Sanken MN2488 and MP1620

The Power of Darlington – Sony amplifiers and receivers

Another DIY project

I have had numerous Sony amplifiers which have been stripped down for spares, mostly power supply, power transistor driver chips, the NEC UPC2581 and the then popular Sanken MN2488 and MP1620 power Darlingtons. Now Sanken has been around for some time and when one refers to audio technology we think of TI, On Semi, National Semiconductors. But Sanken has made some great chips as well, the STK series like the 435. We had the 10W, SI-1010G, 20W SI-1020G, 30W SI-1030G and the whopping 50 Watt the SI-1050G. How about their transistors?

Vintage Sony

Older Sony equipment, especially the Trinitron and audio amplifiers can make one nostalgic, never known for great audio possibly, but bringing out the Sony TA-N77ES in the mid 80s made the audio world sit up and notice.  This was a juggernaut, 200W per channel and absolutely gorgeous.  Possibly one of the best looking amplifiers ever (next to McIntosh of course).

Puff the Magic Dragon – Bang goes Aiwa

The Sony takeover of Aiwa I thought was a f****up of note. Aiwa made some really great stuff and now they are non-existent. I even wonder whether Sony gleaned anything of value from  this once great company. At one stage there wasn’t a household in South Africa which didn’t have at least one Aiwa product, or so it was perceived. Oh yes, Aiwa also made better component systems. But I am not here to ‘diss Sony, we still have many die-hards out there and I really would like to see them make a comeback. What, you didn’t know that they had pulled out of South Africa?

The popular STR-DG600 and STR-DG500  (7.1 and 5.1)

The Sony STR-DG600 7.1 was brought out to some rather dismal and really vague reviews. Most of this I shoved to the back of my reptilian brain and did my own testing, with cheap Sony speakers. Note that this amplifier can be purchased from the USA for less than R 2 000.00 new. In SA, second hand about R 1 500.00 or less. This is not break the bank stuff and gauging by some of the reviews one would have thought they had purchased a Levinson series amp for  3000 quid.

It’s clean, very clean. It cannot power more than two speakers (the front) at full throttle without moderate to severe distortion. At 2/3 volume with SACD or CD input it’s remarkable, for the price.  What makes it tick? Here we stick to the analogue parts of importance – the output stage.

Darlingtons, high gain circuits and 8 Ohm loads

The amplifier uses a very non-unique amplification stage to their entry level range, utilising a NEC chip-amp (IC) driver stage and a Sanken Darlington pair in the output.  Because of the +50V/-50V supply rails, the SOA of these transistors and configuration lends itself to 8 Ohm loading and driving two 8 Ohm speakers in parallel is inviting trouble – just don’t do it. By removing the other channels from the supply and adding an extra pair of darlingtons to the output stage of the L + R channels I do believe will allow for 4 Ohm loading. The transistors, with a current gain of over 5 000, paralleled, should not provide undue loading to the driver chip. Remember the 0.22 Ohm emitter resistors.

So why the focus on this configuration? With the hype on gainclones and the like here is an opportunity for those home tinkerers to build a 200W amplifier with a minimum of components. These amplifiers are very popular. Their failures almost always came down to possible “micro-controller” problems, blown output transistors, mains transformers and the ever popular  “not switching on” after a lengthy period switched off at the mains to allow for condensation.

Building your own stereo amp minus the DSP

To build your own home stereo amplifier with components from these amplifiers (the STR-DG500 is similar BTW), I would recommend one transformer per channel or better still, a toroidal rated at about 300VA, secondary windings 42V-0-42V. If you are going to stick to 100W per channel the transformer may do it.  Don’t hedge any bets though.

There are some things one needs to remember if you do get your hands on some parts to build the modified circuit:

• I have not built this. It’s on my to do list 🙂  which includes many, many things.
• The article is based on the presumption that you can get your hands on an old Sony 500/600.  You should be able to pick one up for about R100.00 if it is faulty. Because of the pre-driver/driver NEC chip it greatly simplifies the build. The transistors are Darlington which simplifies it further.
• Use the components in Fig 2. for Fig 1.  Paralleled transistors need the 33 Ohm resistors at the base as well as 0.22 Ohm emitter resistors.
• This is a very high gain circuit and oscillation can and will occur if the board layout is not properly thought out.
• Power output should theoretically be about 150W / 8 Ohms.
• You can parallel two transistors on the existing board but remove power to the non-used amplifiers.
• You will need a cooling fan on the existing heatsink.
• You will need to add a speaker protection module.  (Velleman is just one such supplier).
• Please let me know if the publication of the partial schematic is inappropriate.
• The original schematic can be found on the internet incl. Hi-Fi Engine.

There are near equivalents for these now obsolete transistors, possibly 2SD and 2SB types. Because this is a from scratch build (possibly) make sure you have current protection across the  (both) fuse holders on test.

Article: Loudspeaker Protection

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