Audio amplifier output capacitor value - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 03:57:53 pm
content: Hi,I need to drive a speaker with impedance down to 4R and am using the OPA547 (datasheet attached here).Not familiar with audio applications but want to keep the signal as clean, noise-free and as close to the input signal as possible. I was planning to use it in a simple inverting or non-inverting configuration with only two resistors setting the gain in the classic fashion.Two questions:1) How do I choose the output capacitor so that I can drive speakers as low as 4R?2) What is the logic and the calculation for choosing such value?3) I saw some designs have capacitors in the feedback loop. Do I need any? Why?Many thanks

author: tooki
author_rank: Super Contributor
timestamp: June 07, 2020, 05:27:44 pm
content: Caps in the feedback loop make it an active filter.I’ve been tinkering with audio amp circuits for a few years and have come to the conclusion that, unless you expressly seek the thrill of DIYing it, it’s cheaper and better to just use a good speaker amp IC. Depending on your needs, linear or class-D will make more sense.

author: Benta
author_rank: Super Contributor
timestamp: June 07, 2020, 06:21:33 pm
content: No magic about the output cap. It determines the bass cut off frequency and is easy to calculate.It's just a CR high pass filter. With a 4 ohm-loudspeaker and a 2200 uF cap, the cut off is at ~18 Hz.Caps in the feedback loop are normally for stability, but need to be chosen carefully (meaning proper circuit analysis,notgut feeling).

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 06:29:35 pm
content: Thank you BentaQuote from: Benta on June 07, 2020, 06:21:33 pm1) How do I find the correct value for the OPA547 driving a 4R speaker? What is the easiest and safest way to determine it?2) if I change the speaker to a different impedance one, do any feedback loop capacitor need to be changed to?Thank you

author: Benta
author_rank: Super Contributor
timestamp: June 07, 2020, 06:52:44 pm
content: You don't need a cap in the feedback loop. Dunno where you got that from. The OPA547's frequency/phase response is excellent.My comment was of a general nature.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 07:02:15 pm
content: Perfect, thank you Benta!!

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2020, 04:26:03 am
content: Quote from: ricko_uk on June 07, 2020, 03:57:53 pmYou have three questions.QuoteThe output reactance should be a fraction of the load impedance both for good low frequency response and to limit distortion products. In lower performance applications, the output capacitor may be used to set the low frequency cutoff.Because of the low impedances and frequencies involved, output capacitors are big and for this reason some designs take feedback *after* the output capacitor so that a smaller one may be used.QuoteA feedback capacitor is used to limit bandwidth however it has the side effect of extending recovery from clipping which may be undesirable so bandwidth should be limited in other ways. A feedback capacitor also requires unity gain stability from the amplifier which further limits performance.A series capacitor may be used to reduce gain at low frequency and remove DC offset. Note that the same rule mentioned about about fraction of the reactance to limit distortion applies here also.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 08, 2020, 08:16:03 pm
content: Thank you David!