Nakamichi 730

Commentary

A stereo receiver with a compact auto-tuner and high output amplifier.

A pure complimentary service push-pull circuit is used in the power amplifier section. The 730 has been specially designed to reduce power loss and provides a high output of 105W + 105W while suppressing heat generation.
In addition, a double NF circuit that applies NF to the input capacitor is adopted, and by reducing distortion, the same effect as that of a DC amplifier is obtained.

In the power amplifier section, power amplifier circuits are densely integrated on a diecast substrate with independent left and right channels, and further integrated with a heat sink.
This eliminates interference between the left and right channels and improves self-compensation for temperature changes.

The power supply section is equipped with a large toroidal orient-core power transformer weighing 5.2 kg and a large smoothing capacitor.

The basic circuit of the preamplifier section has a luxurious circuit configuration with three transistors connected in parallel to achieve a phono input conversion noise of -137dB. In addition to the NF circuit of the whole circuit, the input capacitor is also equipped with an NF circuit to provide the same effect as a DC amplifier.

The preamp section uses a C-MOS analog switch to electrically switch functions, which avoids the sound quality degradation caused by the routing of wires.

With a built-in recording amplifier, when the input gain of the tape deck is insufficient, the volume of the rear panel can be used for 10 dB push-up.

The tuner section uses a motor drive mechanism and a self-lock tuning mechanism to make an auto tuner from an orthodox tuner using an air varicon, thereby avoiding the sacrifice of reception characteristics associated with automation.
The 2-stage low-noise dual-gate MOS-FET and 4-stage air varicon are used in the front end section to reduce distortion and improve the accuracy of the relation between the tuning stages.
The IF stage consists of two stages, SAQ (surface acoustic wave filter) and ceramic filter (4-element one wafer), and high selectivity is obtained without deteriorating the distortion factor.
A PLL circuit is used in the MPX section to maintain stable separation characteristics.

The self-lock tuning circuit detects the deviation of the local oscillation frequency and forcibly suppresses the tuning deviation. The optimum tuning point can be obtained even if the pointer moves slightly on the dial scale, and the optimum tuning is maintained because the pointer itself moves in conjunction with the motor drive mechanism.

An attenuator is mounted on the rear panel to suppress the occurrence of excessive input distortion in a strong electric field.
Can drop by -20dB with one touch.

Preset tuning of 4 stations is possible.

If you want to receive only broadcasting stations with high antenna input sensitivity, turn the threshold switch off. If you want to receive broadcasting stations with high antenna input sensitivity, turn the switch on (20 dBf).

Equipped with an FM mute switch, noise between stations can be reduced during channel selection.

It features -14dB audio muting and tone control, and contour volume to compensate for the lack of high and low tones for human ears.

Speaker selector and wireless remote controller sold separately can be used.

Model Rating

Type	Receiver
Amplifier Unit
Effective output (5 Hz to 20 kHz)	150W + 150W (4 Ω, 0.1%) 105W + 105W (8 Ω, 0.02%)
Total harmonic distortion factor	1 kHz or less 10 kHz or less 20 kHz or less Phono: 0.004% 0.006% - Aux, Tape : 0.004% 0.006% 0.04% main amp: 0.004% 0.008% 0.02%
Intermodulation distortion factor (60 hz : 7 khz = 4 : 1)	0.004% or Less (8 Ω, Aux-Spout, 105W)
Power bandwidth (8 ω, 50W)	110 Hz ~ 20 kHz (Distortion 0.01%)
Damping factor	100 or More (8 Ω, 1 kHz)
Frequency characteristic	Aux, Tape-SP out : 10 Hz to 30 kHz + 0. 3-1dB (8 Ω) Main in-SP out : 10 Hz to 30 kHz + 0 -1dB (8 Ω) RIAA deviation : ± 0.3 dB
Input Sensitivity / Impedance	Phono : 2mV/100k Ω Aux, Tape : 100mV/100k Ω Main in : 1V/47k Ω
Phono maximum allowable input	120 mv (1 khz, 0.1% distortion)
Tone control	Bass : ± 12 dB (20 Hz) Treble : ± 12 dB (20 kHz)
Output Level / Impedance	Rec out : 100 mv ~ 300 mv Variable / 330 Ω Pre out : 1V/1.2k Ω
S/N ratio (IHF-A)	Phono : 83 dB or more (-138dB input conversion ratio) Aux, Tape : 94 dB or more Main amp : 115 dB or more
Residual noise	0.3 mV or less
contour (Volume max)	-12dB at 20 Hz, -23dB at 3 kHz, -14dB at 20 kHz
Channel Separation (1 kHz)	Aux, Tape-SP out : 70 dB or more (Volume -20dB) Phono-SP out : 70 dB or more (Volume-20 dB)
Headphone output (8 Ω)	60 mw (at rated output)
Tuner Section
Receiving frequency	76 MHz to 90 MHz
Practical Sensitivity (IHF, 84 MHz, 300 Ω)	Mono : 2.2 μ V/12dBf
S/N50dB sensitivity	Mono : 4.5 μ V/18.3 dBf (300 Ω) Stereo : 45 μ V/38.3 dBf
Image interference ratio (84 MHz)	85dB
IF Interference Ratio (84 MHz)	85dB
Spurious interference ratio (84 MHz)	90dB
SCA Rejection	70dB
AM suppression ratio	55dB
Capture ratio	1.5dB
Effective selectivity (± 400 kHz)	70dB
Signal-to-noise ratio	Mono : 75 dB or higher Stereo : 68 dB or higher
Total harmonic distortion factor	mono:0.1%(1kHz) stereo:0.15%(1kHz)
Stereo separation	45dB(1kHz) 30dB(10kHz)
Frequency characteristic	30 Hz ~ 15 kHz + 0.5 -1.5 dB
Muting threshold	5.4 μ V (300 Ω, 20 dBf)
<General>
Power supply voltage	100 VAC, 50Hz/60Hz
AC outlet	100W max.
Power consumption	400W
External dimensions	Width 500x Height 90x Depth 370 mm
Weight	17.2kg
Sold Separately	Speaker Selector RS-730 (¥ 6,700) Wireless Remote Control RM-730 (¥ 35,000)