2. amps-preamps
  3. 142371-120446

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

   I have McIntosh MC8207, the first unit I bought from an authorized dealer came with a loud buzzing coming from the left transformer, and was replaced with a new unit which came with even a louder buzzing. The buzzing can be heard from 8 feet away. Then I was told to have install new 20 amp outlet that has its own isolated grounding.
   That was done professionally by an electrician who installed two isolated 20 amp outlets, two 20 amp circuit breakers, two copper polls for grounding for each outlet, each outlet has its own neutral and power line. After all this done the buzzing sound was still there.
   I was then told to buy a power conditioner which I did (Audio Quest Niagara) which was like $4000 and that did not help. Called back McIntosh and was told that I might have DC offset in my AC line and was told by McIntosh that I would need a DC Offset Blocker/Killer to which when I asked them where to buy one they told me to go on the internet and search to find one, to which I cannot find one.
  This bothers me a little bit, if you as a company think that I have dc in my ac and i need a dc blocker wouldn't you need to sell one as well. I brought this amp to my friends house and it was the same no improvement, so my guess is that he has dc in the ac line as well.
   So If anyone of you knows where to buy a DC Offset Blocker/killer please let me know, but even if this helps kill the buzzing wouldn't you guys think that this expensive somewhat hifi amp/brand should be silent from the factory. I mean this is two units in a row all purchased brand new.

My house is 5 years old, everything is brand new, the whole neighborhood is about 8-9 years old, my electrician says that I have perfect power coming to the house and everything looks fine.

Thank You

tomiiv30
jea48
2,944 posts
 
tomiiv30 OP16 posts01-03-2019 12:48pm@jea48


In the manual it says "To compliment the design of the MC8207, there is a high current power supply for the five power amplifier channels". And this is a 7 channel amp so what about the other 2 channels. And why is there two boxes, what’s in them if there is only one transformer. I am trying to understand what is what.

@ tomiiv30

The box on the left houses the power transformer. The box on the right houses the DC power supply. Rectifiers and electrolytic caps.

Scroll down page to photo of amp.
http://www.hifishock.org/gallery/electronics/mcintosh/power-amplifier/surround-sound/mc8207-1-mcinto...

tomiiv30 OP
63 posts
 
Link to the video I uploaded on youtube where you can clearly hear the MC8207 buzzing loud.

Also I have measured for DC offset in my AC line the the result was 0.9mV, which is not enough to cause the transformer to buzz

https://www.youtube.com/watch?v=ucbBaD2eIgg
jea48
2,944 posts
 
tomiiv30 OP17 posts01-04-2019 9:41pm
Also I have measured for DC offset in my AC line the the result was 0.9mV, which is not enough to cause the transformer to buzz

How did you measure for it?

The correct way.

Re: Not trying to start a flame but....

"If your not comfortable with a meter get an electrician or tech that knows his way around test equipment. "

If you read the comments in the AA archives, you would have seen the ones from real engineers (Jon Ricsh, John Curl, etc)who have measured this. They report that most hand held DVMs cannot measure this (John Curl tried three different Fluke models with mixed results).

If you really want to try and measure the DC off-set on your AC line with a cheap hand-held DVM, try the following:

Put a 100K resistor in series with a 100µF cap (this is called an integrator). This now goes in parallel with the AC line. Measure the DC voltage across the cap.

Even a few tens of mV DC off-set can make a toroid buzz, especially low priced ones.


http://db.audioasylum.com/mhtml/m.html?forum=tweaks&n=140383&highlight=integrator+djk&r=...

.


Here is another Link for measuring DC on AC mains.
http://sound.whsites.net/articles/xfmr-dc.htm#dc1
However, if you must (and PLEASE take extreme care), you need a 100k resistor and a 10µF non-polarised capacitor, wired in series. Connect this circuit across the mains (power off!), and connect a DC voltmeter across the capacitor. This attenuates the AC enough to prevent the front-end of the meter from being overloaded, and the DC voltage is easy to measure. Expect to see the DC vary around the zero voltage, with a normal variation of ±25mV or so (typical - residential areas). The alternative method is to measure the DC across the diode/capacitor network in the circuit of Figure 3. Do not connect or disconnect the meter with the circuit live, and use alligator clip leads to make the connections.

.
tomiiv30 OP
63 posts
 
@jea48 When I called Chuck at McIntosh and told him my problem he said and these were his words" Get a multi meter and stick probes one into the neutral the other into the phase and measure for DC mV, and that's what I did. I am not going to do anything more than what they suggested. 

I would try the other methods too but I feel like I have done everything they told me to do or was told to do, and I don't feel comfortable to spend any more time and money on this matter. I have other equipment in the house that does not buzz, hum or make any sounds out of the ordinary. I think that their amp is poorly design and that they should be the ones making sure this amps are not buzzing by maybe installing their own DC blocker/killer or whatever you want to call it in their amps so none of us consumers would have to deal with this what I am dealing right now. Not many have my patients.

Also I have turned off all the breakers except the one for the whole house and the one for the outlet where the amp is plugged into, the buzz was still there and you could still hear it 6-8 feet.


erik_squires
5,268 posts
 
Get a multi meter and stick probes one into the neutral the other into the phase and measure for DC mV, and that’s what I did. I am not going to do anything more than what they suggested.

That’s pretty smart. :)

I never thought of that, as I was worried about the meter being damaged, but reading around the net it seems safe to do with a digital, and with a mechanical, just set it to the highest range first, then come down.

Glad you tried turning everything off.

One other thing you could do, is try moving your amp to another phase. Usually the circuits breakers alternate phase going down.  So if your amp is now on the top breaker, try the second fourth or sixth down.
More to discover