# Impedence Matching

Discussion in 'The Projects Forum' started by bouncetherabbit, Feb 10, 2010.

1. ### bouncetherabbit Thread Starter New Member

Oct 25, 2009
10
0
Hi,

I have a Class D amplifier with a 50mVp-p sine wave at the output. The minimum output load of the class D amplifier should be at least 4ohm.

However, I am placing a transformer at the output of the amplifier instead of a speaker. Since the resistance of the wires of the primary side of the transformers is around 0.4ohm, is there any way the 2 resistances can be matched?

I'm unable to place a resistor in series with the transformer as it "isn't efficient".

2. ### SgtWookie Expert

Jul 17, 2007
22,198
1,800
What is it that you're actually trying to do?

Speakers are rated for impedance at a certain frequency.

If you're considering using a transformer designed for household AC, it's not going to work very well. You would need a broadband transformer.

3. ### bouncetherabbit Thread Starter New Member

Oct 25, 2009
10
0
50mVp-p is actually modelling the outout earphone connected to the mp3 (I think. The person telling me about it was not speaking in english so some stuff may be lost in translation).

What I'm supposed to do is to tap this small voltage to produce a VCC supply for another part of the project. But for me to do dc rectification, 50mV is too small, so I was thinking to use a transformer to increase the voltage before I put the signal through a voltage multiplier circuit.

Would the broadband transformer really help? Assuming i need to increase the voltage by 1o times.

Thanks!

4. ### Audioguru Expert

Dec 20, 2007
10,789
1,212
The impedance of a transformer is at an AC frequency and is determined by its ratio and by its load.
The DC resistance of a transformer is not used for an AC signal.
I don't know the ratio and i don't know the load of the transformer so I don't know if its impedance is 4 ohms. It is probably a lot higher than 4 ohms.

Instead of stepping up the signal level with a transformer why don't you turn up the volume control of the amplifier or use a preamp?

5. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
If the 50 mv into 4 ohms is correct, that gives us 12.5 ma current. The power available is only 625 microwatts. No amount of voltage increase can deliver more power than that.

6. ### Audioguru Expert

Dec 20, 2007
10,789
1,212
We use an opamp circuit, not a transformer to amplify low level signals.

7. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
This is true -
- but the OP wants

8. ### Audioguru Expert

Dec 20, 2007
10,789
1,212
It is silly to power a circuit from a rectified and amplified signal. The circuit should be enabled by the rectified and amplified signal.

He did not say that there is no power supply for an opamp.

9. ### bouncetherabbit Thread Starter New Member

Oct 25, 2009
10
0
Using an RLC meter, the ac impedence of the primary side of the transformer would Be around 8-8.5ohms. I can't use an op-amp to amplify signal because the transmission side and the reciever can't be physically connected.

Transmission side would consist of the class d amplifier, a low pass filter, primary I go sleep le. Night of the transfomer.

Reciever side consist of the secondary side of the transfomer and a recitfier circuit.

If I connect the transformer to the clas d output, I'll just get many sine waves at the input of the transformer.

If I don't use a trasformer to inductively couple the voltage over, are ther any other ways?

10. ### bouncetherabbit Thread Starter New Member

Oct 25, 2009
10
0
The picture shows the input to the primary coil of the transfomer.

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