balanced audio line. This is where it really gets fun, as there are two standards for AES/EBU multi-channel I/O—and manufacturers choose which one to use.

The first is known as the Tascam wiring standard. The Tascam wiring standard is the same as the analog standard at the DB-25 connector end. The wire, however, must be different, as it is not passing analog sound. The AES3 specification requires 110-ohm balanced cabling for AES/EBU signals. Unlike analog snakes, AES/EBU snakes carry two digital channels on each balanced line. This means through one DB-25 snake, the device can send eight channels and receive eight channels simultaneously. If you are using a digital snake that breaks out to XLR connectors, it will have four male and four female XLR connectors instead of four like connectors on the analog snakes. Avid and Universal Audio are two companies using the Tascam standard. Avid’s ProTools HD I/O uses AES/EBU via a DB-25 jack.

Companies such as Apogee and Mackie, among others, have adopted the Yamaha wiring standard for their AES/EBU I/O. The wire is the same as that used for the Tascam digital snakes but the pin configuration is much different. In this case, the ground wires are on one side of the connector, while the other side gets the conductors. The Lynx Aurora 16 AD/DA converter uses AES/EBU with the Yamaha standard.

Note that if you are connecting two digital devices, you must make sure to use the correct pinout for each. If one of your devices uses the Tascam standard for its AES/EBU I/O and the other uses the Yamaha standard, you can still use them together. You must, however, use a snake with the Tascam pinout on one end and the Yamaha pinout on the other. Analog to digital is not as simple. You cannot use a DB25 snake to interconnect AES/EBU and analog signals. This would require a separate interface, which is a topic for another discussion.

The next time you turn to the back of your device for cable answers, remember the connector type is not everything—especially with DB-25. Your first question should be whether you’re looking at an analog or digital connection. If it’s digital, the next step is to figure out the wiring standard the device uses. Taking these steps will decrease headaches down the road and ensure you purchase the right DB-25 snake the first time.