Power line filtering

[oshifis]

I heard that the mains has a lot of higher order harmonics, and if one filters them, audio will be better. Here came my idea (possibly not new):

Let's use a low-pass filter that passes 60 Hz (50 Hz outside US), and suppresses 120 Hz (100 Hz) and beyond. This filter could be connected in line with the mains input of the audio device.

Another approach would be to put the filter after the rectifier diodes to filter 120 Hz (100 Hz) ripple and beyond.

The best filter for this purpose could be a low-pass elliptical (Cauer) filter. The L and C-components are already present in choke and filter electrolytics form after the rectifier diode bridge. They just need proper tuning.

I just played with the free evaluation version of Tonne Software's Elsie, and came to the result that a 20 H choke after the rectifier diodes can be tuned with 0.125 uF parallel capacitor to suppress 100 Hz. The electrolytics before and after the choke do not really affect the tuning, just the stopband attenuation - the larger the better.

Has anybody tried this arrangement? It seems an easy and effective way of reducing ripple on the DC after the PSU.

[woody]

Are you talking about for a 10ma preamp power supply or for
a 10A power amp supply? I would expect the DCR of that 20H
choke would have to be rather low if you used it in a hi current
circuite like for a power amp and any 20H choke with a low DCR
is likely to be huge.

[oshifis]

About the size of an OPT. I have seen designs using a choke in the PSU of a tube power amplifier. But specifically, I intend to use it in my tube preamplifier, with Vpp = 300 V and total current about 40 mA. I think a DC resistance below 50 ohm is acceptable.

[Tweeker]

A lot more DC resistance than that should be ok in a 40ma supply, consider the power transformer has resistance too. You would do much better with a regulator following a moderate slightly higher voltage psu than using massive iron. As an example the 153Q which is 10H and 53R is very largest medium current choke Hammond makes, and weighs 21 pounds. I would not want less than 10H if choke input is intended, less would be ok in a CLC filter.

A common mode choke on the line may help, more so for the heaters than the B+. Heaters are sensitive to common mode noise. Using a screened power transformer will also help. The common mode choke most be rated for peak power, which is much higher than the average current in a capacitor input supply.

Resonated chokes were used much more way back in the day when capacitance was much more expensive, you are giving up filtering at other than the stop band by using it. This is not desirable if you are concerned with higher order harmonics. It is mostly used in the power supplies for single side band short wave transmitters in order to allow for a choke input supply of low inductance now.

[oshifis]

So you think a resonated choke (i.e. a Cauer filter) would not be a good idea? It seemed to me it is an almost free tweak to suppress 120 Hz hum in a sensitive tube preamplifier, like a RIAA correction unit. The 20 H choke was just an example, I can go with 10 H or even smaller. Important is the parallel resonance at the hum frequency.

[Tweeker]

Im saying its a bad idea if the harmonics were a concern, as you mentioned in the first post. Its not free as only the fundamental gets filtered now, the harmonics pass.

If its ultra sensitive you should consider more stages of filtering, one or more RC stages following the (possibly resonated) LC stage.