Why choose Lundahl Chokes?
This is a text page only, but an important one if you intend to use chokes.
First let me say it here: Chokes are the most chosen way to build a low hum, high voltage power supply, and a source of frustration when you buy low quality types.
Residual hum is something you never get used to, and often the reason why such an amplifier is sold after some time. Sometimes people hope to get used to residual hum, but this is often not so. Instead of that you start to focus on hearing the hum, and your ear gets sort of trained for it. The effect comes, when a music piece is over you already know what is coming. It sounds like "hummmm". It may seem unlogical to feel disturbed by this, since there is no music playing at that moment, but my personal experienc is, your need a dead-silent break in between, and then the next music piece can strike your feelings again. The effect is when there is residual hum, you get tired more quickly from hearing.
So it's much better to make a good beginning with a good quality mains transformer and good quality chokes.
There is no single factor which makes a Lundahl choke "better". There are several factors, of which some are unique. Adding these factors all up, makes that the choke is doing what you expect it will do. Here is a list of what is a good idea to expect from a choke. After reading this list, you say: of course, of course, you expected this, but ...... very few chokes really HAVE it.
- No Mechanical hum from the windings package. This is the #1 requirement when winding a choke, and a major source of disappointment with low cost chokes. reason is that the electrical field on the choke windings is extremely large. It has to be because over the choke itself is a very (dirty) kind of high voltage wavesahape. What goes in is double rectified (100Hz with many harmomics. What you expect to come out is purest DC. It means the difference in voltage between those is over the choke electrical connections. The finest way to archive this is by laying all windings exactly in parallel to each other. This requires a computer controlled winding machine, with a "finger" that guides the wire exactly where it should come. The worst are toroid chokes, but also hand wound coils are noisy, or those made by simpler kind of winding machines. Also (believe it or not...) you expect the choke to stay low-noise after several years.
- No Electric field radiated: This is a hard one. Chokes have a non-changing magnetic field. So magnetic radiation is low per definition, and no issue for any choke. So what is the issue? Chokes can radiate an electric field. This is of the kind that stray into high impedance parts of the pre-amp. Its a higher kind of harmonics hum. So 100Hz + some 150Hz. You try to fight against this kind of noise, by using bigger and bigger capacitors, and nothing helps. In the end you accept some kind of residual noise from your amplifier. Use a Lundahl choke and you reduce this effect as the best way. Reason is, the stray effects are as minimized as can be, due to the extremely short distance from the windings to the core. This can only be done by the special windings technique Lundahl has. Suppose you do decide to buy a Lundhal choke... In that case remember these words, and look how extremely close the windings are to the core iron. It will not surprise you that this leads to lowest stray fields.
- Fully symmetrical construction: What is this, and do you need it? For this, we recommend to go the Emission Labs tubes pages, at this website, and read the application note #1 that is linked on the 274A rectifier datasheet. A direct link is here
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