LUXMAN   Vintage Audio   website®

model SQ1220 solid state integrated amplifier

 

General:

Generously build amplifier with good performance, made by Luxman in 1968/1970.

The SQ1220 amplifier is an early transistorized stereo amplifier for home and professional use.
It has many features Luxman made the difference with and this model has generous power capability.

 

 

 

Special futures:

L&R separate tone knobs with switchable crossover frequencys, like many Luxmans have.
Power- and pre-amp can be used separately.
Early 2x 50 Watts RMS stereo HiFi amplifier.

 

User Manual:

 

 

Schematic:

Click here to download the service manual of the Luxman SQ1220 in PDF format

 

Top view ^

 

Bottom view ^

 

Early silicon Toshiba power transistors ^

 

Power pcb's ^

 

Modifications:

Be aware of the fact that this amplifier was initially designed for the use of 16 Ohms loudspeakers,
it works well with 8 Ohms speakers, but an impedance of 4 Ohms will break the power transistors.

This amp had all electrolytic capacitors far out of specs, so all need to be replaced.
Since this amp is quite
peculiarly constructed and some parts of the amp are hardly accessable, like the phono stages,
and the amp is not built very service friendly, the replacement of all caps is a lot of work and is very time consuming.
There are some parts, like the pots for idling current, that may be not usable anymore and need to be exchanged.

If you have a multi-voltage unit, set it to 240 Volt AC mains voltage if you reside in Europe,
due to the detrimental voltage rise in EU.

The schematic diagram does not comply with the actual circuit in this amplifier,
also there are some parts that are obsolete and must be removed or changed in better values.
The power supply needs to be changed and updated.
Replace the 2 power diodes, the original diodes are 1.5A RMS current (!), I used 10A 200V general purpose diodes.
The power amplifiers have some serious flaws in the design.
In fact some parts of the schematic-diagram have to be redrawn to get it right, see the picture below.
The damping factor switch is obsolete, as we want maximum damping factor and all
the other settings of this switch are useless and have a very negative impact to the sound.
The feedback of the power amplifiers has to be changed.
The zobel and compensation circuits of the power amp have to be changed.
The 3D circuit and the connection of pre & main amp has to be updated or removed.
Change the low-cut switch setting into a usable subsonic switch.
The crowbar circuit has a negative impact on the sound and
can better be exchanged by a modern protection circuit.
The wiring of the amplifier can be sanitized: some wiring is not needed and some wiring can be improved.
Replace the pilot lamp by a led and install a led circuit with rectifier, resistor and small capacitor.

 

The output stage redrawn ^ All obsolete parts left out, the power supply must be upgraded to use this design.

 

All caps way out of specs ^

 

Some 50% value left ^

 

The 3 old out of specs power capacitors removed ^

 

All three together just over 800 µF capacity left. ^

 

New capacitors, new diodes, new wiring  ^

 

Replacement of the 3 caps by one high quality 10000 µF 100V LL cap. ^

 

New caps and some new wiring ^

 

 

Old vs new ^

 

The 4 output capacitors replaced by 2 modern caps, make ELNA.

 

Restore of the phono RIAA section ^

 

Restored and upgraded power amplifier pcb's ^

 

Pilot lamp replaced by a LED ^

 

 

Sound:

The sound is very typical for an amplifier of this kind, powerfull and warm.
Due to the +80 Volt single power supply the amplifier has lots of power, but it lacks
a little the finesse of a more modern amplifier of Luxman, like the SQ505x/SQ507x.

 

 

 

 

© PE1MMK Hans Hilberink last update:  25-09-2020.