Audio limiter circuit or device - Page 1
Forum section: electronics / beginners

author: Compo
author_rank: Newbie
timestamp: April 24, 2017, 08:28:16 pm
content: I'm looking for something to limit my headphone volume when listening on my PC, when it goes over a certain threshold. I have seen expensive commercial devices but would like to know if there are any DIY or build-able circuits to mute when the audio is too loud? I have tried software audio compressors but they add too much latency.I read that a pair of zeners can do audio clipping and there are these chipshttp://www.njr.com/semicon/PDF/NJM2761_E.pdfbut it says that the limit range is 200mVrms to 1Vrms. My multimeter shows the line out of my PC during playback is at 0.005V on a comfortable listening level and 0.012V when excessively loud. Would the actual output from my PC be higher than my multimeter is able to capture on AC?Thanks

author: sokoloff
author_rank: Super Contributor
timestamp: April 24, 2017, 08:56:40 pm
content: Those multimeter readings are suspect (almost certainly wrong in the "too low" direction).I'd look at the output on an oscilloscope as you'll get a better reference there (and you'll see what effect the zener diodes will have). I'd expect that your measurements are about a factor of 30 too low. (Rather than 5mV RMS being comfortable listening level, I'd expect it would be more like 100-200mV RMS.)The zener diode clamps will limit the audio level at the headphone, but it won't sound pleasant when they are active. (It will distort significantly as it operates.) If the intention is just to protect hearing and the user response will be, "Oh, I need to turn this down" that's OK.

author: sokoloff
author_rank: Super Contributor
timestamp: April 24, 2017, 09:04:23 pm
content: Also, if you just want it to work for one PC and one set of headphones, you could try just a plain resistor or resistor network voltage divider and see if that takes the maximum output for the headphones down to an acceptable level. (That will have the side-effect of reducing the sounds across the board, so you may need to play around with it and see if that gives acceptable results.)The advantage to this is that you may already have the parts on hand and it should have less distortion to the sound, though has the disadvantage of making everything quieter rather than just clipping the loud parts.

author: Seekonk
author_rank: Super Contributor
timestamp: April 24, 2017, 09:11:44 pm
content: You could make a compressor with an opamp that rectifies the signal and feeds that into a transistor or fet as the lower end of a voltage divider.

author: nugglix
author_rank: Regular Contributor
timestamp: April 25, 2017, 07:30:37 am
content: For a DIY solution you might want to see:Have fun!

author: BrianHG
author_rank: Super Contributor
timestamp: April 25, 2017, 08:29:31 am
content: Quote from: Compo on April 24, 2017, 08:28:16 pmUse the audio compressor, 'Loudness Equalization' built into Windows & set it's response setting to 'Short' instead of 'Long'. It's really reactive and makes all Youtube videos sound at 1 volume without soft volume portions, then, a blast during a commercial interruption, or another Windows app. Setting it to 'Long' gives that annoying short loud burst delay before it kicks in, then when the sound is low, it takes a few seconds to volume up instead of instantly...Remember to un-install all crummy third party volume peak limiters which don't respond according to how the human ear perceives actual loudness, not signal peaks.And remember to set each set of speakers you want the feature on. For example, you can set you headphones output to Volume Equalize while simultaneously set the HDMI audio out not to do so your surround movies play properly instead all at 1 volume. You can always do both if you like.Except for FFDshow's internal audio limiter (an old obsolete Direct show Audio&Video decoder), all other software limiters I've heard don't respond quickly enough, though they may be good enough for music. Setting Window's built in version to 'Short' is lousy for music since everything always comes out at exactly 1 monotone fixed volume, though it's excellent for News, talk radio and regular TV.

author: danadak
author_rank: Super Contributor
timestamp: April 25, 2017, 12:28:52 pm
content: Here is a free PC scope you can look at your audio -https://www.zeitnitz.eu/scope_enGoogle "protect sound card inputs" for ways to make sure you do not blowout your sound card, its input.Regards, Dana.

author: JanJansen
author_rank: Frequent Contributor
timestamp: April 26, 2017, 02:54:19 pm
content: It seems for best audio quality they use a LDR instead of transistor.

author: Compo
author_rank: Newbie
timestamp: April 26, 2017, 08:33:55 pm
content: Thanks all for the replies. The Windows Loudness Equalization just increases volume output across the board for me. It definitely brings up the quiet parts though.Would a sound card oscilloscope be able to read input voltage?I found this pagehttp://sound.whsites.net/articles/vca-techniques.htmldescribing compressors and limiters but a lot of it has gone over my head. Which type would one of these chips be?http://www.njr.com/semicon/PDF/NJM2761_E.pdf

author: Audioguru
author_rank: Super Contributor
timestamp: April 27, 2017, 12:17:17 am
content: Why isn't a recommended circuit shown for the audio limiter IC??Digikey has thousands of them in stock today.

author: stevelup
author_rank: Regular Contributor
timestamp: April 27, 2017, 08:56:49 am
content: The BBC fit limiters to pretty much all their headphones. They are matched to each pair of headphones and calibrated by cutting wire links.Electrically, vaguely speaking, I believe they contain a small transformer connected across the audio. The secondary of this transformer is connected to the gate of a FET which then clamps the output. They have been using the same design since the 70's and they are still manufactured to this day.There's some more info here:-http://www.bbceng.info/EDI%20Sheets/10480.pdfhttp://www.canford.co.uk/HEADPHONE-LIMITERSI couldn't find a schematic anywhere though

author: BrianHG
author_rank: Super Contributor
timestamp: April 27, 2017, 10:37:11 am
content: Quote from: Compo on April 26, 2017, 08:33:55 pmYes, now just lower the system master volume and everything will play at that now 1 lower volume. Now it you want to hardware limit headphones so this may work on any playback device, IE your smart phone, you'll need a device to do so.

author: Compo
author_rank: Newbie
timestamp: April 27, 2017, 08:53:27 pm
content: I found a thread describing the BBC / Canford one with a dead link to a photo of onehttp://freestompboxes.org/viewtopic.php?f=1&t=8456And another site which mentions a transformer being used to trigger a limiter:http://theaudiophileman.com/getting-wires-crossed/QuoteThere's also an ancient photo of one:http://www.bbceng.info/EDI%20Sheets/10597.pdf. I see a transformer and perhaps 2 FETs?