Millett Hybrid Max is Working

Guess where this is going…
(Click for much larger version…)

Yep, that’s the (now working) Millett Hybrid Max with recently installed 3.5mm headphone jack, connected to my Sennheiser HD570 headphones and a recently acquired SCPH-1001 Playstation. The amp isn’t set up yet, but I’ve got the problem figured out. Unfortunately I used up ‘s set of MOSFETs in the process due to some misguided advice, so I’ll have to order another set tomorrow.

Want to know what the problem was / is? In short, the designer(s) screwed up. Again. This time it’s with the power supply / fuse requirements.

Earlier this evening I posted the following here in the Head-Fi forums as a follow-up to the post mentioned yesterday.

Here it is. I hope I wasn’t too harsh:

Tonight I went ahead and replaced all four MOSFETs in the board, using shorter screws this time. I still am not convinced the touching, isolated screws caused the problem, but I figured that I may as well install shorter screws since I was removing the MOSFETs anyway. (Here’s an image of the MOSFET area with the new, shorter screws fitted.)

Unfortunately as this was my only spare set of MOSFETs, it means that my friend won’t be getting his kit for a little while longer. It also didn’t solve the problem.

After a bit more poking I did get the problem solved, though. Well, sort of.

So, what was it? A combination of things… Most notably, the suggested fuses and power supply. The BOM specifies a 1A quick-blow fuse and a 1A power supply, but as can be seen in this photo the inrush current was over 1.7A. (That’s with the MOSFET bias (RB12L/R) turned all the way down and with 12FM6 tubes. With 12AE6A tubes and the bias equally low the max was around 1.6A. I haven’t yet tested it with the MOSFET bias set for normal use.)

I found this after running out of the straight-wire fuses I was using and giving things one last go with one of the new 576-0217001.HXP fuses (1A, 250V) I received from Mouser this afternoon. This fuse happens to have a zig-zag pattern element (photo of the fuse up close), but I figured I would hook up my meter, try one more fuse, and see exactly what the inrush current peaks at before the fuse blows. When I powered the amp on I didn’t see the telltale flash of a blowing fuse, but I instead saw the wire twitch and look as if it almost broke, except when I glanced at the meter I saw a ~1.7A maximum reading. After 15 seconds or so things settled out at about 300mA.

I think that the fuses I was using were just a bit too sensitive and couldn’t handle the inrush current. I’m not sure why it took a few hours to start happening, but once it did, things were pretty consistent. I only wish I had tried this before. (It may be because the room the amp was sitting in before the problem arose was about 15°F warmer than the basement where the amp was first built. If I recall correctly MOSFETs consume more power as they get warmer. Maybe things were right on the edge? I’m not sure about this part of it yet…)

Going forward I think that it would be best to recommend both much higher amperage power supplies and higher amperage fuses. Realistically the power supply and fuse should be able to handle the full inrush current. I suspect that many individuals out there who have built Millett Hybrid Maxes and are using a 1A supply without fuses are very briefly (1-2 seconds) overloading their supplies and risking cooking them.

I personally am going to order some slow-blow 1A and 2A fuses next time I place an order with Mouser. I’ll use the 1A fuses for now and just wait for the day that the wall-wart power supply fails, then I’ll replace it with a 2A supply and 2A fuses. There’s a good chance it may be able to take the sudden spike, and I’ll stick with it, although I’m still not completely comfortable with its sizing.

Oh, two other things…

One, the Kobiconn 161-073SP-EX from Mouser seems to be a really nice gold-plated snap-in front panel 3.5mm (1/8″) jack. It’s an attractive black plastic lump which snaps solidly into a 3/8″ hole drilled in one of the Hammond 1455T1601 metal front panels. I’ll post photos of it later once I’ve got everything set up and ready to use.

Second, if the Millett Hybrid Max boards ever undergo a revision, I have one design suggestion: Set the groudplane back a bit from the sides so that there is less of a risk of the grooves on the sides of the suggested enclosure (Hammond 1455T1601-series) scraping through the soldermask and making contact with the groundplane. I did this on the MIDIbox SID-NUXX mainboard PCBs which were designed for the same enclosure and it worked out great (scan of top of PCB here).

Anyway, thanks for the suggestions, everyone. I’m just glad it’s working. Now, time to go wire in that new 3.5mm front panel jack and get it ready for initial listening / burn-in / biasing.