Today were showcasing a rare Kenwood Model 650 Integrated Amplifier. The Model 600/650 were made during 1973 to 1979 from Kenwoods limited special order only Supreme series. More often sold over-seas or select European/Asian markets these are incredibly rare Kenwood machines! The Supreme series comprises the Model 500/600 and 650 Integrated Amplifiers, a 600T/650 and 700T Tuners and a 700C/700M Power Amplifier. With less than 1000 produced of any given series the KA-907 is the latter 600 adaption.
Power Supply & Rectification/Protection X00-1700-10 / X00-1700-10
The 650 power supply is unique in its monolithic topology using two transformers and a regulated dual mono configure PCB. The stabilized X00-1720-10 assembly produced a dual -/+ 28 V and 40V rails. This PCB unlike any other is secured via a vertical slide mount thats secured to the chassis.
This PCB has a large amount of glue used to mount the large 220MFD capacitors that eat away at the Dz9 diode and associated resistors. These resistors were replaced with KOA carbon film 1/2W and a new modern 4148 diode. All the glue was watched off and cleaned.
All of the TO-220 regulating BJT’s were upgraded to heavy duty On-Semi MJE TO-220 devices along with the 415 to a modern TO-126 Fairchild device and TO-92 low noise devices. All electrolytic were replaced with high temp (105C) long life Panasonic FC and Nichicon PW capacitors with an increase in operating voltages. along with two VP Nichicon BiPolar capacitors.
The protection assembly is located next to the PS PCB and shared with the PS topology featuring a DC deviation detection and fused overcorrect protection. A MY4 Omron was installed onboard and new Vishay X1Y2 Safety capacitors.
Amplifiers PCBs X07-1440-00
The Model 600 650 utilizes an extremely unique and reliable front end. Comprised of a JFET 2Sk58 IC and a NPN BJT’s designed as a “Butler Front End’” which is a very low noise, high linear front end using JFET’s and BJT’s.
This front end is encompassed in a shielded and thermally isolated area! The front end differential BJT’s were replaced with modern extremely reliable ZTX TO-92 devices. The Bias, ASO and constant current BJT’s were all replaced with modern TO-92 and TO-126 Fairchild devices. New precisions Bourns potentiometers installed and all film were upgrade to high grade, audio type WIMA polypropylene film capacitors. Electrolytic’s were replaced with a Nichicon FG Fine Gold and low impedance PW with an increase in operating voltages.
**I did however go back to the original 2SB/SD driver pair compliment for reliability and discrepancies I noted during output tests.
AF Stages X11-1310-00 & X08-1560-10 & Filter X12-1130-10
The Model 600/650 boast an exceptional 44-stepped ALPS Potentiometer featuring
plastic resistive volume and co-axial balance control.
The entire AF stage is built of JFET compliments seen in many of the high-end topologies of the time. The RIAA EQ is a differential JFET stage with a Class A/Buffer JFET and output stage for an extremely linear and quite stage. All of the elelectrolytics were replaced with a low impedance Nichicon PW and audio grade FG Fine-Gold and KT capacitors with an increase in operating voltages. . New WIMA polypropylene films were upgraded as well.
The Tone control (X11-1310) is a flat amp (19.5dB Gain) and BAX type control using shunt regulated push-pull differential and constant current circuit, using a Class A FET output. As above Nichicon FG/KT audio capacitors were installed with an increase in operating voltages.
The Filter stage is a NFB filter with a constant current circuit with a LoPass @ -12dB/Oct 40Hz and -12dB/Oct above 8Khz. Capacitors upgrades here with WIMA polypropylene film upgrades and Nichicon Fine Gold FG audio grade capacitors with an increase in operative voltages.
Photo showing just how fun the Kenwood Model 650 can be.... (sic)
The dual mono supply utilizes two per side 18,000MFD capacitors which were replaced with modern quality Nippon/Chem-Con 18,000MFD 75V capacitors for a 72,000microfarad potential storage total