The Mjolnir 2 thread

That's really insightful, and I've spent a little time dwelling on it. I think a lot of stereophilia probably plays to a vulnerability and obsessiveness. Tube rolling, cable switching, and other component tweaks are very subtle, if they make any difference at all, yet are emphasised and exaggerated in forums like this one and lure us into spending time and money on the wrong things.

There's a lot to be learned from others experience of simplifying and dumping audio gear in the "The decision to clear the decks" thread.

 

There are warnings online not to use 6N1Ps in a MJ2.

 

And that is definitely worth knowing about... So in some settings people say that (some) 6N1Ps are direct drop-ins for the 6922, and others say only the 6N1P-EVs drop in. I’m not too sophisticated yet - the rub is that the required heater current is too much. So what does that mean would happen - could I already have harmed my MJ2? Or if I am running ok now, can I just not do it again? Anyone want a pair?

 

One other noob question, this one I’m almost sure about, but given my lack of erudition on the 6NP1’s I figured worth asking...

1. Are 12AU7’s & 12AX7’s ok in MJ2 with the right adapter? These have been among my favorite tubes... reading thread seems like clearly yes, but since it’s bridging 2 pins in the adapter I wanted to make sure.
   
2. How about variants like 5814A and 6201? These have really been my favorites. understand that these may be higher gain. Not sure if that poses any issue.

Would be a shame if I was doing damage by rolling variants that I didn’t understand very well.

Thanks. Trying hard here.

 

Hopefully someone with a background with electrical engineering or maintenance will answer. Since they haven't yet, my guess is that running hot indicated stress on components that, over time or in a warmer environment, could cause them to burn out or catch fire. It doesn't sound like either of those happened. I would continue to use the MJ2 with conservative tube choices unless there is a sign of damaged components like a burning smell.

 

@Johnny Opps Here's a definitive post by Schiit about the max current for tubes in MJ2.

"It's about a 600mA limit--it'll do all the popular E88CC, 6922, 6DJ8 style tubes, but 6N1Ps or 6CG7s may be iffy."

https://www.head-fi.org/threads/sch...bable-start-up.701900/page-4579#post-16050211

 

Tube heater current for the 6N1P varies. I just did a test for 5 tubes of various markings, plain, E, EV etc.

The results in mA:
582
615
625
609
630

The earlier triple mica tubes are towards the high end, testing was done with a DC voltage source and measured 6.3V +-5% at the pins.

That's scary for underpowered transformers, they start at over 1A of current and need 5-10 seconds to reach steady state while the heater heats up and resistance increases to nominal value, this implies you're overloading by about 50-100% for start-up for a 600mA source.

I don't know how the MJ2 heats up the tubes, current source may be OK if you have a lighter heater pair of about 550-580mA. Otherwise it's not ideal, best practice is 30-50% headroom, 5-10% is less so because you have winding tolerances as well. Could go very wrong with a pair of tubes at the high end of current requirement and a trafo at the low end of supply.
Catch the magic smoke and put it in a bottle for good luck.

 

Super helpful. All my googling went for naught, you got me the key stoddardism. Thank you.

 

I usually run my MJ2 on high gain (no feedback), which means comfortable volume has the knob at around 8 o'clock, and 9 o'clock is maximum. Am I missing out by not having headphones that demand what MJ2 can give?

 

If you want to buy more power hungry headphones purely out of curiosity to try new things, then by all means.

But if you enjoy what you have, you're not missing out on anything. Don't buy audio gear just to "use up" all the numbers that your gear can provide because you are utterly chasing the wrong thing.

Personally, I'd use low gain just to have more room on the pot and possibly have better SNR.

 

That's what I was doing when I sat near the audio gear, @Armaegis

But recently I got a 5M balanced headphone cable, which has been awesome because I can listen in my home office without tying up the limited space there with an audio system. It also means I change the volume remotely on the digital source - so the high gain / no feedback setting has been equally convenient.

 

@Armaegis

You might be interested in a recent interview with Jason of Schiit that discussed the MJ2.

It has a discussion of high gain vs low gain on MJ2 at 19 minute mark, which led me to try high gain for now.

The interview also gives some insight about how MJ might be revamped - he discusses MJ2 as having an older approach of linking the hybrid amp stages with interstage coupling caps. He seems to favor the 'more advanced way' of Lyr 3's direct coupled DC design. Personally, I'd like to see it pick up functionality from the Freya.

 

@Woland Can you reduce the output from your source? Or use inline attenuators? This is simply to give you a bit more play on your volume control... but if you're happy with it, there's no reason to change.

