Next SINUS project??

[sinus trouble]

Hello Necks!

I have pondering on some new projects?

As i am a big fan of NRG, there is one design that has very little info available? Yet has intrigued me for many years!

The NRG audio limiter!

However the circuit does have problems that would need to be addressed? Mainly the obsolete 15Khz filter!

Apart from that, the circuit does have a lot of potential!

Firstly, there is a spare Op Amp on the TL074 which could be used as an active 15Khz filter?

The circuit could also theoretically be full SMT!

Anyways, i think it is a highly underrated piece of art! And has a cool LED meter!

As i will be sharing the PCB and and all associated data! Feedback would be great!

Schematic below

ver_lim-4.png

[Frequent Lee]

I have a few of these pre 15khz filter limiters stephen designed & made. They're from the late 80s early 90s period. Not seen many of them about in the wild as it was only used in pirate equipment and most of his old stuff from then ended up in the hands of the dti.

They still sound really good now and have that distinctive veronica sound and can handle being overdrive really well.

[jvok]

Only issue I can see is those Tokos were 24dB/octave but a single opamp stage will only get you 12dB/octave. I'd look into switched capacitor filters like the MAX2408. Not the cheapest part in the world but probably cheaper than a dual opamp and all the passives you'd need if you go down the active filter route. And it's just a single chip so no bigger than the Tokos were. Just remember they need a 5V supply, and you need to keep the drive level high for low distortion.

https://www.analog.com/media/en/technic ... AX7415.pdf
https://uk.farnell.com/maxim-integrated ... /2799305RL

[sinus trouble]

WOW!!!!

Thank you Frequent Lee for posting that! I absolutely love that old gear!

Its hard to tell whats going on there? But i would guess Stephen is using "Differential pair" transistor configurations throughout!

An early form of Op Amp technology or comparator?

Nice!!!!

[sinus trouble]

Only issue I can see is those Tokos were 24dB/octave but a single opamp stage will only get you 12dB/octave. I'd look into switched capacitor filters like the MAX2408. Not the cheapest part in the world but probably cheaper than a dual opamp and all the passives you'd need if you go down the active filter route. And it's just a single chip so no bigger than the Tokos were. Just remember they need a 5V supply, and you need to keep the drive level high for low distortion.

https://www.analog.com/media/en/technic ... AX7415.pdf
https://uk.farnell.com/maxim-integrated ... /2799305RL

Cheers Jvok!

I cannot find ANY data those 15Khz filters at all!!

As a passive RC filter is not really viable either, its good to see there are alternatives!

[sinus trouble]

Just had a quick browse through the MAX7408 datasheet!

Looks daunting at first? But they aint too complicated! Even has built in oscillator for simple clock modes!

[jvok]

Yeah I had a skim of the datasheet and it looks like all you have to do is pick the right clock frequency to set the cutoff.

TI make the TLC04 which is a butterworth filter so in theory slightly better and will run off 12V, but they don't give any numbers for distortion which always makes me suspicious

https://www.ti.com/lit/ds/symlink/tlc04.pdf

[sinus trouble]

Another excellent find Jvok!

Supply voltages are not too critical as the limiter contains 13.8V, 9V and 4.5V rails to consider in the design so there is plenty of options

Aslong as the filter is not to complicated and does not produce too much gain or unwanted noise (Unity gain if poss) then its all good!

[Albert H]

In a nearby thread I gave the circuit for a 15kHz lowpass filter, using op-amp gyrators to emulate inductors. It works really well, and can be made to work from a single supply (like all the NRG stuff). I also have the circuit for the modified version of the NRG Limiter for stereo. You could just use two mono limiters, but the stereo image would wander around!

NRGlimmods+.png

I'll put the active LPF circuit up here shortly, suitably modified for single rail.

[Albert H]

Also, here's the circuit of the unreleased Pro IV Limiter that Stephen and I were working on shortly before his death. It eliminates the need for FET matching by means of using PWM for the gain reduction. The prototypes sounded very good for a single band limiter.

Pro4Limiter.gif

[sinus trouble]

Also, here's the circuit of the unreleased Pro IV Limiter that Stephen and I were working on shortly before his death. It eliminates the need for FET matching by means of using PWM for the gain reduction. The prototypes sounded very good for a single band limiter.

