PIRA Simple Limiter

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PIRA Simple Limiter

Post by sinus trouble » Thu Mar 24, 2022 11:23 pm

Hello Necks! :)

I built some of these many years ago but never used them as such, Nor did i pay much attention to how they worked!

So i decided to share my experience and opinion on this subject! I will try to cover in detail and answer any questions!
20220324_153906.jpg
Due to a lack of knowledge and correct components in the beginning, I experienced a few problems! I substituted some of the 'Plastic' capacitors for electrolytics!

The limiter worked well but would occasionally heavily dip in volume, Like a really hard compression!

After tracing the source of the problem, I found i had fitted an electrolytic backwards in the audio path! (I will explain why later)
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Re: PIRA Simple Limiter

Post by Albert H » Sat Mar 26, 2022 12:30 am

It's a ridiculously crude design, based on the "Oz Compressor" that was built as a (rather poor) guitar effect box....

The LM386 is a noisy little beast - designed as a low voltage speaker-driving amplifier for use in cheap transistor radios. It's entirely the wrong part to use for a quality compressor.

If you want to do an analysis of a compressor / limiter, take a look at the "THAT" series of audio processing ICs:

I use their 2180-series voltage-controlled amplifier ICs in my limiters. The sidechain uses ordinary TL074 op-amps and LM339 comparators. I use LM833 (or NE5532) dual op-amps for input balanced to unbalanced conversion, and for unbalanced to balanced conversion at the output. The noise and distortion specifications are excellent, and there are "insert" points provided before the gain-controlled stages to allow introduction of delay-lines, so that the limiter can exhibit (effectively) a zero attack time!

I also make multi-band limiters (it's what the market wants), and I've found that it's best to filter off all the audio from 225Hz down and separately process that - a two-band limiter can be very effective.

If anyone's interested, I can show them how I achieve the magic.....!
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Re: PIRA Simple Limiter

Post by sinus trouble » Sat Mar 26, 2022 12:58 am

It is very interesting that you mentioned a 'Guitar' processor Albert! :)

The more i test this circuit, I find it responds differently to general 'On the market' Limiters! Many commercial products are heavily focused on the strongest signals (Which would be bass notes for electronic music) The PIRA has a more mid range response which i did not expect!

I would have to agree that multi band processing gives a more uniform response to what is encountered!

You could be on to a good theory Albert? Maybe commercial manufactures could incorporate a 'Band adjust' into designs??
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Re: PIRA Simple Limiter

Post by sinus trouble » Sat Mar 26, 2022 1:20 am

So for those who are not familiar with the circuit! Here is the diagram!
limiter2.png
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Re: PIRA Simple Limiter

Post by sinus trouble » Sat Mar 26, 2022 1:21 am

Here is my interpretation of how it works!
limiter22.png
The LM386 is just an audio amplifier which i wont go into here!

RED = This is the feedback, filtered and fed into the Limiting stages!

GREEN = This is where the peaks are sampled and held for a duration determined by C7 and R4! Known as 'Release' in Limiter terminology!

BLUE = This is where the gain control happens! :)
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Re: PIRA Simple Limiter

Post by sinus trouble » Sat Mar 26, 2022 1:37 am

Now the YELLOW highlighted parts are where i cocked up! :lol:

I have to mention that if you follow PIRAs instructions! It will work!!!

I however did not! Now generally? An electrolytic in a purely AC circuit should work ok? But in this case, I did not take into account that a DC voltage was present via R2 which caused undesirable results!

Once i had flipped the polarity, All work as it should! :)
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Re: PIRA Simple Limiter

Post by Albert H » Sat Mar 26, 2022 11:33 pm

Your interpretation of the function of the circuit is largely correct.

The things that are wrong with the design:

The LM386 is designed for cheap transistor radios. It's meant to drive a loudspeaker, and has no efforts made to reduce noise and distortion.

The rectifier that samples the instantaneous levels is asymmetrical, so either side of an audio signal will be squeezed differently.

There's no distortion reduction circuit built around the attenuator - so at -6dB attenuation, the distortion will be about 22%!

The time constants around the attenuator mean that the attack will be very long (for a broadcast limiter), and overshoots will occur.

In all - this circuit is - just - better than nothing, but the results from it will be horrible.

I'll put a simple single-band limiter circuit up here later that in its simplest form uses just two ICs, and can include a clipper (if necessary). It's stereo-capable, and can include pre-emphasis. It has been used on innumerable stations and has tightened up their modulation enough to sound loud without over-deviating.
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:34 am

I completely agree Albert! :)

The LM386 is a very basic amplifier designed to squeeze as much power as possible out of a small package! I would not class it as 'High Fidelity' :lol:

Also you pointed out the time constant of the release is quite lengthy which does cause the 'Attack' time to be sluggish! Especially when transitioning from silence to a sudden sound!

