Also, if you want a truly amazing over sampled stereo coder design, I have one that I've been working on for a while. Unfortunately it uses a lot of ICs and precision resistors for the high degree of oversampling, but the results are well worth the effort.

Yes Albert!

By all means, please share! It would be great to see!

I have not had much time lately, but heres a quick update!

So with a little adjustment, the main audio path stage is looking better!

Simulation below!

As can be seen, the gain is a bit low? With an input of 1V the output is around 900mV (Just below the 0db threshold)

However, the roll off @ 15Khz looks pretty decent!

I think more adjustments will be required once i add the limiting circuitry!

So, I have an update on the limiter simulation!

The circuit is now complete and seems to be operating somewhat correct? With an input amplitude of 1V, the output is around 500mV (@ say 1Khz)

Which at first sight seems a bit low? But now the added compression is fighting gain increase!

The active low pass filter is also acting a bit odd since the extra circuitry has been added?

Infact, I am not sure what the output gain should be? (Possibly 750mV) and how that calibrates with the LED meter?

Anyways it is early days and i am sure i will work it out!

Pics below!

After a second look at simulation results? It is not as bad as i thought!

One thing that i noticed with this circuit is that compression is applied constantly! VR1 is merely a small adjustment of compression rate and will not cut it completely!

The simulations have also shown no distortion whatsoever!!

I guess the only way to find out is to build a prototype!

It is quite a complex circuit so bear with me! i will be back soon!

VR1 is NOT to vary compression - it's to allow FETs with slightly differing pinch-off voltages to be used together as a stereo pair. Stephen had a couple of approaches he used to connect them as a stereo pair, but I pointed out that it would be better (and simpler) to use a common rectifier and timing circuit to make the stereo channels work properly together:

Nice one Albert!

I have been looking at the stereo version and it has some neat features! "Stereo expand" and selectable release time constants!

I was thinking of etching a test PCB? However, proto board will most likely be used!

Also that allows easy adjustments and does not need to look pretty!

Albert!

I noticed you have added a 12V regulator to your schematic? Coincidentally, i was thinking of adding the same alongside a 9V regulator to replace the 9v1 zener!

On the other hand? I want to keep it as original as possible? Plus the whole circuit will likely be fed from a regulated power source of some sort in its final installation!

We used 12V regulators to try to keep the noise conducted along the supply rails away from the sensitive parts of the limiter.

I did a prototype version (on Veroboard) that used a couple of un-buffered CMOS gates as enhancement mode FETs for the attenuators. I had to invert the control voltage (which added an extra op-amp), but the FETs were certain to be very well matched! The CMOS IC was also much cheaper than a couple of FETs.....

The way forward is to get a prototype built!

The simulations give a rough guide? However, they are quite abrupt! Typically LTspice will initiate power up and sweep in a matter of milliseconds!

In reality? That is not the case with most projects! Especially when timing is a contributing factor!

Hopefully i can knock up a functioning prototype soon! Even with Veroboard, utilising the space available best can be a challenge!

Apologies for the delay!

Here is a preview of the first draft in easy EDA! Footprints for the components need setting up but overall the layout is pretty much done!

Also the +15V is terminated wrong

That can be ignored for now!

Another point to mention is the "Signal" ground and "Chassis" ground!

Whilst both are important! I will be focusing ONLY on the signal ground for the PCB itself!

Chassis ground components will depend on how the PCB is finally mounted in its enclosure!

For example? The PCB could be mounted via insulated stand off pins! This would allow for chassis ground components to be terminated from PCB to enclosure ground points easily!

Sinus - how about a really cheap and simple stereo limiter? This is one I've thrown together, and it sounds pretty good for a single band job.
It's cheap as chips, and just needs a gain reduction indicator and (possibly) input and output metering.

Here is the basic sketch:

The time constant circuit could do with some work, but it works well as it is. Just three cheap chips - 4049UBE, NE5532 or LM833 and 4558 or LM358. The 4049UBE just provides accurately matched enhancement-mode FETs (for about 40p!), and the 4558 isn't in the signal path. Only the high quality 5532 is affecting the audio.

With the addition of a pseudo-ground midrail, some biasing resistors and capacitors, it would be possible to make a single rail version too. However, I've powered it from a 12V AC supply with two diodes - one for the +ve rail and the other for the -ve rail. I used two 470µF electrolytics for the supply smoothing, and put 100n capacitors to ground from each IC's power pins.