What is a 2-coil choke? It is a choke with 2 windings wound onto the same iron core. The advantage of 2-coil choke is that it can be configured in different ways, according to the user desired effects!
Above is how a 2-coil choke symbol looks like. It is having 2 coils wound on the same iron core, as shown in the above picture.
There are four ways of connection that I can think of right now for tube amplifier power supply usage.
- Series mode
- Parallel mode
- Common mode noise rejection
- Differential mode noise rejection
Let's take a 10H 100mA 50 Ohm DCR choke as example:
|Mode 1 - 10H with 100mA capacity and 50 Ohm DCR|
|Mode 2 - 2.5H with 200mA capacity and 25 Ohm DCR|
|Mode 3 - mH in inductance, 100mA, usually use in AC circuit only|
|Mode 4 - 10H with 100mA capacity and 50 Ohm DCR|
Such 2-coil power supply chokes are made to couple well at low frequency, usually at double the mains line frequency of 50/60 Hz (results of full-wave/bridge rectification). Mutual coupling between the coils will be low at higher frequencies, and will act as 2 different chokes at higher frequencies. This behavior can be used in advantage to different applications.
What are differential mode noise and common mod noises? I think you guys can find that out using Google. There is no need for me to reinvent the wheel again over here.
Mode 3 will reject common mode noise and mode 4 will reject differential mode noise, simple as that. You can treat them like a differential mode choke (DMC) and common mode choke (CMC) commonly used in power supplies. The only difference here is that the standard CMC or DMC do not pass DC where it works in the AC region before rectification process. For our DC chokes here, it works with DC in mind.
|Common Mode Choke|
|Differential Mode Choke|
Do note that for connection #3 - common mode noise rejection configuration, it is usually used in AC circuit mostly. You can apply this in DC circuit too but it does not act as a power supply choke anymore in such configuration. That is, it is no longer a capacitor-input filter in the above diagram. You will need to add another choke to provide the usual filtering function to complement the "common mode choke".
Using such 2-coil chokes will be beneficial to gain extra edge on getting a quieter power supply for your amplifiers. Do not forget this when you are designing your next amplifier. An example of application would be as follow:
- 3 stages: rectifier -> 2-coil choke in series or parallel mode -> 2-coil choke in common mode config -> 2-coil choke in differential mode config.
- 2 stages: rectifier -> 2-coil choke in differential mode config -> 2-coil choke in common mode config
- And many more!
Just for your reference, an open square frame refurbished Permalloy double C core choke of 1-coil choke of 8H - 10H 300mA is U$120/pc now, and a 2-coil version is only U$140/pc. The price difference is minimal yet the benefits are many folds!
Now, go order a 2-coil choke and replace all the chokes in your amplifier to lower the noise floor and improve the sound!
J&K Audio Design