Pro4Limiter.gif

That looks like a neat design!

Forgive my incompetence! Although i know how PWM works? I dont see how it fits into this type of circuit?

Could you elaborate on how you implemented it?

PWM does have excellent efficiency parameters for driving heavy loads? Limiters are not what i would class as "Heavy"

[Krakatoa]

Also, here's the circuit of the unreleased Pro IV Limiter that Stephen and I were working on shortly before his death. It eliminates the need for FET matching by means of using PWM for the gain reduction. The prototypes sounded very good for a single band limiter.

Pro4Limiter.gif

That looks like a neat design!

Forgive my incompetence! Although i know how PWM works? I dont see how it fits into this type of circuit?

Could you elaborate on how you implemented it?

PWM does have excellent efficiency parameters for driving heavy loads? Limiters are not what i would class as "Heavy"

PWM for audio gain control works by chopping the signal at ultrasonic speed, the more duty cycle the more signal gets reconstructed by the output filter and the more volume you get. Going with less duty cycle, you get attenuation.
Download the Inovonics 718 David III manual and you will read a comprehensive explanation on how it works.

By the way, the pro-iv limiter Albert has published, I tested it years ago and as it is, it does not compress nor limit correctly. The sidechain is unfinished. It should have an exponential converter in the control signal after the rectifiers (or implemented in the clock signal waveform special shape, as Inovonics is doing).
If I had to design it today I'd pick a microcontroller with fast PWM output and implement the compress-limit behaviour from a look-up table in the memory (like the autor of the texar prism clone does).

[sinus trouble]

Also, here's the circuit of the unreleased Pro IV Limiter that Stephen and I were working on shortly before his death. It eliminates the need for FET matching by means of using PWM for the gain reduction. The prototypes sounded very good for a single band limiter.

Pro4Limiter.gif

That looks like a neat design!

Forgive my incompetence! Although i know how PWM works? I dont see how it fits into this type of circuit?

Could you elaborate on how you implemented it?

PWM does have excellent efficiency parameters for driving heavy loads? Limiters are not what i would class as "Heavy"

PWM for audio gain control works by chopping the signal at ultrasonic speed, the more duty cycle the more signal gets reconstructed by the output filter and the more volume you get. Going with less duty cycle, you get attenuation.
Download the Inovonics 718 David III manual and you will read a comprehensive explanation on how it works.

By the way, the pro-iv limiter Albert has published, I tested it years ago and as it is, it does not compress nor limit correctly. The sidechain is unfinished. It should have an exponential converter in the control signal after the rectifiers (or implemented in the clock signal waveform special shape, as Inovonics is doing).
If I had to design it today I'd pick a microcontroller with fast PWM output and implement the compress-limit behaviour from a look-up table in the memory (like the autor of the texar prism clone does).

Cheers Krakatoa!

The concept of PWM is genius and in audio applications such as "Class D" amplifiers give a reasonable analogue waveform?

However, adding ultrasonic square waves to the equation seems counter productive to me?

Whilst not perceived by the ear, it still needs complex filtering!

Anyways, back to the original design! I hope to keep it as authentic as possible!

I am currently building the whole circuit in LTSpice!

I also have most of the parts to build a real prototype which i will construct when i get some free time!

[Albert H]

"Krakatoa" is partially right. The sidechain in the "Pro IV" circuit does need more work. The voltage-controlled oscillator doesn't convert DC voltage to pulse width in a linear manner, and that non-linearity gives some measure of the exponentiation that's required. However, the conversion ratio isn't all that it should be....

I've been working on another implementation of oscillator, and this is giving a much better control law. I'll put the new circuit up here in the near future. Incidentally, I consulted on the "David" series for Inovonics some years ago. The original reasons for going for the PWM level control were twofold - accurate channel-to-channel balance at all degrees of attenuation, and significantly lower distortion than that introduced by most analogue attenuator configurations.

The "Blackmer" configuration (as used in the "THAT" ICs) isn't bad, and is based on the attenuator configuration designed at National Semi back in the '70s. The THAT2081 (if you can get them) is a fabulous chip, but outrageously expensive (especially when you consider just what's in them). I just built a three-band stereo limiter using six of those ICs, and the results are excellent - especially with the delay lines between the sensing points and the attenuators, giving (effectively) a zero-attack limiter. Unfortunately, there's just too many ICs in the design, too many calibration points, and it's too expensive, so it's not a truly viable design, but will remain the starting point for a future design.

