dScope III Performance Upgrade

[Tranquility Bass]

This thread is about improving the performance of an already great bit of test gear. If you don't have the necessary skills to do these mods or your instrument is still under manufacturers warranty then we don't suggest you perform these mods.

According to the specs the Prism dScopeIII has an equivalent input noise voltage of about 1.2uV which is equivalent to about -116dBu and which was also confirmed by NewAvGuy on his blog. We also confirmed this on our own dScope III by running a similar test by shorting the inputs and measuring the noise level over a 20kHz bandwidth and we also got close to -116 dBu which is only a few dB lower than the Audio-Precision analyzers which sell for a lot more than the dScope III



Is it possible to improve on this ? The ADC converter use in the dScopeIII is a AKM AK5394A which has a S/N 123dB so there should be some room for improvement. Removing the top cover of the dScope III reveals that all of the ADC and DAC circuitry is contained on a riser board that plugs into the switched attenuator board. On further observation we note some of the IC's are plugged into sockets which would indicate that the manufacturer was contemplating replacing the chips with better devices as they became available or simply they may have been trying different chips from a batch until they found the ones that met their specs. Bear in mind the design of this analyzer dates back to 2001 so it is quite remarkable that they were able to achieve these specs with the silicon they had at the time ! We note that the two IC's closest to the ADC are standard NE5532 dual opamps which would be part of the buffers/level-shifters used to drive the ADC. As these devices have an equivalent noise voltage of 5nV/rtHz it would be a simple exercise to replace these with devices with better noise an distortion performance. As high quality opamps with DIP packaging are slim pickings these days we ended using the tried and true LME49720 which is a drop in replacement with much improved characteristics over the venerable NE5532. The LME49720 has an equivalent noise voltage of 2.7nV/rtHz and an order of magnitude lower distortion so we could expect at most an improvement of noise reduction of 5.35dB but in reality it would be somewhat less. Lets see what happens



And from the new noise measurements we see an improvement of about 3dB from -116dBu down to -119dBu so well worth the few dollar investment of these opamps The question arises could we improve on this yet again ? Since the DIP style of package is becoming very rare for audio opamps, to get a lower noise floor in a dual opamp configuration would mean that we'd have to go to an SMD package and use some sort of SMD to DIP adapter so probably not worth it for chasing down a few extra dB's. Maybe an experiment for a rainy day but the current mod is quite easy to achieve for an hours work and a few extra dollars for the opamps