Figure 1: Sony Modification – 150W / 8 Ohms audio amplifier – TinyCad

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, preferably a matched MN2488 and MP1620 pair at 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 Darlington’s 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.  (mains transformers usually open circuit temp sensors which can be bridged out).

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 minimum, 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.

Sony HT Amplifier – original schematic (DG 500 / DG 600)

Figure 2. Sony – section of original schematic STR-DG500/600

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).  See below.
• 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, the MP1620 and MN2488 which are possibly going to be 2SD2488 and 2SB1620 types. Because this is a from scratch build (possibly) make sure you have current protection across the  (both) fuse holders on test.  I had a Harmon Kardon amplifier where I blew one channel and used the MP1620 and MN2488 as a replacement as I had quite a few in my junk box.  The HK AV151 uses 2SB1559 and 2SD2389 in the output stage.

Article: Loudspeaker Protection

The Velleman K4700 loudspeaker protection kit.

The Velleman K4700 loudspeaker protection kit.