How to PASS a little tme ...

That is what my kids would call a "dad joke" - sad life it is

I thought I would start a thread detailing the trials of building a pair of PASS (Nelson Pass) inspired amps.

This evolved as a late night momentary lapse of reason. I have been wanting to get back to the massive Class A beastie but it has been so long since I looked at it that I honestly cannot recall what I have replaced and what still needs to be replaced. That together with a mad urge to mod Adcom amps recently (designed by NP), I found myself browsing the Pass Labs section on DIY Audio and got completely drawn into the SIT amps that people have been building. I didn't fancy my chances of finding old stock SITs and incidentally saw an M2X kit come up for sale. The original board group buy was long sold out so I plunged. [ since then, the board sets have been made available at the DIY audio store - so plenty of access for anyone interested ]

The M2X is essentially an M2 but with the ability to roll in various input buffers.

A couple of days later this arrived



I cleaned it up a little and added sockets for a critical resistor which may need to be tinkered with in order to get the offset down.

What I haven't mentioned is that I got into a conversation with the guy selling these boards just after I had bought them and it turned out he was giving this build up for the SIT project that was getting momentum. Well before I knew it I had not only one set but 2 sets of NOS SITs - THF-51s beasties coming in from 2 different e-bay sellers ( the trusted guy didn't have anything listed and I tried my luck with a guy in China who had recently surfaced selling stock , then the original, NP approved, supplier listed a pair so that made 2 sets enroute to me )

If you haven't seen these things yet - they are seriously hefty transistors [ SIT = static induction transistor ]



I then figured that the HK775 that had set the faery dust free might be an ideal home for this project and went to work stripping everything out and test fitting everything



and that is pretty much were I have gotten to. I need to build out the power supply , put together a few options for the input buffer and then wire it up for a test run.


In the mean time, I tested both sets of THF-51s to make sure they were good and fortunately both pairs tested very well.

I rigged up a quick test circuit to test vGS with a voltage supply 19V at 1.8A current running.




I am not sure the HK heatsinks will handle the SITs, so I am going to build the M2X first to see how hot they get.

Plan ultimately is to use these to drive either the Maggies or OBs

..dB