If sound is always present? It does not seem to suffer as much from overshoots?

Any ways it would be great to see the version you mentioned! :)
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:45 am

I done some tests earlier today! :)

Hooked up to the oscilloscope and function generator, Tests were performed @ 100Hz with an input voltage of 250mV, 500mV, 1V and 2V!

The RED Trace is the input signal and the GREEN is the output!
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:46 am

Limiter 250mV.PNG
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:47 am

Limiter 500mV.PNG
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:48 am

Limiter 1V.PNG
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:49 am

Limiter 2V.PNG
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 12:57 am

As we can see from the data, The output voltage stays pretty constant @ around 500mV throughout the tests!

However we have to take into consideration that real life dynamics are not as simple as this!
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Re: PIRA Simple Limiter

Post by jvok » Sun Mar 27, 2022 11:59 am

If you set your scope in single shot mode with a long timebase (100ms/div should do it) then suddenly apply a loud signal you'll be able to see the attack time in action.

Rod Elliott over at the ESP site published a design for a peak limiter a few years that does what Albert was talking about. Look at how audio is added to the control voltage on the fet gate to reduce distortion.

https://sound-au.com/project67.htm

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Re: PIRA Simple Limiter

Post by Albert H » Sun Mar 27, 2022 7:27 pm

The limiter on the ESP page is pretty good - fast attack, reasonably low distortion, and an adaptable decay (recovery) time. I've used versions of this type of circuit for years. The only real problem you're likely to encounter is matching FETs for a stereo pair, and commoning the sidechains so that the stereo image remains consistent when the gain reduction kicks in.

The other issue with a simple limiter like this is that a big bass thump will dip out the midrange and treble. This will make that audio "pump" - really not too good with most modern forms of music!

The "pumping" can be mitigated by splitting the audio into bass and the rest, and separately processing them - you're going to need four matched FETs! The outputs of the bass and mid / top limiters (for each channel) can be combined after processing, and the pumping problems will be largely eliminated.

Here's one way of doing it.....
Split-Band Limiter.png
I'm thinking a doing a PCB for this circuit, with a few additional options provided on the board: -
  • Pre-emphasis at 50µs or 75µs on the way in,
  • An option to introduce delay lines to the audio paths to allow "zero attack time" processing,
  • Audio filtering to get rid of content above 15 kHz,
  • A post-process clipping option.
The 15 kHz filters and the delay lines would be on additional plug-in boards, and I've designed a very high quality stereo coder and an optional RDS board to work nicely with this unit. I also planned to add LED bargraphs to show "gain reduction" (the uninitiated love blinking lights), using more LM339s, and even a silence alarm.....

The whole package would be functionally equivalent to something like the Inovonics "David" processor / coders, and would sound a whole lot better than most UK commercial radio ststions!
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Re: PIRA Simple Limiter

Post by radium98 » Sun Mar 27, 2022 7:45 pm

Sinus does this amplify the weak signal and attenuate really the high one , i built one 10 years ago , but did not work , if you can make a little video later and pass on youtube channel .

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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 11:43 pm

radium98 wrote: Sun Mar 27, 2022 7:45 pm Sinus does this amplify the weak signal and attenuate really the high one , i built one 10 years ago , but did not work , if you can make a little video later and pass on youtube channel .
Yes! It gives quite a large audible boost to the quiet sections, This can also be seen in my measurements!

At an input of 250mV the gain is nearly double! (500mV) :)
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Re: PIRA Simple Limiter

Post by sinus trouble » Sun Mar 27, 2022 11:55 pm

jvok wrote: Sun Mar 27, 2022 11:59 am If you set your scope in single shot mode with a long timebase (100ms/div should do it) then suddenly apply a loud signal you'll be able to see the attack time in action.

Rod Elliott over at the ESP site published a design for a peak limiter a few years that does what Albert was talking about. Look at how audio is added to the control voltage on the fet gate to reduce distortion.

https://sound-au.com/project67.htm
Thank you Jvok :)

That was an interesting read! It is always great to see other designs!

The PIRA Limiter is what it says it is! 'Simple' So i dont want to get this circuit confused with more complex designs.

Anyways i took your advice and measured the overshoot! :)
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Re: PIRA Simple Limiter

Post by sinus trouble » Mon Mar 28, 2022 12:06 am

Here is the measurement of how quick the Limiter reacts to a sudden signal! (Attack time)

Test parameters were 1Khz input signal @ 5mS per division!
Limiter Overshoot 1KHZ.PNG
Again RED is the input signal and GREEN is the output! As we can see, The overshoot peaked at around 3.5V but settled within 5mS!

I have to say that 5mS sounds short?? But is still recognisable by the ear! The Limiter kicking in can be physically heard!
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