Heater Supply Buzz
The switching heater supply has been very carefully designed and works very well. However, the size of the first filter capacitor has turned out to be too small. Even though this cap value is in range as far as the National Semiconductor design sheets, some buidlers were finding that 120Hz switching noise was finding its way to the output of the amp. The Excel BoM and PS Parts List have been changed.
To remedy this, cfcubed determined that replacing the 47μf C3H with a 330μf capacitor eliminated this source of noise.
It is recommended that all builders make this change regardless of tube selection or other factors. This Nichicon capacitor, UPW1H331MPD, will fit in the small space allotted for C3H. Here are instructions from cfcubed:
- Remove existing C3H (47uf). Replace D5H (1N5820) with 1N5818 to gain room for larger C3H if necessary. Watch polarity.
- Replace w/330uf* 50v cap that fits. Low impedance preferred. Use 35v rated cap if space demands it. Watch polarity.
- If space below PCB insufficient for cap, follow "BoM case" procedure above.
- If space below PCB sufficient for cap, supplement existing C3H with 270uf -> 330uf* 50v cap that fits. Low impedance preferred. Use 35v rated cap if space demands it. Watch polarity.
An example retrofit from wiatrob:
Additional comments from cfcubed:
“My personal listening tests indicate 247uf of low impedance capacitance is sufficient to eliminate any audible hum/noise contributed by CTH heater supply (in my setup). Scope showed minute improvement over this for 330uf AFAIR. Scope showed no improvement going to 470uf & above AFAIR.”