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PostPosted: 18 Jul 2016, 03:21 
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Joined: 28 Dec 2010, 22:07
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I'm thinking of an idea that might have a practical use.

Simple amps using a tube in class A, using a transformer for the plate load, are common for beginners and preferred by some. I am thinking of a way to make one using transistors.

Where tube transformers require large turns ratios to step a speaker up to plate load impedance, transistors like to see less impedance for useful power out. With an 18v supply I could get 1.5w if I had a collector load of 80 ohms or so. That's a turns ratio of 3.2-ish. Sadly, I'd probably have to make my own transformer.

I'd have to find a small transformer where I could scrap the existing windings and wind on a new set for audio. I'd have to impart a small gap in the core for it to work well. If I could do that, it's possible I could make such an amplifier. It would be very simple and would make for a decent amplifier using minimal parts. A student of http://www.runoffgroove.com could find it a useful amp for making small preamps while being a little more substantial than something like a Ruby or Little Gem.

My interest in this could turn to modulators. If I could make an amplifier capable of putting 2 watts into a class C power amplifier, I'd be able to modulate the thing and have AM transmission. Using transistors, thats probably the easiest way of doing it.

Ed

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PostPosted: 18 Jul 2016, 15:03 
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18V... 1.5W of power?

Here:
http://www.edcorusa.com/xsm_series

Remember that transformers work off ratios. Doing a ratio for an 8 ohm load, won't spec out as the datasheet, but will work.

If it's a modulator for a tube class-C final stage, you'll need proper OPT's from their Class-X section.

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PostPosted: 18 Jul 2016, 15:09 
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That's another thing I considered. Rather than spend 14 for an experiment, I'm thinking of getting an alarm clock at a junk store, hitting it with a hammer, and rewinding the small transformer just to see if it will work.

If you have a transformer less amplifier running off 12v, the most power you can see into 8 ohms is just under 1.5v...if you run it rail to rail. Most amplifiers won't do that. Someone who wanted to build a transistor amplifier would do well to use 18v or higher.

Ed

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PostPosted: 18 Jul 2016, 16:10 
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Which is why car speakers got standardized to 4 ohms.

Fortunately a transformer doubles your swing, allowing for four times the output (minus losses).

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 18 Jul 2016, 18:18 
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If you can find a really old car radio it might have the trannie you want and in the early days of transistors they actually did use output transformers as the transistors did not mate well with the load.

Good listening
Bruce

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PostPosted: 18 Jul 2016, 19:08 
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That's right!

Look in a scrapyard for old AMC anything, Jeep CJ, Ford Pinto's, etc. with AM-only.

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* Ratings are for transistors - tubes have guidelines*
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