Here's the second ever prototype of the NRG Pro IV stereo coder. I built the first one on Veroboard, and this was the second one for which we photo-etched the PCB. The only thing I missed off the layout was one wire link that we found slightly improved the effectiveness of the output filter. I plugged it into an exciter earlier, and it still sounds great!

The spurious resistor hanging from the output socket was tacked on to allow easy connection of the 'scope probe. I really should remove it!

That's a very good stereocoder.

Ruud - It was meant to be easy to build, and had to have a single-sided PCB. The synthesis of the 19kHz pilot works well, and the oversampled channel switching is very precisely synchronised with the pilot, and therefore with the suppressed subcarrier as well. It has low noise and distortion, and very good channel separation. Its performance is better than many of the commercial offerings, and actually quite significantly out-performed an Innovonics "David" coder. Not bad for a CMOS crystal oscillator, CMOS switch, CMOS data multiplexer and a few dual op-amps. There are some nicely 19", 1U boxed versions of this that you'll find on broadcast sites all over the world. The only additions to those are the gyrator LPF, electronically balanced 600Ω inputs and a line driving output with a 50Ω output impedance.

I never liked the use of the Toko 15 kHz filter blocks, but they worked well enough - certainly better than the simple op-amp based Sallen and Key type that was the other easy option. There was a prototype with a gyrator-based filter, but I can't find it. It's in one of the many boxes of gear here.

@Albert H,

May I ask you what do you think about Pira stereo-encoder?
I have built few of them and they work good as I can see. But, to be honest I have not built other serious encoders to compare it with, so I don't know how good Pira encoder really is. Thanks in advance.

The Pira encoder is quite good. The phase accuracy between the various components of the signal is pretty good (not as good as my NRG coder, but good enough) and it's reasonably low noise and has low distortion. As I recall, it's quite expensive to build.

Albert H wrote: ↑Wed Mar 08, 2017 11:06 pm The Pira encoder is quite good. The phase accuracy between the various components of the signal is pretty good (not as good as my NRG coder, but good enough) and it's reasonably low noise and has low distortion. As I recall, it's quite expensive to build.

Thank you Albert.

The most expensive is PIC1220 or 1320 as in my case. Its about 5 USD. There are also decent amount of other valuable components, which all together brings it up to, lets say, 30 USD.
I would like to try other good encoders, so as soon as I have schematics of such thing, I plan to try it.