Luxman SQ 503 (X) integrated amplifier

General:

Around 1970 Luxman company came with one of its first "high quality" integrated amplifiers on the European market.
This amplifier was a modern design with excelent characteristics.
It had a full complementary output stage and a differential input stage of the power amplifier.
It is symmetric supplied and the power supply stage was generously built for a 2x30W amp.

The SQ503 without the X is an earlier version that lacks the modern electronic design of the X version, but looks quite the same.

Luxman SQ 503X

After 40 years + the outside is looking very good.

 

Before the update the inside looks like this.

 

Inside the amplifier, top view, new transistors, drivers and output after the update.

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Bottom view.

 

Special futures:

Specifications in txt format click here.

 

Schematic:

Download the schematics in jpg of the SQ503X here.

Download the schematics in jpg of the SQ503 here.

 

Modifications:

This amp needs to be updated as the transistors and the capacitors are outdated.
Many capacitors can be replaced by recent types.
Set the power amp DC offset and the idling current of the power transistors.
The transistors of the pre stages are often noisy. Use BC550/560C or equivialent,
you can get the noise figure below the original specs of Luxman.
The drivers are often defective and the power transistors of the output stage may be obsolete.
The final power transistors are TO-66 cased NEC 2SA616 and NEC 2SD155, they are not available anymore.
The ledgendary 2SC1445 TO-66 transistors and PNP equivalent 2SA765, of Sanken
are of exceptional quality and are the perfect replacement, but hard to get today,
the only plastic replacement is plastic transistors like 2SB595/2SD525 of Toshiba or equivalent,
you have to change the mounting on the heatsink somewhat when you use plastic transistors,
and you may find in some cases that the sound quality will not be the same anymore.
I used BD139/BD140 and BD 292/BD437 of STMicroelectronics and 2SA1694 & 2SC4467
of Sanken as you can see on the photo: works great.

 

The pcb of the main amplifier.

 

Sound:

The sound of this amp is a pleasure to listen to, as the basic schematic diagram design is good. 

 

 

© Hans Hilberink - PE1MMK ® 2003 - Thanks to: Rob Schonkeren - Last update: 30-06-2014.