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Audio Amplifiers

One was a very popular 125W RMS into 4 Ohms unit, pretty much commonly known as the Autona 125W. This was marketed as the Autona according to the magazines I read at the time). This had 4 x 2N3055 transistors in the output stage and with an 80V D.C. supply could push out 120~125W into a 4 Ohm load. Compared to modern day audio the sound quality was terrible – the standard 12″ drivers were made by Fane and with anything else, besides the Novik range ,they sounded even worse. Believe me, I tried some pretty upmarket speakers on these and they were just simply awful. I think I’m being overduly critical – for the money spent they were excellent. They were capactively coupled to the speaker… ummm, this was obviously part of the problem but they did the job. In most cases I think DJs used to run these only into an 8 Ohm load and they were sufficient. But if you still have a module or two lying around what you should do is regulate the power supply to 60V – the sound quality improvement was very noticeable both in high end and low end – driven into an 8 Ohm load.  Into a 4 Ohm load make sure the output devices and their NPN/PNP driver transistors were matched otherwise they destruct – man, the linear operating specification of these devices was something to behold. But really great old work-horses. I have put two of these devices in a jar with 2 old EL84s which I want to be buried with one day.

Then along came an amplifier straight out of the Elektor magazine, using the tried and trusted 2SJ49/2SK1344 – I couldn’t get these in Cape Town by the way – both had to come from Candy’s in Durban. I wonder if they still exist? Very friendly, very helpful and very efficient. The two amplifiers I built blew my loudspeakers (Fane 100W) but what fun. Although the amplifier was rated at 150W RMS into 8 Ohm (can’t remember now) the quality difference between the old 3055s and MOS devices was like comparing the old Ford XR3 to the Focus.  It was unbelievable.  But then again they told you so. Eventually the amplifiers were borrowed and never returned but I really knew then that we were in for great changes in the audio industry.

There’s really a lot going on the web when it comes to audio – Class D amplifier circuits, plenty of Bipolar and Mos amplifiers plus of course some valve amplifiers for the guitar amplifier enthusiast. I wonder if there’s anyone out there wanting to drop us a line regarding old radio grams they want selling. (I don’t know what the American equivalent name to this was – any takers?). My neighbour does wood restoration and they had a gram which belonged to some South African guy immigrating to Australia – the changes he wanted to make were obscene. Keep it the same chap! I don’t see these old ‘Radiograms’ advertised anymore but can guarantee you that there are con men out there wanting to rip the public off if they do get them in. So be careful.

When I was still tinkering with audio amplifiers I built an amplifier from scratch and according to my calculations the amplifier should have been able to deliver about 40W into an 8 Ohm load. I tested them on my dad’s Akai speakers (he had an M8 Akai reel-to-reel) really upmarket stuff in those days.  Being a mono amplifier I paralleled the two speakers and did some tests with Ozzie Osbourne and blow me down if there were no high notes. Actually what had happened was the amplifier delivered nearly 100W into a 4 Ohm load, the speakers were only 25W each – the bass drivers could easily handle the power, they were that under-rated but the old tweeters were only 3 watts each. The speakers (tweeters) were designed for the M8 class A 6W amplifier so they didn’t really handle the power that well. I quickly packed everything away but alas was only to be found out the following week-end when the old boy decided to play some of his classic stuff. Classic stuff doesn’t sound so good when there’s no high range speakers to add to the ‘quality’ of the sound. And believe me, only 6W per channel but the sound was awesome.  Did I get it in the ear that day. Pun intended so included.

And the time I ran a magnet next to our Panasonic TV to see the result…

Yep, young boys aren’t good for any father’s blood pressure.

5 Replies to “Audio Amplifiers”

  1. Good evening.

    I recently purchased a kit using 2n3055 on the output, fter assembling everything it look very similar to the autona 125 amplifier. I was wondering if you have a picture of the autona and maybe a circuit diagram. this kit did not come with a circuit diagram only the pcb layout. I am almost certain it is a copy of the autona.

    I found that if you increase the output cap to 4700uF the amp sounds much better and you can easaly drive 4 ohm loads.

    I bought the kit to play with and built into a 10″ cabinet to have some small speakers that i can just pop in the boot. I do have 2 mosfet amps of 1Kw that i use on my 18″ base bins and 15″ tops.

    I would realy appreciate i if you can send me the circuit of the autona.

    kind regards

    1. Hi Willie

      The Autona modules were very popular in the 70’s and 80’s, ran on an upto 80V DC supply. There was also a 50W version which was similar, only using 2 x 2N3055s. I drew out the schematic years back and will see if I can find it. These modules were great for disco in the good old days 🙂 but really the quality is bad compared to modern units without coupling capacitors. The worst part is the switch-on thump, which I am sure caused more damage to speakers and the 2N3055s than anything else. I always used Toshiba devices, I found them to be more reliable (otherwise RCA). To get rid of switch-on thump use a 5W 100 ohm resistor across relay contacts attached to timer. (in series with load).
      I’ll see if I can get the circuit and attach it to the OPiC Technical Library. Have fun!

      1. Know its some time from May till now I Am also looking for autona 125 circuit Dia. any chance please.

  2. i have a diagram im busy messing around with one now not sure how to set the bias give me your e mail adress and ill mail you a diagram

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