A modular synthesizer usually uses unbalanced signal transmission – i. e. the modular system is not interference tolerant and is limited in its maximum size. Irradiations, potential differences and voltage errors add up in the signal chain and lead to audible problems depending on the size of the system. The more important is the quality of the power supply and the distribution system.
The PSU provides you with the source material for the sound. The modulation of this circuit is your sound. It makes no sense to connect your precious module collection to an inferior PSU.
The alternating current applied to the socket must be converted into direct current. In the case of Eurorack in a voltage of +/- 12 volts and for some modules in +5 volts. This conversion can take place inside or outside the housing.
If the transformation takes place inside the housing, a grounding design must be available. That means all three lines (phase, neutral, protective conductor) from the socket lead into the housing. The protective conductor is secured to the central grounding point of the housing. At this point, all metal parts of the housing are grounded and also the 0 volt reference potential is derived from there. This is the optimum scenario in terms of expandability the modular system.
Most offered power supplies for modular synthesizers in Eurorack format are external solutions. The alternating current is converted into direct current outside the housing. This is a power cable with a block, such as those for the notebooks. From there, usually two lines (+/-) go into the housing or into a corresponding module. What happened to the protective conductor is hidden in the block. From a safety point of view this is ok. Since no alternating current is fed into the housing, a protective conductor connection is not necessary.
The advantage of an internal power supply lies in the existing grounding design, which makes the system easily expandable. A common 0-volt reference potential for several interconnected systems can be easily derived from the central grounding point. This prevents too much potential differences across several cases – which means less system noise. Furthermore, a symmetrical signal transmission is possible with a grounding design. Hinton Instruments sells In / Out modules for balanced patching.
The advantage of an external power supply is its simplicity. You do not have to worry if you have everything properly connected and grounded. For small, non-growing systems, this is definitely recommended. Depending on the quality of the PSU and the other components of the distribution, this can also work well for larger systems. However, if several PSUs are involved in the supply of the entire system, it will be somewhat more difficult to overcome any system noise that may occur with the aid of reference potential equalization. The central protective conductor connection on the housing is missing.
Transformer power supplies have a lower efficiency, higher weight compared to switched-mode power supplies and are barely produced today. However, they are the first choice for analogue sound generators since the mains voltage is not converted into a high switching frequency voltage (15 – 300 kHz), as is the case with switching power supplies. The high switching frequencies of the switching power supplies can lead to disturbances in the electrical circuit, which are noticeable in the signal chain of the modular synthesizers in the form of noise or unwanted modulations. However, this is very dependent on the quality of the installed components and the downstream electronics for smoothing the signal. The A-100 PSU2 from Döpfer is a classic transformer power supply. The newer version A-100PSU3 is a switching power supply – but the proven linear control of the A-100PSU2 is used to smooth the signal. Both provide a clean output voltage.
So it is less the type of power supply than more the quality of the installed electronics. I can only recommend not to go cheap on the power supply. What works for a notebook can cause a modular synthesizer to vomit noise.