AudioSonic  ST-101 amplifier

 

General:

Around 1969 issued amplifier for European Germany market, made by 'Nikko Electronics Inc.' company Japan.

front ^

 

Owners manual:

not yet available

 

interior, very well built and very good pcb, photo made during update process ^
 

special futures:

This amplifier is with RCA input/output sockets. OTL - OCL power amp section.
Generously built amp. Wood case. Well build and good parts used.
High quality amplifier with outstanding specs. All ELNA capacitors employed, some in good shape after 45 years.
This amp employs germanium and silicon transistors.
This amplifier has driver transformers and has a symmetric +22/0/22- Volt power supply.
No output capacitors employed.
Nikko used this main pcb in some other Nikko amplifiers and receivers,
like model TRM-40 and STR-701, also slightly changed versions of this pcb.

 

Nikko receiver with almost the same power amp pcb ^
 

Specifications:

Distortion (THD 20Hz - 20kHz) < 0.1% at continuous output per channel: 15 Watts RMS per channel.
Speaker impedance: 8~16ohms
Signal/noise ratio: 85dB power amp. section
5 x RCA input. 2x RCA for Tape in and Tape monitor.

 

Legendary Hitachi power transistor ^

 

Schematic:

Below a similar schematic diagram of the Nikko SA-500 amp. There are only slight differences. click on the picture to download in good quality.

Below a schematic diagram of another similar amp. (Chicago) There are only slight differences. click on the picture to download in good quality.

PCB ^
 

Modifications:

This amp was designed for the use of Hitachi 2SB337 TO-3 germanium power transistors, with 40V Ucb spec.
The in this amp found Hitachi 2SB337 transistors were 100% within specs! Great quality, great sound.

Datasheet 2SB337: click here.

In the above amp the original ELNA power supply caps of 3300µF - 25V were within 5% of their specified value, this after ~ 50 years.
You may replace some diodes, capacitors and resistors in the power supply and power rail. Wiring can be improved too.
Power amp voltage should be +/- 22 Volts DC, but due to the detrimental power voltage change from 220 to 240V in the EU:
the power supply voltage is now +/- 24.5 Volt, making a total of 49V and that is 5 volt above max. specification of 44V.
There is no 240 V coil on the transformer. This means that EU users of this amp should be carefull with max. output power levels.
Because of overheating of the driver transistors due to the too high supply voltage, I exchanged the drivers for more power capable transistors.

After modification this amp is capable of producing 2x 20 Watts RMS.

 

Fully updated and upgraded amplifier, as you can see I have elevated the main pcb, for about 1 cm higher from the chassis ^

 

Pre-amp & main-amp section upgrade. The elevation of the pcb with copper spacers is to be noticed here. ^

The old and new driver transistors ^
 

Same model amp. interior after update, (internet photo) ^

 

Great rugged speaker output terminals employed, after 45 years fully functional and in top shape ^

 

^ Strange is that the text of the rumble switch and high-cut noise switch (rumpeln & rauschen) are in the wrong place:
they have been mixed up and exchanged. It is a factory error and most of this batch of this model ST-101 amps have this.

 

Nice vintage amplifier ^

The German import company "Multitechnik" in the town of Emmerich. ^

 

Sound:

The sound is very good for this vintage amplifier.

 

© Hans Hilberink - PE1MMK ® 2003 - Last update: 21-11-2018.