Fostex 750 manual Technical Notes

Model 750

Don’t misunderstand our position: Ex- tended dynamic range is great for music recordings. We love listening to terrific classical, jazz, and, when available, pop recordings. But for the applications served by the Model 750 dynamic range is not really a factor. Go ahead, prove us wrong and use the Model 750 to record the London Symphony Orchestra. Then you should turn the limiter/compressors off!

Technical Notes

¼-Inch Plugs and EIA RS-453

An incompatibility problem lurks between some ¼-inch 2-conductor and 3-conduc- tor phone plugs and the jacks found on professional audio equipment. While all the plugs seem to “look” the same, some do not comply with the industry standard, called EIA RS-453. This standard defines the physical dimensions, including the shape of the plug’s tip. It seems that some plug manufacturers don’t bother to make the tip comply with the standard.

Why is this relevant to you? Because the phone jacks used on the Model 750 do comply with the standard, and they expect to be mated with plugs that do, too. Should a connection seem “flaky,” sound noisy, or make intermittent contact, the most likely problem is a nonstandard phone plug. Replace the plug if this is the case.

You should find that all plugs from Switch- craft or Neutrik will work correctly, specifi- cally Switchcraft No. 297 or Neutrik NP3C. In our experience, headphones are the most likely place to find nonstandard plugs—we even found them on some “world-class” headphones that we use in our lab!

Definition of Level

Studio Technologies has opted to use the dBu designation as it seems to be quite rational. Using dBm was fine when all audio outputs were terminated with 600 ohm loads. In this way it was easy to say that “0dB” was 1 milliwatt dissipated in the known load (i.e., 0dBm across 600 ohms will measure 0.775V). Today, an output is rarely terminated with 600 ohms; generally 10k ohms or higher. The dBu designation is better because it refers to dB referenced to 0.775V, with load impedance not a factor. This takes into account the contem- porary audio scene where most equip- ment has a low output source impedance, and a high input impedance.

External Monitor Input 2

This section is buried here because we are almost certain that no one in manage- ment will ever read this far into the guide! Sources connected to the external monitor inputs will usually have a nominal level of +4dBu, so why the –10/+4 DIP switch for input 2? Frankly, we thought that field engineers may sometimes want to con- nect a piece of –10 “consumer” audio gear for their own “personal monitoring.” While operating a remote vehicle can be challenging and fast-paced, it can often be filled with hours of waiting. The ability to connect a compact disk player or cassette “Walkman” may make the time pass a little more pleasantly!

You might recognize that we selected –10dBu, even though the –10 designation usually implies –10dBV. We feel that –10dBu is a better sensitivity when con- necting to “–10” equipment. As –10dBV = –7.78dBu, the extra 2.22dB of sensitivity is helpful.

Issue 3, May 1997	Model 750 User Guide
Page 36	Studio Technologies, Inc.