Radio Shack Optimus CD-3400 portable CD player Measurements

Sidebar 2: Measurements

The Optimus CD-3400's maximum output voltage from its line outputs was very low, measuring just 720mV when reproducing a full-scale, 1kHz sinewave. This is nearly 9dB lower than the standard 2V output level, and, coupled with a measured output impedance of 1370 ohms across the band, suggests that the CD-3400 should not be used with a passive level control. The output impedance from the headphone jack was a much lower 15 ohms, but the maximum level (defined as 1% THD+noise) was still low at 900mV into 150 ohms.

Channel balance was an acceptable 0.11dB at 1kHz. DC levels at the outputs were a low 300µV (left channel) and 700µV (right channel). The CD-3400 is one of the few CD players I've tested that inverts absolute polarity; a positive-going impulse on the CD produces a negative-going spike at the CD-3400's output jacks.

The CD-3400 had an unusual frequency response (fig.1). A peak in the treble, accompanied by a rolloff in the top octave, will add a bit of tizziness to the sound, while the bass, as CG noted in his auditioning, is rolled off early (footnote 1). The rising treble response may have been designed into the product to give it more "clarity." The CD-3400's response with its de-emphasis circuit switched in (lower traces in fig.1) tended to track the unit's frequency response, indicating minimal de-emphasis error. The three programmed equalization curves, which only operate on the '3400's headphone output, are shown in fig.2. [I assume that the treble and bass boosts are achieved digitally, which would explain why no one was much impressed with the unit's sound quality in these modes. A digital filter trades off effective boost against resolution, and to achieve the extreme equalization revealed in fig.2 is playing with digital fire.—Ed.] Interchannel crosstalk (fig.3) was only fair, measuring 82dB at 1kHz, decreasing to 62dB at 16kHz.

Fig.1 RadioShack Optimus CD-3400, frequency response (top) and de-emphasis error (bottom) (right channel dashed, 0.5dB/vertical div.).

Fig.2 RadioShack Optimus CD-3400, programmed equalization curves, from top to bottom at 100Hz: PE-4, PE-3, PE-2, PE-1 (2dB/vertical div.).

Fig.3 RadioShack Optimus CD-3400, crosstalk (top) (right-left dashed, 10dB/vertical div.).

A spectral analysis of the CD-3400's output when decoding a -90dB dithered 1kHz sinewave (fig.4) showed some power-supply noise in the audio signal, and a fairly high noise level overall. Note that this 60Hz noise won't appear in the audio signal when the CD-3400 is run on batteries (all my tests were performed with the recommended RadioShack AC adapter powering the CD-3400).

Fig.4 RadioShack Optimus CD-3400, spectrum of dithered 1kHz tone at -90.31dBFS, with noise and spuriae (AC supply, 1/3-octave analysis, right channel dashed).

Footnote 1: According to John Curl, the '3400's restricted bass is due to two 10µF/6V aluminum electrolytic caps located inside the player. Very experienced modifiers can bypass these with external high-quality caps. Note that, if you write to Stereophile to ask how, you don't qualify as "very experienced."—JA
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