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Re: Advice on transformer hum

Reply #50
Think we might be getting a bit sidetracked. The problem is with a DAC, not an amp. I do indeed have a Classe amp, which does emit some mechanical hum, though that is not a particular problem as it is quiet enough to be inaudible from the listening spot. The DAC hum however is audible and I am still trying to fix this issue.

Further to some of the posts above - I tested the voltage at my house yesterday - and the multimeter was reading 245 and 246V. So I would have thought that the voltage level is OK at least?

Re: Advice on transformer hum

Reply #51
I know this a DAC hum thread, but in regard to the mechanical amplifier hum, what model of Classé amp do you own?

Re: Advice on transformer hum

Reply #52
Think we might be getting a bit sidetracked. The problem is with a DAC, not an amp. I do indeed have a Classe amp, which does emit some mechanical hum, though that is not a particular problem as it is quiet enough to be inaudible from the listening spot. The DAC hum however is audible and I am still trying to fix this issue.

Further to some of the posts above - I tested the voltage at my house yesterday - and the multimeter was reading 245 and 246V. So I would have thought that the voltage level is OK at least?

Its a bit on the high side but nominal.

If we don't have to worry about the Classe, what happens if you apply pressure to the top cover of the DAC?
If that shuts it up, then mechanical vibration through the chassis is the issue, and damping the cover could really help. But as I mentioned earlier, some rubber pads aren't going to sort it out.

Re: Advice on transformer hum

Reply #53
Atmasphere - I do indeed believe that mechanical vibration through the chassis is at least part of the issue.

However, pressing on the  top of the DAC (avoiding the tubes or at least the tube holes if the tubes are removed) does not seem to alter the hum at all. What does appear to alter the hum somewhat is removing the chassis altogether (i.e. leaving only the base and the front plate). When this is done, the hum is less and possibly below the threshold of being a problem. If the casing were then to dampen this sound further it would likely be OK. The problem is, the casing appears to worsen the hum.

Whilst I remain keen to address the root cause of the problem, if there is anything that can be done casing-wise then of course I would be keen to try it! As mentioned earlier, I tried putting a rubber washer in-between the 2 screws that fasten the chassis to the front panel, though this did not alter it. I considered putting washers in-between all the smaller screws that hold the casing to the base panel, though this would be extremely fiddly to do.

Hubert - the amp is a Classe-CAP2100 (2x100W stereo amp).

Re: Advice on transformer hum

Reply #54
However, pressing on the  top of the DAC (avoiding the tubes or at least the tube holes if the tubes are removed) does not seem to alter the hum at all. What does appear to alter the hum somewhat is removing the chassis altogether (i.e. leaving only the base and the front plate). When this is done, the hum is less and possibly below the threshold of being a problem. If the casing were then to dampen this sound further it would likely be OK. The problem is, the casing appears to worsen the hum.
There are two ways that the casing might accentuate the hum:

1. The case has a mechanical resonant frequency that coincides with the vibration in the transformer and is therefore excited by it. Pressing down on the case would alter its resonant frequency so that it is no longer excited. Since doing so does not reduce the hum, this is probably not the mechanism in play.

2. The other mechanism is that the case is acting as a resonator for the noise inside - like the body of a violin or acoustic guitar. Pressing down on the case won't affect this very much, since it's more to do with the shape and volume of the interior space. Given what you have reported, this seems the more likely explanation for what's going on.

Re: Advice on transformer hum

Reply #55
extracampine,

If the hum is really annoying to you, I suggest you cut your losses and buy something less exotic.    You bought something exotic and you got exotic problems.   :(  

Seriously...  vacuum tubes have been obsolete since the 1960s (although guitar players still like tube distortion).    It's super-silly to put tubes in a DAC.   There are DAC chips that don't need any additional active electronics in the signal path, although you might want an op-amp buffer on the output.

I don't have a separate DAC but I've got a $20 USB soundcard and a $300 AVR and there are no "sound quality" issues.   I've got a couple computers with no sound quality problems but they do have audible fans and that might annoy you as much as the hum .

Re: Advice on transformer hum

Reply #56

However, pressing on the  top of the DAC (avoiding the tubes or at least the tube holes if the tubes are removed) does not seem to alter the hum at all. What does appear to alter the hum somewhat is removing the chassis altogether (i.e. leaving only the base and the front plate). When this is done, the hum is less and possibly below the threshold of being a problem. If the casing were then to dampen this sound further it would likely be OK. The problem is, the casing appears to worsen the hum.

