A basic Limiter

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mike123
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Re: A basic Limiter

Post by mike123 » Mon Apr 17, 2017 10:47 am

Hi there,

I build the mono limiter which I created a print layout for.
However, when I put audio into it, the clip LED lights up first and when add more input volume the peak led lights up. The peak led does not blink as the clip led does, but goes just on or off. Depending on the hight of audio input.
It also sound a little distorted and when I put to much audio into it, it just mutes for a while.
I ideas what is wrong?

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Re: A basic Limiter

Post by Krakatoa » Mon Apr 17, 2017 8:59 pm

Hi Mike,

It seems you have built the typical Fet limiter circuit, as I don't find any VCA chip or else in your circuit.
These circuits (Fet limiters) just serve as final stages in audio processing, they have a narrow dynamic range, meaning that if you push them hard trying to get some sensible compression-limiting effect, they enter the non linear region and distort severely.

I'd recommend something more proper, with a low noise and distortion VCA such as the circuit in the following link: http://sbpcb.blogspot.it/2012/12/stereo ... s.html?m=1

This one works fine and compresses the full dynamic range a pro grade mixing table can output. It also can be a good starting point to build an analogue multiband compressor, adding a crossover and some mods....

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Re: A basic Limiter

Post by sinus trouble » Mon Apr 17, 2017 10:39 pm

Greetings Mr 123
Welcome too Mr Krakatoa :) That looks like a nice limiter you advertised and im sure it works very well!
However it doesn't answer Mr 123s question??
Mr 123, apart from the obvious transistors missing/wrong orientation? It looks to me like a split rail PSU is needed to power it? Or is that just another ground point?
You say it mutes for a second or so? Its hard to tell without a schematic? But it could be that the compression release time or sensitivity is too harsh?
I am as stupid as I look! :|

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Re: A basic Limiter

Post by mike123 » Tue Apr 18, 2017 8:33 am

Hi,

I used the schematic Albert H posted: download/file.php?id=509&mode=view

The "missing" transistor is there, but is added on the back of the print as an SMD.
The other transistor looks like incorrectly added, but that's just the misprint of me when developing the layout.
I had to turn the transistor.
Sorry for the confusion.



Mike

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Re: A basic Limiter

Post by sinus trouble » Tue Apr 18, 2017 11:16 pm

Apologies Mr 123! I did not see the schematic from earlier posts! :|

If I am correct? The 2N5459 is the active component controlling your amplitude? The red LED shows the overall clipping of your output signal! The yellow LED should show when the limiter is kicking in and for how long depending on the attack and release timebase!

R-3M9 and 1uf tantalum should control release time (small/moderate delay)

Assuming your PSU is good and youre not over driving? It should work fine? In operation I would imagine youd get alight flicker on the red LED and smooth but slightly delayed pulse occasionally on the yellow LED?
I am as stupid as I look! :|

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Re: A basic Limiter

Post by Albert H » Wed Apr 19, 2017 1:09 am

The 2N5459 is used as a voltage-controlled resistor - as you vary the bias on the gate, the resistance from source to drain varies. A tiny amount of audio is also applied to the gate - this is a distortion reduction trick that was discovered way back in the 70s. Contrary to what Mr Krakatoa asserts, FET attenuators are the preferred audio control method in virtually all commercial compressors and limiters. They have very low noise and distortion, so are widely used in studio audio processors.

Another option is to use an optical control circuit - there are some very well-regarded audio processors that use this approach. If you use an LED to illuminate a Light Dependent Resistor, you can make a low noise gain controller or attenuator. However, the LDRs react quite slowly, so a limiter of this sort will let through transients, and will recover quite slowly. These are great for use in a studio (my preferred guitar compressor uses this technology because it retains the attack of each guitar note), but are not really much use for a limiter. I have used them in a slow-acting AGC in radio broadcast chains that have several sources that can have wildly differing levels.

The next kind of audio control is the OTA - the Operational Transconductance Amplifier. This is a modified type of op-amp, which has a current input pin that allows gain control. There's only really one or two of these devices on the market these days: the LM13600 and 13700. They're practically identical for our purposes. They can be used in series with the audio, which is OK, but they tend to add a bit of noise and distortion, or a better approach is to use them to control the negative feedback around an op-amp. These devices are OK, and for pirate use they're probably good enough. There are two OTAs in each IC, and they're very well matched, so stereo limiters are easily realised.

