Preamplifier Section
Equalizer Stage
One problem that is
not taken care of by many preamplifiers is transient response because their
measurements depend on the input being a sine wave. Unfortunately, this simple
wave is not the same as the comlex signal produced by live music. The real dynamism
of live music comes from its many abrupt and impulsive sounds. It is this
dynamic sound that can cause distortion due to poor transient response in all
circuits if nothing is done to prevent it in the design of the equipment. In
the equalizer stage in particular this problem of transient response must be
handles because this is the input stage for the music signal. If the signal is
distorted in this first stage it gets worse and worse until the distortion
destroys all listening pleasure.
Precise equalization
is one of the very important features of a preamplifier. To avoid any
distortion that might come from saturation in this equalizer stage model 725
has a big phono overload capacity of 200 mV at 1 kHz and 930 mV at 10 kHz. The
phono overload capacity gets larger with the rise in frequency so that it is
the same as that of the RIAA
equalization curve. RIAA deflection is ±0,5 dB from 30 to 15,000 Hz. The
signal to noise ratio is better than 70 dB.
Tone Control
The Tone Control is
put in model 725 so that each individual can adjust the tone to fit his own
taste in music. The Tone Control has click stops and each stop represents 2 dB
up or down. High grade components have been selected for the Tone Control so there
is always exact response, low distortion and high stability.
Transient Killer
Model 725 has been
equipped with the Transient Killer to prevent the usual disturbing noise that
you hear when an ordinary amplifier is switched on or off. The Transient Killer
in this model guarantees that there will be no irritating sounds to disturb your
music enjoyment every time you turn the set on.
Power Amplifier
Section
Model 725 is the
fruit of Onkyo's tireless efforts to reduce distortion of any kind in the
amplifier section of its equipment. Distortion is reduced in model 725 because
of the quasi complementary SEPP, direct coupled and differential amplifier
circuitry. The computer was used to analysis the very complex music signal and
this analysis extended outside the audible range. This extensive measurement
was necessary because Onkyo found that the very low frequencies have a distinct
influence on the music even though they cannot be heard by the human ear.
Ordinary amplifiers do not take these very low frequencies into consideration
and consequently cause a lot of Intermodulation Distortion.
The statistic for
this amplifier are very excellent. It is capable of delivering a continuous
power output of 22 watts per channel at 8 ohms. The Total Harmonic Distortion
of model 725 is rated at less than 0,05% at 10 watts, 1 kHz.
whether used at full power or very low output
the same low distortion rate is maintained. Stability is enhanced by the use of
a differential amplifier, instability in an amplifier will put DC voltage on
the loudspeaker terminals which will causes excessive distortion.
Fluctuations in the
power supply voltage and temperature can also cause distortion but these
fluctuations are solved by the differential amplifier. The OCL circuitry is an
added reason why this model has less distortion, a flatter response over the
whole frequency range and especially good damping in the lower frequency range.
Top Panel Connectors
and Large Heat Sink
Model 725 has all
its connectors centralized on the top panel so that connecting and
disconnecting is simplified. This is especially true if the amplifier is placed
against a wall. Instead of having connectors in the rear panel this model uses
the rear panel as a large Heat Sink which helps to protect the power
transistors as well as the rest of the amplifier. This is important because of
the change in the capacity of the transistor. However, because of the large
Heat Sink in model 725 it always stays cool no matter how long it is used at
full power.
Multi step-up
terminals
The design of model
725 is unique in that its preamplifier and main amplifier can be used
separately. This design opens up new possibilities for the music lover because
model 725 can easily be the base of a multi-amplifier stereo system if either
model 624 or 613 is added. These two model are dividing amplifiers and they do
the job of separating and amplifying the signals before they are passed on to
the tweeter, mid-range speaker, and woofer. If only one extra unit is added
your system is up-graded to a bi-amplifier system. If two units are used your
system will become a tri-amplifier system. The top panel of model 725 makes
connecting and disconnecting very easy because all the terminals are located on
the top of the unit and not in the back
as is the case with most units.
