What makes a Lundahl mains transformer so unique?

A HiFi Mains transformer is not just there for the output voltage,  but there are also requirements for low mechanical hum,  low magnetic field radiation, low electric hum fields, voltage stability and voltage precision. Here we mention some of the problems that you can have with low cost mains transformers, and how Lundahl has solved those problems.

Problem sources can be: 

1. Mechanical hum
2. AC Electrical Field
3. AC Magnetic field
4. AC current signal flow through the ground capacitance

All three can be very disturbing, and not only with tube amplifiers!

For instance with a KT88 design, voltage stability and precision means a 6,6 Volts KT88 filament winding should not rise to 7,2 Volts when a 250 Watt transformer chassis is "only" loaded with 100 Watt.  In case you find the voltage of one winding depends too much on the total transformer load, you have a quality problem here. Not only is the unstable voltage by itself a problem, but most likely this transformer is constructed by a low cost concept. It means magnetic paths inside the core have too much magnetic resistance. The result of this is what you measure with your voltmeter. What is harder to measure is the stray radiation caused by this. What you expect is one thing, but what you get after buying the transformer  is reality. And as we all know,  once you have too high filament voltage you expect the tube tolerance to digest it.  Poor tubes.

We found the Lundahls to be very honest and precise in their specifications.  The correct voltages applied to the tubes will increase the lifetime of your tubes a lot. Mostly this is an issue with DHT tubes, but in the end it will become an issue with any tube.

. 

Ground Capacitance problem, and solution by Lundahl. With normal transformers (Top picture) there is allways a certain capacitance between the mans winding and the chassis core. This capacitance is larger then you think, because it adds up from each single winding! Over this capacitance is the mains voltage applied. With some simple math it can be calculated what AC Current will result when 220Volt AC is applied over a capacitor. What is hard to say how it will flow exactly. , which makes this current to tricky. Will it cause hum, or not? With Lundahl transformers they use the so called method of electrical compensation. It means they transformer is splitted in two symmetrical, identical sections. Each will have it's own AC ground current, in this drawing called I,1 and I,2. Because of the special windings technique, these parasitic currents flow in the opposite direction, and effectively cancel compensate each other that way. This takes some time to understand how it is done, but it explains why the whole transformer package is present two times! So if you have your transformer primary and secondary voltages, there are TWO transformers like that on the C-Core. These TWO are paralleled or serialized in the right way to give the required voltages, at ZERO radiation. . What you see on the bottom drawing is only the primary winding. Each half winding is on one side of the C-Core. Since the primary winding is physically split in two, all secondary windings must be present two times also. This must be done for reasons of perfect symmetry, otherwise the target of zero radiation is not reached.

 

THE PROBLEM:

• Mechanical hum can reach the preamplifier tubes when the transformer is mounted on the chassis.   This can not always be "heard" by the ear, but still is conducted to the pre-amp tube.   So you start to increase power supply capacitors, etc.,  but never solve this problem that way.
• Electrical field will stray into any high impedance part of the amplifier. Electrical fields radiated by bad transformers can be very high gradient fields. Luckily the "impedance" of those electric fields is high. Impedance is not the right word for it, buy by this we means an electric field can only generate a signal on loose wires when those wires are connected to some circuit of high impedance.... like in tube amplifiers.
• Magnetic field radiation is harder to deal with. These represent much more energy, and they will find their way into low impedance circuits too. They do tend to generate lower signals in loose wires (exposed to the field) but these signals are very lower impedance, and they will make currant flow in almost any circuit. Besides they are hard to shield. Magnetic fields can pass shielding easily. .Any wire will pick up an AC signal when it is held into a AC magnetic field.  Already a straight wire can do this.    There is no good way to prevent signal pick up. Take a magnet at home, and try it what it takes to shield the magnetic field. You will see it passed through metal easily.    So the best is not to have LOWEST magnetic field radiation (stray radiation) coming from your transformer.  The one and only way for this is to have the windings tight together.  Hand wound  mains transformers do not fulfill this requirement. Next is, you need to have a possible flat copper package.  So the outer windings are still close to the iron.   Check a Lundahl, and you will see no transformer  has such short distance of the outer windings to the magnetic core.  Also each wire is nicely in parallel to any other.  These are HiFi requirements for mains transformers.
• Ground Capacitance. This is explained in the above picture.  With all mains transformers there is a significant capacitance from the primary winding to the core, and by this to the chassis ground, and to the secondary windings as well.  The problem is often that the AC current resulting from this is hard to measure, and it is hard to say how it will flow.  It will follow the path of lowest resistance,  but we can't say what that is in a given circuit diagram.

THE SOLUTION:

With classical transformers (Not Lundahl) you have a primary winding package, and a secondary winding package.   Lundahl has a brilliant way to make their mains transformers. This results in a fully symmetrical construction.   They use  no separated primary winding and a separated secondary winding.   On the LUNDAHL mains transformers,  the whole configuration is present two times. So on a 250 Watt core,  are actually wound TWO identical transformers of 125 Watt.  Each copper package has it's own primary.   The windings of those are serialized or paralleled in order to get the required voltages,  and by this it is possible to eliminate most of the parasitic current, flowing through the ground capacitance.  So ground capacitance can not be avoided,  but the current trough can be reduced.