Nakamichi 620

Commentary

A 600 series stereo power amplifier.

A newly developed complete mirrored push-pull circuit is used for the circuit configuration.
The first stage of this circuit is a current mirror load differential, but the output stage has been modified. The first two stage transistors and the second two stage transistors are combined with transistors with opposite characteristics. The plus and minus sides are also symmetrical. It is named Complete Mirror because it has a structure that looks like a mirror.
It is generally believed that the characteristics of NPN and PNP transistors are symmetrical. However, this is not the case. N-type and P-type transistors have slightly different characteristics because they are produced by different manufacturing methods depending on the addition of a small amount of impurity element. This causes crossover distortion and switching distortion when the amplified positive and negative waves are connected. To solve this problem, the CompleteMirror configuration has been developed as a method to cancel the difference in characteristics by skillfully combining NPN and PNP in the circuit.
This circuit configuration realizes a low distortion factor of 5 ppm with an output power of 100W + 100W.

The complete mirrored push-pull circuit reduces the idle current by using the transistor characteristics without waste or effort.
As a result, the temperature rise is small and the characteristics hardly change with time.

Two 39,000 μ F capacitors and a toroidal transformer are used for the power supply.
A tinning copper plate 2 mm thick and 1 cm wide is used to connect the power transformer and the capacitor. This makes the internal resistance of the wiring close to 0, eliminates the potential difference between them, and prevents noise due to ripple.

A peak power indicator is embedded in the heat sink.
It has two lamps, green and red. Green can be switched to 1W/5W/25W, and red can be switched to 25W/50W/MAX (110 to 130W).
Lights up for 3 ms with 0.1 ms input.

A protective circuit is provided at the output stage.

There was a bridge adapter BA-100 as an option sold separately.
Bridge connection is possible by connecting the BA-100 between the preamplifier and the two 620 s.

There was a system rack for the 600 series as an option sold separately.
This rack mounts the 600, 610, and 620 600 series and is equipped with digital timers.

Model Rating

Type	Stereo power amplifier
Output	50W/50W (16 Ω, double-channel drive, 5 Hz to 20 kHz, THD0.01% or less) 100W/100W (8 Ω, both channel drive, 5 Hz to 20 kHz, THD0.01% or less) 350W (8 Ω, BTL connection, when using BA-100)
Power bandwidth (IHF, both channel drive)	5 Hz ~ 50 kHz (Distortion Rate 0.1% or Less) 5 Hz ~ 20 kHz (Distortion Rate 0.01% or Less) 5 Hz ~ 10 kHz (Distortion Factor 0.005% or Less)
Damping factor	100 or More (1 kHz, 8 Ω Load)
Total harmonic distortion factor	0.002% or Less (1 kHz or Less, RMS) 0.005% or Less (10 kHz or Less, RMS)
IM distortion factor	0.002% or Less (60 Hz : 7 kHz = 4 : 1, 100W Output, 8 Ω Load)
Frequency characteristic	5 Hz to 100 kHz + 0 -1dB (RMS)
Input impedance	10k Ω
Input sensitivity	1V
Residual noise	0.05 mV or less (IHF-A Network) 0.1 mv or Less (Linear)
Signal-to-noise ratio	120 dB or more (IHF-A network, input short)
Crosstalk	-70dB or more (1 kHz)
Accessory circuit	Peak power indicator Green : 1W, 5W, 25W, selectable Red : 25W, 50W, max (110 to 130W), selectable
Semiconductor used	IC : 3 Transistor : 46 Diode : 41
Pwer	AC100V/117V/220V/240V, 50Hz/60Hz
Power consumption	50 VA (No Signal) 700 VA (both channel drive, maximum output, 8 Ω load)
External dimensions	Width 400x Height 189x Depth 248 mm
Weight	12.5kg
Sold Separately	Bridge Adapter BA-100 (¥ 13,000) 600 Series Rack System-One (¥ 25,000) Timer (¥ 23,500)