@10229 Two-prong plug devices are usually either double-insulated, or there is a SMPS that has additional isolation that protects the chassis from potential AC faults. I have several CD players like this, but I also have 30 year old preamps that only had 2-prong plugs but had low voltage transformers.
@jea48 Grounding schemes have been a matter of debate for many years in the audio world. Some builders believe grounding the chassis to earth causes hum, others do not. I agree that a Class II device with no need for chassis earth ground should have a 2-prong jack.
I agree the schematic shows signal and circuit grounds go to chassis, and that the sequence shows fuse, switch, and transformer. It would have been nice to show the jack connection first, rather than just the source AC symbol. Also, RCA jacks and speaker jacks are generally connected to the chassis, would you agree?
@erik_squires The photo shows me nothing to believe this is a Class II double-insulted circuit on the primary of the transformer (or anywhere else). It is also fused incorrectly as described previously.
The power supply has a primary side at incoming AC voltage which is what is being discussed here. The secondary side of the transformer is the other part of the power supply. If the AC side has a short, for example the power switch as atmasphere has stated, or a broken line (hot) connection, etc. touches the chassis, there should be a low resistance path to ground, but sometimes designers rely on the neutral connection at the service panel to provide that ground. However, properly connected, the fuse will blow. This is true of solid state gear as well, which can have much higher current draw but lower voltages.
On the secondary side with the HT voltage going to the tubes and circuitry, if there is a short, it will blow the fuse(s) or in circuits with virtual center tap connections through 100 Ohm resistors to ground, the resistors. Properly designed and fused, the unit is unlikely to damage the interconnects.
High-gain signal grounds (preamp, VAS, driver, etc.) are kept separated from high-current grounds (output tubes, output transistors) in most amps to minimize hum or there is some sort of star grounding scheme. Signal grounding schemes are quite varied and hotly debated as well, but they are not safety grounds.
There is ample information available that details the need for proper chassis AC safety grounding, but there may be some particular reason why this unit was considered to be safe without it - I just don’t see it based on the schematics. I’m not pro or con Carver, just commenting on the photo and the schematic.
Again, it would be very nice if Carver would provide their reasoning which may answer all these concerns.
