New project "500mW Retro AF Amplifier + experiments"

Admin wrote:... they are 1:1 ...

Hi Harry,
the link should point to 1300:8 ohm transformers. I probably made a mistake. The same supplier has also 600:600 ohm ones, which you probably have in mind. I ordered both types, I'll see what they are worth.

VBR Ivan

Hi Ivan,
These transformers you quoted would probably work really well. I see that they are 1:1, so the two windings could be connected in series to form a 1:2 auto-transformer.

Thank you very much for the input. Really appreciated.

Very best regards from Harry - SM0VPO

Most of the tasks can be accomplished also by a simple A-class amplifier with an output transformer. Suitable transformers are e.g. these ones. They do not have the center tap of the primary, so they do not suit into a B-class amplifier.

VBR from Ivan

Ivan wrote:Hi, the "HX108-2 AM Radio Kit" from AliExpress is a good source of rare parts. It contains two AF transformers, a 0,5W loudspeaker and a suitable PCB. Besides that you get a variable capacitor, three canned coils (oscillator and 2 IF 465 KHz), 7 transistors, passive components and a box. The price is reasonable. Several sellers offer it.

VBR from Ivan

HX-108-2, I will have a look at that. I have seen loads of kits (I was in a meeting when I wrote that / the boss was calling !! That's why the answer was so short).

I saw on AliExpress and BangGood that there are several AM radio kits. I saw reasonable prices in the region of $7 upwards. All the brand-new components not soldered. There are also AM radios for about $2.50 that you have to de-solder. I think it is strange that a bag of bits can cost more than the same components already assembled.

In the last two months I see that the new transformer sets have disappeared and IF transformers have also become difficult to find. I have a good stock of those, enough to last me my lifetime   The only things I have NOT been able to find are the red oscillator coils. I have a few radios I can cannibalise if I ever get the need for a MW oscillator coil, but I also have the coil-winding data.

Thank you very much for the response Ivan. Really appreciated, as always.

Very best regards from Harry - SM0VPO

Ivan wrote:

Admin wrote:If you have any ideas for children's projects then please give me the suggestion and I will see what I can do. Any suggestions have got to be fun, cheap(ish) and nothing that can entice the in-built smoke in semiconductors to escape.

Hi Harry, Radio NIVEA is an easy Czech QRP CW TRX, designed to start interest in electronics, homebrewing and hamradio. Maybe it will interest you?

VBR from Ivan

Hello Ivan, I will take a look at that. At the moment everything is of interest, but I have five ideas on the desktop.
Thank you very much, I will check it out now :-)

Very best regards - Harry

Admin wrote:If you have any ideas for children's projects then please give me the suggestion and I will see what I can do. Any suggestions have got to be fun, cheap(ish) and nothing that can entice the in-built smoke in semiconductors to escape.

Hi Harry, Radio NIVEA is an easy Czech QRP CW TRX, designed to start interest in electronics, homebrewing and hamradio. Maybe it will interest you?

VBR from Ivan

Hi, the "HX108-2 AM Radio Kit" from AliExpress is a good source of rare parts. It contains two AF transformers, a 0,5W loudspeaker and a suitable PCB. Besides that you get a variable capacitor, three canned coils (oscillator and 2 IF 465 KHz), 7 transistors, passive components and a box. The price is reasonable. Several sellers offer it.

VBR from Ivan

Ruud wrote:"any potentiometer of about 4.7Ω through to 25KΩ"

I suppose that should be "4.7 KΩ through to 25KΩ"

Of course 4.7Ω will work(!), but I doubt if potentiometers with such a low value exist!
Also the input impedance would be so low, that the input could only be driven from a loudspeaker output.
(But that would eliminate the need for an amplifier... )

Oops!! Sorry Ruud. This is exactly the reason I have previously asked you or Ivan to review a project before publishing. Ob this occasion I thought it was so simple a spelllchack would be ok 

Thank you for the correction and I shall update it. It is too late, but probably in the morning.

Thank you very much for your comment. I really appreciate it.

Have nice evening, at least that bit remaining. Harry - SM0VPO

Hello John,
Thank you for the reply. I just love the simplicity myself.

As regards using a mains transformer, that is easy. Here is a miniature 1W transformer. Use a 12-0-12v - 230V transformer. Costs about US$6 for one, unless you choose China, then the price drops.

The two windings are modular so you can remove the 230v winding and rewind it for 6v (8-Ohms), then reassemble. With this design there is no standing DC to saturate the core, so the calculations without standing DC work: the small transistor bias current is equal and opposite in each half of the transformer. You just need to make a test winding to find the turns/volt. Also the PCB will not work, but there is not much on it to start with.

Thank you for your comments. Really appreciated.

Best regards from Harry - SM0VPO

Great project. The transformers may be a little old fashioned now but no matching NPN/PNP devices, no bootstrapping and no need to look for that elusive VCC/2 at the output, or one output transistor running hotter than the other.  I wonder if a small cheap mains transformer could be rewound for the Output transformer? I saw an article somewhere for an AM modulator using a ferrite core trafo like the ones used in SMPSUs....

I did some experimenting earlier this year with low power af amplifiers and found the sziklai pair configuration worked best for me, but I love the simplicity of the transformer coupled amplifier.

"any potentiometer of about 4.7Ω through to 25KΩ"

I suppose that should be "4.7 KΩ through to 25KΩ"

Of course 4.7Ω will work(!), but I doubt if potentiometers with such a low value exist!
Also the input impedance would be so low, that the input could only be driven from a loudspeaker output.
(But that would eliminate the need for an amplifier... )

Hello people,
I think that this is a really interesting project that dates back to a project I built back in 1961 or 1962. It was one of the first multi-transistor projects I ever built. An audio amplifier using inter-stage and output transformers. Before you say "poooo-old-hat, you are showing your age, Harry", yes I am showing my age.

A modern complementary amplifier does not need transformers, but there are a couple of things you can do with a transformer amplifier that is a lot more difficult to do with a more modern design.

For example, light-beam telephone using a torch (pocket lamp) lamp, or using a LASER, AM transmitter modulator, long-distance microphone, and many others. If, like me, you can be a little childish and like to prank-people then I have included something for you. I manage to make a mother of two quite hysterical, but you can read about it in the article.

I started this project about 9 months ago but it sat on the back shelf waiting for "a rainy day". But I think that -9°C and snow can count as a rainy day. Took me only a few lunch-breaks to finish it.

If you have any other ideas, corrections or suggestions, then please drop me a line, preferably via this forum.

There is another project I started about a year ago after I got a suggestion for communication/technical projects for children.

I immediately came up with a couple of ideas and I will work on one of them in my lunch-breaks, as long this snow lasts. If you have any ideas for children's projects then please give me the suggestion and I will see what I can do.

Any suggestions have got to be fun, cheap(ish) and nothing that can entice the in-built smoke in semiconductors to escape. I don't want to be accused of coercing young people to exterminate semiconductors. I will also entertain anything that is remote to radio and electronics, but it must be fun and hopefully, educational.

Very best regards from Harry - SM0VPO