Massdrop x Eddie Current ZDT Jr. (Official Thread)

Unfortunately, it's not completely so. There is a lot of space around the small transfomers, that's for sure. The big transformer though... That is so close to the back wall of it's box that I could barely screw the box back after I applied 0.8 mm shield to the transformer.
I have no idea why it is mounted there at all. Magnetic force changes exponentally with distance, so common logic dictates that it should be as far from the small ones as possible to avoid interference. Maybe I will relocate it, though it would mean utter disassembly and drilling a new mounting hole to the housing.

Mu-metal is a great material, but it has a disadvantage: It provides very high permeability only if it's heat treated (annealing). Bending, or any mechanical shock distrupts the material's crystal structure. As a result, it's permeability decreases to the fraction of it's real potential. [Details here]

I don't know if the same applies to Permalloy. If not, then that seems be a better choice. It's relative permeability is similar to mu-alloy, and it's also easier to acquire (at least where I live). It's expensive, but so is mu-metal.

About the necessary length: 1 meter (~40 inches) length of 46 mm (~2 inches) wide strip is enough for two layers around the main transformer, one layer around each small transformer, and one layer of cover inside under the small transformers.

 

There is a barely audible hum in my specimen with HD800. But, the general noise in my apartment (fridge, clock on the wall, faint sound of traffic etc.) is louder than it. When any music is playing I hear none. I've decided not to ruin the little mental health I still possess with such a trivial issue.

 

I don't think permalloy and mu-metal have different properties and I believe it is just trademarks and exact chemical composition varies inside trademark too.
Non-price-prohibitive products on market are in form of thin self-adhesive strips/foil so no real bending is needed.

Also I'd like to point that shielding output transformers is more beneficial as provides protection from external, non-amp-generated fields too. Making the amp less picky about neighboring equipment.

 

My Grade 4 science project method -- a rectangle of sheet steel between transformers and circuit board -- is still going strong and has reduced hum to pretty much unnoticeable levels across HPs and moat of my IEMs. Not sure it's had any effect, good or bad, on overall SQ, which has very good to begin with. There's definitely a family resemblance between this and the ZDT I've used for years, even if the ZDT Jr isn't exactly a blood relation.

 

Could you elaborate? I mean exact placement of the sheet and way you put it there. Thank you.

 

I followed your lead, and cut a piece roughly proportional to the one in your pix in post #503, and placed it almost identically. My build looks a little different, in that the ground wire is black and not as thickly-insulated. I'm on the road now for a bit but can post some internals when I get back (with the caveat that it is sub-sub DIY level workmanship).

 

Very important to remember magnetic shielding doesn't block the field lines but controls how they travel from origin to destination in the magnetic field source.

 

... and because of that it depends heavily on shielding form and orientation. For example, a shield perpendicular to the vector does nothing - shielding around a wire with electric current practically will not affect wire's magnetic field. But it will shield the wire from external fields.

 

Sorry, correction, we are talking about ferrous shields only. You can just block magnetic field with superconductors, of course. And non ferrous highly conductive materials work in different way too, as @murray pointed out.

 

Got my Zana Jr. in and I got the hum also. Not that bothersome right now, but it's definitely there. I will try mods.

 

Hello Everyone!
Just want to thank you for all of the testing and positive work to try and keep these amps.
(Been a bit crazy working on new launches, you'll see a few more roll out soon.)

We have a fix and now need to figure out the path to help. Next week we're meeting to work through the potential paths and have more info for you. Really appreciate the patience and constructive testing you have been doing and sharing here.

Your amplifiers (and you!) mean a lot to us and we will not forget about this, we will find the path forward.

Until next week...
Christian & Team

 

This is one responsible seller.

 

De-humming guide for ZDT Jr.

Part I: Introduction

I have worked approx. 30 hours to get rid of the humming of my ZDT Jr. By now, my amp is completely silent when music is not playing. In this guide I describe all of my findings.

Is your unit affected?
This is the best way to determine if your unit is affected with this problem or not:
- When the amp is cold, find a quiet place.
- Put on your headphones
- Turn on the amp.
If you heard something right after you turned your amp on, it’s affected. When the humming issue is resolved, you won't hear any noise at all. Just complete silence. It will be silent when you turn it on, and remains so until you start playing music. The hum won't return ever.
It's worth mentioning that my unit's hum seemed to be louder after 200 hours of burn in. I don't know why. Maybe just because I became sensitive to it.

Should you be concerned about the hum?
The humming annoyed me, but maybe you're more tolerant. However, I noticed an another change which might be important for you too: The sound quality improved as the hum went away. Especially the bass improved.

