# AC Coupling/DC Blocking Capacitor sizing for speakers

#### SamR

Joined Mar 19, 2019
4,601
Let's use this circuit from the PDF for a LM386.

Why is this capacitor 250uF? I also often see 100uF used. Why such a high value? Is there a proper way to size it?

#### ericgibbs

Joined Jan 29, 2010
17,416
hi Sam,
Consider the capacitor AC impedance compared to the 8 Ohms speaker over the audio frequency range.
E

#### Jony130

Joined Feb 17, 2009
5,445
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
6,513
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.

#### MrSalts

Joined Apr 2, 2020
2,767
Why such a high value?
If you look inside your stereo receiver, you'll see some much larger output caps and you'd be asking "why such a low value on the LM386 datasheet?"

#### SamR

Joined Mar 19, 2019
4,601
K, got it! Thx guys!

#### crutschow

Joined Mar 14, 2008
32,031
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.

#### Audioguru again

Joined Oct 21, 2019
5,816
An AM radio does not produce hifi sounds. Then it does not matter if it does not play bass sounds below 80Hz.

#### SamR

Joined Mar 19, 2019
4,601
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.

#### crutschow

Joined Mar 14, 2008
32,031
An AM radio does not produce hifi sounds. Then it does not matter if it does not play bass sounds below 80Hz.
True is in not HiFi, but I do think it generally broadcasts bass frequencies below 80Hz.

#### Audioguru again

Joined Oct 21, 2019
5,816
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
5,816
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
9,856
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?

#### crutschow

Joined Mar 14, 2008
32,031
Isn't AM radio used for vocal news, sports scores and weather guessing?
Mostly, I suppose.
But here in the Denver, CO area there are a several stations that broadcast music on AM.

#### MrChips

Joined Oct 2, 2009
28,473
The lowest note on my electric bass is 41Hz.
The lowest note on a 5-string bass guitar is 31Hz.
The lowest note on a piano is 16Hz.

#### crutschow

Joined Mar 14, 2008
32,031
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?
The value shown in post #1.

#### LowQCab

Joined Nov 6, 2012
3,064
Just use a "2-way" "1st-Order" Crossover-Calculator like this one ............
Just enter the lowest Frequency that your speakers can deal with,
then use the Capacitor value given,
( which is normally used to protect the Tweeter from low Frequency damage ).
https://www.diyaudioandvideo.com/Calculator/SpeakerCrossover/
.
.
.

#### joeyd999

Joined Jun 6, 2011
4,816
The lowest note on a piano is 16Hz.
And all you hear are the overtones. Your brain fills in the rest.

#### Audioguru again

Joined Oct 21, 2019
5,816
Most pianos have 88 keys. The lowest frequency is 27.5Hz (A0) which anybody can hear.

#### Attachments

• 192.4 KB Views: 3