Polarity on a non-bladed plug?

Wow I didn't think my thread would produce so many responses. All great info and I'm thankful.

Jea48, if you actually measured the 'potential' at neutral or ground, you would find that it it varies from positive to negative at 60 Hz frequency, thus making it symmetrical! Capacitance (and inductance, for that matter)only exists in AC circuits where polarity essentially changes 60 times a second. Can't be more symmetrical than that. Hum, however, can exist due to different potential at the ground or neutral at different places. That is why reversing 'polarity' might make a difference.

Bob P.

I use a high-quality digital multimeter to check. You need to measure for residual ground voltage at the RCA jacks. Make sure nothing is hooked up to the component but the power cord. Try plugging it in both ways while noting the voltage reading between the RCA hole and outer ring on one channel (hole is positive). The lower voltage orientation has the polarity correct.

I did this with my vintage MC240 tube amp that had 1960s lamp cord for its power cord. One way I got like 35mV and the other way I had 6mV. I have to say I couldn't tell a difference in sound though but I am anal so I wanted it correct for peace of mind.

By the way, whenever making a low-level voltage measurement, you HAVE to use the local ground that is as close to the positive as possible. Using the outlet ground (or chassis) will do nothing but screw up the results.

Arthur

10-14-07: Inpepinnovations

while it might not make a difference sound wise, it might make a difference 'hum' wise. If no hum present, then I agree, since AC does not have 'polarity' as DC does.
Inpepinnovations

Bob, when we speak of ac polarity lets not confuse the issue by comparing it to that of DC polarity. In this case of AC polarity I am not talking about Current flow in a circuit.
When referring to AC polarity I am referring to an AC grounded system like that of your average home. Where, to keep it simple, the secondary winding of the utility transformer that is used to feed homes in a typical US neighborhood. The secondary of the transformer is called a split phase winding. Basically the single phase secondary winding is center tapped midway in the winding. The nominal voltage is 120/240V 3 wire power that is brought into your home. NEC, (National Electrical Code), requires that the service be an AC grounded system.

NEC requires the center tap conductor be intentionally bonded to earth ground by a minimum of two earth Grounding Electrodes.
The intentionally current carrying grounded conductor becomes the "Grounded Conductor", the Neutral.
NEC also requires the grounded neutral conductor to be bonded to the electrical panel enclosure. Final product, a single phase 3 wire AC grounded electrical power system. Two ungrounded conductors, L1 and L2, with a difference of potential between them of 240V nominal. With a difference of potential from either of the two Lines to the Grounded Conductor, of 120V nominal.

Now we can talk about AC polarity as it pertains to NEC.
NEC requires all 120V receptacles and lamp holders to be of the polarized type. A standard 120V receptacle, (NEMA 5-15R or 5-20R), is a 3 wire 120V polarized type receptacle. NEC requires the receptacle to have a white, silver, colour terminal screw for the neutral conductor. A brass, or similar colour screw for the hot ungrounded conductor. The equipment ground terminal screw green in colour.
Looking at the faceplate of a 5-15R, 15 amp, receptacle one female slot is longer than the other. The longer of the two is the grounded conductor, the neutral. In the case of the 5-20R, 20 amp, receptacle the longer of the two is a "T" slot, (the neutral). This is the correct polarity wiring configuration.

Understood, Jea48. I understand the difference in the use of the term 'polarity' for AC circuits versus DC circuits. This difference, unfortunately, is not understood by many, however. The incorrect use of the term 'polarity' to describe ensuring that the grounds/neutral are all connected and that the 'hot' side doesn't appear at the nominal chassis ground, always leads to confusion.

The reason that I wish to remind people that AC has no polarity is to rid them of the fallacy that there is only one direction for the current. This is not true, therefore all 'mumbo-jumbo' about assymetrical AC circuits and where the hot and ground leads (which are incorrectly referred to as positive and negative) are located making a difference in sound, such as the contention with the transformer leads, is just that, 'mumbo-jumbo'. Any difference to the sound due to reversing the plug is due to a change in the noise or hum which might appear at the audio line chassis connection, not transformer configuration of hot and ground connections being asymetrical, etc.

Respectfully, Bob P.