Opamp based audio high pass filter - Page 1
Forum section: electronics / beginners

author: Lekke
author_rank: Newbie
timestamp: July 19, 2012, 03:42:23 pm
content: Hello,Me and my friend have built boombox/ghettoblaster. It is built aroundTA2020-based ampand Audio pro stage 4 speakers. Whole thing is powered from 12V 7Ah VRLA-battery and audio input is coming from mp3-player/mobile/etc. Problem is that low frequencies (<80-90hz) cause clipping relatively easily due to poor frequency response. Since improving that is difficult (Its supposed to be cheap and portable) I figured that simplest solution would be to filter those frequencies out.Filtering is most convenient to do before the amplifier so I came acrossthisschematic and I guess I don't need anything more complicated? Do I need double-sided voltages for the op-amps? If so, what would be the best way to do that? Also, what values should I use for R and C? 8.2k and 220 nF would give me 88Hz, which would be perfect but do the absolute values of components affect sound quality/noise pickup/whatever? Does type of the op-amp matter? Would there be any audible difference between cheap generic op-amp and expensive op-amp designed for audio applications?Cheers,Lekke

author: T4P
author_rank: Super Contributor
timestamp: July 19, 2012, 05:04:29 pm
content: Open up commercial equipment that sounds decent. Do you see any burr brown? NO.anyway, if the lack of frequency response is at 80-90Hz, go design your own kit, they got it wrong if it clips at 80-90Hz ( That's mid-bass for you, if you filter that one out, your portable boombox will sound very thin)If it's a inherent flaw in the TA2020 chip, you're SOL

author: olsenn
author_rank: Frequent Contributor
timestamp: July 19, 2012, 05:14:25 pm
content: QuoteI have no problem with Burr Brown; I used one of their op-amps in an audio design before and it sounded great. However, if you're only getting down to 90Hz then you have some other serious problems there... you should be getting down to at LEAST 20Hz.If it actually is clipping, and not being attenuated, then you probably have extra low-bass amplification in your op-amp (a paralel RC circuit in the feedback loop) which is causing the output to try to go above/below the positive/negative rails. Inject a low frequency signal (pure sine wave) into the input and observe the output on an oscilloscope... see if the gain looks to be larger than that of higher frequencies.

author: Lekke
author_rank: Newbie
timestamp: July 19, 2012, 05:29:14 pm
content: Sorry, might have used some wrong words there. Amplifier is working perfectly. What I meant was that speakers don't produce as loud sound as they do with higher frequencies (small elements and enclosure). So If I try keep sound level the same it easily results to clipping due power output limits of amplifier. Cutoff frequency is really rough estimate at this point, If music sounds thin then Ill just lower it until problem is gone.Sorry for confusion.

author: T4P
author_rank: Super Contributor
timestamp: July 19, 2012, 06:05:49 pm
content: Quote from: Lekke on July 19, 2012, 05:29:14 pmI see what you mean, how about dipping the high frequencies instead?Quote from: olsenn on July 19, 2012, 05:14:25 pmJust making a point that there's no need to use a burr brown or any audio-specified opamp

author: jahonen
author_rank: Super Contributor
timestamp: July 19, 2012, 08:09:35 pm
content: Based on my experience for these things, may I suggest at least a 2nd order high-pass filter. 1st order filter might not be steep enough, besides the "knee" is pretty shallow. Here is a simulation of 85 Hz second order Sallen-Key filter. It requires merely few more passive components and needs to be driven from a low impedance source. A garden variety cheap opamp like TL072 will work just fine for the purpose. Since it is a dual opamp, you can use second half to buffer the input.Regards,Janne

author: madires
author_rank: Super Contributor
timestamp: July 19, 2012, 08:33:01 pm
content: If I got that right, you're talking about weak bass. So you need a bass booster or better speakers. I would go for better speakers.

author: olsenn
author_rank: Frequent Contributor
timestamp: July 19, 2012, 08:39:26 pm
content: QuoteThat is good advice. However, as I said earlier, if you just want to amplify the bass more than the treble, place a parallel RC filter in series with your other resistor in the feedback loop of the op-amp.

author: jahonen
author_rank: Super Contributor
timestamp: July 19, 2012, 08:46:50 pm
content: My understanding is that the bass frequencies cause the amplifier to clip. And that happens even if the speaker part is not able to reproduce lowest bass frequencies. Thus it makes sense to filter out those frequencies which are not reproduced properly anyway and leave more room there which can be reproduced. It makes perfectly sense. "better speaker" is a difficult thing since it almost invariably means that the box must be larger or efficiency must be reduced, which is not desired on a boombox application.Regards,Janne

author: madires
author_rank: Super Contributor
timestamp: July 19, 2012, 09:57:51 pm
content: Quote from: jahonen on July 19, 2012, 08:46:50 pmSince bass is weak Lekke increases the volume and the amp starts clipping, I guess.QuoteTo be able to discuss that we need to know the details of the speakers. It's not hard to build decent speakers with a box volume of 2 liters.

author: Psi
author_rank: Super Contributor
timestamp: July 20, 2012, 01:58:05 am
content: Put a scope on the TDA2020 power rail and check that it's not dropping down under the load of base frequencies.The amp maybe having issues pulling high current out of your gelcel. If that's the issue a large powersupply cap may fix it, or a newer/better battery.

author: T4P
author_rank: Super Contributor
timestamp: July 20, 2012, 06:50:29 pm
content: Oh cr*p, that was a mistake ... tired at the timeOkay let me clarify, thou shall check voltagesLow voltages can cause clipping... so check if there's voltage dipping when you raise the bassAnd also put it though a scope and use FFT

author: madires
author_rank: Super Contributor
timestamp: July 20, 2012, 09:09:01 pm
content: Just checked the datasheet. The TA2020 is a modified class D amp. Are your speakers 4 or 8 Ohms? The Cocapacitors of the output stage need to be 0.47uF for 4 Ohm speakers and 0.22uF for 8 Ohm speakers. Might be the cause of your problem.