Whilst I remain keen to address the root cause of the problem, if there is anything that can be done casing-wise then of course I would be keen to try it! As mentioned earlier, I tried putting a rubber washer in-between the 2 screws that fasten the chassis to the front panel, though this did not alter it. I considered putting washers in-between all the smaller screws that hold the casing to the base panel, though this would be extremely fiddly to do.


OK- if you are alright working with the unit while the case is off, if it were me I would be using a plastic straw and probably also a light metal rod, each pressed against the power transformers in the unit, with the other end at my ear. IOW, using them as a stethoscope to see which transformer is making the noise.

Once you've done that, report back with any findings.

Have you contacted the manufacturer regarding this issue?

Re: Advice on transformer hum

Reply #57
My advice is to get rid of that DAC, it's poorly made garbage.  It seems to be far more trouble than it's worth.  I can understand if it was antique or a collectable that is worth something then it maybe worth fixing but this doesn't appear to be the case here.  DACs are a dime a dozen and shouldn't be causing this much grief.

Re: Advice on transformer hum

Reply #58
OK- if you are alright working with the unit while the case is off, if it were me I would be using a plastic straw and probably also a light metal rod, each pressed against the power transformers in the unit, with the other end at my ear. IOW, using them as a stethoscope to see which transformer is making the noise.

I think that it is the smaller transformer which I had identified above that is making the noise. I tried prodding it with a rod and also unscrewing it altogether and placing a bit of rubber underneath it though that didn't really seem to help.

Have you contacted the manufacturer regarding this issue?

Yes, I actually sent the unit back to the manufacturer for testing, and there was no hum evident there. I am going to take the DAC to a music shop or 2 this coming weekend to see if it hums there.

My advice is to get rid of that DAC, it's poorly made garbage.  It seems to be far more trouble than it's worth.  I can understand if it was antique or a collectable that is worth something then it maybe worth fixing but this doesn't appear to be the case here.

Well, as mentioned above this is a very highly regarded DAC and I do like the way it sounds - hence my efforts to address this problem rather than simply swap the DAC. But you are right, changing it for another DAC is always an option.


Re: Advice on transformer hum

Reply #59
That you "like the way it sounds" is very suspicious. A DAC should not "sound" like anything. It should be simply the equivalent of a straight wire, and if you are attributing anything more to it then you are deluding yourself.

Please accept the advice that you have been given and replace this troublesome device with one that works just as well but does not cause the problems.

Re: Advice on transformer hum

Reply #60
Well, as mentioned above this is a very highly regarded DAC and I do like the way it sounds - hence my efforts to address this problem rather than simply swap the DAC. But you are right, changing it for another DAC is always an option.

The statement that the "Limpizator" DACs are "...highly regarded..."  is strictly a matter of a tiny noisy pocket of personal opinon, and requires considerable Cherry Picking to arrive at.

From a technical standpoint, it is a DAC that is intentionally mis-designed and from the pictures, mis-constructed.  It is a blatant example of anti-technology, anti-craftsmanship and anti-engineering.

The most obvious example of mis-design is the the reliance on throwback anti-engineering, one major aspect  of which is the use of an obsolete DAC chip with far less than SOTA performance and elimination of the digital filter and aperture correction that was origionally part of its design. This issue is so strong that it leads to easily measurable frequency response errors that might even be audible.

I could point out that the DAC has tubes, that may look great to some but add expense, complexity, reduce reliability, and possibly hinder sonic accuracy. But this is a level of rationality that I expect to get no traction at all with rabid subjectivists. :-)

Another mis-design element is the avoidance of modern technology for building audio gear that is inherently free of hums and buzzes due to the power line.

The most obvious example of mis-construction is the choice of an inappropriate but inexpensive style of the power transformer, and its location.

If I were to be such a Luddite that any of this weirdness appealed to me, I'd obtain an inexpensive solid state DAC and an equalizer for a fraction of the trouble, size, energy consumption and expense, that had vastly greater flexibility, and the ready ability to remove the equalizer or at least zero out its controls should a burst of sanity ever enter my brain. :-)




Re: Advice on transformer hum

Reply #61


I think that it is the smaller transformer which I had identified above that is making the noise. I tried prodding it with a rod and also unscrewing it altogether and placing a bit of rubber underneath it though that didn't really seem to help.

Have you contacted the manufacturer regarding this issue?

