Nakamichi CA-70

Commentary

A control amplifier designed to simplify the amplifier circuit without compromising sound quality.

To achieve pure signal transmission with a simple circuit configuration without relying on a large amount of NFB, each section uses a low-noise FET at the input stage and the final stage consists of a simple emitter follower with a current bootstrap with a large current supply capability.
In addition, high-precision discrete parts are used except for the DC servo, and all stages are DC amplifiers to achieve both high linearity and simplicity.

The MC head amplifier uses a parallel push-pull input configuration based on ultra-Hi-DC gmFET instead of conventional differential input, which is difficult to achieve low noise. In addition, the MC head amplifier has an excellent S/N ratio by minimizing the signal resistance.
In addition, a five parallel push-pull input configuration per one channel, an output stage with an emitter-follower with a current bootstrap is adopted, and a transformer winding dedicated to the MC head amplifier section is installed to eliminate interference from other circuits.

The equalizer amplifier and line amplifier section take advantage of the linearity of the differential amplifier and adopt a two stage differential amplifier configuration in which the first stage is differential input by dual FET and the second stage is differential amplified by bipolar transistor to enhance the bare characteristics.
In addition, the emitter follower with current bootstrap at the last stage supplies sufficient current even when the load impedance fluctuates. Therefore, high linearity is maintained even when high-frequency impedance drops due to the influence of the connection cable to the RIAA element or power amplifier, and high-precision signal transmission is possible.

The last stage of each amplifier section employs an emitter follower with a current bootstrap, which is simpler than the conventional cascade connection but provides a high current. This circuit provides a high current while maintaining full class A operation.

A multi-regulated power supply system and an integrated ground system are used in the power supply section to eliminate mutual interference between circuits.

The multi-regulated power supply has a unique structure with a two stage regulator.
First, the power transformer windings and rectifiers are separated into two types, one for the equalizer amplifier / line amplifier and the other for the MC head amplifier. The power supply is first regulated by the master power regulator and then regulated again by the local power regulator installed for each L/R channel of the MC head amplifier, equalizer amplifier and line amplifier. Then, the power supply is supplied to each amplifier section.
This structure prevents voltage fluctuations caused by the operation of one amplifier section from adversely affecting other amplifier sections.

In the opposed ground method, a buffer with an FET is incorporated in the power supply line. The current used to flow from + → ground → - in a general amplifier is directly passed from + to -. Thus, the ground only functions to provide an accurate reference potential to the amplifier, and any mutual interference around the ground is eliminated.

It is equipped with AFT (Acoustic Fine Tuning), a newly developed sound field correction system that suppresses quality degradation.
The AFT can be adjusted within ± 5 dB in 0.5 dB steps at three points : Low (25 Hz), Mid (250 Hz), and High (30 kHz), so that the response is close to a flat response.
In order to minimize the effect on sound quality, AFT does not use general volume but consists of a highly reliable 21-step rotary switch, metal film resistor, film capacitor, low-noise FET, and transistor discrete configuration.

To switch the program source, a crossbar twin-type relay with the input terminal installed at the shortest distance is adopted, making the signal flow simple and straight.
In addition, the internal construction is L/R twin mono structure, and excellent separation characteristics are secured by eliminating the interference of the left and right channels.

Thorough measures are taken to separate the logic and amplifier systems.
First, in order to prevent the microprocessor clock noise from contaminating the music signal, ten types of photocouplers are inserted between the relay part and the logic circuit to electrically isolate the logic circuit and relay.
There are also 30 types of photocouplers between the system remote control terminal and the microprocessor to prevent interference caused by electrical connection of the ground line due to pin cords of other components connected to the CA-70 or remote control cables.
In addition, the logic parts and power supply parts are completely sealed in an aluminum shielded room, and aluminum panels are used extensively to ensure complete shielding.

The volume resistor uses a high precision detent type to eliminate channel interference within the volume unit.
In addition, to enable volume adjustment by wireless remote control, we have developed a method unique to Nakamichi to solve the disadvantages of the general motor drive method. In this method, a sufficient distance is provided between the motor and the volume resistor, and the shaft of the knob is separated from the shaft of the resistor. The shafts are connected with a special flexible joint to shut out the influence of motor noise and vibration and to absorb external vibration.
The balance volume consists of 10 discrete metal film resistors for each channel, and this resistor network is switched by a rotary switch of 21 clicks and 0.5 dB steps.

