I recently breadboarded this audio signal limiter circuit from ESP (see figure 2 in the link):
Fast Audio Peak Limiter
It's very subtle and it sounds okay, but only if you don't adjust the limiter threshold. I added a 10k POT in series with the 4.7k resistor R6, and when you adjust the POT it takes a second or two for the level to adjust - so there's a really annoying dip in volume, and sometimes some nasty clicks (which are mentioned in the "editor's notes" at the bottom of the page). Any ideas/suggestions how to overcome these? Can someone recommend a simple limiter schematic that would be better? Thank you!
Fast Audio Peak Limiter
It's very subtle and it sounds okay, but only if you don't adjust the limiter threshold. I added a 10k POT in series with the 4.7k resistor R6, and when you adjust the POT it takes a second or two for the level to adjust - so there's a really annoying dip in volume, and sometimes some nasty clicks (which are mentioned in the "editor's notes" at the bottom of the page). Any ideas/suggestions how to overcome these? Can someone recommend a simple limiter schematic that would be better? Thank you!
In general, limiters work 'best' with fast attack and fast release times. Fast attack prevents peaks from passing through unchecked, and a fast release prevents holes from being punched in the program after a peak above threshold comes along.
The standard tradeoff is that fast release will prevent holes from being punched into the program, but will "eat up" low frequencies, and turn them into distortion, since the gain reduction circuit will be fast enough to unwind the LF components. Similarly, fast attack will catch even short peaks, but can eat up HF components purely by the limiting action, and replace them with modulation artifacts. So, with an analog limiter, it's a tradeoff between conflicting goals - signal quality and 'naturalness' vs. effective level control.
Study the classics if you can, and focus on how the attack and release time constants were handled. There have been a lot of tricks over the years, some of them suitable for different types of program material and gain reduction goals. Nonlinear networks have been used which can vary the time constants along with the amount of gain reduction. For example, decreasing the release time with increasing amounts of gain reduction can allow the limiter to be more subtle near threshold, and get more aggressive with higher input signals.
Have fun!
The standard tradeoff is that fast release will prevent holes from being punched into the program, but will "eat up" low frequencies, and turn them into distortion, since the gain reduction circuit will be fast enough to unwind the LF components. Similarly, fast attack will catch even short peaks, but can eat up HF components purely by the limiting action, and replace them with modulation artifacts. So, with an analog limiter, it's a tradeoff between conflicting goals - signal quality and 'naturalness' vs. effective level control.
Study the classics if you can, and focus on how the attack and release time constants were handled. There have been a lot of tricks over the years, some of them suitable for different types of program material and gain reduction goals. Nonlinear networks have been used which can vary the time constants along with the amount of gain reduction. For example, decreasing the release time with increasing amounts of gain reduction can allow the limiter to be more subtle near threshold, and get more aggressive with higher input signals.
Have fun!
If I may ask why do you need it?
I'm using circuit based on https://lunainc.com/wp-content/uploads/2016/08/Audio-Limiter.pdf
Cheers
I'm using circuit based on https://lunainc.com/wp-content/uploads/2016/08/Audio-Limiter.pdf
Cheers
What did you use for the optocoupler? I have discovered that the VTL5C4 specified by Rod is no longer manufactured. Perhaps a NSL-32SR3 would work, but I don't know if it's as clean.I have used somthing very similar to Rod's P137 and for light PA it works great. You can hear that the limiter ia active but after a few beers you dont notice it
Thanks for the input guys!!
@Monte McGuire.. Thanks for the great tips and info! When you say "holes" do you mean like audible gaps (because the longer release time didn't allow the signal to change)? So that might be the weird gaps, clicks, and volume dips I'm hearing.
In terms of the tradeoff you mentioned, I'm more interested in effective level control, rather than signal quality and naturalness. Do you have any suggestions for which classic limiters to study? I'm a bit of a noob when it comes to limiters.
I was also reading through Douglas Self's "Small Signal Audio" book, and there's a section where he talks about limiters, and in one of his JFET circuits he replaced the 10uF cap on the drain of the JFET with a unity gain op-amp buffer, so I'll have to try that out.
@dotneck335.. interesting circuit, thanks for the link! I might try this out, but I need to get a vactrol/LDR - which I'm trying to avoid right now.
@andrewlebon.. I did a quick search for the QSC 5050 limiter, and it looks like a beast! And I actually found the schematic (on schematicsunlimited.com) - it's a little confusing and complex, so I have to look at it more carefully.
@Tajzmaj.. I need a limiter because the output of my distortion circuit can get pretty really high, which then just sounds like a mess.
@Monte McGuire.. Thanks for the great tips and info! When you say "holes" do you mean like audible gaps (because the longer release time didn't allow the signal to change)? So that might be the weird gaps, clicks, and volume dips I'm hearing.
In terms of the tradeoff you mentioned, I'm more interested in effective level control, rather than signal quality and naturalness. Do you have any suggestions for which classic limiters to study? I'm a bit of a noob when it comes to limiters.
I was also reading through Douglas Self's "Small Signal Audio" book, and there's a section where he talks about limiters, and in one of his JFET circuits he replaced the 10uF cap on the drain of the JFET with a unity gain op-amp buffer, so I'll have to try that out.
@dotneck335.. interesting circuit, thanks for the link! I might try this out, but I need to get a vactrol/LDR - which I'm trying to avoid right now.
@andrewlebon.. I did a quick search for the QSC 5050 limiter, and it looks like a beast! And I actually found the schematic (on schematicsunlimited.com) - it's a little confusing and complex, so I have to look at it more carefully.
