What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Pioneer SPEC-4

Pioneer SPEC-4
Stereo Power Amplifier in “DC” Configuration with Independent Left/Right Dual Power Supplies and Circuitry.

Think of the Pioneer SPEC-4 Power Amplifier as two independent mono amps in one chassis. Our stayahead reputation in hi-fi technology is upped another big step here by achieving real independence for the essential circuits in both the left and right channels. Physically separated signal paths and dual power transformers, four big capacitors (two per channel) to prevent power dry-up, separate left-right relays – everything to prevent crosstalk and distortion that erode stereo clarity and tonal quality. Because the SPEC-4 is of Pioneer's new generation of “DC” configuration power amps, its performance is never compromised by capacitors in the negative feedback loop. The results; a continous power output of 150 watts per channel, min. RMS, at 8 ohms from 20 to 20,000 Hz with no more than 0,1% total harmonic distortion.  
Why go for a separate component for power amplification? Pioneer's SPEC-4 clears up that question, and your music, with indisputable proof that separate power amplification is really worth it. Massive power reserve is available throughout the spectrum, including the super-low and ultra-high ends. Pulsive signals – i.e. musical signals – are reproduced with transparency and clarity at all levels, at all times. This high quality is achieved with Pioneer's  stay-ahead technology, as explained below.

the Pioneer SPEC-4 uses the latest ciruitry for power amplification, as follows. The input is formed by a dual PNP transistor differential amplifier , loaded by a current mirror circuit. This is followed by a constant-current-loaded predriver and a 3-stage Darlington connected driver. The final output is in the parallel SEPP (Single Ended Push Pull) configuration. Here are the advantages one by one:
(1)        Dual PNP Differential – Zero DC voltage is constantly maintained at the input.
(2)        Current-Mirror Loading – Reduces the load to the dufferential input to achieve a high gain which, in turn, permits the application of sufficient negative feedback for the effective reduction of distortion. DC stability is also improved.
(3)        3-Stage Driver – Against loads are reduced so that the transistorsin the circuit are called on to operate only at their most linear operating curve. Distortion is reduced and gain is high.

(4)         Parallel SEPP – This output configuration reduces by half the amount of current flowing through the output (power) transistors, again allowing the utilization of only the most linear parts of their operational curves, reducing distortion without sacrificing output.
Better 2-channel performance all around. That's the result of physically separating the left-channel and right-channel power supplies and signal paths. Paired power transformers are employed, one for each channel. The advantages show up in steady-state as well as  dynamic conditions. We've even separated the power relays for the two channels. No matter how complex the signal handled by one channel , the other is always ready to perform independently for more musicl clarity. Think of the SPEC-4 as two independent mono amplifiers housed in one stereo unit, both as perfect as we can make them.

Power dry-up? It takes an acute ear to detect it from most amps when tey are playing ordinary music. But it shows up easily during dynamic stareo passages in rock, classic and other music unless definite measures are taken to prevent it. The Pioneer SPEC-4 takes those measures. Because it operates on a “DC” basis, with no capacitors whatsoever in the negative feedback loop, and because it features a truly dependable power supply formed of two huge power transformers and four 22,000uF filter capacitors, power dry-up is entirely avoided. Your music is reproduced with vivid, full-power lows and natural transients for high resolution.

Impedance plays a vry important role in amplifier design and behavior. High impedance in the grounding and hot lines can result in significant losses of power and increases in distortion. The SPEC-4 avoids this in the following ways:
(1) Pure Copper Grounding -  Low impedance prevents the generation of distortion and safeguards power loss. We've achieved it with the use of pure copper plates, 20 mm wide by 2 mm thick, in the ground path of the dielectric capacitors.
(2)        14-Gauge Cables – Again, low-impedance performance is assured with the use of very thick (2,03 mm or 14 gauge) stranded cables in connecting the power output assembly to the output terminals and throughout significant large current circuits. Reproduction never suffers, even with large current flows are suddenly introduced. These “coat-hanger” cables also permit the dielectric capacitors to charge quickly and efficiently.

(3)        Copper-Foil Circuit Boards -  Conventional printed circuit boards use thin layers of copper foil. Ours use layers twice as thick – 70 u – to further reduce circuit impedance even when large current flows are required.
The logarthmically compressed scales of the twin power-output meters on the front panel of the Pioneer SPEC-4 make it easy to read peak power output in both channels accurately. No sensitivity changing is necessary when large or small output levels are chosen. The large, outer-magnet type meters indicate the full output peaks within 70 milliseconds, aided by an advanced detection circuit for instant-by-instant accuracy. Their direct-readout scales indicate from 0,01 watt to 300 watts.


Also on the froont panel are independent level controls for each channel's input. Optimum input level sensitivity is thus easy to achieve regardless of the output of the connected preamplifier. Each control is an attenuator with 22 stops, calibrated precisely in 1 dB increments from 0 to -20 dB. Gang error and indication error are minimum so that you may adjust inputs delicately and accurately.
The SPEC-4 circuit for speaker and power transistor protection is formed  of a quick-response power relay and a dependable electronic detection stage. Detection of DC components and overload currents is instant, triggering the relay and electrically disconnecting the amplifier from your speakers and its own output stage. The protection circuit also serves as a muting circuit which eliminates click noise otherwise heard when you switch the power on or off we've taken extra steps to protect stereo separation with the use of two independeny relays, one for each channel, with twin contacts in parallel. Surge current, introduced as you switch the power on, never reaches the large power transformer/capacitor block thanks to this extra protection.

Rear Panel Trminal Protection -  Rear terminals are protected from damages with a terminal protector.

Rack-Mounting Ready -  All dimensions/specifications suitable for use in standard EIA equipment racks.
Circuitry:  Current mirror loaded differential amplifier, 3-stage Darlington parallel push-pull, direct-coupled OCL, (DC amplifier construction)
Power Output:  Continuous power output of 150 watts per channel min, at 8 Ω from 20 Hz to 20,000 Hz with no more than 0.01% total harmonic distortion of 180 watts per channel at 4 Ω from 20 Hz to 20,000 Hz with no more than 0.03% total harmonic distortion.
Total Harmonic Distortion (20 to 20,000 Hz, 8 Ω) :
 No more than 0.01% (continuous rated power output)
No more than 0.01 % (75 watts per channel power output)
No more than 0.01 % (1 watt per channel power output)
Intermodulation Distortion (50 Hz : 7,000 Hz = 4:1, 8 Ω )
No more than 0.01 % (continuous rated power output)
No more than 0.005% (75 watts per channel power output)
No more than 0.005% (1 watt per channel power output)
Frequency Response:  5 to 100,000 Hz, +0 dB, -1 dB
Input Sensitivity/Impedance:  1 V/50 K Ω
Output Speaker:  4 to 16 Ω
Damping Factor:  100 (20 to 20,000Hz, 8 Ω )
Hum and Noise:  115 dB (IHF, short-circuited A network)
ICs:  2
Transistors:  61
Diodes:  62
Power Requirements:  110/120/220/240 Volt 50/60 Hz (switchable)
Power Consumption:  760 watts/950 VA
Dimensions (W x H x D):  19” x 17 ¾” x 7”

Weight :  54 lbs unpacked 

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