Changing the capacitor values of an amp for a subwoofer  - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: June 29, 2017, 05:58:09 pm
content: hello friends..I often sees amplifiers using various capacitor arrangements for filtering the audio spectrum for passing specific frequency levels and I never knew how to change it according to our need. So to learn about it I thought of using a LA4440 amplifier circuit in bridge mode (which is capable of delivering 19W at the output) to drive a sub woofer. On looking at the datasheet I saw the description about the capacitors used in the circuit but I found it very hard to understand as I'm not good in electronics. So could anyone help me with the capacitor values & the method for its calculations?? I'm attaching the circuit and the description in the datasheet.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2017, 07:24:51 pm
content: None of the capacitors in the circuit shown have anything to do with frequency filtering, at least not directly. It obvious that coupling capacitors will impact the low-end rolloff, of course.For a subwoofer, you'll need additional low-pass filtering, active or passive, with well-defined cutoff frequency and phase response.

author: abraxa
author_rank: Frequent Contributor
timestamp: June 29, 2017, 09:53:42 pm
content: Quote from: Benta on June 29, 2017, 07:24:51 pmSorry, but I'll have to disagree - C7 and C8 are in series with the load and certainly act as a high pass. For a subwoofer amp, their value shouldn't be too low or else they'll use the energy to keep warmOn the topic of caps, I would suggest using a (much) bigger cap for C11 (the one connected to pin 11).Quote from: Benta on June 29, 2017, 07:24:51 pmAgreed. I suggest you research opamp-based low pass filters that you can use on the line signal before feeding it to the amp.http://www.circuitstoday.com/low-pass-filter-for-subwooferis an example.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2017, 10:06:55 pm
content: Please quote me correctly. C7 and C8arecoupling caps and will of course influence low-end rolloff, just as I stated.Thank You.

author: Adhith
author_rank: Frequent Contributor
timestamp: June 30, 2017, 06:51:35 am
content: Thank you Benta and abraxa for your help. I'm planning to use an active 2 way crossover for the filtering based on LM833 IC which you can see in the attachment. So would the problem be solved if I change the value of the caps C7 & C8 from 1000uf to 2200uf?? What about the feedback caps C1 & C2 and the bootstrap caps C3 & C4 ?? should I leave its value as it is in the schematics??

author: Adhith
author_rank: Frequent Contributor
timestamp: June 30, 2017, 06:55:53 am
content: Quote from: abraxa on June 29, 2017, 09:53:42 pmC11 deals with the power supply ripple rejection right?? I'm already providing a regulated supply from a 7812 regulator IC with 10uf caps at its input & output as filtering caps. So do I need to change the caps at the pin 11 ??

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2017, 08:01:38 am
content: Quote from: Adhith on June 30, 2017, 06:55:53 amWhy?The datasheet says nothing about it requiring a regulated power supply and the LM7812 might limit the current to too lower value, causing the power supply voltage to dip.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 03, 2017, 06:31:32 pm
content: Thank you Hero999 for your suggestions. Please correct me if I'm wrong ,I was under the assumption that an unregulated power supply could increase the noise level of the output of an amplifier and I keep on using regulator ICs for almost every amplifier circuits that I make. I'm also intrested to know the purpose of 1000uf cap in the power input side??So unless specified there is no need to regulate the voltage and the circuit could work fine??

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2017, 09:20:26 pm
content: Quote from: Adhith on July 03, 2017, 06:31:32 pmA properly designed amplifier's output is unaffected by changes in the supply voltage. A property known as the supply voltage rejection or ripple rejection ratio.A large capacitor on the power supply, is all that's needed to remove the ripple before it gets to the amplifier.

author: Adhith
author_rank: Frequent Contributor
timestamp: July 04, 2017, 06:28:52 am
content: thank you again for your help. In that case I'll use a 2200uf cap at the input. In my previous work on the same LA4440 IC I noticed a lack of power than specified in the datasheet. Since I used a 7812 for the amp I doubt that it might be the problem. Let me check this time without 7812.