 

Yes, that's what I'm doing. I'm controlling the source volume in Audirvana or BubbleUPNP -> Volumio.

In other news, I have plugged the MJ2 and Gungnir Multibit directly into wall outlets. That's based on what others described in the power regeneration thread. A benefit to clarity seems to be there. It's small, but more than I was able to achieve with tube rolling.

 

Just throwing some data out here. TLDR is MJ2 is already a Continuity™ amp. I got interested in circlotrons and push pull pairs lately, so I did some measurement with MOSFET . I don't have a MJ2. From what I can gather from the internet, Mjolnir 2's output stage is a circlotron out of a pair of IRF610 biased around 75 to 80 mA, and degenerated with 1 ohm resistors. Then I plotted how the transconductance of the MOSFET blend together. Hey, it's already an over-compensated constant gm amp! (All the Continuity™ amps are probably all over-compensated) You should be able to juice out Mjolnir 2 about +- 1A without hitting the gm drop. Schiit can probably slap a Continuity™ badge on it without any refresh.

As of how this happens, it's a little complicated. Basically the square law, but FETs aren't really square law device below a certain threshold. If you run them too hot, they can get under-compensated again. It doesn't have to be circlotron. The traditional push pull pair just requires a little more measurement and tinkering. Temperature plays a role too, so I made my measurements extra hot

 

Awesome analysis!

Since you seem to understand these things, can I ask how does Schiit circlotron look with a few years of hindsight? Circlotron was the defining feature of the Mj2 and used in other Schiit Flagsips like Rag 1 and Freya.. Also, why would Jason be so concerned about removing coupling capacitors?

 

I'm probably too young to comment on this. MJ2 came out before I really understood amplifiers. Plus I don't have experience with MJ2, all my analysis came along as I'm diving into circlotron. I can only comment on circlotron in general. The Achilles' heel about circlotron is that it's not built for SE output. I'm not sure what Schiit did to get the SE out, but from the pictures available it's not going to sound nearly as good as the BAL out. The SE out can probably take a Continuity™ refresh though. There are trade offs going from traditional push pull to circlotron. I think circlotron is the sweet spot for BAL all the way amps.

The lack of Nexus™ might make it slightly worse handling SE inputs. I think MJ2 still has feedback, so technically it shouldn't matter much. Jot 2's tuby flavor is tasty, but hey you got actual tubes on MJ2. Perhaps getting rid of the feedback could make it sound better, but everything else will get iffy.

It's the necessary evil. The tube output sits at 100V, and you have to bring it down to 0V for the circlotron. There are only 3 other ways I can think of right now, none of them is as good as coupling capacitors for a $849 amp.

1. Interstage transformer - best sounding, but out of the budget. A good pair of Interstage transformer easily costs $200. Cheap ones might still sound good, but can have consistency issues,
2. Add a buffer stage to bring down the voltage - more complicated to design, and might sounds worse than coupling capacitors. The general rule is more stages more higher order distortion.
3. Let the circlotron sit at tube voltage - most transparent sound, and instant electrocution when you touch the output. What's even worse is the mismatch in tubes could damage your circlotron stage!

So yeah, coupling capacitor is the way to go. The capacitors in MJ2 might not be as good as the paper oil ones, but Schiit certainly didn't cheap out. Those big red fat WIMA caps around tube sockets look delicious to any amp makers.

 

If you watch the interview linked higher up on the page, Jason explains MJ2 only uses feedback in low gain mode. High gain is no feedback.

 

That's very interesting. Then I guess the input tubes could be in the long tail pair configuration, and take voltage feedback at low gain. Probably handles SE inputs well, but sound less SE than Nexus™ this way. If it's not long tail pair and take current feedback at low gain, it would be pretty much Nexus™ except poor with SE input. I lean toward the former since it's less troublesome. It would be interesting to see Nexus™ with tubes, but the tube mismatch might be too much PITA to deal with. Also it requires pretty high feedback, might be hard with tubes.

 

I think matches the common experience. SE input is fine, SE output is a little weak. It was added compared to the MJ1, and is summed from the balanced signal.

What you've written is fascinating. It suggests there isn't much that can be done to improve the current design in terms of a balanced, differential, circlotron headphone amplifier. I'd see the most promising enhancements to be improved functionality as a pre-amp. So relay-switched stepped attenuator, remote control etc. I'd also like to see the MJ2 pitch of "solid state or tube" implemented better with a switch to solid state circuitry instead of implemented with solid-state tubes like MJ2.