TRIO KA-900
¥ 79,800 (around 1980)
Commentary
Pre-main amplifier with Σ drive.
In the Σ drive system, the conventional concept of NFB, which was completed in the case of the amplifier, is extended to the input terminal of the speaker. The speaker cable is also considered as one of the internal wiring of the amplifier, and the performance at the input terminal of the speaker is guaranteed.
First of all, we have installed a unique Σ cable that connects the + side of the speaker and the feedback point of the power amplifier to control various types of distortion at the input terminal of the speaker, including reverse distortion caused by the magnetic circuit of the speaker.
In addition, by separating the ground point and the power loop on the - side cable, the potential difference between the input terminal of the speaker and the ground point is completely eliminated from the concept of one point grounding.
In the amplifier section, we further advanced high-speed technology and developed and mounted a new high-speed that extends the open-loop gain beyond the audible band. By making the internal impedance and feedback amount constant in the entire audible band, we suppress the occurrence of TIM and switching distortion.
In the past, in order to apply NFB stably, various phase correction circuits were used, and the cutoff frequency of the open loop was within the audible band. Therefore, the absolute amount of NFB was insufficient in the high frequency range. In the KA-900, all stages are A-class with a cascode, and the output is 3-stage Darlington connection. This improves the so-called bare characteristics before applying NFB. Moreover, the required amount of NFB is sufficient. In addition, the high-ft type multi-emitter structure is used for the final transistor, and ultra-high-frequency transistors are used for the voltage amplification stage, eliminating factors that require phase correction in the circuit.
As a result, we succeeded in extending the frequency characteristics at open loop time (upper body without NFB) from around 1 kHz to several 10 kHz. Since NFB is uniformly applied in the audible frequency band, high-frequency distortion such as switching distortion is greatly reduced.
Focusing on the damping factor around the "fo" (lowest resonance frequency) of the woofer, we measured the damping factor at the tip of the 10m cord of a normal amplifier. The damping factor at 55 Hz is about 10, but the KA-900 has an amazing value of 500.
The power amplifier section is a straight DC configuration from the Tuner (AUX) input to the SP terminal without any capacitor.
A newly developed IC is used for the equalizer amplifier section, and a gain switching method is adopted for input switching of MM/MC.
Unlike the head amp system, the circuit can be simplified and the sound quality is improved.
A heat pipe with high thermal conductivity is used for the radiator to minimize the electromagnetic wave jump loop generated between the final transistors and to suppress the adverse effect on the signal system.
Equipped with a recout selector, the signal to the recout can be freely selected regardless of the input selector.
By using a non-magnetic structure that minimizes magnetic materials near the signal system, such as resin materials for the bottom plate and side plate, we have improved the magnetic distortion that is noticeable to the listener.
The faders allow you to preset the volume. If you set the volume, you can fade in and fade out simply by touching the faders.
.JPG)
.JPG)
Model Rating
| Type | Σ drive amplifier | ||||
| Rated Output (20 Hz to 20 kHz, Both Channel Operation) | 80W + 80W (8 Ω, Phono → sp out) | ||||
| Total Harmonic Distortion Factor (20 Hz to 20 kHz, 8 Ω) |
|
||||
| Intermodulation distortion factor (60 hz : 7 khz = 4 : 1) | Tuner, AUX, Tape → sp out : 0.005% (8 Ω at rated output) | ||||
| Frequency characteristic | DC ~ 400 khz -3dB (Tuner, AUX, Tape → sp out) | ||||
| Damping factor (100 Hz) | Amplifier unit : 500 Σ Cable termination : 500 |
||||
| Input Sensitivity / Impedance | Phono MM → sp out : 2.5mV/47k Ω Phono MC → sp out : 0.2mV/100 Ω Tuner, AUX, Tape → sp out : 150mV/47k Ω |
||||
| Signal-to-noise ratio (IHF-A) | Phono MM → sp out : 86 dB Phono MC → sp out : 66 dB Tuner, AUX, Tape → sp out : 105 dB |
||||
| Tone control |
|
||||
| Loudness control (Volume -30dB) | +10dB(100Hz) | ||||
| Subsonic filter (-3dB) | 18 Hz, 6dB/oct. | ||||
| Rise time | 0.9 μ s (Tuner, AUX, Tape → sp out) | ||||
| Slew rate | ± 120 V / μ s (Tuner, AUX, Tape → sp out) | ||||
| Phono maximum allowable input (Phono → Tape rec, 1 kHz, distortion rate 0.003%) |
MM:270mV MC:15mV |
||||
| PhonoRIAA deviation | 20 Hz to 20 kHz ± 0.2 dB | ||||
| Output Level / Impedance | Tape rec (PIN) : 150mV/330 Ω | ||||
| Power supply voltage | 100 VAC, 50Hz/60Hz | ||||
| Rated power consumption (Electrical Appliance and Material Control Law) | 220W | ||||
| Power outlet | Power switch interlock : 2 systems Power switch not linked : 1 system |
||||
| External dimensions | Width 440x Height 123x Depth 375 mm | ||||
| Weight | 10kg |