@Tajzmaj.. I need a limiter because the output of my distortion circuit can get pretty really high, which then just sounds like a mess.
Why do you want to avoid the vactrol/optocoupler? Does it make for worse sound, do you think?
You *don't* adjust that value after building. It is set for about 90% of of the JFET's Vgs(off). Once set, don't mess.
You do want to change the signal-path threshold. You do this by varying gain in front of the core limiter. (There's other ways, but not great ones in one TL072.)
Exactly.
Leave as is, circuit has already been optimized (thanks Rodd Elliott) , in any case possible threshold variation wpould be small and not a useful tool.
If you need to reach a certain audio level and not further, fine, leave the variable gain block as is, then to suit your needs vary gain/attenuation before and after it until triggering and levels are what you need.
Leave as is, circuit has already been optimized (thanks Rodd Elliott) , in any case possible threshold variation wpould be small and not a useful tool.
If you need to reach a certain audio level and not further, fine, leave the variable gain block as is, then to suit your needs vary gain/attenuation before and after it until triggering and levels are what you need.
What about simple clipper/limiter. " diodes or 2 led's in anti parallel one resistor before that and pot to adjust level? It might work for you. But it's going to be really badly ''squashed''.
That circuit I posted the link works fine. It might be slightly complex to build as pedal unit since you need symmetrical power supply. You could use different types of optos and you could fiddle with values on sidechain to get desired threshold range and speed. I'm using for my stage wedges amps.
Cheers
That circuit I posted the link works fine. It might be slightly complex to build as pedal unit since you need symmetrical power supply. You could use different types of optos and you could fiddle with values on sidechain to get desired threshold range and speed. I'm using for my stage wedges amps.
Cheers
> That circuit I posted the link works fine.
Yes; as do a hundred other photo-resistor audio limiters.
The problem is that photo-resistors contain Lead and Lead in electronics is being restricted in large parts of the world. The old-time photo-resistor makers have quit the business. Photo-resistors are still available from China but the consistency is poor.
Yes; as do a hundred other photo-resistor audio limiters.
The problem is that photo-resistors contain Lead and Lead in electronics is being restricted in large parts of the world. The old-time photo-resistor makers have quit the business. Photo-resistors are still available from China but the consistency is poor.
Thanks for all for your input and suggestions!!
@PRR and @JMFahey.. Thank you for clarifying! Sounds like I'm be thinking about the entire thing incorrectly then. So I should be putting a variable gain opamp before the limiter. I will heed your advice
Here's a side-note question.. Do you see any issues using a POT as the feedback resistor on an inverting opamp configuration? Maybe I'm overthinking it, but I've never actually seen a circuit with a POT as the opamp's feedback resistor. I know there's other ways to do this, but I've always wondered if there's an issue in doing it this way.
@dotneck335.. I'm hoping to use parts I have lying around home - which are MOSFETs, JFETs, LM13700 OTAs, and optocouplers (like CNY17F-2 and PS2811) - but I've never used optocouplers in audio. And I don't have any experience with vactrols, but I also know their tolerance is poor (as mentioned by PRR), they're basically obsolete, difficult to get, relatively expensive parts that contain Cadmium. So I'd just rather just avoid them.
Thanks for that Silonex schematic. Btw, do you know why they use 2 different opamps (TL072 and NE5534)? I wonder if it would work using TL072 for all 3 of the opamps.
@Tajzmaj.. I really want the ability to control release time with a POT, and not just the limiter threshold - so that's why I don't want to use a simple clipper/limiter. As for optos, I've never used optocouplers to control audio signals. But I'll have to try it. A little complex circuitry is okay (even double the size/parts-count of the ESP limiter I initially linked to), I just don't want to make an 1176 with transformers and stuff.
@PRR and @JMFahey.. Thank you for clarifying! Sounds like I'm be thinking about the entire thing incorrectly then. So I should be putting a variable gain opamp before the limiter. I will heed your advice
Here's a side-note question.. Do you see any issues using a POT as the feedback resistor on an inverting opamp configuration? Maybe I'm overthinking it, but I've never actually seen a circuit with a POT as the opamp's feedback resistor. I know there's other ways to do this, but I've always wondered if there's an issue in doing it this way.
@dotneck335.. I'm hoping to use parts I have lying around home - which are MOSFETs, JFETs, LM13700 OTAs, and optocouplers (like CNY17F-2 and PS2811) - but I've never used optocouplers in audio. And I don't have any experience with vactrols, but I also know their tolerance is poor (as mentioned by PRR), they're basically obsolete, difficult to get, relatively expensive parts that contain Cadmium. So I'd just rather just avoid them.
Thanks for that Silonex schematic. Btw, do you know why they use 2 different opamps (TL072 and NE5534)? I wonder if it would work using TL072 for all 3 of the opamps.
@Tajzmaj.. I really want the ability to control release time with a POT, and not just the limiter threshold - so that's why I don't want to use a simple clipper/limiter. As for optos, I've never used optocouplers to control audio signals. But I'll have to try it. A little complex circuitry is okay (even double the size/parts-count of the ESP limiter I initially linked to), I just don't want to make an 1176 with transformers and stuff.
I know this is DIY audio but.... this isn't the 1970s. You can buy a very fine musical limiter cheaper than you can buy a nice box and jacks.
FMR RNC is the high end of the budget limiters and is very fine. I've used others, generally with satisfaction. Many now just copy the THAT Corp app-notes, and THAT/dBx has been in the gain-change business a very long time.
FMR RNC is the high end of the budget limiters and is very fine. I've used others, generally with satisfaction. Many now just copy the THAT Corp app-notes, and THAT/dBx has been in the gain-change business a very long time.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.