Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Transformers offer some filtering, but not as much as chokes.

Posted by Al Sekela (A) on July 3, 2007 at 14:22:41

In Reply to: RE: Agree, a 3KV or higher cap would be better. posted by wheezer on July 3, 2007 at 11:08:01:

The Hammond choke tweak works, I believe, because the large inductance together with the parasitic coil capacitance makes it a wide-band capacitor. I have a 193H, a smaller 5-henry unit in the same Hammond series as the 193L, and it has a maximum impedance versus frequency at 5 KHz. This is likely due to the self-resonance, as typical transformer cores are useful up to 100 KHz in some cases.

Transformers have much less stray inductance than the 5-henry chokes, so they function as transformers over a wider frequency range. They would not offer filtering in themselves over their working frequency range, but you can load the secondary with an R-C network that will present a resistive impedance to the AC line over most of the transformer's working range. The upper frequency limit, whether due to primary leakage inductance and stray capacitance, or core magnetic domain switching speed limits, is a threshold above which the transformer shunts noise on the AC line regardless of what you attach to the secondary.

My DIY transformer shunt filters are designed to present 120 ohms to the AC line. Impedance transforms as the square of the turns ratio, so a 120-to-12-volt transformer would need 1.2 ohms load on the secondary to appear as a 120-ohm load at the primary, for example. The problem then is that a large capacitor is needed to keep the 60 Hz power out of the resistor, but allow the device to load the AC line over most of the audio band. It is better to use transformers with lower turns ratios.

Different transformers have different levels of performance as filters. I'm lucky to have a local store with a surplus room, and got a 1 KVA 14-volt transformer for cheap. A choke of similar size would cost a fortune to buy new. If the transformer filter is used in the vicinity of the audio system, it is important that it be mounted firmly and not have parts that can vibrate, such as end-bells. Shaking steel in the presence of strong magnetic fields is not a good idea.

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