Owners of Carver Crimson 275 -Caution/Warning--Potential Increased Risk of Electric Shock

Set aside who designed the amp and look at it from a design standpoint. I’m not an EE but have built about 22 tube amps and repaired many others, and have built and repaired a lot of solid state gear. I rely on current information on safety from books and qualified engineers either in person or online.

First, as has been said before, proper installation of the fuse is in the hot (line) leg of the primary winding of the transformer along with the power switch. In years past, the neutral was fused but this is incorrect because it could blow, leaving the hot connected to the circuit through the switch, waiting to complete a circuit through the operator or internal mechanical failure. This is a strange holdover from early tube amplifier and radio designs that I’ve run across, and it gets replicated to this day, which is incorrect.

Second, I don’t see how the unit can be classified as Class II, where it is double-insulated. Even if it had a completely non-conductive outer shell, the circuit, which is connected to the chassis per the schematic cited earlier, could be connected through the input/output jacks, exposed screws, etc. That means the unit should have a safety ground connection - a dedicated, short piece of green or green/yellow wire connecting the ground lug on the IEC jack directly to a dedicated nut/bolt through the chassis.

This is so that if any hot part of the wiring comes into contact with the chassis, it will blow the fuse and/or the service panel breaker. In the US, neutral is tied to the ground buss bar in the service panel, so hot leaking or making direct connection to neutral or ground trips the safety devices.

Some manufacturers consider the transformer to be a safe isolation between AC wall power and the circuit and chassis, but that does not account for a potential failure at the primary side of the transformer, or the transformer itself which could short internally (some output tube failures can cause this), coupling the circuit with AC and the chassis.

Hard to tell from the photo, but if I read the comments properly, the rectifier board is glued to the chassis? If that is correct, it’s really bad design/construction. That is a high voltage part of the circuit with lots of current and should be very well attached to the chassis.

It will be very interesting to hear what the company provides in response to the questions by the OP!

The 275 is the entry-level amp. It would be interesting to see the schematic of the Raven 350 monoblocks.

https://www.bobcarvercorp.com/crimson-raven-features-specs

Btw, I'm a Bob Carver fan.

So just to play devils advocate, and to potentially obtain some useful knowledge, I own both a Marantz NR1200 and a Cambridge AXR85…both amplifiers house in metal casework and neither have a ground pin on their power cords…why is the Carver amp any different?  
 

Matter of fact after further inspection, I have lots and lots of electrical devices that don’t have 3 pin power cables plugged directly in to outlets…should each of these devices also get dedicated threads publicly telling everyone that they are potentially deadly and to contact their makers for comment otherwise?

@gktaudio

As for changing the incorrect AC Power wiring it would be an easy fix for someone with a soldering iron and basic soldering skills.

Note, the schematic wiring diagram for the amp. Carver shows the correct wiring sequence. Fuse >> switch >> transformer.

Note, the R57 4.7M resistor from the neutral conductor to the chassis.

Also note, "CIRCUIT GROUNDED TO CHASSIS" I think there he is referring to the circuit ground of the amplifier. Therein B- and signal ground.

Schematic wiring diagram of the power supply for the amp.

As for the EGC, (Equipment Grounding Conductor), I agree with what you said in your post. But... My point that I was making, my post, is that user doesn’t know the EGC pin on the 3 pin IEC connector is not connected to the chassis. It should be the users choice if he/she wants to lift the EGC with a ground cheater.

From what I have read Carver doesn’t like the EGC. It does nothing for the sound of the equipment. If anything the EGC can harm the sound. Especially if the signal ground is connected directly to the chassis. And it appears Carver did on this amp. That can leads to ground loop hum if an EGC is connected to the chassis.

There are things carver could of did in his design to solve the problem and connect an EGC to the chassis for electrical safety. But apparently he chose not to...

Here is the schematic wiring diagram for the amplifier section. Note the RCA jack ground is grounded directly to the chassis.

@10229

The AC power wiring inside the equipment is double insulated. (NO safety EGC , Equipment Grounding Conductor, is needed)

Look on the back of the units. It should say Class ll wiring and or may display a square inside of a square.

It costs more to make Class ll equipment, than it does to make equipment that uses the EGC from the AC mains wall outlet.

CLASS II power wiring