Nakamichi 410

Commentary

A control amplifier with improved distortion characteristics and SN ratio.

In the 410, new ideas were pursued for the basic circuit structure, and a special circuit called a triple transistor circuit was devised and adopted for the first stage circuit to improve the SN ratio.
This circuit focuses on the problem of the thermal noise en generated by the base input resistance hie. In an actual circuit, the source impedance Rs is added to hie in series. In this case, the value of Rs is larger than that of hie, and the thermal noise generated by Rs is often larger than that of hie, so the value of en tends to be ignored. However, when Rs is very small, such as in MC cartridges, the effect of hie on the SN ratio is large. Therefore, the hie is suppressed to 1 / 3 and the en is reduced to 1 / √ 3 by a triple transistor circuit.
In addition, the SN ratio has been improved by approximately 10 dB by adopting a transistor with less current noise in.

Both the first and second stages are made of common-emitter amplifier, and the output stage is made of a class A complimentary service push-pull circuit.
A differential amplifier is often used as a pre-amplifier because it is easy to construct an NF circuit. However, since the internal resistance of the differential transistor causes non-linear distortion at low volume and noise build-up, a buffer amplifier is used for the 410. This is based on the idea that the characteristics at low output power are more important than those at maximum output power.
In a differential amplifier, the differential transistor outside the NF loop has an internal resistance Ri. Since this Ri varies depending on the level, nonlinear distortion occurs when Ri becomes larger than the resistance inside the NF loop at a low volume. In a circuit of 410, the differential transistor can be assumed to be removed, and Ri becomes 0 so that the playback sound is not affected by the level change.

Noise and distortion are reduced by using luxurious parts such as metallic film resistors and high-precision capacitors, which are more expensive than transistors, 3mm-thick tinning copper plates for input / output pin jack boards, and epoxy-based plastic for printed circuit boards.
In addition, the part layout is also very low in the influence of the flux by taking advantage of many years of experience.

A power transformer using a toroidal oriented core is adopted to reduce leakage to the outside.
A level in which no magnetic flux leakage is detected at 410 measurement has been achieved.

The tone control circuit and the amplifier circuit are completely independent on separate boards and operate only when the tone switch is pressed. This realizes both the completeness of a flat amplifier and a practical device.

Equipped with a subsonic filter.

Equipped with a phono input sensitivity selector switch, it can be adjusted to the output of the cartridge.
Impedance is 100k Ω constant, and direct connection of MC cartridge is also possible.

Equipped with contour volume as a loudness function.
Unlike general loudness, this method automatically changes the compensation amount of frequency balance as the volume is reduced.

High-precision detent volume is used for volume.
This volume has 22 contacts and 2 dB steps with a left / right error of 0.5 dB, so that the left / right balance does not become noticeable at low volume.

Model Rating

Type	Control amplifier
Input Sensitivity / Impedance	Phono : 1 mv, 2 mv, 5 mv Switching Type / 100k Ω Aux, Tuner, Tape : 100mV/50k Ω
Maximum input level	Phono : 250 mV (1 kHz at 5 mV switch) Aux, Tuner, Tape : 15 v (-30dB), 50 v (-40dB)
Output Level / Impedance / Load	Output : 1V/500 Ω / 10k Ω or more Rec out : 100mV/1k Ω / 50k Ω Headset : 40mW/4.5 Ω / 8 Ω
Maximum output level	Output : 5V/50k Ω Rec out : 4V/50k Ω Headphone : 300mW/8 ohm
Frequency characteristic	Phono (RIAA deviation) : 30 Hz to 15 kHz ± 0.3 dB (2 mV) Aux, Tuner, Tape : 20 Hz to 50 kHz + 0 to 1.5 dB
S/N ratio (IHF-A network) (reference level) / input conversion ratio	Phono : 80 dB or More / -140dB (at 1 mV sensitivity switch) Aux, Tuner, Tape : 102 dB or more / -122dB
Residual Noise Level (IFF-A)	Output : 4 μ V or less (Vol min) Headphone : 8 μ V or less
Distortion factor (Vol max, Outpu2V)	Phono : 0.003% or less (10 kHz or less) Aux, Tuner, Tape : 0.003% or less (10 kHz or less)
Tone control	Bass : ± 9 dB (20 Hz) Treble : ± 9 dB (20 kHz)
contour	contour Volume 8 3kHz:-30dB 20Hz:-14dB 20kHz:-25dB
Subsonic filter	30Hz:-1.8dB 20Hz:-8.5dB 10Hz:-45dB 5Hz:-38dB
Semiconductor used	Transistor : 63 Diode : 12 pcs Zener diode : 1 unit
AC outlet	Switched : 2 systems (350 VA)
Pwer	100 VAC, 50Hz/60Hz
Power consumption	Up to 20 VA
External dimensions	Width 400x Height 80x Depth 215 mm
Weight	Approx. 4 kg