Transformer question

[Enzo]

We mentioned that a couple times already, and I even pointed out an example schematic to check out.

[AkisTzortzis]

Actually I just fuond your #17 link Enzo, thanks (320/330). I apologise for sometimes askign same/similar questions, I also have a day job as well as family and kids at home :-) It's all getting too much.

Before I do anything I need tools. An inspection of the workshop downstairs over the weekend revealed an old 20MHz oscilloscope, seems to be working fine. It has its own square wave output for testing purposes, I seem to remember there was something odd about it, need to spend some time on it to see if I'd need a newer one. Else, I am looking on ebay for something perhaps more modern. I also discovered a bench power supply, I could not even lift it off the bench! I can vaguely remember buying it about 17 years ago and never plugging it in... An old digital multimeter - definitely 20 years old - I would need a more modern one for sure. My old "electronics" box revealed a good selection of resistors, capacitors, transistors, LM317s, op amps TL081s, most still on tape and sealed in plastic bags. The old soledring iron is in shambles, I tried to use it a few years ago to put a microchip on the kids' PS2... Good lenghts of enameled copper for making coils. Big pack of solder. Lots of tools.

I will probably need a couple of weeks (weekends) to clear up the space on my workshop (currently used as storage space), and make sure my equipement works. I'd probably need a signal generator, a dummy load... Something to test/match transistors - something to show me Ib vs Ic...

I'd need some means of building up a test rig (say a handful of transistors and resistors) without destroying the components, ie without cutting the wires off and soldering. Any ideas?

And I would need a way of making / testing transformers. Did we say that there is no way to use a transformer with DC flowing through it? Because if I could do just that I could then use such a coil to act in place of the emitter's resistance.

I plan to start from a simple 12V circuit, test ideas and see how it pans out, before venturing into higher voltages and currents.

But whereas I can simply go out and buy a number of capacitors, the same does not apply to transformers, I'd probably need to buy a lump of ferrite and start winding it myself, so I need to learn about transformers

Any other ideas welcome.

[Enzo]

My 20 year old Fluke meter sits next to me and I still use it every day. If the meter works, it should be fine.

If you plab to make something remotely conventional, there are lenty of commercial power transformers ready to go on the shelves of suppliers. Unless you just want to be a pioneer, I see no reason to wind one. Determine the voltage and current you need.

I don't know that the Acoustic 330 is what you'd want to build, it was just an example of a comercial design using single ended power and an output cap. Personally I'd stick with the more modern split supplies like everything in the world uses.

RG has a transformer tester on his Geofex site. SImple to make and use.

[R.G.]

...Any other ideas welcome.

OK, I've read the list of posts and I'm going to give you the advice you need to hear, but which you haven't been given yet.

I suggest that you shelve this project until you have much more theory and practice than you have now. If you proceed as you've suggested, you'll certainly spend a lot of money and time, come up with little to show for it, and possibly get injured in the process.

You appear to have had some background in electronics decades ago. That's good. Go buy yourself an amplifier kit and build it. The tricky questions of what works with what and within what limits have been thought of by someone who's currently practicing and is familiar with them as they exist today. Removing the design onus from yourself will let you get your electronics skills back on track, and start you on learning what does and doesn't work.

I don't mean to criticize you; you're merely being enthusiastic (I think) and forging ahead based on what you can pick up by some little reading and forum advice. However, you're placing yourself in the position of the Sorcerer's Apprentice, where a little knowledge is a dangerous thing. You're asking questions which betray a serious lack of knowledge about power electronics, transformers, speakers, safety practice, and probably a lot of things that you haven't mentioned. For instance, it's possible to do small but useful welding jobs with the power supplies you would have to use with your original approach - not to mention starting fires. It's not just high voltages that are dangerous.

Then there's the issue of money. At the power levels you've implied, you can lose US$100 in semiconductors in a few milliseconds. Winding a transformer sounds easy until you price copper wire, and note that you may need quite a few different sizes out of the hundreds that exist; any selection of the wrong size means the stuff you've already bought is useless.

It is not reasonable to expect that you can get the education you need from even a long series of posts here on this forum or on a number of them. I urge you to go buy some electronics kits and learn by doing, taking small bites until you build up the background to understand why the mainstream of amplifiers are built the way they are. It's not just because no one ever thought to do things differently before now.

Above all, be safe. That does not only imply learning to deal with high voltages correctly. High currents can be quite dangerous too. Don't endanger you and your family.

[AkisTzortzis]

Hi

For my first experiment I made a simple bridge rectifier (a bridge and a few caps) which I thought was almost perfect, until I plugged it into the oscilloscope and realised that 17V DC output I had a 20mV p2p sawtooth ripple (fast attack, slower decay, 100Hz) and at 35V DC outout I had a 40mV ripple (with a minimal load, 1-3mA).

So my first project will be, I think, a proper bench power supply, I am thinking something like a 30-0-30 x 2 at 6 Amperes. I will upload the schematics I have drawn soon so you can give me clues

Thanks

[Enzo]

Unless you have a regulator for the circuit, 40mv of ripple on a 35v supply rail is not much - about 1/10 of a percent.

In a push pull output stage, small amounts of ripple will tend to cancel out anyway. A commercial amp with 40mv of ripple would be a darn nice power supply. Where you want nice clean regulated power supplies is in the preamp stages. Having a nice bench suppply is always useful, but a plain old rectified and filtered supply is just fine for a power amp.