DC Offset Blocker/Killer - where to buy in the USA

@OP,
I think you have done everything you possibly could have to resolve the problem. I'm glad the dealer is refunding your money, but I think they should have been more active in resolving the issue. Especially having you install wiring, though a good thing in the end, should not have been something you had to do.
Hopefully, you can find another amp to your liking. 
Bob

Here are the DC offset measurements using a low pass filter. (DMM probes across capacitor leads)

I used a 1875 watt hair dryer to create the DC offset on the AC mains.

Test # 1
Dedicated Audio room.
Low pass filter plugged into dedicated audio outlet. 120V, 20 amp branch circuit. Branch circuit wire is #10-2 with ground Romex. Length of Romex from wall outlet box to electrical panel is approx 75ft.

Low pass filter plugged into wall duplex outlet. No other loads connected to the outlet.
AC mains voltage at outlet, no load, measures 122.3Vac (DMM, Fluke 87 True RMS)
**EDIT: (I just checked mains unloaded voltage again so I could check for the hair dryer loaded voltage drop. Unloaded mains voltage at the outlet now measured 121.5Vac. Loaded, hair dryer turned on, high heat, low blower, measured 119Vac).

DC offset voltage measured across the low pass filter cap measured 1.6 mVdc, avg reading. (Fluke set on Dc mV scale).
I also used a Radio Shack DMM. Meter set on DC volt auto scale. It also read 1.6 mV avg.

With the hair dryer plugged in, heat on high, blower on low. (This combo setting gave the highest DC measurements).
Fluke measured 0.734Vdc (meter set on auto DC volts. Dc mv meter setting is for 400mV max.)
Radio Shack measured 0.726Vdc.

0.734Vdc (734 mVdc) is more than enough to saturate the core of a moderately sized torid and cause it to buzz loudly. From what I have read 50mVdc will cause a torid to buzz.

Just for the heck of it I ran a few more tests.
Test #2

Low pass filter plugged into same audio dedicated wall outlet.
Hair dryer plugged into a wall convenience outlet in the same room. Branch circuit is separate 20 amp, #12 Romex, for several conv outlets in the room. Approx length of the romex from the wall outlet, the hair dryer is plugged into, to the electrical panel 80 + ft.

Hair dryer turned on. High heat, low blower.
Fluke measured 112.6 mVdc - 113.3 mVdc (Meter set first to DC auto, then moved to mV setting)
Radio shack measured pretty much the same, 112.3 mVdc - 113.6 mVdc.

Here is where, hopefully, Al (almarg) chimes in.
With the hair dryer plugged into the same outlet as the low pass filter the Fluke measured 734 mVdc. With the hair dryer plugged into a different outlet fed from a different branch circuit the Fluke measured 113.3 mVdc.
Just a guess on my part, two things are at play. Both involve the total combined length of the two Romex cable branch circuits, (Approx 75ft + 80ft). One is the inductance of the cable and the other the resistance of the cable. Inductive reactance? One other thought to consider is the utility power transformer. Just going from memory harmonics will travel back on the neutral conductor to the source. Some of the harmonics are dissipated by the secondary winding in the form of heat.
If Al is still following this thread I am sure he will have a better technical answer.

Test #3

Hair dryer plugged into wall outlet in dinning room. Wire is Romex, #12 , 20 amp circuit. Approx length? Just a guess 80ft or so. (Up, down, and all around).
Low pass filter, still plugged into the dedicated audio wall outlet circuit, audio room.

Hair dryer on, high heat, low blower.
Fluke measured 106.7 mVdc.
Radio shack 105.8 mVdc.
Voltage at wall outlet unloaded measured 122.3Vac. With the hair dryer turned on voltage dropped to 116.2Vac.

Test #4

Hair dryer plugged into an outlet directly below the electrical panel.Fluke measured 115.6 mVdc
Radio Shack measured 115.4 mVdc.
Note, here only the audio dedicated 20 amp branch circuit wiring length is at play. Approx 75ft, #10awg wire.
One other thing...... I will have to go back this afternoon, maybe tomorrow morning, and Check for the Line, Leg, (in the electrical panel) that each of the branch circuits are fed from. (For tests 2, 3, and 4). There in, Line 1 or Line 2. I believe that would have some relevance. There’s a 50/50 chance, LOL, some are on the same Line as the audio room dedicated audio outlet circuit that the low pass filter was plugged into for all the tests.
Jim

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jea482,863 posts01-15-2019 12:14pm


One other thing...... I will have to go back this afternoon, maybe tomorrow morning, and Check for the Line, Leg, (in the electrical panel) that each of the branch circuits are fed from. (For tests 2, 3, and 4). There in, Line 1 or Line 2. I believe that would have some relevance. There’s a 50/50 chance, LOL, some are on the same Line as the audio room dedicated audio outlet circuit that the low pass filter was plugged into for all the tests.

They are all connected to breakers that are on the same Line. Line 1, leg.
What are the odds?

The "Humdinger" from Audio by Van Alstine.

https://avahifi.com/products/humdinger-dc-line-blocker

If Al is still following this thread I am sure he will have a better technical answer.

Not necessarily, Jim :-)

Your finding that having the hairdryer plugged into the outlet where the measurement was taken resulted in a DC offset of around 730 mv, while measuring at the same outlet but with the hairdryer plugged into various other circuit branches on the same leg resulted in around 115 mv, is indeed a bit of a headscratcher.

But not being familiar with the design of hairdryers I did a little research and found this interesting paper:

http://www.idc-online.com/technical_references/pdfs/electrical_engineering/MEASURING_ACOUSTIC_NOISE_EMITTED_BY_POWER.pdf

As stated on pages 4 and 5 and as depicted in Figure 13, at least when hairdryers are operated "at lower power" they place a half-wave rectifier diode in series with the load they place on the AC, or at least a substantial part of the load. So DC offset results from the difference between the amount of current that is drawn during the positive half-cycle of the AC waveform and the amount of current that is drawn during the negative half-cycle, and the differing voltage drops that occur in the resistance of the AC wiring between the two half-cycles as a result of that current difference.

In tests 2, 3, and 4 essentially zero current was being drawn through the dedicated wiring between the service panel and the outlet where the measurement was taken, so no voltage drop would have been occurring in that wiring. And the DC offset that was measured would have resulted essentially from the voltage drop differential between half-cycles that was occurring in the panel and in the outside wiring, since the additional voltage drop in the wiring between the panel and the outlets where the hairdryer was plugged in would not have been in the path to the outlet where the measurement was being taken. And presumably the outside wiring is considerably heavier gauge than the 10 gauge Romex used for the dedicated line, and therefore it would present a lower source resistance (per unit length, at least) for DC offset to develop across as a result of the asymmetrical current draw.

Whereas in test 1 that asymmetrical current was being drawn through an additional resistance in the path to where the measurement was taken, corresponding to the sum of the resistances of the dedicated line’s two 75 foot 10 gauge conductors, which amounts to about 0.15 ohms.

Looking at it quantitatively, if my theory is correct the difference in the amount of current drawn by the hairdryer between the positive and negative half-cycles would be:

(730 mv - 115 mv)/0.15 ohms = 4.1 amps.

In the context of the large current draw of an 1875 watt hairdryer, and one that is placing a diode in series with much of the load it presents, I suppose that is consistent with your findings.

Best,
-- Al