The final type of voltage-controlled attenuator is the specialist chip, like the "THAT" series of high quality audio processing components. I've used the 2018 in my commercial limiters with great success, but the ICs are over £10 each. They are also susceptible to supply noise and they really don't like RF about! These ICs are capable of really superb audio performance, but the sidechain often limits their speed of response, necessitating a clipper after the limiter, which rather defeats the point of a low distortion gain control stage!

My latest commercial audio processor uses the MN3007 clocked so as to eliminate any possible beat products with the 19kHz and introduces about 1.5ms of delay into the audio path. This gives the sidechain time to operate, so this limiter has (effectively) zero attack time and also no overshoot. There is no need for a clipper, and the use of high quality audio attenuator ICs, guarantees fidelity. It allows you to be the loudest thing on the band, without nasty clipping or pumping. It's NOT cheap or simple, but if you want the best, it's the only game in town. It makes BW's processor sound nastily distorted, and it outperforms Orban's offerings too! We're shipping lots of them to the 'States and South America, and many station engineers describe our processor / coder combination as their "secret weapon" !
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Re: A basic Limiter

Post by mike123 » Wed Apr 19, 2017 12:20 pm

Thanks for all the information guys.
I do not have that much knowledge of electronics, so all input is welcome.
I used an electrolytic cap of 1u, instead of the tantalum version, but that should not make a difference right?

What part controls the attack time of the limiter? Maybe I can play with that a little bit.
Is there a way to control (or temporarily disable) the clipper also? I think clipping is too brutal and/or kicks in too fast.

@Albert H: Where you selling your audio processors?

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Re: A basic Limiter

Post by mike123 » Wed Apr 19, 2017 12:55 pm

In operation I would imagine youd get alight flicker on the red LED and smooth but slightly delayed pulse occasionally on the yellow LED?
@sinus trouble: Yes. Although I expected the smooth slightly delayed pulse from the yellow led to lighten first and the red led to flicker second (after more input of volume).
But I have a feeling the clipper is activated first and then the limiter. Like it's working the other way around.

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Re: A basic Limiter

Post by Albert H » Wed Apr 19, 2017 3:09 pm

Increase the 1k going to the top of the clipper - that'll reduce the sensitivity a bit (try 2k2 and see if that works for you). You could also add a pair of 1N4148 diodes in anti-parallel to the top of the clipper chain. This will make the clipper begin to work at 0.65V (of audio) more.
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Re: A basic Limiter

Post by mike123 » Wed Apr 19, 2017 3:45 pm

Hi Albert H,

Could you explain where to put the extra diodes (from where to where)?

thanks

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Re: A basic Limiter

Post by Albert H » Thu Apr 20, 2017 2:05 am

Modified diagram for you:
AH_Mono_Processor+.png
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Re: A basic Limiter

Post by mike123 » Sat Apr 22, 2017 9:04 am

Hi guys,

I disabled the diodes to test the limiter only.
The volume still increases, even when the peak led indicates it's limiting.
And it also mutes the audio for a a second, when I increase it further.

Any suggestions?

Mike

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Re: A basic Limiter

Post by Albert H » Sat Apr 22, 2017 10:27 am

You have to bear in mind that it can't handle massive over-level - the impedance through the FET gets to a minimum point, and then it can't go any further! The slope is 1:1 for lower levels, then, as the sidechain starts to feed voltage to the FET gate, the slope reduces until it's almost 1:20 with the indicator fully lit. Beyond that point it will begin to get louder again! The clipper will take care of wildly over-level signals and really rapid transients that are too fast for the sidechain to catch.

If you take a look at http://sound.whsites.net/project67.htm to see a limiter that's nearly the same - Rod Elliott obviously went to the same school as LB! Rod's article explains the operation of the gain cell and the steps taken to minimise distortion (I think that LB pinched Rod's values when he did his PCB).

