Audio transformer (LT700) behaviour [newb]

[abstractjuz]

Hi, please forgive the very newb question…

Taking my first steps into Class A amp design using the single transistor common-emitter config show here:

An externally hosted image should be here but it was not working when we last tested it.

My question: Under normal conditions, should the output at the collector (8V PP on my breadboard) be the same as the output on the secondary windings of the transformer connected to the speaker?

I'm using an LT700 with the outer pins of the primary connected to the collector and 9V+ (middle pin unconnected) and the 2 pins of the secondary collected to my speaker.

Thanks!

 

[jerluwoo]

Transformers Part 1 - Beginners' Guide to Electronics
A bit of reading about transformers would help you answer your own question I think.

 

[abstractjuz]

Thanks for the link jerluwoo - I learnt some interesting stuff but it didn't really answer my question - or not that I could see.

This is the first time I've used an audio transformer so I'm really not sure what I should be expecting to see and the data on the LT700 seems pretty thin.

So the question stands - on the config in the OP, from a 1V PP input, I'm seeing approx. 15VPP (between approx. 1V and 16V) at the output of the collector and approx. 800mV at the 'positive' connection of the transformer to the 4R speaker…

…so - loads of gain - but what appears to be a big drop on the secondary side of the transformer. Happening on breadboard and simulation.

Is this normal and is there a way of decreasing the loss or increasing the V PP signal the speaker is seeing?

Thanks.

 

[Mooly]

An LT700. I've used these in the past for a headphone amp. 800mv output sounds about right tbh. These are only a 0.2 watt transformer so V = √W* Ω which is 0.89 volts.

Running the secondary open circuit will see much larger voltages.

This is the kind of transformer a tiny pocket radio might use... you are not going to get anything more from one of these I'm afraid.

 

[abstractjuz]

Thanks Mooly,

800mv output sounds about right tbh. These are only a 0.2 watt transformer so V = √W* Ω which is 0.89 volts.

0.2 Watt is the power rating? So theoretically a transformer with a larger power rating could provide more gain (+/- the rest of the transformers specs…)?

Running the secondary open circuit will see much

Sorry - can you explain what you mean by 'running the secondary open circuit'?

This is the kind of transformer a tiny pocket radio might use... you are not going to get anything more from one of these I'm afraid.

Yeah, I figured as much. Not a problem for now - I really just want to get my head round the circuit to start with.

Thanks again - much appreciated.

- J

 

[Mooly]

Running open circuit just means with no load attached. If you really want a transformer coupled circuit like this to drive a 4 ohm speaker to useful levels then you need something much larger. Valve amps use such things but they also have very different impedances and consequently voltages used (such as 300 to 500 volts rather than say 12 volts for a small transistor circuit.

If you wanted to try a small Class A amp then the JLH69 10 watt amplifier is as good as any. Using no audio transformers it needs around 17 volts supply at a couple of amps but the design is such that you can run it at much lower current if you only want a couple of watts output. That means you can get away with much smaller heatsinks etc. Its a great design to start with and can hold its own against kit costing thousands when it comes to sound quality, and all from four transistors.

 

[sreten]

Hi,

The JLH can be run at lower current and voltage for lower power outputs.

rgds, sreten.

 

[Mooly]

Thanks, yes it can indeed run on a lower supply if needed.