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Amplifying Bluetooth phone signals

Recently, I purchased a Bluetooth hands-free unit to take phone calls while driving. It is nowhere near loud enough so I built the CHAMP amplifier (from Jaycar’s Short Circuits Volume 3) and wired an electret microphone to the input with a 10kΩ resistor to power the electret and a 0.22µF capacitor to couple the signal to the CHAMP. That works well but I wanted more volume so I built the high power 12V amplifier from the same book.

I wired the output of the CHAMP to the input of the high power amplifier but I get lots of hiss coming from the speaker even with the volume all the way down on the CHAMP. When I turn the volume up, I don’t get much more volume than from the CHAMP; then it starts to motor-boat. I am bench-testing the set-up with a 12V jump starter pack so the supply is not noisy or weak.

If I disconnect the CHAMP and feed my audio oscillator into the high power amplifier, the thing is as quiet as a mouse and happily gives clean sound when I turn up the oscillator from a low setting. Coupling the electret directly into the high power amplifier doesn’t give much gain either as the article states it needs nearly 1V as a signal input.

I have tried all sorts of resistors and capacitors to interface these two amplifiers but I am obviously missing something simple. Can you please give me some advice on this? (G. C., Toormina, NSW).

• Project 13, the preamplifier from Jaycar’s Short Circuits Volume 3, would have been a better choice and should have been used ahead of the power amplifier rather than using the Champ. The Champ is not as quiet as the preamplifier would be.

But since you already have the Champ, it should be modified by adding a 100Ω load resistor at its output to allow the 470µF output capacitor to charge. Also, the GND (ground) wiring should be first run from the negative terminal of the 12V battery supply to the power amplifier and then to the Champ to prevent problems with motor-boating.

We suspect the excessive noise is due to high-frequency oscillation of the Champ. The electret supply via the added 10kΩ resistor should be connected to a decoupled supply consisting of a 220Ω resistor from the switched 12V supply. The free end of the resistor connects to a 100µF capacitor, bypassing the supply from the 220Ω resistor to GND.

Reversing facility for electric car

Some years ago I built my grandkids an electric car powered by a 24V 250W motor. It has done years of reliable service and now is the time to add a reversing feature. I thought of wiring in a 2-pole 3-position (centre off) switch but I could not pass 20A through those that are readily available.

Would you consider designing a solid-state external reversing system with the following features: (a) reverse not selectable until the motor is at rest; (b) an internally adjustable speed limit on reverse; and (c) LED warning lights to indicate FWD and REV? In applications such as wheel chairs etc full speed reverse would be dangerous while in dinghies etc full speed reverse might be useful and safe. (D. V., via email).

• Our Railpower projects from October to December 1999 and the September/October 2008 versions had reverse lockout and lockout indication, plus forward/reverse indication. There was no reverse limit setting. The H-drive circuit for the motor was not rated for the current required to drive a 24V 250W motor but this could be upgraded by using TIP35C (25A NPN) and TIP36C (25A PNP) transistors. The current limit would also require changing, with a lower value current sensing resistor.

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