Speakers

[Brainteaserfan]

I have a set of speakers plugged into an apple AirPlay unit. When the AirPlay device boots up and successfully connects to the network (when the light turns green) there is a "pop" or a noise like your ears popping. Is there a reason for this, or is it a sound effect maybe that apple designed the AirPlay device to produce when it was ready for use?

Edited October 31, 2011 by Brainteaserfan

[Xittenn]

Most audio amplifiers operate in what is known as class A/B, biased push pull. Class A/B operates in nearly Class B mode.

 

In class B mode an amplification circuit is constructed so that the positive half of the waveform is driven by half of the topology and the negative by the other. The problem with this class is there is an excessive amount of crossover distortion where the one half of the circuit relinquishes and the other takes over. This is partially caused by transistors natural operational values where they do not maintain linearity in their first ~volt, but this is another story.

 

In class A/B a small bias is introduced to offset the output above this distorted operational zone. In effect each stage is biased so that they operate, on avg. 5% to 10%, into the opposing swing of the waveform. The crossover zone is then canceled when recombined and put through the coupler.

 

The final coupling stage tends to be a rather large capacitor. Because passive components tend to have variating values based on frequency a sudden introduction of direct current will produce an oscillation. The circuitry is trying to level out, and this oscillation will take a few RC time constants to dissipate. So essentially when you turn on the amplifier all the circuits are charging up and the process causes the oscillation that you hear. If this were a purely class B amplifier, chances are probable that you would not hear it. If you are making a comparison to other devices, either the effect was too small to notice or the device was driven using a different circuit topology(of which there are many.)

 

My own stereos and sub-woofers did this all the time. I think most of the mid range to mid-hi range stereos do this, where some hi-hi range systems utilize soft start circuitry to help protect the longevity of the internal components. Class A tube amps, where the waveform is completely biased into a given potential and where half of the output power is consumed across a coupling stage resistor, I'm sure made a rather large thump :/ Any old people in the forum to corroborate? I hope to go 300B someday . . . . . . even though old age has taken >15khz away from me :<

[Externet]

It can also be poor design.

A properly made unit would mute audio during switching on and ramp up the level after.

[Brainteaserfan]

It can also be poor design.

A properly made unit would mute audio during switching on and ramp up the level after.

Bad design of the AirPlay device?

[Xittenn]

It can also be poor design.

A properly made unit would mute audio during switching on and ramp up the level after.

 

Unloading the driver from the amplifier during the initial charging phase would only serve to delay the inevitable discharge over the driver. Any additional circuitry used to minimize this effect presents debris in the path of the outgoing signal. Expensive equipment minimizes this effect at the power source. The rails are brought up to operational potential gradually as opposed to immediately. Unless done properly such circuits also only serve to add debris . . . . limiting this phenomena, generally produces more undesirables than it is worth in eliminating the effect. The effect is generally considered benign with exception to equipment where the investment becomes more apparent.

 

Just for example I used to have a Yamaha AX570 amplifier, which was one of the better ones made by Yamaha. I used it to drive a pair of Energy 5.1e tower speakers, also one of the best produced by it's respective producer. A quick google shows the many reviews of each . . . . My friends used to find my thump impressive!