Wiring the Speaker to the LT700 Transformer

At some point, all hobbyists come across a single transistor amplifier circuit with a LT700 transformer driving a loudspeaker, and they will want to try it. Beginners often find it difficult to use this transformer, hence I have wired up the secondary side to give them a good head start.

This particular speaker is a very good quality vintage one made in Japan. It came out of a 1980s stereo and has a great sound. If you are looking for a small cheap speaker, then I recently saw some at the pound store. They had small portable stereo speakers for connecting to an mp3 player, and they contain an ordinary conventional speaker inside!

The LT700 has an impedance of 3.2 Ω on the secondary output side, and therefore you need to match this with a speaker of equal impedance. Ideally, you need something closer to 4 Ω but it is not critical, and you could use any recycled 8 Ω speaker that you might already have. I am using this recycled 10 cm diameter speaker. It has an 8 Ω impedance, but more importantly, it will save me a fiver from having to buy a speaker.

As you can see, the red wire connects to the positive terminal, and the plain grey wire to the negative. These wires connect directly to the secondary side of the LT700 transformer. As you can see, the wires connect this way.

To the uninitiated, this is just a drainpipe clip; however, with the magic of imagination it now becomes a loudspeaker holder. It is perfect for the job and even better was the price of 40p! I will take three!

Of course, you might be using a different loudspeaker, and therefore you will need a different solution.

LT700 Transformer Mounting and Wiring

Mounting the LT700 transformer is very simple in this build. The one I got was from eBay, and it was a PCB mount type with short leads, however, I discovered that if you bend the leads by approximately 45-degree angle then it is more than enough to be used with the screw cup system. In order to do this, you will need the screws 1-inch apart in a perfect square for the best fitting. In many ways, a precise fit is better than long leads.

Without the screws and their cups, you can see that the leads are long enough to reach the centre of the holes, and they will make adequate connections.

For more information about the LT700 component, please follow the link.

This shows the view from the top.

The wires from the speaker connect this way. Pay special attention to where the red wire connects.

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