What's the source of the humming?
I think it’s a magnetic field caused by the main transformer, which is at the back of the long rectangular box. If interacts with the output transformers, which are in the smaller boxes. For some reason, the main transformer’s magnetic field is quite strong, and the output transformers are sensitive for it. The output transformers are seem to have large production deviation. With my unit, the left channel was much more affected then the right: about 33dB of hum vs. 25dB.

Should you fix it by yourself?
Massdrop announced that soon they will have a solution. I advice you to wait for that! Maybe that will be quick, easy, and perfect. I present my solution only as a second option. Use it only if their solution is not good enough, you like DIY, or you don’t want to wait any more! My solution eliminates the problem perfectly, but it requires DIY work.

How can this problem be solved?
Possible ways:
1, Replacing the transformers (especially the power tr.) with parts that have lower magnetic field. For example, transformers with nu-metal core barely create magnetic field at all. This path is hard to follow, because we know nothing about the parameters of the transformers in the ZDT Jr. If someone can measure them, please write the results to me!
2, Magnetically insulate the transformers with iron and high permeability metal. This is what I did primarily.
3, Increasing the distance between the transformers. This was my final move, because it’s the most difficult to execute. However, this was the one which shifted the hum to the completely inaudible levels.

What are the recommended modifications?
I did four changes. In order of time:
1, Putting insulator below and around the output transformers
2, Putting insulator around the main transformer
3, Lifting up the output transformers
4, Relocating the main transformer to the other end of it’s box
Every change decreased the hum by approx 1-6dB, but in a silent environment it remained noticeable until the last step. That change dealt the final blow: after that the hum was completely gone. Maybe some of the previous changes are unnecessary.

The result after each mod
Note: Take these values with a pinch of salt, because I have no instrument to measure such low sound levels. This chart shows what I heard. I estimate my hearing threshold to be around 17dB at 180Hz. The chart shows the hum level of the left (worse) channel.



I my next posts I will explain the details of the changes further with pictures.

 

Part II: The output transformers

The first difference I noticed a few weeks ago between my unit and the image of Marv’s pre-production one is that in his unit the output transformers were over the ZDT’s main chamber, while in my unit the protruded in. So I tried to isolate them magnetically a bit by putting some curved metal plate under them. You can see the results in my earlier post here. Ugly, and not too effective. In fact, I didn’t notice much change. Maybe one decibel or two – but it might have been just wishful thinking.

Later I realized that the OTs can fit to their boxes easily, so they don’t have to protrude in at all. So I decided to raise them:


The original screws are not long enough, so I had to replace them. The optimal size is approx M3x45. It’s not necessary to use metric screws. Any screw will do, provided it is thin and long enough and you have fitting nuts for it. Some washers might be useful too.
I think I overdone the lifting a bit, because my replacement screws were too long. This is why you see unnecessary screws on the top too. Later I cut the screws to size, so they are's not there anymore. Doesn't matter.
I used larger plastic screws as spacer. I think one would have been enough. Any other hard non-conductor material would be fine too: a sheet of plastic, wood, etc.
This raise mattered much more than I expected. I have no idea why. But the decrease of the hum level was audible.

The wires of the transformer are long enough for the lift. You can see them better on the following picture:


The wires are more fragile than I expected. Maybe this is the reason why this transformers are so sensitive. The coil winding is made of many rounds of rather thin wire.

After the lift was done, the output transformers didn’t protrude to the main chamber anymore, so I could make a cover for them much more easily. I used 3 layers of 0.4mm thick iron plate, plus 1 layer 0.6mm permalloy plate. Not because many layers has any advantage, but because I had only these materials at hands. I don’t think the permalloy is necessary at all. It’s expensive, and I think my plate wasn’t annealed, so it’s permeability was not much higher than simple iron’s. I think, a simple 4-5 mm thick iron plate would do the same as my whole layered structure. That's cheaper, and easier to work with.



Using the same layered structure, I created a closed cover around the transformers. It fits easily, because there is more than enough space between the transformers and their boxes.


Finally, I screwed back the boxes to their original places. As you can see on the chart in my previous post, these changes reduced the humming significantly. It was still noticeable, but much less obtrusive than before. This was the first time when I noticed that the SQ improved. Please, don’t misunderstand: the SQ was quite good already in it’s factory state. It just improved further to the direction I liked.

 

Part III: Fixing the main transformer

I’m quite sure about that this is the source of the whole humming issue. It’s a toroidal transformer, with one input and two output coils. I believe replacing it with a good mu-metal based one might solve the whole issue without doing any of the other changes I have described. However, this is a strange transformer. According to my measurements it’s output voltages are 175V and 7.5V. It seems odd to me. Maybe I measured it wrong. But if I measured it right, then the replacement should be tailor-made because I don’t think it’s a common arrangement. It should also be small, because it’s on a very unfortunate location in it’s box. More about this later.
So instead of replacing the transformer I went for isolating it magnetically. It’s not as easy as I expected, because of space limitations.

