LUXMAN vintage audio
Luxman SQ 507X / SQ 505X integrated amplifiers
Around 1970 Luxman company came with its first "high quality" integrated amplifier 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 was fully symmetric supplied and the power supply stage was generously built.
The amplifier was very well built and specially the power amplifier stages were an example of compact and slick electronic design and good amplifier building practices.
The 505X is a lower powered version of the 507X.
The most remarkable thing about this amplifier is that the driver- and power- transistors were also directly soldered to the poweramp-pcb: so no extra wiring was needed. That was quite remarkable in these days. The specifications were very good as you can see at the picture below. It was one of the first Japanese transistor amplifiers starting a new audio era. It incorporates the first generation transistors designed in Japan specially for audio purposes. Later Luxmans were built with transistors specially designed for audio. This Luxman SQ 507 amp was built to be a fast and powerful audio machine with good speaker control. It had good tone-control possibilities on front.
The schematic diagram shows a 3 stage amplifier: RIAA amp, control amp, output power amp.
The RIAA amp is a DC-coupled 3 transistor amp with low noise PNP silicon transistors, with RIAA correction network in the NFB, it is of quite conventional design. It is capable of taking input signals of up to x100 above the average phono-cartridge levels without causing overload problems.
The tone control unit consists of 2 stages, plus buffer stage and allows extensive tone control, with the control taking action on selectable frequencies. The tone control can be switched off by selecting the "defeat" position.
The Power amp is a DC coupled class AB amp with differential input stage and a full complementary OTL / OCL output stage, a compact and modern design. NFB is moderate. It is fully symmetric supplied.
The power supply stage is generously built, with a stabilized section for the preamps.
Distortion and noise levels of this amp are low.
The transistors used were modern high speed transistors specially made by NEC and Toshiba in Japan for audio applications.
Reliability of this design appeared to be very high. There is no serious speaker protection employed in this design.
Download the usermanual + schematics in PDF of the SQ505x here.
The sound of this amp is good and stable. A pleasure to listen to.
This amp is greatly DC-coupled, unfortunately the coupling capacitors are on some places not of high quality (tantallum) and started to give problems in the control amp section after several years, due to DC-leakage through the variable resistors and switches. This problem could easely be solved by replacing the capacitors by polypropylene's of the same value and voltage.
Also the low cut switch can easely be modified into a subsonic filter switch, by modifying the capacitors of the low cut section. A subsonic filter is indispensable when playing vinyl records.
The power supply main capacitors value can extended together with the rectifier diodes to make the supply even more rugged that it already is, the low frequencies benefit from this, 10.000 uF same voltage is a good value, splitting up the main psu is also a good idea: this is using 2 diode sections and 2 capacitor sections, for each power amp one.
For better speaker control the speaker switch can be omitted and hardwired with good quality stranded copper wire, the low frequencies and the sound image benefit from this.
Photo: Arthur de la Court
This is a picture of a faulty electrolytic capacitor that was found in the power suppy unit (psu) of a Luxman SQ507X. As you can see it is heavely corroded and got very hot, one connection lead is completely missing, it was corroded away by the acids coming out of the cap as it started leaking. The cap did not function anymore and consequently the psu started causing hum in the sound of the amp.
© Hans Hilberink - PE1MMK ® 2003 - Last update: 10-01-2010.