Notch Filters and Release 1.821

Do you plan to make the oscillator/notch available? It looks really nice.

Hi 1Audio, I don’t think there’d be enough volume to justify a full product but a few will be built for the lab and perhaps a few more could be built for hard-core experimenters.

The next PCB is shown below. Area A is USB CPU and relay drivers. Area B is +/-12V. Area C is oscillator. Area D is oscillator output +0 / +12 dB section. Area E is 0…63 dB atten in 1 dB steps. Area F is passive notch. This has fixed +12dB output, but there’s a relay there so you can bypass the notch to make an easy comparison to the original signal.

So, this should give a killer 1 kHz at +12 dBV to -63 dBV output, and a passive notch. There are test pads so that you can easily measure combined R and Cs and tune if needed. For example, the nominal C in the notch is 44.44 nF, which comes from 2 22nF + another 440pF. But if you have a really good DVM that can measure C, then you can measure the two combined 22nF and then pick a cap (390pF, 440pF 470pF or anything in between) that will let you hit 44.44nF. All caps are 1%, all R are 0.1%.

The caps are doubled value in this version so the R’s can be cut in half to help with the noise in the previous version shown in the blog post. All opamps are OPA2156 or OPA1656. The hope is that this rev is 4-6 dB quieter than the last. And the last was about 4 dB quieter than the QA401 (-115 dBV 20 kHz bw)

There are no timelines on any of this, but the boards are back. But they’ve been back for 2 months it’s just finding the time to get them built and brought up. There will probably be another revision needed too for cleanup. So, nothing will be ready anytime soon