Amplifier Output Capacitor Values

[sp10mk11]

Hello
I have seen transistor amplifiers with varied value output de-coupling caps, 470uf, 1000uf and 2200uf.
Does this value relate to the impedance of the speaker to be connected, ie 470uf for 4 ohms etc.
Thanks
Gary

[ukcol]

One of our members with design experience will no doubt give you a definitive answer but in the meantime:

There is not a direct relationship between the value of the capacitor and speaker impedance because there are other factors involved but there is a general one.

The capacitor and the speaker form a high pass filter so, everything else being equal, you need to increase the value of the capacitor as the speaker impedance goes down.

However (as an example) you would not chose the same value of capacitance to feed a small 4ohm speaker in a pocket transistor radio as you would to fed a large 4 ohm speaker in a HiFi system. The small speaker would likely be very inefficient at low frequencies.

[dseymo1]

I think you mean coupling capacitor.

[paulsherwin]

Yes, it blocks DC from the speaker.

Lots of early 70s Japanese amps only used a 470uF cap in this position, which does seem rather small, though I've never done the maths to calculate the high pass effect.

[Radio Wrangler]

If you mean the output coupling capacitor used to block DC from reaching the speaker, the value has to be bigger for lower impedance speakers, and bigger if you want to go lower in frequency.

The 3db attenuation frequency is 1/(2*Pi*R*C) where R is the impedance of the speaker, assumed to be purelt resistive for simplicity

Say 1000uF and an 8 Ohm speaker.

then F is 19.89 Hz. So it will have lost 3dB of its bass by 20Hz. For hifi with good speakers, you might want 2200uF. For comms speech 100uF might be enough and a 200Hz cutoff acceptable. for 4 Ohm speakers, double the C value.

David

[Diabolical Artificer]

I always wondered what the purpose of that capacitor was, thanks for clarification. Why isn't there one on complementary OP stages? Is it because they use split rails and the OP is adjusted for DC offset?

Andy.

[tony brady]

it's not needed on split rail designs but is needed on single rail designs as the output will be sitting at roughly 1/2 the rail voltage

[sp10mk11]

Thanks all for your help this makes some sense now.
Sorry when I used the term "decoupling" I meant in terms of decoupling the speaker from the DC on the output.
Thanks
Gary

[Radio Wrangler]

The terms tend to get interchangeably, depending in what flows you are thinking of....

A decoupling capacitor on a power rail couples noise and ripple currents to ground. A signal coupling capacitor, along with the input impedance of the following stage makes a crude highpass filter to block DC, so the two stages can be biased independently.

David

[G8HQP Dave]

Typically, a decoupling capacitor acts as part of a low pass filter to stop an AC signal from passing from one stage to another while DC gets through OK.

Typically, a coupling capacitor acts as part of a high pass filter to enable an AC signal to pass while blocking DC.

[Mooly]

This shows graphically the result of four different coupling cap values (220, 470, 1000 and 2200uf) when feeding an 8 ohm load from a voltage source (read amplifier). Note the -3db points for each.

Also shown is the squarewave response for the same four values at 20Hz.