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NIC1138 2nd May 2007 04:16 PM

Transformer on feedback path
Hello folks. This is my first post... And I hope an interesting one! :)

I'm doing this project, a scientific experiment if you will, and I need fo feed a low-impedance load (1.5 ohms) with about 2 amps. It's a very experimental stuff, nothing very certain yet.

I was building a simple AB push-pull with a pair of TIP41/42 and a TL071 to feedback and eliminate the crossover distortion, but then I started to feel like using a transformer before the load (I'm also starting to believe the current might be too high for the transistors)

Well, I also want to measure the current somehow... I want to sample the voltage and current going through the load. So I was thinking about putting a resistor in series with the load, on the other side of the transformer, and feeding back the voltage drop on it using a INA114. So the input is the current we want through the load, and the output is the voltage on it.

So, I was looking for suggestions, criticisms... Has anybody ever seen a feedback with a transformer on the way for current gain? No big deal?...

PS: If you are curious, the experiment is to pass a current through a GUITAR STRING with a large magnet nearby, to move it!... =) Yes, it works, I already attached it to the power outlet using transformer to take the voltage down to 6v, and controlling the power with a dimmer. I tuned down the E string, and it vibrated!... Now I'm going to control the frequency from the computer... Yes, this might become an electric hammerless piano in the future! :)

jcx 2nd May 2007 04:50 PM

the tip41/42 won't have a problem with 2 A, Xfmrs in a feedback path suck - they are bandpass filters and severely limit the feedback that can be applied before phase shift turns the amp into a oscillator

the likely problem is that the op amp won't have enough drive current to drive the tip base currents, you should add a buffer stage, look for "diamond buffer" schematics

NIC1138 3rd May 2007 03:40 AM

Hmmmm... Right, I totally disconsidered thecurrent at the ampop. :( Perhaps MOSFET could be a better choice??... It's just that I'm not too used to it. :rolleyes:

I don't need a great bandwidth... Just 1kHz should be just fine... And 10W maximum...

Guess I better at least run a simuation before asking for more help! :D I'll be back when I have something more substantial...

NIC1138 9th May 2007 06:41 AM

Just for the record, I gave up on the idea, for now, and I'm trying a TDA2050 amp, which might be just the power I needed... Thanks for the (wideband) feedback! :)

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