Yes, I actually sent the unit back to the manufacturer for testing, and there was no hum evident there. I am going to take the DAC to a music shop or 2 this coming weekend to see if it hums there.


It does sound like the problem is line related.

If you reinstalled the mounting screws, they would have transmitted the vibration past the rubber pad.

My suggestion would be to isometrically mount the noisy transformer. If it employs mounting screws, obtain some rubber grommets from the hardware store. Grommets have 4 dimensions- the inner diameter that accepts the mounting screw, the overall outer diameter, the hole size in which it fits, and the material thickness that it can accommodate.

You will need some longer mounting screws as well as 2 washers for each screw and a self locking nut for each screw.

Select a grommet, you don't need a big one, maybe the outside diameter might be 1.5cm. Measure the hole size it needs and select a drill that is the same diameter. You will use this drill to drill out the mounting holes in the chassis for the transformer (assuming there is room; the examples that I have seen do have room). Having done that, install the grommets. Install a washer on a screw, feed the screw through the grommet from below, install the second washer on top, so it is between the grommet and transformer resting on top, then install the self locking nut to hold the transformer in place. You don't want it particularly tight, but it should be snug. Do this for the other mounting screw as well.

If this is done right, the transformer will be mechanically isolated from it mounting and thus unable to transmit vibration to the chassis.

Re: Advice on transformer hum

Reply #62
Thanks for the further replies.

I understand that there are many differences of opinion regarding DACs (and indeed other components, leaving alone cables), and tempting as it is to behave otherwise, my intention here was not to get drawn into a debate on the relative merits of the various implementations. I would rather stay on track with the issue of the hum/buzz if possible - though thanks for the food for thought.

Atmasphere - thanks for the interesting thoughts. I have no experience of isometrically mounting anything. What you describe sounds like it may be worth a try, though it also sounds like a reasonable amount of work. I had already removed the transformer mounting screws and placed the transformer on top of a relatively thin bit of rubber - which made little difference once the case was back on. Is the technique which you described likely to yield substantial improvements over this?

I was planning to take the DAC to an audio store for further investigation tomorrow, though a snowfall has meant that sledging with the kids may take priority and this will have to wait until next week :)

Re: Advice on transformer hum

Reply #63
Atmasphere - thanks for the interesting thoughts. I have no experience of isometrically mounting anything. What you describe sounds like it may be worth a try, though it also sounds like a reasonable amount of work. I had already removed the transformer mounting screws and placed the transformer on top of a relatively thin bit of rubber - which made little difference once the case was back on. Is the technique which you described likely to yield substantial improvements over this?


No. That suggests that the toroid is making the vibration, not the smaller EI core.

Stay warm!

Re: Advice on transformer hum

Reply #64
The EI core may actually be a choke. There are connections on the PCB which are labelled CHOKE.

Lampizator may have constructed a choke-filtered power supply. Chokes are designed to tolerate DC, but they may hum, too, particularly when they saturate.

Re: Advice on transformer hum

Reply #65
Interesting further thoughts. It does indeed have a choke power supply. But it seems that the hum is coming from that area rather than the larger round toroidal transformer. Alternatively, I suppose that it could be coming from something under the PCB (which is mounted upside-down), but then I would have to remove this also which might be difficult. If you hear hooves think horses, and so I had assumed that it was the EI transformer that was the culprit.

Re: Advice on transformer hum

Reply #66
People often use separate heater transformers, so that they can switch on tube heating before they energize the rest of the circuitry. In the old days, when the rectifiers were also tubes, that happened automatically, because the rectifier tubes had to heat up before they passed current. But these days there will be silicon rectifiers, which need no heating to pass current.

So maybe there's another transformer hidden somewhere, for the tube filaments.

Re: Advice on transformer hum

Reply #67
Perhaps there is. The DAC does have a tube rectifier as you mention.

Re: Advice on transformer hum

Reply #68
Conducted a further experiment. I plugged the DAC in in a music shop some 35 minutes drive away to see if it would still hum whilst on a different part of the grid - and it did. So it would seem to be less likely that it is something else in my house, or the power supply to my house, which is causing the problem.

Re: Advice on transformer hum

Reply #69
If that's the case, I'd send it back to Lampizator with instruction to fix it this time.

 

Re: Advice on transformer hum

Reply #70
OK, I had it checked by a local electrical engineer - he seemed to think that the level of transformer hum was within manufacturers tolerances.

 
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