If you've bought a top-grade stereo system during the past two or three years, you probably aren't getting nearly all of the sound you paid for.

The makers, I must quickly add, are not to blame. If you own a modern well-matched audio system, it is probably capable of almost perfectly reproducing everything that has been molded into the grooves of any record album. I mean everything -- noise as well as music.

There have been tremendous advances in hi-fi technology over the past 20 years or so, but record-making techniques have lagged far behind. If you looked at a record -- even a new one -- under a microscope, you'd see many imperfections on its surface. You'd find scratches, bits of dust and hunks of vinyl left over from the disc-stamping process. In the days when record players did not mercilessly expose every flaw the imperfections didn't matter much. But they come through loud and clear on virtually any good system you buy today.

And the flaws you can't see are almost as bad. As the stylus glides along a record its interaction with the groove walls generates a low-level but clearly audible (if you listen for it) noise that lies like a thin veil over the recorded sounds. The interaction also causes a high-frequency gremlin called tracing distortion.

Engineers have been searching for years for a way to manufacture a noiseless record. Now, apparently, they have succeeded. A Boston-based company called dbx, manufacturers of noise-reduction equipment, is now in the record business and has started releasing LPs that are virtually noiseless and distortion-free. The new discs also have a far greater dynamic range (the difference between the softest and loudest recorded sound) than conventional albums. Jerome E. Ruzicka, dbx vice president, calls it "the most significant advance in record technology in the last 25 years."

The new albums are being produced and distributed by dbx and a number of record companies. About 20 noiseless discs have been released to date and dbx hopes to have about 100 on the market by this time next year at prices from $8 to $20. To play them you need a special decoder made by dbx and now in hi-fi stores at a$109 suggested retail price.

To produce a noiseless disc, dbx engineers go back to the master tape of an album and cut a new record from it while processing the signal through a special encoder. The encoder compresses the range of sound on the tape at a 2:1 ratio over a 100-decibel spectrum. The compressed signal is then rerecorded at a level higher than the ambient level of record noise. In addition, high-frequency reemphasis is used to reduce the low-level noises such as minor pops and scratches.

When the record is played back through a home decoder, the music is linearly expanded by a 1:2 ratio so that the original dynamic range is restored. During the expansion process, surface noise is reduced to an inaudible level, with ticks and pops also made inaudible by a mirror-image high-frequency deemphasis action in the decoder.

I have heard dbx noiseless discs compared with conventional recordings of the same material in carefully controlled A-B testing. Record surface noises-snaps, pops and the whooshing sound of tracking noise-were apparent when the conventional records were played but inaudible on the dbx. And noises on the conventional albums became much more noticeable when judged alongside the noiseless records.

Within three to five years, audio writers say, the vinyl phonograph album we know today will be replaced by an entirely new kind of record, perhaps a digitally recorded disc, that will produce no electromechanical noise because it will be tracked optically by a laser beam. But meanwhile, if you'd like to treat yourself to some virtually noiseless vinyl records for an initial investment of just $109 you might want to try out a dbx. And who knows? As audio expert Len Feldman recently observed, with the kind of sound you can get from a dbx disc, the world may never even need a laser-tracked digital record. Super Slewing If you're a perfectionist, hi-fi is your kind of hobby. Perfection is what it's all about; audio engineers are always seeking the ultimate system, one that will at last deliver true sound.

One achievement that has brought hi-fi one step closer to perfection over the past few months is something called high-speed amplification. A high-speed amplifier is built with transistors that allow it to react quickly to sudden changes in input signals, such as a sudden switch during a musical passage from a string bass to a violin. This faster transient response, say the proponents of high-speed amplification, means better sound quality.

One major component manufacturer introduced a line of high-speed amplifiers a few months ago and met with much success. An executive explained: "You can easily determine which of two amplifiers of comparable power and distortion specifications is high speed by its superior sound quality. The quality is manifest in the exceptional clarity of the mid and high frequencies, without the loss of stereo definition that previously plagued amplifier performance during powerful, complex or percussive musical passages.

Strain and distortion appear in amplifiers when the dictates of the music require the slew rate (measured in volts per microsecond) to be higher than the amplifier can deliver.

You can get an idea of the speed capability of an amplifier or receiver by looking at the slew rate figure on its specification sheet. A slew rate of, say, 230 volts per microsecond is said to be four to 10 times better than the slew rate for first-class components of conventional design.