Next SINUS project??

[Albert H]

Above, We can see the two 1Khz signals!

RED is the input signal @ 1V peak to peak

Green is the output signal @ 16V peak to peak

This explains why the limiter is too loud to feed any sort of exciter! So at first glance, it would seem that the limiter is not functioning?

Sinus!!! The problem is that if you reduce the gain of the output op-amp, you'll never reach the limiting level, because the rectifiers won't see enough signal.....

You're going to have to change the gain of the first rectifier op-amp too. The output amp has a gain of 56 - determined by the value of the feedback resistor divided by the inverting input to ground resistor - in your case 56k / 1k = 56 times.

Suggested experimental "fix":

If you change the 56k feedback resistor (R11) to 28k (just solder another 56k in parallel) and reduce the 47k (R16) to half its value by soldering another 47k in parallel, you should have pretty much the same input level for onset of limiting, but your output level will be halved. You will have halved the output stage gain, and doubled the gain on the input to the first sidechain op-amp (IC1.2)

Give that a try, and see if you now get a squeezed output at half the level....

Incidentally the gain of an op-amp stage doesn't change with type of op-amp - it's entirely determined by the resistors around it!!

You really do need to read Horowitz & Hill!!!

[Albert H]

If you try the suggestion in my last post, and you're getting closer to the levels you want, let me know, and we can recalculate the relevant resistors to get exactly the levels you require.

Incidentally, it's good to have plenty of level on the way into your exciter - you can attenuate it when it arrives! If you think about it, if your wanted audio is at (say) 4V p-p, the noise on the cable might be as little as 50mV, so it would be relatively insignificant. However, if your wanted audio was only 1V p-p, your 50mV noise would remain the same, but would appear to be four times worse as a proportion of what's going into the rig....

In professional audio, if you're sending audio over long cables, you use the lowest source impedance you can manage, and aim for the biggest undistorted audio p-p Voltage you can achieve - 20V p-p isn't uncommon in stage set-ups. Again, you're trying to make the interfering signals as small a proportion as possible.

[sinus trouble]

Doh! I am such an idiot!

My apologies to all following this thread! It seems i had accidentally switched my scope probe from 10x to 1x whilst handling!

The real output of the limiter is 1600mV peak to peak! NOT 16V as i originally stated!

So the measured output is pretty much on point with the specified output of 775mV RMS (Approx 1600mV peak to peak)

Thank you Albert for the advice on OP AMP gain control and behavior!

I am learning alot more about the circuit still

[sinus trouble]

Also here is a quick video to illustrate how the limiter corrects the output amplitude with a changing input signal!

[sinus trouble]

I hope to verify the spectrum soon and how effective the 15Khz low pass filter is?

I suspect that the filter will be -12db ish as others have mentioned previously?

We shallsee?

[radium98]

i ask you to try calibrate it with an old deck casette player . the output of comp / lim can go to input REC record of the deck , then move the wheel to 0db and see , i think is also a technic to set it , thanks for sharing , and also put to many input level and make a video i like to see that .

[radium98]

continue with the procedure
below.
Fit the Jumper to the two sil pins on J2. Adjust
VR1 to mid position. Connect a regulated power
supply that is between 12 and 13.8V DC to the DC
lead, RED to +, Black to GND or -. DO NOT EXCEED
16V DC.
Connect the limiter audio output socket to the
transmitter, stereo coder audio input or audio
amplifier using a phono to phono lead. Apply an
audio source from an HiFi CD player,one with
phono/RCA outputs. Start to play a good quality
pre-recorded audio CD. Adjust VR1 slowly and
carefully on the limiter so that LED3 just flashes
on the music peaks. VR1 is a sensitive adjustment
for the limiter gain circuit. Abrupt adjustments to
VR1 may cause a large temporary gain reduction. If
the audio appears to disappear, wait a few seconds
for the limiter gain to re-settle. This procedure
calibrates the limiter DC conditions and LED3 to
+6dBu.
Using an FM radio, monitor the audio from the
transmitter. Adjust the variable resistor on the
transmitter or stereo coder audio input to give the
correct level of FM deviation.
You can now apply any audio source between -12dBu
and +24dBu to the limiter and the output will be
held to a maximum of +6dBu.

[sinus trouble]

So i will be releasing the Gerber Files below! Also an updated schematic to follow!

There are a couple of things to consider though?

Firstly, I have tested the limiter on a number of different exciters and they all have a different input sensitivity! Most have a potentiometer to preset this? But some may not?

Unsurprisingly, The limiter worked best with the NRG PLL PRO!

Secondly, The 15Khz low pass filter does indeed achieve approx -12Db! However, Heavy drive (limiting) causes the low pass filter to be less effective due to the AGC levelling off ALL signals to around 0Db!

This should not be a massive issue, especially in MONO situations? But if you are concerned about Stereo disruption? You can simply add more filtering to suit your needs!

Anyways, I will try to answer any questions related to the build!

Good Luck!

[sinus trouble]

Here are the Gerber Files!

Gerber_PCB_Sinus Audio Limiter.zip

[sinus trouble]

20230218_153456.jpg

[sinus trouble]

Above is the completed limiter PCB

D5 and R46 should be mounted approx 10mm above the PCB to allow adequate air flow as these components get HOT!! (not so much @ 12V)

C3 and R3 form the pre-emphasis!

C3 = 1.8nF and R3 = 27K for UK (50uS)

C3 = 2.2nF and R3 = 33K for USA (75uS)

R7 was also not needed!

[sinus trouble]

Schematic_Sinus Audio Limiter_2023-02-26.png

[radium98]

Sinus thank you , i wish one thing if it was to me single side,but thanks ,another thing did you see it on dev meter to see 75 khz ,and did you see what i post above , thank you for this gift.one day it will be built.

[sinus trouble]

Sinus thank you , i wish one thing if it was to me single side,but thanks ,another thing did you see it on dev meter to see 75 khz ,and did you see what i post above , thank you for this gift.one day it will be built.

Thank you Radium!

There is not much going on with the top layer, It is only DC connections to the ICs and a few other places which could easily be fixed with wire links!

I did not check deviation as this will vary with different exciters, The limiter maintains around 0.775mV output irrespective of the audio input volume!

[sinus trouble]

So i thought id take an actual measurement of the low pass filter after processing by the AGC!

Whilst most of the audio band sits @ 0Db! The signal @ 20Khz sits just shy of -10Db!

Ofcourse, this is no comparison to Stephens performance! But will offer a degree of protection!

And in practical use? Signals in the realm of 20Khz would generally be a lot lower than my test!

Results below!

[sinus trouble]

20Khz Test.PNG

[sinus trouble]

There is also no reason why the low pass filter cannot be adjusted to suit your needs?

The PCB will support that!

The components i selected were a compromise between audio quality and high frequency filtering!

[Bazz]

Nice work look forward to building this
Aron102

[sinus trouble]

Nice work look forward to building this
Aron102

Cheers Bazz!

As this project has far more applications than just broadcasting! I would suggest adding it to your chain of general audio listening to get a feel of how it operates!

I have had the limiter connected to my mixer for a few weeks now and not needed to adjust my headphone levels once!

Good luck!

[radium98]

nice words .what to say more...