CTH switcher info
Won't have time to tinker for a bit, but wanted to put up info.
At issue here is that it seems common that w/sensitive, lo-Z cans & 6,3v heated tubes, a noise/hum can be heard. For most, this does not intrude on music but is heard between songs. Use of design-center 12au7 variants lessen the noise.
I'm giving design info here, but if/when I get time these are the experiments in order that I'd conduct:
- disable switcher by disconnecting its supply. lift output of 3uH & use clean 6V heater supply (e.g. batt) for pins 4/5. See if noise disappears, just to be sure switcher is involved.
- if this confirmed switcher is causing the noise, restore 3uH & introduce resistance between lifted switcher input leg & connect. Resistor should handle load (e.g. 5W?)
- Perhaps alter cap values.
If switcher is found the culprit (1st experiment), I'd suspect we're asking a lot of it to cut from its supply (about 33V, its max is 40V) all the way down to 6.3V. Its min is stated as 1.2V but I'd suspect an ugly waveform at that voltage.
So, the 2nd experiment would be to alleviate some of the load through a resistor.
Here are National's Simple Switcher results at 6.3V & 12.6V:
WRT above, note that our input (Cin) & output (Cout) caps are higher than required, a good thing I'd suppose.
Note also that ideal main coil values vary between the two (220uH, 330uH), but in this next snip of the datasheet, 220uH appears best compromise value (& much smaller as well):
Now the ideal ripple filter cap may
be a bit higher (180uF) than we are using (100uF). BUT we are getting very nice ripple reduction vs switcher w/o ripple reduction (see scope image from spec above). And in P2P proto I used a "worse" switcher (LM2575 - 52kHz) with NO ripple filter & found it to work well. So what may
be happening @ 6.3V is the actual chop freq (or a harmonic of it) is entering audible range. More research would need to happen to reach that conclusion. Alleviating some of the switchers job through resistance OR altering assoc cap values might then address this.