retard coil

[LAJ]

Does anybody know what the specifications are for a retard coil? Its basically an audio frequency choke. It allows audio to be impressed upon a DC voltage without the reactance of the power supply killing the audio (or affecting the response). Thanks.

[bob91343]

It's simple enough. The impedance is what matters. The choke needs to provide sufficient impedance at the lowest frequency of interest to avoid loading the source. So you need to know the source impedance and the lowest frequency you are using. The formula is two times pi times the frequency times the inductance; that will equal the reactance.

[wolf_teeth]

L = R/(2*PI*F), you mean that formula?

Then R is for resistance, not reactance.

Later,
Wolf

[janneman]

Originally Posted by LAJ

Does anybody know what the specifications are for a retard coil? Its basically an audio frequency choke. It allows audio to be impressed upon a DC voltage without the reactance of the power supply killing the audio (or affecting the response). Thanks.

My 2ct worth:

Generally, one uses the rule that at the lowest freq you want to isolate, your coil impedance can be equal to the source impedance. Any lower freq, or lower L, and you start to load the source and impact the audio response.

[Because of the phase shift of the coil, at that point, (impedance of L = source impedance), this is also the point where your response is 3dB down.]

So. L impedance = source R. L impedance = 2*pi*f*L = R so if you rework that you get the spec for L: L = R/(2*pi*f) as mentioned by Wolf above. Example: for a source R of 1k and a lowest freq 20Hz you get: L = 1/(2*pi*20) (with R in k and consequently L in mH) so L = about 8mH. Since you only deal with signal levels that are relatively low the L can be wound with thin wire.

jan didden