Avoiding Hum From Power Supplies

If you are buying a second-hand rack, or if you already have a Doepfer modular rack with a cheap alternative (non-Doepfer) power supply you should be aware of how to avoid the (minor) transformer hum that cheap PSUs generate.

In the area near those power supplies (bottom right) no modules should be assembled that are sensitive to transformer hum.
Doepfer recommends to assemble only control voltage modules in this area (e.g. LFO, ADSR, trigger delay, clock divider/sequencer, sequencer, slew limiter, MIDI interfaces and so on)
,but no audio modules (VCO, VCF, VCA, VCP, frequency shifter, ring modulator, spring reverb and so on)
Also don't put the A-178 Theremin modules near the power supply.

Doepfer Power Supply I ( PSU I )

This matters for some of the cheap alternative non-Doepfer power supplies, and for the old Doepfer versions with PSU I 
For new A-100G3/G6 cases (with original PSU II) this does not apply. 

Find out how to distinguish the old transformer (PSU I) from the new version (PSU II) at http://patchpierre.blogspot.nl/2012/07/doepfer-power-supplies.html

More info at http://www.doepfer.de/a100p_e.htm#A100P6

Doepfer Power Supplies

There are 2 types of transformers used in the existing standard Doepfer A-100 racks and cases.

The old Doepfer power supply with a
rectangular transformer

If you are buying a secondhand A-100 rack, it might be useful to know how to distinguish the old transformer from the new version.

The newer version is a better version, more reliable and provides a steadier voltage that is better for your system, although i personally never heard any complaints about the old transformer besides the longer warming-up time.

The old power supply has a black or blue rectangular transformer, where the new power supplies have a round silver transformer.

The new Doepfer power supply with a round transformer

For a period during the transition (about 2 years) the customers could order both versions.
In any case all A-100 cases manufactured later than early 2005 are equipped with the new power supply, and all cases manufactured until about of 2002 are equipped with the old version.
For cases manufactured in 2003 and 2004 it depends which version has been ordered by the customer.

In any case the new power supply can be identified by a small label outside the case near the mains inlet.
It says "Power Supply II" unless it was removed by the user.

Source: Dieter Doepfer