Specifications
Type: All Silicon Transistor Integrated Stereo
Pre-Main Amplifier
Circuit: Direct-Coupling and Differential Amplifier,
Quasi-Complementary SEPP System, Transistor 29, Diode 18, Therminator 2
Main Amplifier (8 Ω load in case of no indication)
Dynamic Power:
70 W (IHF) zt 8 Ω
100 W (IHF) at 4 Ω
RMS Continuous Power
22 W + 22 W (8 Ω , 1 kHz) Single Channel Driven
27 W + 27 W (4 Ω , 1
kHz) Single Channel Driven
Total Harmonic
Distortion
At Rated Output (1
kHz) : less than 0,1%
At 0,5 W Output (1
kHz) : less than 0,05%
At 10 W Output (1
kHz) : less than 0,05%
Intermodulation
Distortion: less than 0,05% at 10 W
output (SMPTE 70 Hz : 7 kHz = 4 : 1)
Frequency
Characteristic: 15 Hz - 70 kHz ( +0 -1
dB at normal output)
Power Bandwidth: 10
Hz - 100 kHz (IHF -3 dB T.H.D. 0,5%)
Signal-to-Noise
Ratio : better than 100 dB (IHF-A
network)
Damping Factor: 60 (DC
- 30 kHz)
Residual Noise: 0,001 µW (IHF input shunt)
Load Impedance: 4 Ω
- 16 Ω
Input Impedance: 100 k Ω
(10 Hz - 50 kHz)
Gain: 22 dB
Rated Input
Voltage: IV
Input ?Output
Characteristics: In-phase
Pre-Amplifier
Input
Sensitivity/Impedance
Phono 1 : 2,4 mV / 50 k Ω
and 100 k Ω
Phono 2 : 2,4 V / 50 k Ω
(1,03 kHz)
Tuner, Aux-1, Aux-2 and Play :
100 mV / 100 k Ω
Total Harmonic
Distortion : less than 0,05% at 1 V
output (1 kHz)
Intermodulation
Distortion : less than 0,05% at 1 V
output (1 kHz) (SMPTE 70 Hz : 7 kHz = 4 : 1)
Frequency Response:
Phono 1, Phono 2
: 50 - 15,000 Hz (±0,5 dB RIAA
Characteristics
Tuner, Aux-1, Aux-2,
Play : 10 - 60,000 Hz (+0 -1 dB at 1 V
output)
Max. Allowable Input
(Phono): 200 mV (at 1 kHz); 930 mV (at
10 kHz)
Signal-to-Noise
Ratio (IHF A Network Input Shunt)
Phono 1, Phono 2
: 75 dB
Tuner, Aux-1, Aux-2,
Play : 90 dB
Output Voltage?
Impedance
Pre-Out 1, Pre Out 2 (100 k Ω at load) :
IV (rated output);. 4 V (max output) 2,5 k Ω
Rec Out: 100 mV (rated output
open terminal voltage) 30 k Ω (phono)
Tone Control (2
dB/step switch type)
Treble : ±10 dB at 10 kHz
Bass : ±10 dB at 100 Hz
Filter:
High : 6 dB/oct
at 8 khz
Low : 6 dB/oct at 70 Hz
Muting: -20 dB
Loudness: 6 dB at 100 Hz (VR -30 dB position)
Input/Output
Characteristics: in-phase
Transient Killer
Operation Time: 3 sec when SW is turned
On
0,5 sec. When SW is
turned Off
Power
Requirements: AC 110/120/220/240 V,
50/60 Hz
Power
Consumption: 30 W
AC Output: Switched, Unswitched
Dimensions (W x H x
D): 313 x 366x 136 mm (12,%" x
15,6" x 5,4")
Weight: 8 kg (17,6
lbs)