Equipment > Phono Preamps
Fred Nachbaur's high precision tube phono preamp
lshin80:
I was looking for a good quality tube phono preamp and I stumbled upon this:
http://www.dogstar.dantimax.dk/tubestuf/sapinde1.htm
It looks very interesting! :D It seems to me well designed and quite different from most tube preamps. Check out the sound sample. What do you guys think?
lshin80:
It's complete! I need a wooden outer housing to enclose the metal inner chassis, but everything else is done.
The first testing was OK: everything worked, it is just a bit low on voltage because one of the transformers supplies slightly less than the nominal 117 Volts, but the volume is good. There's a barely audible hum in the background, possibly due to not perfect ground solderings, I'll investigate that.
Soundwise I will judge it after the break in period of around 300 hours; the first listen was hardly indicative. It sounded very closed in and muddled, not airy at all. I guess the coupling capacitors need to open up. I'm going to buy Intertechnik Audyn Caps Plus if the current capacitors won't satisfy me.
Compared to my solid state preamp, it's quite soft, but equally fast. I didn't notice any loss in the highs; bass is more recessed and soft, but definitely more meaty. The mids are more pronounced, and the thing that surprised me the most, is the added dept in the sound stage: with my solid state amp it's like you are in the first row or directly on the stage, while with this phono preamp you are almost in the middle of the hall among the public.
lshin80:
Now, it may look crappy, but it sounds like heaven ;)
The case is an old multipurpose pre-drilled chassis my dad had since the '60s. The circuit looks messy, but it was built not to use long pieces of wire, in order to avoid any possible noise or hum. There is a light hum in the background, so we'll have make a central ground (star) point. Otherwise, it has no issue. It is possible I will get a better looking steel chassis, so consider this as the working prototype.
Pic #1 - front view.
Pic #2 - rear view with power switch, AC input, fuse holder, line in & out.
Pic #3 - tubes detail: 2x ECC83 Philips Miniwatt, 1x ECC81 Siemens.
Pic #4 - circuit overview.
Pic #5 - power supply detail. F&T and Hitano Electrolythics. 2x NOS Wima TFF series caps. Ultra fast rectifier diodes.
Pic #6 - input and output caps. Bottom: 2x NOS Russian KBG series paper in oil caps. Top: Solen Fast Caps bypassed by KBGs (Solens will be replaced by Intertechnik Audyn caps.)
Pic #7 - RIIA circuit detail. 4x Icel MPL series caps.
BSD2000:
I think it looks good! :)
I wish I could hear it. This is giving me the bug to want to build one. :D
First, I need to buy a new set of tubes for my phono preamp. One of the tubes went bad last year and I'm using a spare. They really should be replaced all at once with all matched tubes since the left channel has slightly higher gain than the right. :(
lshin80:
The Intertechnik caps have arrived. They are now plugged to a frequency generator for the break in. A frequency sweep of 100 Hz to 3 kHz @ 10 Volts is applied. Next step will be a sweep of 1 Hz to 100 Hz. I expect much from these caps, especially when compared to the Solen Fast Caps, which I feel to be the weak spot in the audio path. Well' see :)
Since we have to remake the ground point, we'll possibly have to dismantle everything. That could be a pretext to get a better looking chassis. I was thinking to this:
http://www.hammondmfg.com/dwg20.htm
in the black version:
http://www.hammondmfg.com/jpeg/1441BKU_B.jpg
This chassis would resolve the problem of a wooden case, there is in fact this version which looks great:
http://www.hammondmfg.com/dwg20CWW.htm
However that would cost almost 60 Euros, while the simple steel version costs 10 Euros...so I could get the simple version and then apply two pieces of unused wood that I've got.
What tubes do you need? I looked for a long plate Philips ECC83 to couple with the one I've got (no matter if they're not matched, my solid state preamp has independent volume controls for each channel), but nothing under the 100 Euros mark :(
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