230V 50Hz to 15V Switching Transformation

Question

I have an audio circuit which is supplied by a voltage of 9V dc and 12V dc. These voltages are produced by means of a conventional transformer which transforms the mains 230V ac 50Hz to 15V ac 50Hz. After the transformer there is a rectifier (diode bridge and capacitor) followed by two regulators, one for the 9V dc and one for the 12V dc.

My problem is that the transformer produces a heavy magnetic field, which couples with my circuit producing a low hum. I am thinking about substituting the linear power supply with a switching one, thus avoiding the 50Hz magnetic field.

Now:

• Is there around on the market a switching power supply with the requested characteristics?
• Is it better to use a power supply giving directly the 9V dc and 12V dc, or is it possible to Step down the 230V ac to 15V ac (without using a Conventional transformer) and keep in place the rectifier and regulators?

Answer 1

• Stray E or H fields are called Common Mode , meaning common to both {signal and ground} or {power and ground} and when floating or high impedance can cause the unbalanced input to convert this to a differential voltage.
• This may be conducted or radiated interference. Each can require similar or different solutions.
• In some cases an unregulated DC supply such as you describe injects conducted noise into the output stage which draws from this voltage.

The most common solutions are;

• Identify if it is magnetic radiated noise by moving the source.
• Identify if is conducted by measuring the AC ripple voltage on the DC supply.
• Shunt the common mode field by earthing or ground connection to a nearby ground.
• Add ferrite CM and differential chokes to the DC input. ( Many wall supplies have CM ferrite clamshells molded inline on the DC cable)

• shielding of high impedance signals may be necessary in some cases.

• A linear low drop out regulator (LDO) offers the lowest noise rejection but low in efficiency.

• SMPS are common and 50mV ripple is common as well but more efficient.

A PC PSU has 12V. THis can be used for external applications from an unused HDD Molex plug.

Answer 2

I am sure the hum is caused by the magnetic field of the transformer coupling to the coil of the inductance inside the wha wha pedal - I made many cross-checks. In fact, even when powered by a battery, the wha picks up the magnetic field of the transformer when the latter is connected to the mains. I'm sure it is the wha because the noise vanishes when it is turned off.

That makes perfect sense.

Figure 1. The popular Dunlop CryBaby wah-wah pedal. Image source: ElectroSmash.

Figure 2. The Cry-Baby PCB with the inductor clearly visible in the centre of the PCB. Image source: ElectroSmash.

You seem to have a preferred relative positions for your setup. If everything else is OK (and assuming the construction of your pedal is similar to this one) then one option is to demount the inductor from the PCB, solder on extension leads and reorient it to minimise hum. Once the optimum position is found you can fix it in position using a silicone sealant or similar.

Answer 3

Solution: I substituted the conventional transformer by a small toroidal transformer, supplying 14 V ac at full load (more precisely, it has two secondary windings at 7 V ac, which I connected in series). I left in place the rectifiers and regulators. This completely eliminates the hum - a toroidal transformer produces much less magnetic field in its neighbourhood than a conventional one.

For those who are interested, I bought a transformer produced by Nuvotem Talema.