Here’s a close-up of this transformer:


As you can see at 3 o'clock and 7 o'clock, the wires aren’t seem to be equally distributed. May this be the reason why one channel is more affected than the other?
I have read that the more windings a toroid has, the stronger magnetic field it will have. Might this explain why this transformer has such an strong magnetic field? I don't know.

What's more important, is the transformer’s position:


You can see the perimeter of the transformer's house by the dust. It seems to me that the transformer is on the worst position possible. Magnetic field strength decreases by the square of distance. Therefore the transformers should be as far away from each other as possible. We see the opposite here: the input transformer is almost as close to the output transformers as possible. It’s definitely the reason why they affect each other. Seems like someone warned the assembly line to put the main transformer to the end of it’s box, but forgot to mention which end…

So the transformer is very close to the wall of it’s box. There isn’t enough space for thick iron plate. This is why I bought permalloy metal strip instead. I had read that permalloy is 10 times more efficient in the same thickness in shielding. So I thought 2x0.6mm will solve the problem for good. I tried it, and it turned out that I was wrong. The hum barely changed. Then I read the comments of @atomicbob and @Dzerh about saturation (thank you!), and I realized that might be the problem here. The magnetic field is to strong for this amount of permalloy if I put it right to the transformer.

Next step: I put 5x0.4mm iron plate to the transformer first, and the permalloy layers after. Unfortunatelly, I forgot to take a photo of this step. Sorry about that. The ring looked like something like this, just nicer:

(Yeah, that’s my blood. Sorry for that too. These thin iron plates cut like knife, especially if you have only scissors and your hand to form them.)

This arrangement provided much better result. The hum decreased significantly, to the level where it was noticeable only if I paid attention to it. I guess it’s volume at this state was approx. 19dB – a few dB over hearing threshold. It was a bit stronger right after I turned on the amp, as you can see on the chart in my first post. This solution had a disadvantage though: the metal ring was too thick, so the cover of the transformer couldn’t be screwed back to it's original place.

I’m a perfectionist though, and by this time I became really annoyed by the humming. So a few days later I decided to execute my ultimate idea: moving the transformer further. I dissembled the amp completely, drilled a new whole as an exit point for the wires, and used the previous as a new center for the transformer:

This was the move that finally killed the hum for good! After this step, the amp became completely silent. Not quiet, not “hum barely audible”, but completely, and utterly silent even in a silent environment. When no signal, no audible hum. Not even right after turning the amp on. You can never tell by hearing if it's on or off. In fact, this last fix worked so well, that now I’m not sure if the other changes are necessary at all. I left them in their places because why not, but this mod seemed to make the biggest change. This is why I have no doubts anymore that the culprit for the hum is the main transformer. It's position seems to be a simple design flaw. It would by interesting to know, where the transformer was in the pre-production “humless” models.


Finally the amp looks exactly like it did initially. Too bad that this last fix which seems to be the most effective, is the most difficult to execute. But I didn’t promise easy solution… I promised perfect solution. It’s not extremely difficult though. Sure, it needs the amp to be disassembled. New wires should be soldered to the main transformer’s wires to make them longer, and one new hole should be drilled. Let’s hope Massdrop’s solution will be easier to execute, and similarly effective.

Did the work worth it? For me, definitely. It’s because this amp has wonderful sound. After the fix, it retained it’s character, but became even better. If I had to nitpick, I would have said that in it’s humming state it had a bit lean sound, and mediocre bass. The shielding and relocation made the bass cleaner, more pronounced. The middle region became more lively. How is that possible? I don’t know, I’m not an amp designer. Maybe the tubes were affected by the magnetic field too. Anyway, I’m happy for this amp now, because it’s sound with high impedance HPs is way over it’s price bracket. If it was four or five times more expensive I would say that it is reasonable price for this sound. Clean, detailed, well balanced, with great 3D headstage. Worthy for the legendary Eddie Current brandname.

 

Lot's of good news all around, I'm glad this thread exists. I've modded mine similarly to what was suggested (wrapping the main power transformer in iron sheet metal). There is an audible difference but it did not completely fix the hum.

Looking forward to the MD fix, thanks @CEE TEE. @AudioFriend in the future I would recommend something like this. I cut myself on the metal sheet also. I did 3 wraps around the transformer, I'm going to wait to see what MD has in mind but, I may try 5 like you suggested.

 

Any updates on this?

 

It has been nearly 2 months. Any updates?

 

Yeah doesn't seem like an official fix is going to materialise at this point. It's a shame such an excellent product appears to have been abandoned in such a fashion.