Audio Limiter - Page 1
Forum section: electronics / projects

author: Michaelaudio
author_rank: Regular Contributor
timestamp: December 10, 2020, 07:35:42 pm
content: Good evening Guys,i recently thought about designing an Audio Limiter but i would need lots of help with this.I want a limiter, that limits the sine wafe in the Input of an amp but NOT that the wave just gets cut off. Lets say i have 2Vptp on my input, that would couse an Output ptp of 48V what would couse the max rms Power of my connected speaker. Now, when i wanna turn the input higher the voltage should be still at 2Vptp. I thought about it and all i could think of was something with VCAs and opamps.Is there a not too complex methode to do it analog?Since i cant program anything, digital would bei a bit Tricky i guess (besides of ADCs or DACs)

author: bdunham7
author_rank: Super Contributor
timestamp: December 10, 2020, 07:45:17 pm
content: https://www.analog.com/media/en/technical-documentation/data-sheets/AD8336.pdf

author: Benta
author_rank: Super Contributor
timestamp: December 10, 2020, 07:55:41 pm
content: https://www.onsemi.com/pub/Collateral/NE570-D.PDF

author: Michaelaudio
author_rank: Regular Contributor
timestamp: December 11, 2020, 03:25:18 pm
content: That was easy, thanks😄.So it seems that the ne570 is exactly what i need, i guess it wont affect the Audio quality since its a rather old ic?

author: Messtechniker
author_rank: Frequent Contributor
timestamp: December 11, 2020, 03:32:33 pm
content: I use the attached simple FET-limiter.Of course it affects the audio.Distortion is around 0.05 % at levels below limiting.

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2020, 06:59:32 pm
content: Quote from: Michaelaudio on December 11, 2020, 03:25:18 pmIt's about the same age as the NE5532, which even by today's standards is still an extremely good audio op-amp. I'm not too sure about the TL072...As you only intend to use the compressor part, I see no problem.The issue is with the expander, where at low signal levels noise gets amplified as well.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 11, 2020, 09:12:19 pm
content: Many years ago, Clairex corp manufactured a lamp-photocell combo inside a black tube. Under the trade name Vactrol.Some of the models were very useful as gain control devices in the audio chain.You can still find them on Ebay.

author: TimFox
author_rank: Super Contributor
timestamp: December 11, 2020, 09:41:41 pm
content: In audio, a "limiter" usually clips the waveform instantaneously (which you don't want). This is required, for example, ahead of an FM transmitter to ensure that the instantaneous frequency deviation does not go outside the legal limit.A "compressor" is slower, and reduces the gain at high signal levels. However, it does not guarantee that the voltage does not instantaneously exceed the desired limit.A good compressor should not introduce excessive distortion, but it does reduce the dynamic range which is part of "audio quality", at least for music that goes from soft to loud."Vactrol" lamp-photoconductor modules were popular for both gain control and compression, and exploited the relatively slow response of the photoconductor (typically CdS) to average the response. The incandescent lamps, with thermal inertia, also gave a useful response to AC signals. LEDs can be used with appropriate circuitry, but photodiodes are not suitable.Before that, broadcast compressors used "variable-mu" pentodes (or similar tubes), with an appropriate grid bias voltage derived from an audio rectifier and low-pass filter. If you want to try that, I suggest a 6L7 heptode.Be sure to understand what is meant by the power limit of your speakers. Actual peak power with a low duty cycle (typical of music) may be less destructive than the true average power of a continuous tone ("rms power" is a misnomer).

author: Non-Abelian
author_rank: Contributor
timestamp: December 11, 2020, 09:47:06 pm
content: Try this:http://thatcorp.com/Analog_Engine_Dynamics_Processor_ICs.shtmlLook here for design examples:http://thatcorp.com/Design_Notes.shtmlAnd Spice models:http://thatcorp.com/Device_Models.shtml

author: Michaelaudio
author_rank: Regular Contributor
timestamp: December 13, 2020, 08:52:33 pm
content: @Messtechniker but it just cutts the wave right?@Timfox id rather so it with "normal" ICs who are available in SMD package too. And yes ur right, im looking for a compressor not a Limiter. I know that s compressor doesnt attack imidiatly but it doesnt need to. I Just want a (ideally) adjustable compressor, so i can adjust IT to a input voltage that doesnt get Dangerous over time in the Output for the soeaker.@non-Abelian i read those Sheets and realized my technical english isnt as good as i thought😄 But this must (also) be an IC i was looking for. What is the difference between the THATs and the NE570(besides the Features)? I can use both ICs for my needs cant i?.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: December 14, 2020, 06:07:01 pm
content: My limiter basically does this classic knee thing. See pic.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 14, 2020, 08:16:16 pm
content: Quote from: Michaelaudio on December 11, 2020, 03:25:18 pmThis is an odd statement. Why wouldn't it affect "audio quality" because it's an old IC?Anyway, as said above, of course it "affects" audio passing through it. The question is, are the characteristics meeting your requirements, and what your requirements are to begin with.As also said above and shown with this IC, what you need to implement a limiter is some kind of dynamic range processing. There are a number of analog solutions (THAT being famous for them) you can choose from.You could also implement it with basic components if you're so inclined, although getting "better" and/or "cheaper" than even a NE570 is not necessarily as easy as it looks. It will be close to a compressor, although a limiter only requires to operate one way (ie. lower the gain if the input signal's amplitude gets above some threshold).What you need is typically:* A VGA (variable gain amplifier) - which can be a VCA, or something else;* A signal amplitude estimator;* A thresholding circuit.For the amplitude estimator, you could either estimate the RMS, or the peak amplitude. So either a RMS estimator or an envelope detector, basically.For the thresholding circuit, you may want something that can implement an "attack" and "release" time depending on your needs. A circuit that would react too fast may oscillate in the frequency range of interest, which could be nasty.

author: Michaelaudio
author_rank: Regular Contributor
timestamp: December 15, 2020, 11:29:22 am
content: @Messtechniker whats the threshold voltage and how can i adjust it? Attack time?Okay some explanation: i Just want the IC to have a linear frequecy reaponse, i somewhere read that the NE570 isnt that linear.Okay then it seems some of the THAT ICs is what i need, they have the RMS estimator and the Others things. Now i have to fugure Out how to build the circuit of it.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: December 15, 2020, 01:40:46 pm
content: As its a soft knee there is no precise threshold.If you want it to limit beginning at lower levels simply increase gainahead of the limiting circuit. Within reason of course.Or as we audiots say: blow at it harder.

author: Non-Abelian
author_rank: Contributor
timestamp: December 15, 2020, 09:50:33 pm
content: Quote from: Michaelaudio on December 15, 2020, 11:29:22 amIf you look at all of the design notes on the THAT website, they have examples of how to use the chips. If you want to adjust attack and decay time, look at design note dn115. If you want an example of how to do it using an opamp instead of an fet, let me know and I'll post something. Basically, you need the 4305, a few opamps, a compartor and some transistors for current sources.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: December 17, 2020, 04:31:01 pm
content: That NE570 is a expensive part and not in DIP.When i go build a compressor and dont care about the quality i go for a OTA.