It may be hard to believe, but the earliest hi-fi amplifiers had no readymade or built-in facility for direct hookup with a tape recorder. Sound buffs used to wire their own signal jacks and switching arrangements to feed signals into a recorder, and to patch them back for playing through the amplifier and speaker.

As tape recorders became more popular, the "tape monitor" feature began appearing on amplifiers and receivers. Now a standard on all home sound components, this feature permits integrating a tape deck with a steteo system in a way that is electrically correct safe and convenient. In fact, it encourages the use of tape since, at the push of a bottom, the system is set up to record or to play. The tape "takeoff" point (for feeding signals into a recorder) is standardized in terms of signal level and impedance match with respect to the "line-in" jacks on the recorder. The European DIN connection uses a different signal level, but as long as the signal cable runs from one DIN receptacles to another (between) the amp or receiver and the tape recorder) everything will work out correctly. It would be a mistake, however, to connect from a DIN socket on one component to a normal "tape-out" or "line-in" jack on another.

So far, so good. But lately there has been a proliferation of "add-on" audio devices (equalizers, reverb units, adapters, volume expanders), most of which are intended for hookup in the tape-monitor loop of a stereo system. Many of these provide a substitute tape-monitor connection for the one preempted by the hookup, but if you use several of these devices in one system (an avid tape recordist or "sound experimeter" easily might own three or more such add-on devices), the original signal transfer into and out of a tape recorder could become degraded - not to mention the "rat's nest" of wiring you get from such a complext setup. And if you own more than one tape recorder, the profusion and confusion may make the whole system unmanageable.

Stepping into the breach is a relatively young hi-fi company, Russound/FMP Inc., with two ingeniously designed accessories that essentially expand an existing tape-monitor facility to handle just about any number and degree of complexity of add-on devices and signal paths in a sound system. The SP-1, for stereo setups, serves as a control center for up to four tape recorders and over a dozen sound-processing devices. Everything plugs into the SP-1; operation involves switching and patch-cord handling on the front panel. The degree of signal "interfacing" possible with this gadget is unprecedented outside a professional studio patch bay, and includes tape copying, noise-reduction encoding and decoding, sound-channel mixing, and so on. Twenty-two pages of instructions accompany the SP-1, so be prepared for some new learning.

Even more staggering is the model QT-1, which can do for a four-channel system what the SP-1 does for stereo. This one comes with a 24-page instruction manual, and has no fewer than 72 signal jacks at the rear, 48 at the front and 13 switches. It costs $250.

The nice thing about these devices is that in lending studio-like professionalism to a home sound system they do not add distortion to it.