Massdrop x Eddie Current ZDT Jr. (Official Thread)

This is similar to what I suspected, though I was planning to put the rudimentary shield to a different place. But let's start from the beginning.
Today I removed the bottom plate of the amp to see if there is any obvious problem inside. I found nothing strange, so I screwed it back and continued my day. Later I wanted to check something, but I was lazy to download the photo from my camera, I rather used purr1n's picture (here).
This is when I noticed that it's different to what I saw in my amp, so I downloaded my own image. This is it:



Do you see the lots of differences? One very obvoius of them is that on my ZDT the small transformers are not shielded from the internals of the amp. Does it matter? Maybe, I don't know.
I didn't check what's behind the PCB. May the main transformer be also not shielded?
There are many other differences between my (humming) ZDT and purr1n's pre-production (silent) model, but I'm not an electrican, so I don't know which difference is meaningful, and which isn't.

I also noticed on your chart that the frequency of the humming is around 60Hz, and 108Hz. Taking into consideration that you're in the USA, where the power outlet's voltage is around 110Hz, it's seems reasonable to believe that in Europe we hear 120Hz + 230Hz humming instead. Being a higher frequency, it might be more obtrusive.

 

The greater amount of enclosed shielding for the output transformers in the prototype may be just enough to keep power transformer coupling lower. This deserves attention.

Minor correction. The US mains are 120V and 60 Hz. So hum will be 60 Hz and harmonics. In this case 3rd harmonic which is 180 Hz. Europe will often be 50 Hz so expect 50 and 150 Hz frequency components. The 3rd harmonic is more noticeable as our hearing has greater sensitivity to 150 and 180 Hz than 50 and 60 Hz. See Fletcher-Munson curves.

 

Follow up thoughts. If this is a magnetic field issue the solution will involve steel or mu-metal covers for the exposed transformers. Copper is useful for electric field issues but is not effective for magnetic field problems.

 

@atomicbob, you are a well of knowledge. I initially had reservations about getting one after reports of humming units were coming out since all my cans at the moment are pretty sensitive to things like that, and my listening station is basically a mess of gadgets and crap, but this is immensely reassuring.

@CEE TEE I know you probably get asked this a lot but will there be another drop? Ideally in a few months when I can afford one, haha.

 

I think this explains the hum nature

 

I have not been following this thread, but has anyone checked to see if the heaters are AC rectified or if they are AC heaters.

And has anyone checked the voltage on the heaters.

The reason is that I found that my Darkvoice 336SE also had hum issues with certain tubes, but sometimes would go away after awhile. It was determined that the heater voltage was 7V instead of the 6.3V...yikes!

With an increase in heater voltage, some tubes are fine, some tubes are not and that hum can come through.

I am not saying this is an issue, but just a possibility. Someone with one of these amps can verify. You can even test without any tubes in.

I would also check to see how that LED is wired. Disconnect it and see if it continues. There is a right way and a wrong way to implement an LED on the heater voltage rails.

The other issue might be grounding scheme. And if that is the case that is no good because you can't really just tie a ground off anywhere, it is within the circuit.

 

Today I removed the bottom plate again to check if the main transformer is shielded from PCB or not:



As much I could peek under the PCB, it has it's own, separated compartment. So that cannot be the source of the annoying hum. I also took a better picture of the parts. Maybe there is a electrotechnician among us who can determine the source of the problem by looking at it.



(I noticed that the labels of the RCA's are swapped. The one closer to the power inlet is marked as bypass, but as it can be seen on the picture, that's the input. It doesn't really matter though.)

Tomorrow I'm going to buy some iron sheet and create a makeshift cover for the small transformers hoping that it will kill the hum.

 

The other odd thing in those pictures is that there is nothing protruding from those holes. Almost as if they cut the holes for venting. Otherwise it could just sit on top without those square holes altogether.

And they could also rotate the transformers 90 degrees so they are not also interfering with one another and be in the null zone.

 

I opened up my unit today and it looks identical to AudioFriend's as far as I can tell. I took pictures but his came out better than mine.

 

I thought the transformer closest to the power socket is humming? No?

 

Coming in with late additional impressions, as usual.

Store demo conditions, fairly busy day but was reasonably quiet (measured LAeq ~45dB) towards the end. Was closer to 70dBA ambient during peak people, louder when someone demoed some bluetooth speakers not a meter from me. The fact that I was using semi-open cans to evaluate (i.e. Klipsch HP-3 and Fostex TH-900) helped alleviate ambient noise problems somewhat.

