AC Coupling/DC Blocking Capacitor sizing for speakers

Thread Starter

SamR

Joined Mar 19, 2019
5,060
Let's use this circuit from the PDF for a LM386.
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Why is this capacitor 250uF? I also often see 100uF used. Why such a high value? Is there a proper way to size it?
 

Jony130

Joined Feb 17, 2009
5,516
This capacitor together with the speaker impedance form a high-pass filter. Thus, this capacitor "sets" the desired low-frequency response of the circuit.
Fc ≈ 0.16/(R * C) = 0.16/(8Ω * 250μF) = 80Hz
So, a signal with a frequency greater than F > 80Hz will be "heard in the speaker".
 

BobTPH

Joined Jun 5, 2013
9,152
250uF feeding 8Ω is a high pass filter. The capacitor is chosen to pass the lowest frequency we want to hear. For a full range speaker, 250 is too low.
 

crutschow

Joined Mar 14, 2008
34,689
Since it is labeled an AM Radio Amplifier, perhaps the designer thought it okay to roll off the response at 80Hz.
Normally an audio amp goes down to below 20Hz.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,060
Yup, I noticed that. Would need to go 2200uF to get down to 10Hz. But with a primitive AM radio I wonder how much of that is noise... I'll find out soon. I just didn't recognize the speaker impedance as part of a filter. Now I do.
 

Audioguru again

Joined Oct 21, 2019
6,758
Humans cannot hear sounds below 20Hz, just feel vibrations and hear any distortions of the vibrations.
Few speakers can produce 20Hz, a few produce 30Hz and many speakers can produce 50Hz.

My 3" computer speakers produce 60Hz well but 50Hz is at a low level and 40Hz is not heard.
I have a cheap clock radio that I increased the capacitance of its coupling capacitors and replace its cheap 3" shrieker with a ported 6.5" woofer and dome tweeter that I built. It sounds like hifi on FM stations but still sounds bad on AM.

I have a cheap Amazon 4" woofer in a ported enclosure that produces 60Hz well but 150Hz is much too loud due to a resonance.
 

Audioguru again

Joined Oct 21, 2019
6,758
Isn't AM radio used for vocal news, sports scores and weather guessing?
Most guys cannot produce vocal sounds below 80Hz and of course women also cannot.
 

DickCappels

Joined Aug 21, 2008
10,230
Most guys cannot produce vocal sounds below 80Hz and of course women also cannot.
This is 2023! Genders do not exist in public anymore!!!! Sorry, could not let that go :- )

If one wants an 80 Hz low frequency cutoff, and assuming the speaker enclosure allowed an 80 Hz cutoff, what would the capacitor's value be for a typical (non-large) 8 PM speaker?
 
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