Millet Hybrid MOSFET-Max or MiniMax for HD600 headphones

This has nothing to do with preference, the Senns DO sound better on the Minimax due to the way the BJTs work. Someone can correct me if I'm wrong, but the Mofsets put out more current, which is preferred for low impedences. At any rate, I'm listening to HD650s on my Minimax right now, and they sound wonderful. I don't think the amp looks too flashy either, with the tubes recessed halfway into the amp. The only reason I'm selling mine is because I purchased a Speedball Crack.

There is a simple physical limitation with the MOSFETs and voltage (regardless of what amp they're in).  There is specific turn-on voltage (gate voltage) that's required for MOSFETs to operate.  For the MOSFETs used in the MOSFET-MAX or M3 (Z24 and 9Z34), that's anywhere from 2 to 4 volts.  Anyone who's built both types can easily see the difference in operation on their DMM's when measuring Class A bias - the MOSFETs are actually dead until a certain threshold is reached when adjusting voltage - then they start registering.  BJT's on the other hand, are alive from the start - with some little bit of voltage and current regardless of the bias setting.

 

At any rate, that turn-on voltage threshold for MOSFETs means that the available voltage swing from the power supply is decreased by that amount.  With low impedance phones (like 32ohms), that voltage decrease is insignificant compared to the current impulse capability that the MOSFETs offer.  Conversely, with high-impedance phones, current is secondary compared to the voltage swing needed.  For instance, at 300 ohms a 500mW signal would require only 41ma but 12.25V.  At that same power level, a 32 ohm headphone would require 125ma but only 4V.

 

Unfortunately, it almost gets into an after-market cable discussion when trying to determine how much power is needed by a headphone.  Certainly, steady-state voltage and current levels are very, very low for most headphones to reach levels that will damage your ears.  However, audiophiles are interested in every nuance of the recorded music and are thus interested in every peak, regardless of how transient the signal.  Then it becomes much more difficult to say how much instantaneous power is really needed for a headphone's given impedance and efficiency (ever heard of "music power" ratings?).

 

At any rate, from the simple calculation comparison it should be easy to see that MOSFETs are optimized for low-impedance and BJT's for high-impedance.  The comparison is perfectly analogous - M3 to PPAV2 vs. MOSFET-MAX to MiniMAX.  Do they do well with most headphones regardless of impedance?  Of course.

Yes, the MiniMax has a STEP like power supply (correct me if I'm wrong). Owning both the MOSFET-Max as well as the MiniMax, I can tell you that Sennheiser HD6XX definitely sounds better on the MiniMax, it was VERY obvious with my HD650 :)

 

BTW to speed things up even more (also saving some dough), you could just pick up Bob's MiniMax that is for sale . It was essentially built by Tom so you know it will be of good craftsmanship.

 

Well, some people simply like building things. 

 

I don't own the crack yet, but there was a group buy recently and I'm waiting for them to ship it out (maybe another week or two). From what I've read, the Speedball Crack might be better suited for the HD650s (but it costs quite a bit more with the Speedball add on and new tubes), but I really just want too build a new amp. If it sounds better, cool. If it sounds worse, I'll sell it. Just a word of advice, If you don't plan on owning 5-10 different amps by the end of next year, buy one that's already built. DIY will have you building many different amps.

 

Also, the power supply in the Max Is very high quality. I wouldn't worry about PSU upgrades.

I can personally attest to the wonderfulness of the minimax, it is a very well put together kit, and the one I built my step mother as a gift went together very quickly, was fairly easy as diy kits go, and sounded phenomenal. I just finished a MOSFET max, and while it does not sound quite as good as I remember the mini sounding, it is improving with burn in, so I am hopefull it will settle into the same signature. I am using a pair of M-50's at the moment, witch are low impeadance if I recall. The Millett design seems to be quite well refined, so while I have not heard any of those other amps, I feel confident in recommending it.