First to make things clear: The Synvoiz has an excellent signal-to-noise ratio. I don’t have calibrated measurement utilities but can tell you that the unit is silent when it needs to be. Compared to other analog or digital synthesizers it’s in the same ballpark.
Not so my lovely Retroverb. It’s a spring reverb and open to a lot of influences beside the input signal. Additionally it has a muffled sound so you tend to grab an equalizer and push those higher frequencies. Doing this last week I heard a noise which I know to good from bad wiring in cheap recording setups. Today’s CPUs change their power needs thousand times a second dependent on the load it is given. Even with modern plugins you won’t stress out your CPU to the max so it has idle times where it tries to save power (and heat emission). So it is likely that this emits to your audio cabling in some way. Here is an example of the Retroverb’s noisefloor with the interfering noise of the CPU:
It’s easy to exclude other sources by giving the CPU something heavy to do like the Prime95 tool. With the software running the noise floor of the unit sounds like this:
Please mind that the sound examples are heavily equalized so it’s clear what I am talking about.
The troubleshooting started. Replacing the audio cables didn’t help. Also I realized that it’s not input dependent which means the interfering noise comes from within the unit. Changing position of the PSU also didn’t get me any further but moving the whole unit removed the unpleasant sound. After switching off unit by unit nearby the Retroverb I found the culprit: The power supply cord of the TFT! At the beginning of my investigation I thought about the PSU of the computer or that of the Retroverb itself – which isn’t a standard DC one so not so easy to replace (12 v AC). After superseding the old cord with a new one the noise floor isn’t as polluted as before but you still hear it:
With a proper gain staging the noise is on a level I can live with for the moment as there is no better place for the Retroverb in my small room.
By the way when I activate the filter of the unit the signal-to-noise ratio is immediately reduced and the computer noise only barely noticeable. Filters tend to emit noise that’s the reason why in every synthesizer the filter comes before the VCA.