Ron's Audio
                                             Sound for the regular guy


  SS1 Transistor 180w Amplifier


    I'm sure like me all of us have a lot of parts laying around. Ideas are always running around and I decided to make use of  an output transistor array that was pulled from a vintage Realistic Sta-2290 receiver.

So my goal was to re-create the vintage sound from those late 70,s early 80's receivers I remember using a combination of vintage and new parts.  I notice the vintage late 70's Pioneer SX-1980 receivers are going for up to $4000.00 on ebay.  The Realistic Sta-2290 early 80's receiver I found was trashed when I came across it. It had been butchered by someone  damaging a good part of one of the main boards that looked like it was on purpose. I salvaged the output array which contained pairs of Sanken 2SC291 / 2SA1215  Silicon Epitaxial Planar Transistors rated for 15 amps at 150 watts dissipation.
       I am suspicious of those tiny amplifiers advertising a zillion watts using a small power supply.  As far as I have found, any amplifier whether it be a vacuum tube, transistor, or a chip amp that puts out a respectable power needs a respectable power supply.  Can today's new receivers keep up with the vintage ones? Iv'e read where the SX-1980 can go toe to toe with some of the new equipment in tests. When purchasing new equipment today and the sales specs are 300 to 400 watts per 7 channels. Ask one question. If seven channels are supposed to be putting out 2100 watts, cant happen. A good solid 120v 20 amp circuit can supply 1800 watts of power so how can you have more out than in. Even a well designed efficient digital amp has losses and its power supply has losses.  So a good sized power transformer / power supply is first required.
        I used an Antek AN-4435 400va toroidal power transformer that was actually a pull from a CNC machine. Then a combination 3-band tone/preamp circuit  utilizing 2604 op amps, and finally a driver board from Elliot Sound to drive the Sanken transistor output array along with a speaker protection board also from Elliot Sound.  All this is coupled to some vintage Realistic Mach Two 3 way speakers that each contain a 15" woofer for that 70's / 80's sound. This amp was a little to much for my Bowers & Wilkins vintage 602-S2 speakers that I use on my tube amps, as they tend to distort at the high volume that this amp can produce.


    Dare I say that the lights briefly dim when powering up your amplifier? The solution is a simple soft start circuit as you can see in the schematic.  When power is first applied it ramps up the power supply for a split second through the three 68 ohm 5 watt resistors in parallel, thus limiting the current inrush. When the voltage rises the relay closes and bypasses the resistors. Signal ground (0V) is isolated from chassis ground through the bridge diode array and parallel resistor capacitor combination. If there is a short to chassis the diodes will conduct blowing the main fuse, and the capacitor helps shunt any high frequency noise to ground. Speaker protection is accomplished using Rod Elliot's speaker protection board. It is a compact design that will protect speakers from any DC potential appearing on the output of the amplifier and quickly disconnecting the speakers and shunting them to ground. Also there is a short turn on delay as the power supply ramps up then the relays are activated connecting the speakers to the amplifier. The output transistor array has a factory temperature safety limit which I decided to connect to the speaker protection board. If the heat sink overheats the limit will disconnect the speakers. An option here would have been to shut down the amplifier in case of an overload, but I figure I would rather protect my speakers as fast as possible in any overload situation.


Here you can see where I mounted Rod's driver board to the back of the output transistor heat sink.  The thermal protector is mounted in the middle between the output transistors. The output transistors are vintage Sanken 2SC2921, and 2SA1215 transistors.


Heat Sink front view during testing.


Top view of the amplifier with everything mounted including the power transformer and power supply pc board. Additional power supply circuitry is to the front of the pc board.


Bottom view of the chassis shows location of the relays  preamp/tone board and speaker protection board.