Do note that I'm coming off a decidedly mid-fi, if that, home rig: Bifrost Multibit is a decent baseline, but while the Fostex HP-V1 amp is damn good for low-z biodynas and I admit having enjoyed the HD800 out of it, it's not the last word in power and control even at the sub-$500 tier (never mind that it retailed for $700 when it debuted back in 2014). That said I can vouch for its ability to resolve sonic textures and that it does a not-horrible job presenting room cues and gross detail. I also have a G1217 Sunrise III on loan, but if I'm being honest I've not had much time to listen to it because of school and crap, and am still far more familiar with the Fostex hybrid amp.

Since I couldn't well tote the Bifrost Multibit around I ran the amp out of the iFi Micro iDSD (silver), and later a Chord Hugo v1. I've had the iDSD for a few years now and am intimately familiar with it both as an AIO and as a DAC; though I technically sold it to the girlfriend a while back she doesn't mind me keeping it around since it's technically under joint custody. The Chord Hugo I'm reasonably familiar with because a friend who'd gone off to the weeaboo homeworld for events loaned it to me for in excess of a week; this is the same guy who owns the ZDT Jr and who was kind enough to bring it to the store for others to demo (I know you lurk this thread, so thanks, ZB!).
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Okay, long-ass preamble over, actual impressions here. Please note that the amp did not exhibit the humming issues documented in this thread and that I demoed this out of my music by lugging along my heavy-ass laptop around:

I thought I knew soundstage, I really did, and maybe I actually do in the context of not-expensive components, but the ZDT Jr. honestly surprised me here, and I've been seeing the amp's ability to present depth and width hyped to hell on this page. $400 for this is ridiculous, you get an unprecedented amount of content projected around you in a laughably spacious spread, especially on well-recorded classical and small ensemble jazz. I actually giggled a bit listening to my music collection, which I'm sure would have gotten me booted out if I wasn't a sorta-familiar face and my friend was friends with the shopkeep.

The Bloodborne soundtrack and Vivaldi's Four Seasons as performed by Anne Akiko Meyers were ridiculously good on this amp. I'll resist the temptation to fall into hyperbole and start preaching about how I felt like I was sitting about fifteen rows back from . Lateral movement across the soundscape was smooth and cohesive, no 3-blob effect or disjointedness in the projection to take away from the illusion of a soundscape.

Imaging on the iDSD isn't necessarily a strong suit, at least relative to the Bifrost, but presentation when paired with the ZDT was reasonably crisp. Not the best I've heard, but I'm guessing comparing this to the iCan Pro is just a wee bit unfair since it's a hybrid topology and has a bit more control on more efficient cans as a result. Both the TH900 and HP-3 are great at imaging to begin with, though, so the amp had its work cut out for it.

As an aside, friend tried the Beyerdynamic T1s off the ZDT Jr. and came to the conclusion that the amp fared best when paired with less sensitive dynamics. Didn't bother trying that combo out for myself, but I'm inclined to take his and the shopkeep's word for it since I got to try the Susvara (yes, that one) out of the amp and wasn't necessarily overwhelmed, but I wonder how I was outvoted two-to-one when the both of them said that the TH900 had more refined highs than the Klipsch. To be nice about it, I disagree. Emphatically.

There's a definite warmth to the amp that alleviates treble issues, but the HP-3 is still occasionally unpleasant and the TH900, which is the sizzly to the Klipsch's tizzy, remains just about unlistenable on some music, most prominently harsher songs like the Foo Fighters' White Limo, Panic! at the Disco's Golden Days, or Paramore's Idle Worship, all of which made me want to yank the pretty red urushi off my head and run it over with a car. I used both stock pads and the pads off a TH-610 that were lying around on the TH900, the latter supposedly helping with tonality, but I still wanted to practise my football punt on them after listening to the cymbals on Signe off Clapton's Unplugged. This is a shame as the Fostex's massive soundstage and clarity would otherwise go excellently with the amp, but mods exist for a reason.

Midrange isn't something either the HP-3 or the TH900 do particularly well, the Klipsch outdoing the Fostex by a fair margin at the expense of bass control and clarity, but the amp's presentation brought some much-needed body back to vocals. Sinatra's Wee Small Hours was passable on the Fostex, though it had more of an obvious honky, reverb-y cupped-hands effect relative to the Klipsch, which to its credit doesn't exhibit much of the same regardless of source. Stevie Nicks and Billie Holiday were likewise nice on the Fostex, though accompaniment was sometimes painfully strident, especially brass and percussion.