I have been reconsidering the PWM approach to audio attenuators. Incidentally - you're not "adding ultrasonic square waves to the signal", you're just chopping out vanishingly small amounts of it. At the moment, I'm playing with a "split-path" balanced attenuator, which should (at least in theory) be able to provide distortion cancellation. The mathematical models suggest that this might be the way to go, but I've never really trusted Spice, and until I've got it built and debugged on a prototype board, I won't be saying any more about it.

[sinus trouble]

"Krakatoa" is partially right. The sidechain in the "Pro IV" circuit does need more work. The voltage-controlled oscillator doesn't convert DC voltage to pulse width in a linear manner, and that non-linearity gives some measure of the exponentiation that's required. However, the conversion ratio isn't all that it should be....

I've been working on another implementation of oscillator, and this is giving a much better control law. I'll put the new circuit up here in the near future. Incidentally, I consulted on the "David" series for Inovonics some years ago. The original reasons for going for the PWM level control were twofold - accurate channel-to-channel balance at all degrees of attenuation, and significantly lower distortion than that introduced by most analogue attenuator configurations.

The "Blackmer" configuration (as used in the "THAT" ICs) isn't bad, and is based on the attenuator configuration designed at National Semi back in the '70s. The THAT2081 (if you can get them) is a fabulous chip, but outrageously expensive (especially when you consider just what's in them). I just built a three-band stereo limiter using six of those ICs, and the results are excellent - especially with the delay lines between the sensing points and the attenuators, giving (effectively) a zero-attack limiter. Unfortunately, there's just too many ICs in the design, too many calibration points, and it's too expensive, so it's not a truly viable design, but will remain the starting point for a future design.

I have been reconsidering the PWM approach to audio attenuators. Incidentally - you're not "adding ultrasonic square waves to the signal", you're just chopping out vanishingly small amounts of it. At the moment, I'm playing with a "split-path" balanced attenuator, which should (at least in theory) be able to provide distortion cancellation. The mathematical models suggest that this might be the way to go, but I've never really trusted Spice, and until I've got it built and debugged on a prototype board, I won't be saying any more about it.

I really appreciate you posting it Albert!

Whether it needs tweaking (As most prototypes do) does not matter to me!

I have learned something i would never have thought of!

[sinus trouble]

So!

I have been thinking on which route to take with this project? With the great options you have all provided, i am still undecided?

However, as a compromise on the 15Khz filter i thought i would do some simulations!

As Jvok pointed out, An active filter would only give a 12Db reduction (Which is correct)

I guess it is simple to implement and better than no filter at all? Simulations below!

[sinus trouble]

So here is the first simulation of a simple active filter!

Firstly, i could not get it to respond because i used the wrong type of Op amp!

After adding a dual DC supply, the circuit fired up!!

15K LPF.PNG

[sinus trouble]

I also added this to the existing NRG circuit!

Whilst initial results looked promising? The frequency response was almost opposite of what i wanted!

Anyways, as it is the first draft? It needs a lot of work!

NRG Lim Stage 01.PNG

[jvok]

As Jvok pointed out, An active filter would only give a 12Db reduction (Which is correct)

Just to be clear you can get as much roll-off as you want from an active filter, you'll just need more stages, same as with a passive filter. But if you only have the one opamp to spare then the best you'll get is 12db/octave. Two opamps gives you 24db/octave, three would be 36 etc. Not really much point going beyond 24db normally though

[Albert H]

Here's a lowpass filter that uses gyrators to emulate inductors. This elliptic filter has been used in several commercial designs, but is cheap and simple enough for anyone to build. If you want a single-supply version of it (in case you wanted to integrate it into the NRG Limiter), I'll put it up here if you ask. It provides a suitably steep cutoff after 15kHz, so that there's almost no audio content by 19kHz, and it's very phase accurate in its passband, so that stereo imaging isn't compromised. I have a PCB design here somewhere for an NRG Pro IV Stereo Coder with this filter added to each channel - again, I'll put it up if anyone's interested.

15kHz_Filter.GIF