Fixed...but was it the "right" solution?

[jewishjay]

I recently built a small SE hifi amp. At first it had a 120hz buzz. I must have made a mistake, so I played with lead dress, swapped tubes, tried all kinds of tricks but it still buzzed. Finally, I added a small filter choke to the power supply and - now it sounds great, no more buzz! Fixed right? ...or did only mask the problem? Do some power supplies just REQUIRE a choke? Why?

There was another time on an earlier build, with a 60hz hum, I thought I did something wrong, tried everything, finally what fixed it? Elevating the heater center tap to 20v. Quiet as a church mouse after that. Again, was that the "right" solution? Or did it only hide the problem? Do some amps NEED heater elevation? Why?

Just curious what you all will say.... thanks!

[Dave]

The answer is in the name "Filter Choke".



Sounds, to me, like your power supply needed a little more filtering. You might have attacked the problems a couple of different ways, but this worked, so you're done.


Dave

[shooter]

you can't "hide" hum, it's either there or it isn't.

[Latole]

I think your power supply didn't have the correct ESR.
The choke restored the resistance, which a simple resistor would have done.

[Merlin]

How old was this amp? Maybe the PSU caps are drying up.

[tubeswell]

small SE hifi amp… had a 120hz buzz. I added a small filter choke to the power supply and - now it sounds great, no more buzz! … Do some power supplies just REQUIRE a choke? Why?

Yes, SE amps tend to be hummier than PP all other things being equal, and so SE benefits from better filtering. A CLC filter provides better filtering than a CRC filter for 2 reasons, namely;
1) the self inductance of a choke resists changes in current and so removes virtually all AC (leaving pure DC), and
2) a choke winding has low DC resistance so won’t drop much from the B+ voltage.


A CLC filter in the power rail between the rectifier and the OT primary works better for SE amps because a SE OT primary winding lacks the inherent hum-cancelling properties that the opposing halves of a PP OT primary winding has.