I also like how it was surprisingly easy to pick out tiny nuances in vocals relative to other amps I'm familiar with. I'm not talking about Michael Jacksonesque hiccoughs, but lip smacking, the sound of inner cheeks parting from teeth, and tiny breaths in between passages. Heck, if I tried hard enough and listened for more than a few hours I'm reasonably confident I could have fooled myself into thinking I could trace the passage of air through a vocalist's teeth!

It'd be irresponsible to make any hard calls on overall resolution or low-level detail retrieval considering I could hardly hear myself think at times even with the headphones off, but I suspect it'd do quite well based on how well it does gross detail.

Provenance and assorted other piss-poor snapshots (I hesitate to call these photos) in the link. I came across some nice tables that I really wanted:

http://imgur.com/gallery/Ir4Uh5L

P.S.
Yes that is a Cayin HA300 amp. No I did not get to listen to it because I needed to get a haircut before the mall closed and I was starving besides. The fact that I'm also tired of pining over gear I can't have might also be a factor.

That said, I might go back in a few days just to play with that one because DEAR f'ing GOD IT'S PRETTY.

 

PS @Lyer25 I cannot see your googleusercontent so I suppose others might not be able to either.

 

Thanks! Bother, but I'm just going with links to the imgur ones now. Ah well, easier to load stuff on here, haha.

 

I had the same problem initially...once you zoom in & back out the pics were revealed...at least for me.

 

I checked the hypothesis about lack of magnetic shielding for output transformers from power line.
Looks, or rather sounds, like it was correct. I added a sheet of metal about 0.5mm thick and 5cm wide above transformers from side to side. Humming is still there but greatly reduced. I have to listen very carefully to notice it.
I have no idea about permeability of my shield - I cut it from some decorative part from broken microwave, I just checked that it was magnetic. So proper magnetic shielding will likely eliminate humming completely.
On other thought may be just rerouting wires from the power socket can reduce humming too.

 

Well, it's interesting, because I also tried, and my experiences are different.
I bought 0.4mm thick iron sheet, cut it to shape, and put it over the transformers like this:



No change in humming that I can hear. I also tried to put a double layer to one side - also no change.
Sometimes it's volume seemed to change, but I eventually realized that it's because of the change of environmental noise, not the humming. At night, in silence, the hum is much more noticable - obviously.

How did you relocate them? The wires are short. Could you please provide a picture?

 

I haven't relocated wires, I suggested that rerouting may help, or may not.

I guess we need someone with some measuring rig and humming ZDT Jr. to perform modifications too.
Also I'm curious what is best form of the shield taking into account that you not shielding transformer's magnetic field but wire's, if it is indeed power wire to blame.

I put shield just bellow the power socket and I have to remove and put back power socket and case grounding to install the shield. So may be shielding has nothing to do with hum reduction on mine unit but 'touching' grounding did? If you look at Marv's photo the case grounding is different too.

I'll try to provide pics tomorrow.

 

Further investigation confirms residual hum is caused by AC magnetic induction from the ZDT power supply transformer. External magnetic induction is also possible, implying the ZDT Jr should not be located next to other sources of strong magnetic fields.

A proposed solution from Massdrop incorporates additional shielding of the power transformer. High permeability steel band is wrapped around the transformer as shown:





Measurements

Residual hum left channel before additional transformer shielding:


Residual hum left channel after additional transformer shielding:


Residual hum right channel before additional transformer shielding:


Residual hum right channel after additional transformer shielding:


Efficacy of the proposed solution in my ZDT Jr. appears to drop residual left channel hum by 10.5 dB, now very close to -75 dBu threshold of perception in my quiet acoustic lab using HD800 headphones. -75 dBu will produce 26 dB SPL on my HD800. Right channel hum was already low but is further reduced by 5.2 dB.

Adding a 50mm x 155mm x 1mm plate of steel between power transformer and output transformer covers reduced left channel hum another 2 dB.


The additional steel shield achieves below perceptual threshold in my acoustic lab during a period of 21.1 dBA residual noise, very quiet. Most listeners will not achieve such a low ambient level. Hum will be below masking level of their respective ambient environments with the proposed Massdrop solution.

My acoustic lab residual noise level:


To provide context additional room ambient measurements are shown below.

Water boiling in kettle on range at 1 meter distance:


Quiet footsteps, running shoes on hardwood floor at 4 meters distance:


Further experimentation will occur with 3M High-u magnetic shielding sheet 1380 when I receive it.

 

I have no dog in this fight, but I do have a Rega Brio R that I think has the same issue. Atomicbob, I’m going to try and duplicate what you did to the Zana on the Brio and see what happens. I’ve opened it before to rotate the transformer and there should be room. Thanks for the idea.

 

They make actual toroidal transformer shields that are not too expensive.