John, thanks for your findings and post.
The chokes idea is very interesting but I don't understand couple of thins - please pardon my ignorance:
* how do you determine the required choke resistance (I am using 1500uF FE caps)?
* why bigger than 20mA current is required and does it need to match the one the chokes are specified at (156M - 3H @ 100ma)?
Thanks,
Ivo
All good questions Ivo, and I had to do my homework on this before the building as well:
Explanation
When you put the choke before the first capacitor you are creating an inductor capacitor series circuit. It has a natural frequency of resonance which needs to be damped to prevent it singing away like a tuning fork provoked by the regular kick from the incoming mains pulses. Damping is achieved by getting the critical amount of resistance into the circuit. Some of this is already there in the choke and a small amount of ESR in the cap. Thankfully there's a pretty easy calculation for this as follows:
Calculations
For critical damping i.e no ringing the series resistance R = 2 times the square root of L/C. So for the 1.5 Henry choke and 0.001 Farads of capacitance we get R = 2 times the square root of 1.5/0.001 which comes to 77.45 ohms. That's why I added 22 ohms to the choke which already had 56 ohms resistance (makes 78 ohms).
There's also a calculation that gives the minimum current draw needed to keep a choke supply working as it should. This minimum current is just dependant upon the value of the choke (bigger means a lower minimum current) and the mains frequency. For 50 and 60 Hz it approximates to the following:
The minimum (critical) current in milliamps = the RMS secondary voltage divided by the choke inductance in Henrys. For my 1.5 Henry choke and a 55 volt transformer secondary that came to 55/1.5 mA or 36.7 mA.
The 100ma quoted for the 3 Henry choke 156M is the maximum rated current for the choke. It is either the current limit bofore it overheats over time or before the core saturates. As long as you don't exceed it in the long term you'll be OK.
Your setup calcs
With your 1500uF caps and the 3H choke you are looking at a needing a minimum of 89.4 ohms of damping resistance and a minimum current draw (in milliamps) of whatever your transformer secondary RMS voltage is divided by 3. The damping resistance in the choke is specced as 86 ohms and probably close enough on its own. If you have one 40-0-40 transformer serving a pair hackernap front ends then the minimum current draw of 13mA is already exceeded by the 20mA or so the front end uses, so no bleeder resistors are needed. From a 40-0-40 transformer you will get a minumum of 36 volts to the front ends which will be enough for evaluation of the benefits.
I hope that helps get you there. I'm keen to share the better sound this has brought me.
John