I found the artwork for the PCB that we made for this circuit. It's on another computer, but I'll liberate it later today and post it here. It's a neat little single-sided PCB that was designed to fit behind the front panel of a small link rig. There's also a Veroboard version that was the first prototype that I'll add too.
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Re: A basic Limiter

Post by Krakatoa » Sat Apr 22, 2017 2:59 pm

Hi Mike, these audio cuts can be caused by the gain control element (the Jfet) that may have a very low internal resistance when it turns on, and eats all the audio in its path, that should otherwise be able to control proportionately.
Try with a lower Idss Fet from the same family such as the 2N5457 or an equivalent.

Look as a reference the limiter portion of a commercial product such as the schematic I have attached. Try if with the minimum mods you can get it working as expected.

If you have access to a signal generator and an oscilloscope, apply signal to your circuit and look at the same time at the input and the output signals. Adjust them so they look equal amplitude. Start with a low level signal of around 1kHz and go increasing the amplitude, and try to find if your limiter circuit starts to attenuate the output signal at a certain input signal level. That is your threshold, and your audio peaks should get into the limiter threshold gently. Don't expect that it will keep attenuating with large input signals, as I stated in my other post.
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Re: A basic Limiter

Post by mike123 » Mon Apr 24, 2017 3:45 pm

Thanks again!
I know someone with an oscilloscope, so we can measure what's happening.

A little of background information:
I was planning to use this limiter for a stand-alone setup (transmitter), with an MP3 decoder board + 32GB USB stick filled with pre-recorded music.
I was testing the audio with a Pira Analyzer and saw it easily overshoots the maximum frequency deviation of 75Khz.
So I want to add a limiter that can level the audio of the different MP3 files.

But reading the title of this page, this design is a more of a Peak Limiter than leveling the audio.
http://sound.whsites.net/project67.htm

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Re: A basic Limiter

Post by Albert H » Mon Apr 24, 2017 10:34 pm

Sounds like you're in the market for a compressor, set up as an AGC before you feed your limiter. This is often a problem - especially in proximity to a transmitter (ask any offshore pirate!), since you'll bring up the noise as the AGC gain increases.

A better way to do this would be to run your MP3 files through "Audacity" or another audio tool that can do "levelling". I do this to the music that I play on my programmes - it's pre-processed before I even go into the studio. You can also do away with the need for a site limiter, since you can pre-correct all the files. It's an extra Production stage, but it'll make all the difference, and if you have a reasonably powerful computer (running Linux, NOT f***ing Windoze), you can process an hour of programme in a few tens of seconds.
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Re: A basic Limiter

Post by mike123 » Fri May 05, 2017 9:38 am

Hi guys,

We found the problem of the limiter not working, with help of an oscilloscope.
We re-soldered the SMD FET, that did not make good contact with the print. Next time I won't use SMD components. Too small for me ;-)
Anyway, the limiter is very very good, and can handle a massive audio input!
I can move the mixer fader from 3 till all the way up to 10 without any significant increase in volume or with distortion.
This is a idiot proof design ;-)

It came with a new problem with the clipper not working suddenly, but I think I won't need the clipper part.
Maybe we will fix it later.

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Re: A basic Limiter

Post by LeeCavanagh » Tue Dec 08, 2020 2:56 pm

Albert H wrote: Sat Apr 22, 2017 10:27 am
I found the artwork for the PCB that we made for this circuit. It's on another computer, but I'll liberate it later today and post it here.
Did you post this somewhere else? I don’t see it here , thanks in advance.
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Re: A basic Limiter

Post by Albert H » Sun Dec 20, 2020 12:57 pm

Here's a Veroboard layout that I've used a couple of time when I haven't had a PCB available. The clipper diodes can be reduced to two pairs, or you can make one pair BAT41s (Schottky diodes) to raise the clip point by 0.2V. It's cheap and simple, includes a 19kHz filter to prevent the blinking stereo indicator problem, has a stage of 15kHz filtering just to shave the top end off a bit, and provides 50µs pre-emphasis. Considering its simplicity, it sounds pretty good on the air, and costs almost nothing to make. The only slightly unusual component is the 22mH coil.
Mono Broadcast Processor.png
This has been used by lots of stations, and all seem to like it!
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Re: A basic Limiter

Post by XXL » Sun Dec 20, 2020 2:16 pm

I might try and build the mono limiter and sinus RDS encoder on the same board. Then I could input audio from a cheap mobile phone and not have to worry about clipping.

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