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Improving Sound Of a Portable Phonograph
Q. The sound of my daughter's portable record player is thin, hard, wiry, and reedy. Is there any reasonable way (under $25.00) to significantly improve its sound while retaining the original speaker cases, which are made of thin, light plastic? Each speaker has a 6-in. by 9-in. woofer and a small tweeter in an open backed cabinet. Although I can find no impedance designation on these speakers, the long leads suggest 16 ohms to me. If I replace these speakers with 8-ohm speakers, would this change damage the amplifier?
A. It is probable that a combination of circumstances has combined to produce the rather bad total sound you have described. These circumstances may include: (1) an amplifier which is deficient in low frequency output; (2) a mismatch between the ceramic cartridge probably used and the input of the amplifier; (3) an imbalance between the output of woofer and tweeter, and (4) woofers whose resonant frequency is too high to enable bass response to be produced in any reasonable quantity.
The amplifier may either have its frequency response measured or, more easily, be connected to loudspeakers known to have good sonic qualities which will provide you with an audible indication of the performance of the amplifier. If the resulting frequency response is reasonably good, obviously the major problem will be the original speakers. A simple series resistor placed in the "hot" tweeter lead may pad the tweeter level down sufficiently produce a better balance in the speaker system. However, the amp's output must be high enough to drive this less efficient load to acceptable levels.
If the amplifier itself lacks low frequencies, then it is often possible to improve this situation by the use of larger values of both interstage coupling capacitors and emitter bypass capacitors than are now provided. Because the amplifier used in this record player probably has a relatively low power output, the overall audio volume may appear to be reduced from what it was before the capacitors were changed. This is because more of the power developed after the change in the amplifier will be used to produce low frequencies; before, most of the available amplifier power was available as midrange energy, making the system appear to play more loudly.
Where a portable record player employs a ceramic cartridge, it is possible that the bass frequencies are lost because of an impedance mismatch between that cartridge and the input of the amplifier. Ceramic cartridges require impedances which are typically 1 megOhm. If the input impedance of the amplifier is significantly lower than this value, say 100 kOhms, bass response will be lost. If there is sufficient amplifier gain, it may be possible to improve this situation merely by inserting a resistor of perhaps 0.5 megOhm in series with the "hot" cartridge lead of each channel though there will be a significant overall signal loss.
If the resonant frequency of the original woofers is very high, then they should be replaced. The impedance of the speakers should be measured, if possible, since the speakers used are probably 4-ohm types for this application, rather than 16 ohm.
You may be able to learn the impedance of the speakers by writing the maker, but I do not think we can make a good guess just on the basis of the long leads. The thin interconnecting cables ordinarily used with such equipment are not conducive to good performance, and you may improve things by using heavier gauge wire, something on the order of 18 gauge zip cord.
If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.
(Source: Audio magazine, Jun. 1977, JOSEPH GIOVANELLI)
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