I bought this little preamp as a kit last month. I had just finished it before the G2G so it was sounding rough before break in, so I didn't play it then.
Now that it is broken in, I can comment on the sound. It is AWESOME!!! A lot of solid authoritative bass came into the system with it, so once again the Snappers are absolved of blame, this time for wimpy bass. The preamp is very lively and present. It commands your attention, music sounds very important and valuable. You don't want to miss a note. Even some silly waltz on the radio that would otherwise be dismissed as elevator music really caught my attention, the violin sound, the pizzicatos, the gorgeous thick tone made me stop talking to my wife midsentence (happens all the time) to listen. There is a thickness to the tone that reminds me of the luscious EE M156 amps which gave that in much larger doses. You can grab a note and really hold on to it, you can feel the sound physically in your brain. It is a more physical listening experience. There is noticably more euphonic tubular distortion in it than the Minimax with my favorite stock EE tubes. But the minimax parts quality is higher, like the alps blue pot, so some mods should improve GG a bit more. But even as stock it is a very enjoyable little bugger! It is extremely fast and detailed, and alive sounding, with abundant tube warmth. Stock EH 12AU7 tubes, for now.
But it was SO MUCH DAMN FUN building it. So much fun that I didn't want to stop so I worked too late one night, past the idiot hour and made a mistake by soldering in a PS filter cap backwards. After replacing the melted transformer and bulged cap I had a blissful week of incredible sound. But after a few days I started noticing hiss under the music. After two weeks it was definitely getting louder so I took it out worried it might bang. One friend who owned the preamp said his was dead quiet, and the mfg webpage says "no hiss, no hum, no nothing." A tube amp building friend suggested that the Zener regulators might have been damaged by the errant cap, so I replaced those and the hiss did reduce by maybe 25%, maybe more. But it's still there, probably the same as when I first started playing it. The music sounds so good, it is not hard to ignore the hiss. My amps have 31dB gain, so I think that's probably the main cause of the hiss problem. If I could crank up the preamp louder then I would get all 92dB S/N as advertised, or at least "quiet enough"
The mfg's own amps are only 20dB gain, hence the "no hiss, no nothin" claim. The Minimax preamp has no noticable hiss at all, but it is lower gain, I can turn that up to 12 oclock for normal loudish listening. GG is loud at 9 oclock.
If I want to reduce the gain of the preamp, should I pad the input or the output? Goldpoint describes how to reduce overall gain by padding at the volume control with a resistor network. I have seen someone mention putting a resistor on the output, or even an RCA lump inserted into the cabling to attenuate. Which is better in theory? I would probably take 6dB off. The preamp spec claims 12dB gain, but I think mine is more than that. Minimax is 9dB gain which is about right.
If I can cure the hiss, then I am considering modding some of the circuit components. Maybe some of these upgrades will reduce the hiss? The parts in the kit are all decent quality, but nothing extra fancy. Kit signal resistors are already metal film so I doubt this much hiss is coming from them. The kit parts all seem to be sourced from Mouser.
I am considering these upgrade parts for it.
Dueland VSF 1uF coupling output caps
PRP 1/2W and Mills 2W resistors
Cree silicon carbide schottkey rectifier diodes
Power supply caps are 22uF, 350V - any suggestions for these? Xicon electrolytics are installed now.
Goldpoint stepped volume control (50k like stock), OR Bent PRM-1 Solo (only 10kOhm input Z)
Is there anyway to raise the 10kohm input impedance on the Bent?
Goldpoint source switch
JPS solid core hookup wire (should signals be shielded inside the case? My grover ICs are not)
WBT Nexgen RCA jacks
White pine cabinet to reduce the amplitude and happy up the flavor of the chassis resonance.
It would probably be smarter to just build the thing again using these parts, instead of wasting all the stock parts and the effort and mess of replacing everything. Or maybe just build it point to point wiring?? Now that would be fun!!
Open to all ideas and advice!
Thanks
Rich
