Last night I listenedd to a re-press of Mingus Ah Um. Wow. it sounds like they band is right there, so clear. i can hear all the nuances of the horns, like the spit rolling around in the sax as its played. yeah, that's my audiophile benchmark: can i hear spit? if you can hear spit you're doing well. I was feeling that building this PH15 is sort of like building a musical instrument - its all in the nuances and rich deep character when you get to this kind of quality of sound. the cap and resistor selection, making every solder connection count, etc - it just feels like making a fine instrument rather than slapping together an electronics kit. the final pay back is listening to Ah Um and feeling like, the next day, I saw Mingus live at a local jazz club.
I've been messing around with step up transformers. what a black art. having no experience with them its been a big lesson is hooking small coils of wire to larger coils of wire and determining what the heck goes on with that. talk about non-linear. naively i bought a pair of used Jensen 34Ks which have a 38:1 ratio, 31dB. I figured it was just about gain and getting the right load on the MC. partly true. but there is a lot more at play because when you load the MC you lose gain. but you also change the sonic character of the whole MC+SUT system and this is what feeds the phono stage.
My MC is a Benz Ruby 2 with a 40 ohm internal impedance. apparently it sounds best with a 200-400 ohm load on it. so in order to get that on the primary of the SUT, i have to use a 430k resistors because the reflected load on the primary is (reflected primary load) * (turns ratio)^2 = 300 * 38^2 = 433k.
So theoretically 430k on the secondary will give around 300 on the primary, in the middle of the generally accepted range that works best with the Ruby 2. thing is, it sound bad: very bright, airy, lots of detail but smeared instruments, no body and lacking bass. through trial and error I found 100k on the secondary sounds good but that theoretically load the MC with around 70 ohms.
So there is more going on here. Impedance, inductance, coils of wire driving other coils of wire... its a black art. but its fun to learn. "they" say that a 10:1 or maybe 20:1 is better suited to MC's with an internal impedance of <40 ohms and that SUTs like 38:1 are best for < 5 ohm. why? I am not sure but I am sure there is math somewhere that explains it.
For now I am running the 34K's with 100k despite the fact that it should sound like its loading the MC down too much, it sounds really good with the PH15. better than I have ever heard. Its the closest thing so far that I've had where my system sounds like a live gig. The hope is that a "better matched" SUT will sound better. I've ordered a pair of Lundhal LL9226's which can work as 5:1, 10:1 and 20:1 so I will try that out and if they sound better I will swap the 34K's out.
I also rolled in a pair of EH 6922's (low noise) that I burned in for about 100 hours. They sound pretty dark compared to the Amperex JAN's I have in the PH15. I will do more listening and report. I am going to try and get my hands on the reputed best sounding 6922, the Amerpex white lable PQ pinched waist but try and find those for less than $200 a tube, used.
But so far the PH15 is sounding much better than anything I've used in the past. I'd like to do some comparison with other tube phono stages at some point and also a stock PH14/15 with the stock parts compared to mine with the crazy hot rod parts.
Oh and I ordered the cases: 11"x7"3.5" with bolivian rosewood side panels. should have them in a few weeks. I made up the power umbilical last night. that took 2 hours because it was so fiddly getting the pins in the connectors. the parts just for the power cable cost me around $70. but it will be a sweet high current capability, shielded twist lock connection between the PSU and preamp. there is about 0.2 ohms on each line so not too bad all things considered.