PHILIPS LHH A700

Commentary

This is a stereo power amplifier that has been completely simplified.

In the LHH A700, the voltage amplification stage is composed of a wide-band, small-capacity bipolar transistor, and the open-loop frequency characteristic, which is the bare characteristic before applying NFB, is improved to 500 kHz (-3dB). By this, NFB is suppressed to less than 20 dB, and dynamic characteristics are greatly improved.

MOS FET is used in single push-pull for both power stage and driver stage of the current amplifier stage.
MOS FET has characteristics such as high input impedance, high switching speed without carrier accumulation effect, and high thermal stability. However, it has characteristics such as large equivalent input capacitance, and it is important to handle the input capacitance in order to demonstrate the characteristics of a high-speed switching element. The LHHH A700 achieves an overall high-speed amplifier with a slew rate of 200 V / μ s by passing a drive current that is one order of magnitude larger than normal and charging / discharging all at once with a low impedance drive.

Conventionally, the NFB is returned from the output stage to the input, but it is limited to the voltage amplification stage with gain, so that the effect of the back electromotive force from the speaker loop does not reach the first stage.
For this reason, the driver stage and power stage have no distortion improvement effect due to NFB, and the static specifications are not improved as compared with conventional amplifiers. However, the operation of the amplifier is released from the effect of the back electromotive force of the speaker, which is a load, and it shows the overwhelming stability against not only low load but also capacitance load. Moreover, the dynamic characteristics are further improved by eliminating the output coil.

The power supply section is equipped with a 500 VA high-capacity EI core transformer with excellent instantaneous power. In addition, by using plus or minus separate windings and separate rectification, it is possible to supply high-efficiency power that is resistant to fluctuations. In addition, a large number of custom electrolytic capacitors using low-magnification foils are connected in parallel to achieve low impedance and high speed.
A 2 mm thick OFC bus bar is used for the power supply line and power supply ground to minimize a large current loop and to obtain excellent dynamic regulation and clear ground potential.

A gold-plated glass-epoxy double-sided copper foil board is used for the printed circuit board, and by taking a wide ground plane, a high shielding effect and excellent high-frequency characteristics are obtained.

The chassis uses 10 mm thick extruded aluminum material for the front, site, and rear. It is firmly fixed to the bottom chassis of 3 mm thick copper plated steel plate, and is connected and reinforced with each other.
The heat sink uses a chimney type heat sink made of extruded aluminum material with a circumference of 10 mm and fins of 8 mm thick. This heat sink is fixed firmly to a 4 mm thick aluminum top panel and bottom chassis to form a monocoque structure. Combined with a 3-point support system of sintered alloy heavy legs (point contact and contact point can be selected), it realizes excellent vibration resistance characteristics.
Extruded addition, extruded materials have higher aluminum purity than die-cast materials and have the advantage of reducing thermal resistance, realizing highly efficient heat radiation and uniform temperature distribution.

The volume circuit employs a current-driven low-impedance volume in which the signal current does not pass through the carbon resistor or slider contact.

It uses a custom RCA pin jack of brass carving using Teflon insulator with excellent high-frequency characteristics.

The speaker terminal uses a banana plug terminal made by WBT of Germany.

Uses 123g solid aluminum volume knob for volume knob.

Uses high-precision parts such as ± 1% metal film resistance and high pressure acoustic condenser.

Model Rating

Type	Power buffer amplifier
Rated output	80W + 80W (4 Ω, 20 Hz to 20 kHz) 40W + 40W (8 Ω, 20 Hz to 20 kHz)
Input Sensitivity / Impedance	500mV/10k Ω
Frequency response (at 1W output)	5 Hz ~ 350 kHz + 0 -1dB
Output bandwidth	7 Hz ~ 110 kHz (20W/8 Ω, when both channels are driven)
Total harmonic distortion factor	0.2% (20 Hz to 20 kHz at 8 Ω rated output)
Signal-to-noise ratio	105 dB (A-curve correction)
Slew rate	200 V / μ sec or more
Power consumption	175W
External dimensions	Width 330x Height 134x Depth 392 mm
Weight	18kg