A DC Blocker to help stop transformer HUM

[Matias]

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.

 

[restorer-john]

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.

 

[GrimSurfer]

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]

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]

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.

 

[restorer-john]

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...

 

[GrimSurfer]

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...

 

[restorer-john]

I have been known to mix a jug or two of margaritas while music is playing though...

Tends to drown out the low level detail a bit eh?

 

[Eirikur]

[Mixing margeritas] tends to drown out the low level detail a bit eh?

Remember: short, controlled bursts - just like in Aliens.

 

[GrimSurfer]

LET'S ROCK!!!!! (Followed by plasma rifles on full auto)

 

[Count Arthur]

To the Bat Cave shed:

 

[DonH56]

First time I've seen a power resistor with 19.34 GB storage capacity. Guessing it's a WOM?

 

[Omar Cumming]

Emotiva CMX-2 worked for me to cure a humming toroid transformer!

Cheers

 

[Count Arthur]

Guessing it's a WOM?

Almost certainly.

Emotiva CMX-2 worked for me to cure a humming toroid transformer!

They don't seem to be available in the UK.

 

[Count Arthur]

Well, I've built DC blocker and installed it, and what do you know, it works - no more transformer hum. Absolute dead silence from the power amps, even with my ear pressed up against the case, no noise from the speakers either.

Other than stopping the amplifier transformers humming, my system seems to be louder for a given volume setting. Am I imagining that?



Q. Why do transformers hum?

A. Because they don't know the words.

 

[DonH56]

Well, I've built DC blocker and installed it, and what do you know, it works - no more transformer hum. Absolute dead silence from the power amps, even with my ear pressed up against the case, no noise from the speakers either.

Other than stopping the amplifier transformers humming, my system seems to be louder for a given volume setting. Am I imagining that?



Q. Why do transformers hum?

A. Because they don't know the words.

More than likely because you are hearing it without the low-level humming that was adding some masking before. It is possible the rail voltage is slightly higher but feedback should fix the gain regardless. Without the hum everything will sound more better.

 

[Count Arthur]

The hum wasn't loud enough to be heard over music, it was fairly faint.

Also, my DAC/preamp has an accurate volume setting/dBr readout, so I was going by that, not a vague idea of where the volume knob is set.

 

[DonH56]

You might be surprised how much a faint buzz corrupts the sound. It's sort of like the annoying little fly buzzing around you don't really notice until it is gone, then.... ahhhhh!

My unscientific observation is that when the noise floor drops from slightly audible to inaudible the music seems louder not because the peaks are greater but because the silent parts really are.

IMO! - Don

 

[eliash]

The hum wasn't loud enough to be heard over music, it was fairly faint.

Also, my DAC/preamp has an accurate volume setting/dBr readout, so I was going by that, not a vague idea of where the volume knob is set.

Probably pure enthusiasm...just enjoy it!
(self-experience)

 

[tmtomh]

I had the same problem with my Adcom power amp - huge toroidal transformer would gradually build up hum, then stop, then repeat. Not audible with music playing but audible in the silences in between. And not coming through the speakers - purely mechanical/electrical from the amp.

I got a DC blocker from Van Alstine - they call it a "HumDinger" - was $140 and instantly did the trick - no more hum.

In the right circumstances DC blockers are silver bullets. But the effect is not subtle and the situation is quite binary: your equipment's transformer either has audible hum or it doesn't. The DC blocker will do nothing for ground hum (or ground-multiple transformer noise) coming through the speakers, or for baseline electrical noise of equipment that's not related to DC transformer oscillation. It cleans up the power only one very specific way. But I've found my DC blocker approximately 100% (or I guess more accurately, infinitely) more effective and audible than power conditioners and voltage regulators, neither of which has ever produced an audible improvement in my experience.