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A DC Blocker to help stop transformer HUM

Matias

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#21

DonH56

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#22
Emotiva CMX-2 linestrip removes DC offset for $129.
https://emotiva.com/products/cmx-2
That is going to be my solution. Much more expensive than DIY but already done and in a nice box. Waiting for a sale... Last time I tried to order they were out of stock and I never got back to them. At one time they had some B-stock for $69 (years ago) and I stupidly passed them up.
 

Matias

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#23
That is going to be my solution. Much more expensive than DIY but already done and in a nice box. Waiting for a sale... Last time I tried to order they were out of stock and I never got back to them. At one time they had some B-stock for $69 (years ago) and I stupidly passed them up.
I had the CMX-6 and they are built like a tank, really thick and heavy metal chassis. Impressive from the outside and on the inside things look neat too. Never had the CMX-2 though.

BTW, CMX-6 does not have the DC blocking feature.
 
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#24
That is going to be my solution. Much more expensive than DIY but already done and in a nice box.
They look to be a nice, inexpensive but well made product. No good for us in Australia however...

I've got a Australian made KCC "Squeeky Clean" on my bench feed. Must be 30 years old or more with DC blocking etc all in a bomb proof steel case. Another unit on the HiFi.
 
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#25
For folks using the North America power grid: Is DC offset usually a big problem. I've never heard transformer hum from any of my gear...
 

g29

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#26
For folks using the North America power grid: Is DC offset usually a big problem. I've never heard transformer hum from any of my gear...
According to Rod Elliott

"...
There are many misconceptions about the use of DC blockers, with some self-proclaimed 'experts' insisting that they are snake oil, because DC can't get through the mains distribution transformers. This shows a complete lack of understanding of mains distribution, how offset can be created, and just how little DC offset is necessary to cause problems with larger toroidal transformers. This is not something that I just dreamed up - I've been able to measure DC offset, and as described above, a hair dryer that uses a diode for half power can be enough to cause a large toroidal transformer to blow the mains fuse or even the switchboard circuit breaker!

Likewise, I can't bear to hear/ read actual snake oil vendors claiming that you'll get "cleaner highs", "more authority in the bass" or any of the other stupid things you may read elsewhere. The purpose is to stop transformers from growling (usually at no or light load). There is no magic, and it doesn't improve anything other than reduce the acoustic noise from the transformer(s). Anyone who claims otherwise is probably lying.

You need to add a DC blocker only if you can hear power transformers growling intermittently when the amps are running but with no (or very low) power output. Adding one will not 'improve' the sound, it's purely a preventative measure that ensures that mains DC offsets don't cause audible (mechanical) noise. This isn't a panacea, but if you do have issues then it's a reasonably cheap (and effective) way to minimise the transformer noise. There may be cases where the DC blocker may not be effective (transformers can also growl if the mains voltage is higher than normal).
..."
 

DDF

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#27
For folks using the North America power grid: Is DC offset usually a big problem. I've never heard transformer hum from any of my gear...
My toroidal transformers hum, and the mains dc offset is ~ 225 mV during quiet times. With a hair dry in use elsewhere in the house, the humming gets comical.

Removing the steel lid from the amp significantly reduced the hum's audibility as did torquing down the transformer mounting screw.
 
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#28
With a hair dry in use elsewhere in the house, the humming gets comical.
Also many kitchen appliances use half power diodes (blenders, mixers etc) and can produce the same issue. The mains waveform is polluted enough with all the SMPS and high current inverter driven fridges and air-conditioners etc.

It cetainly isn't a pretty looking sine wave anymore...
 
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#29
Fair enough. May not be an issue at my place, as I cannot recall a time when a hairdryer was on during critical music listening. In my locale, fridges and microwaves require a dedicated circuit. It cost me a lot to learn that, having spent more on a kitchen reno that I ever will on audio.

I have been known to mix a jug or two of margaritas while music is playing though...
 
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#33
Hey, Сolin!

The circuit does not make sense, to me anyway.

You are not by any chance operating a 120V equipment on 240V are you?

Or perhaps the equipment has a voltage adjustment switch/link that is not set to the appropriate position to suit your mains supply voltage.
 
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