It may be hard to imagine a generation that doesn't know why a record would be called a "45." If you have a library of old 78s, 45s or newer 33 1/3 LPs, an accurate turntable speed is critical. In the analogue world of audio reproduction, the turntable is calibrated according to revolutions per minute, hence the numbers used to describe different record types. A turntable that spins too slowly or too quickly will alter the pitch of the music heard from the record. You wouldn't want to explain to your kids why the 45 is really only a 44.
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Things you need
- Strobe speed test disc with strobe calibration light
- Test tone record with audio tone generator
Place the turntable speed test disc on the turntable.
Set the turntable speed desired using the speed selector switch on the turntable.
Locate and press the "On" button to turn on the strobe light. Place the strobe light two inches above the test disc so the light from the strobe illuminates the markings as they pass underneath the strobe light.
Watch the markings as the disc rotates. If the disc is playing at the correct speed, the markings will appear stationary. Any "drift" of the markings will reveal fluctuations in speed. If the markings are moving steadily one direction or the other, the turntable is playing too fast or too slow.
Precise Strobe Method
Turn on your audio amplifier so you can hear the record.
Select the desired turntable speed. Place the AES (Audio Engineering Society) Calibration record for the selected speed on the turntable. AES Calibration records are recordings of various test tones and tone sweeps.
Cue the record to play the recorded 1000 HZ tone.
Select a 1000 HZ (1 kHz) frequency tone from the audio tone generator by adjusting the "Frequency" knob to 1 kHz or 1000 HZ, depending on how your unit is labelled.
Compare the sound of the 1000 HZ tone played by the record and the 1000 HZ tone heard from the tone generator. With 1000 HZ tones heard from both the AES calibration record and the test tone generator, the record playing at the proper speed should be in tune with the tone generator. A higher tone from the record indicates the turntable speed is too fast. Lower indicates the record is playing too slow.
Tips and warnings
- Some strobe test discs are designed to work with ordinary fluorescent lights, which flicker at a precise rate.
- Newer turntables often featured built-in strobe lights that worked with markings on the turntable platter.
- When using an AES calibration record, if you have a variable speed adjustment on your turntable you can find the correct speed by adjusting the speed control until the 1000 HZ tone sounds at the same pitch as the 1000 HZ tone on the tone generator. The process is essentially the same as tuning a guitar string, although not as accurate as the strobe method.
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