As for the parts, we use carefully selected ones that are easy to hear and have no unbalanced habit as much as possible.
We use ultra-Hi-gm FET for the MC head amplifier and FET with the same characteristics for the equalizer amplifier / line amplifier. For passive elements, we use metal film resistor, brass gold plated cap / carbon mold resistor, high-quality film compound wound capacitor, etc.
The power supply consists of an amplifier system and a logic system, each of which is equipped with a dedicated transformer. A toroidal transformer with less leakage flux is adopted for each of them.

The cartridge selector on the rear panel allows you to select MM/MC, load capacity in the MM position (100pF/200pF/300pF), and gain in the MC position (36dB/30dB/24dB). In addition, the load resistance in the MC position is fixed to 100 Ω at the time of shipment from the factory, but can be adjusted to 50 Ω / 30 Ω / 20 Ω / 10 Ω / 3 Ω by inserting the supplied high-precision carbon resistance.

Audio mute is installed.

Equipped with two types of tape recording / playback terminals capable of mutual dubbing.

It has a gold-plated input / output terminal with low aging and high conductivity.

A very thick power cord using oxygen-free copper wire with polarity indication is used.

Wireless remote control is included.
In addition, it can be controlled from another room by using the optional remote sensor RS-7.

Model Rating

Type	Control amplifier
Input Sensitivity / Impedance	Phono MC : 40 μ V/100 Ω (gain 36 dB) 80 μ V/100 Ω (gain 30 dB) 160 μ V/100 Ω (gain 24 dB) Phono MM : 2.5mV/50k Ω Tuner/CD/Aux/Tape : 150mV/12.5k Ω
Input sensitivity (new IHF)	Phono MC : 10 μ V (gain 36 dB) 20 μ V (gain 30 dB) 40 μ V (gain 24 dB) Phono MM:0.625mV Tuner/CD/Aux/Tape:37.5mV
Maximum input level (1 kHz, non-clipping level, new IHF)	Phono MC : 5 mv (gain 36 db) 10 mv (gain 30 db) 20 mv (gain 24 db) Phono MM:320mV
Output Level / Impedance	Pre-out : 2V/1k Ω Rec out : 150mV/200 Ω
Maximum power level (new IHF)	7 v (20 Hz to 20 kHz, 1% THD, Pre-out)
Total harmonic distortion factor (20 Hz to 20 kHz)	Phono MC → Rec out, 1 v (gain 36 dB) : (gain 24 dB) : Phono MM → Rec out, 1 v : Tuner/CD/Aux/Tape → Pre out, 2 v : Not more than 0.004% Not more than 0.002% Not more than 0.002% Not more than 0.002%
Frequency characteristic (Tuner/CD/Aux/Tape)	1 Hz ~ 100 kHz + 0 -3dB 1 Hz ~ 20 kHz + 0 -0.2 dB
RIAA deviation	Phono MM/MC : 20 Hz to 20 kHz ± 0.2 dB
S/N ratio (IHF A-WTD, 0.5 v pre-out, input short)	Phono MC (0.5 mV input) : Phono MM (5 mV input) : Tuner/CD/Aux/Tape (0.5 v input) : 87 dB or more (gain 36 dB) 86 dB or More (Gain 30 dB) 84 dB or More (Gain 24 dB) 88 dB or more 94 dB or more
Stereo separation (1 v Pre-out, input short)	100Hz 1kHz 10kHz Phono MC (0.5 mv input, 36 dB gain) : (0.5 mV input, gain 24 dB) : Phono MM (5 mV input) : Tuner/CD/Aux/Tape (0.5 v input) : 95dB 90dB 100dB 120dB 100dB 100dB 105dB 110dB 90dB 90dB 95dB 100dB
AFT (Acoustic Fine Tuning) Characteristics	Low : ± 5 dB (25 Hz), 0.5 dB step Mid : ± 5 dB (250 Hz), 0.5 dB step High : ± 5 dB (30 kHz), 0.5 dB step
Audio Mute	-20dB
Remote power on/off control output	24V DC
Pwer	100 VAC, 50Hz/60Hz
Power consumption	42W max.
External dimensions	Width 435x Height 82x Depth 310 mm
Weight	Approx. 7.2 kg
Attachment	Wireless Remote Control RM-7CA
Sold Separately	Remote Sensor RS-7 (¥ 27,000) Wireless Remote Control RM-7CA (¥ 8,000) System Power Control SPC-1J (¥ 25,000)