Today we’re highlighting a Pioneer SPEC-4 Power Amplifier, we’ll be following up this post with the SPEC-1 Pre-Amplifier as paired. The SPEC-4 was manufactured in the late 70’s into 1980 as one of their higher end amplifiers in the DC series.
The SPEC-4 is an excellent design featuring separated dual power transformers/filter stages/ soft-start and protection relays as well as driver and input paths! just fit into one chassis. Im a big fan of the 22-stepped 0/-20dB attenuator as well.
Power Supply Stage and Rectification & Meter Assembly AWM-113
The Spec utilizes two independent 82B+/- rail power supplies with each its own transformer.
The Driver supplies are each a bridge rectification point featuring 44,000 microfarads of filtering.
The Meter assembly is supplied by a 90V bridge rectification circuit.
Each of the 22,000/80V capacitors were replaced with high grade 22,000 mfd 100V Nippon Chem-Con capacitors. The safety capacitors were replaced with .01 X1Y2 1Kv rated and new .01 ECW PP bypass film capacitors.
The Meter assembly is best accessed by removing the front chassis plate. The electrolytic on here were replaced with high temp (105C) long life Panasonic FR/FC capacitors and high temp (105C) low impedance Nichicon PW’s. Remaining 2473 type diodes with modern 4148 type as well. The regulating BJT’s 2SB536//SD381 are prone to developing faults on the SPEC series, these were replaced with TO-220 MJE15028/29 devices and affixed with thermal compound. Small signal devices were updated with 2690/945 TO-92’s and TO-126 devices.
The driver assemblies are separated symmetric topologies. Initial gain is applied at the dual PNP deferential pair transistors with a constant current mirror network providing sufficient low distortion gain to the class-A pre-driver stage. a 3 stage darlington array to a parallel SEPP output stage. The protection circuit of the SPEC-4 consist of a soft-start type On-Off muting, overload and dc deviation. The PCB relays were replaced and soft start burnished with die tools.
The differential pair package was replaced with a separated pair of 992’ type TO-92’ devices matched to 1% tolerance to ensure stabile dc offset. 2SC1775’s were replaced in pairs with 1845 TO-92’s and the pre-drivers to 3503/1381 TO-126 devices along with potentiometers and cleaned. All of the electrolytic were replaced with high temp (105C) long life Nichicon PW’s and the .22 electrolytic with a ECW polypropylene .22MFD film capacitor. All remaining .033/.01/.1 films were replaced with high grade ECW Panasonic capacitors, the input .1 film was replaced with a high grade MKP polypropylene film capacitor.
New Meter LED’s