Can an ordinary transformer be used for coupling the speaker with a transistor amplifier

Question

Audio transformers are not available in my locality. However, I have bought an ordinary transformer. It has the following specifications: Primary voltage: 230 V Secondary voltage: 6 V Current rating: 300 mA

Apart from these, I measured the impedance using multimeter. For the primary side it is 2 kilo-ohms and for the secondary, it is 11 ohms. These impedance kind of match with what I require for my amplifier load.

Now my question is whether I can use the ordinary transformer or not. I googled and some sites said that these transformers are safe to use in the audio frequency range. At the same time some sites said the opposite. I just want to clarify before I end up messing my transformer.

Also please provide any other means for impedance matching. I tried an emitter follower, but its not very pleasing.

Answer 1

If your transformer has been adequately designed for power applications (50/60 Hz) it will make a poor audio transformer. Ask yourself the question - why does a transformer use laminations - did you know they are insulated from each other so that electrically they don't conduct?

Did you know that if they did all conduct you would have one massive shorted turn?

So, the laminations are there to prevent excessive shorted turns and the thickness might be (say) 1mm and suitable for 50/60 Hz - will that thickness laminate be suitable for 1 kHz - no. You'll get a lot of heating and wasted energy just fighting the eddy currents.

What about coupling factor - a power transformer has (with some hand waving) a coil coupling factor of about ~97%. An audio transformer is much closer to 100%, probably at least 99.5 %. Does this matter? It does at high frequencies because if you added a few tens of milli-henry external to your primary and tried to pass 20 kHz through it you wouldn't get a very good result. That's what happens with leakage inductance - it doesn't couple magnetically to the secondary.

Consider a primary with inductance of 10 H - that's great for 230V AC but the leakage could be 3% of that at 300 mH. At 20 kHz, 300 mH has an impedance of about 38 kohm.

So, if you just want some "near-enough" crappy-fidelity thing with loss of top-end that's fine, use a power transformer.

I measured the impedance using multimeter. For the primary side it is 2 kilo-ohms and for the secondary, it is 11 ohms

Those are the dc resistances of the windings and that means your speaker on the secondary is in series with 11 ohms - a lot of power loss there if you have an 8 or 4 ohms speaker.

Answer 2

Just saw this as I have a similar transformer laying around, but with a ratio of around 10 (or less if it accounts for voltage drop under load), the 50/60Hz transformer would actually drive a bass speaker which is in the region of those frequencies (20-80Hz). So the answer is yes, you can use it to drive a speaker, but only for the lower frequencies like a bass speaker.

If you want to cover more frequencies, you can use a couple of smaller high frequency transformer itself to produce sound. Unlike a regular speaker, there are no moving parts. It just won't make for a very good speaker, but it will make for a fun project. It does have its own sound profile too and would be fun at parties, to keep people guessing where the sound comes from.

Edit: while other people are editing my answer i will explain this t. Taking 2 of the same transformers wont help in producing a range of sounds, you need to use different transformers in conjunction with one another as each of them are designed with a particular frequency range in mind. Its just like with speakers, the size of the speaker and its driver matters for the frequency in which it works best, so sound systems combine different speakers together. The same can be done with transformers as well. Lower frequency optimised transformers are large while higher frequency ones like those used in wall warts are small.

Answer 3

What's the idea to have a transformer to connect a speaker to a transistor amp?

60 years ago low cost transistors couldn't handle more than 20V. If one needed 100 watts to a generally available, say 8 or 4 Ohm speaker, the only way to get it was to rise the output voltage with a transformer. In addition push-pull output stages were possible with a low supply voltage and using only PNP transistors, if one had transformers. But their windings should be something like the low voltage secondary of a mains transformer and the iron core of a mains transformer probably is too lossy at 1 kHz and above.

You maybe have read "impedances should be matched for maximum power transmission". That's an useful fact with radio frequency circuits and with millivolt microphone signals, but in audio speaker amp outputs that's different. Your amp may be rated for 4 Ohm speaker, but that doesn't mean you need 4 Ohm speaker to prevent signal reflection like in radio frequency transmission lines. It means that lower than 4 Ohm can take too high current at the voltage that the amp outputs. You should consider amp output as an AC voltage source. If you for some reasons want to have a transformer its windings should be calculated for the used voltages. I bet 230V winding is useless.

Full audio range covering high power and low distortion transformer is a serious engineering challenge, far beyond what's needed in power supplies at 50...60Hz. All non-idealities will jump out as the frequency increases.