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Forum Index : Microcontroller and PC projects : low pass filter
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| ville56 Regular Member  Joined: 08/06/2022 Location: AustriaPosts: 96 |
I think the carrier suppression is only relevant if you have an amplifier following the filter, so load impedance is several kOhms usually. If one connects headphones, the 44 kHz level is more or less irrelevant IMHO. What is important then is the low impedance of the filter in the range up to 20 kHz to be able to drive 32 Ohm headphones. 73 de OE1HGA, Gerald |
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Mixtel90 Guru  Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
You can't really drive 32R headphones properly from a PicoMite, IMHO. It can't give the current needed so tthe output impedance isn't good (damping factor is non-existent). It does give reasonably acceptable results though. There's not really any point in aiming to get a flat response up to 20kHz when most of the people interested in the PicoMite can't hear much past 10kHz anyway. :) You may as well aim lower and get better rejection at 40kHz with an easier to build, wider filter. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2134 |
>-----choke------choke------> : : : = = = : : : ----------------------------Gnd ? |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
Guy's, please cool down. You are free to design your own filter. This is what my simulation shows, and it's pass band (the audio you should hear) has been verified by measuring frequency response using the PLAY TONE. Note this filter is 14kHz (not 20kHz) since that was giving the best results for me. The 44kHz is really far down in my test setup, and my 2 Picomite VGA 1.7 boards. The 1k termination is not dominant, you still get acceptable frequency response when terminating with the input of an audio amplifier (10k or 50k) and with a headset (600ohm or 32 ohm).  P.S. if you want to build, or simulate yourself, note that the inductor is 4.7mH (mili Henry), not microhenry.... Edited 2023-03-30 05:19 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
... and it sounds *very* nice to my ears. :) Many thanks. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2134 |
I tried the manual filter witha cheap portable stereo and got a scratching noise about every second. Sounded better with cheap logitech amplified speakers. Please try this do for c=50 to 1000 PLAY TONE c,1050-c,10 pause 20 next c loop How do I use play wav file$ to play one of 2 files on sd card, t1.wav and t2.wav? I don't understand the file system. |
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homa Guru Joined: 05/11/2021 Location: GermanyPosts: 351 |
play wav "b:/t1.wav" or play wav "b:/t2.wav" |
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Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Thanks Volhout, that is the sort of response curve that I was expecting. I notice that you do not include the 10 ohms effective series resistance on the inductor that should broaden the notch and reduce it's depth. To what extent? This topic has always included a lot of 'I think' and 'IIRC' and 'IMHO' and my concerns that I raised before still exist (to me at least). Many seem to believe that it is vital to remove as much of the 44.1kHz as possible with warnings of cooking MOSFETs and tweeter damage. But no with proof. Silicon Chip produced a Pico Backpack which I believe has been reproduced in Practical Electronics. The PWM audio in that circuit consists of a single stage RC filter. They say that it is suitable to connect to an audio amplifier. So much audio is reproduced digitally these days with CDs, DVDs, BLU-RAYs and MP3s. I don't recall concerns being raised about any residue carrier signals - although I don't read HiFi magazines or articles much anymore. Bill Keep safe. Live long and prosper. |
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| stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2134 |
I scoped the audio gp pins without the filter and it locked onto a 44kHz signal not the note playing. The filtered sound gives repetitive odd sounds with a cheap amp but not with another cheap amp/speaker. One sound scenario is vga monitor and the tiny speakers/tweeters. It depends on the filter and what inaudible left. Inaudible but gives you a headache? I'm tempted to play 44kHz though amp/speaker and see if it affects me :) |
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| stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2134 |
nice but I get > play wav "b:/t2.wav" Error : The volume has no work area I don't find the manual helpful. |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Do that at your own amplifier's peril Stan. I wouldn't do it. Other posts have shown that you can increase the level of the PWM by switching circuits and feed it to a small speaker to get an acceptable audio. I would still worry about that causing the speaker voice coil to heat up and be damaged after long term use. Bill Keep safe. Live long and prosper. |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
@Volhout. please take this no further. You and many others are very happy with your filter and that is a good thing. Maybe I tend to over analyse and question things too much but while I still have the concerns and opinions that I expressed they are only mine to worry about. I have posted them here and that's that. I'd rather that you put your energy into your next great project. Have you considered a cut down and portable LCD version of the Logic Analyser? Bill Keep safe. Live long and prosper. |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3816 |
Why do you even ask that? It's not what the warnings are about. It won't. Might damage tweeters, as has been posted with reasons. John Edited 2023-03-30 17:05 by JohnS |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
Yes, that is on my list, but will require a bigger screen than the ili9341, not neccesary pixel wise, but diameter wise. You shoul have clear vision. PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
You can't hear 44kHz at all, especially at your age. Your ears simply won't respond to it at all. What *might* happen, depending on many things, is that your speakers simply stop working and you won't be able to get them to work ever again because the voice coils have gone open circuit. It doesn't matter whether your amp and speakers are cheap or expensive. It does matter whether the amp can pass enough power at that frequency to burn out the voice coils. Some can, some can't. Some speakers can stand it, some can't. It really is a toss-up what happens. No filter is absolutely perfect. Those that are close to perfect are expensive, bulky and tricky to set up. That doesn't matter though. The important thing is that at the required listening volume there is insufficient 44kHz from the power transistor driver stage to damage either the output transistors or the speakers. There is nothing new about all this. Just ask anyone who repairs guitar speakers. They are experts at changing high power tweeters that have fried because of harmonics from distortion pedals at high volumes. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2141 |
In a multiway speaker the tweeters are vulnerable to destruction as their power rating is a small fraction of the woofer rating. In music of any type most of the energy is in the lower frequencies so that is a practical design choice. The amp. will often have enough power to push the woofer to it's limit so can easily incinerate the tweeter if there is enough high frequency content. The wide band driver of a single way system is less affected by this as it should be rated to take all the amp can deliver without overheating. The voice coil inductance and inertia protect it from excessive travel. The 44khz should have little effect on the amplifier. It will be pushing the amp to clipping so the output transistors are just on or off and dissipating little power. |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
@Volhout, your LC filter values seem the by set in stone now but other values will resonate at the same frequency. For instance: 2.7mH and 4.7nF = 44.68kHz the same frequency. 3.3mH and 3.9nF = 44.36kHz even closer to 44.1kHz Other values of inductance would also work but may need non standard values of capacitors (two capacitors in parallel) Do you foresee any problems using other values? Bill Keep safe. Live long and prosper. |
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