I've wanted to build an amplifier for quite a while. Initially from the guitar amplifier side, and more recently from a hifi perspective. As I look around, I feel like it should be easier than it is to find something meeting my needs, but I keep coming up empty. I want to build an all tube (including power supply) RIAA phono preamp. I want to use all readily available and currently produced tubes. Coming from the guitar amplifier perspective, to me this mostly means 12AX7/5U4, but I'm not 100% aware of everything in current production. I just know I can hop on down to GC, and pick up those tubes brand new and reasonable quality all day long. Call it future proofing.
Can anyone point me in the direction of a schematic or set of schematics from wall to input to output for such an amplifier? Thanks.
Can anyone point me in the direction of a schematic or set of schematics from wall to input to output for such an amplifier? Thanks.
If this is going to be your first build from scratch, you may find getting an RIAA preamp quiet enough to be more frustrating than you'd like.
Still, this is a relatively popular tweak of a design from RCA.
Yes, you will need to come up with a 250V power supply and an appropriate power supply. If that is overly daunting, then starting with a kit might be a better idea.
Still, this is a relatively popular tweak of a design from RCA.
Yes, you will need to come up with a 250V power supply and an appropriate power supply. If that is overly daunting, then starting with a kit might be a better idea.
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Check with Tubecad, very good quality boards (or make your own) and circuits.
GlasssWare Phono Preamps
GlasssWare Phono Preamps
Free schematic, and if you want, a ready board is available at
The Valve Wizard
there is also lots of advice and some other boards to use as building blocks.
There is this years Bigbottle project on diyaudio.
I believe there are still boards available.
No schematic. Just get the board and parts and build.
I believe there are still boards available.
No schematic. Just get the board and parts and build.
Clarification: I'm looking for something on the order of 40-60dB gain (that seems like a pretty generic phono preamp output gain), RIAA equalization, all tube and passives from the power supply to the inputs to the outputs. No special output impedances or anything, just bog standard. Ideally, I'd like to shoot for a minimum part count with point to point wiring. See the photo below for an example of what I'm after (guitar amplifier). I don't want a PCB, because I have multiple giant piles of exotic hardwoods, and I have ideas for the enclosure that don't allow for a PCB. I have bags full of NOS tubes and a desk drawer full of other passives, so even a kit isn't really ideal since I already have a lot of what I need. Most of my analog amplification experience comes from fixing what's broke rather than making new things, so that's why I'd prefer to start with just a known reliable schematic (so that I don't have problems making it quiet) that will get me there as a starting point for learning the making side.
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There is one in Linear Audio volume 4 that has no transistors or semiconductor diodes at all, but it uses four times EF86, two times ECC83, an ECC81 and an EZ80. I don't know whether the EZ80 is still in production.
The standard gain for an RIAA stage is 34dB to 40dB. A gain of 60dB would be for moving coils.
With no pcb or metal enclosure, noise and hum likely will be a problem.
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34-40dB sounds great. I think my current preamp is 50dB, so that's probably a middle ground. I'm not super picky. I have a MM cartridge.
this is the best you can find with 12ax7, au7
LUXMAN CL34 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
for moving coil you need input transformer.Virtually no valve can provide the SNR level needed by a moving coil cartridge with a 0.1mv output.
LUXMAN CL34 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
for moving coil you need input transformer.Virtually no valve can provide the SNR level needed by a moving coil cartridge with a 0.1mv output.
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The original RCA design (schematic provided), as opposed to the tweaked setup I'm associated with (previously provided), is 100% tubed. The major problem is its WRETCHED drive capability. A cap. coupled cathode follower added to the original will deal with the matter. DC coupling the cathode follower requires working around heater to cathode potential limits. Advantage MOSFET! 😉
100% tubed also implies AC heating. The original RCA setup calls for 7025s, not 12AX7s, for exactly that reason. Low voltage DC, without SS, is highly problematic. The 7025 contains the same triodes as a 12AX7, but the heater is spiral wound and hum bucking. The current production Sovtek 12AX7LPS is a true 7025 equivalent. FWIW, LPS = long plate spiral.
5AR4s and (IIRC) EZ81s are currently being produced. The only current production 5U4 worth a damn is the ElectroHarmonix (EH) 5U4GB. All the rest are guano.
BTW, it is well established that a 24 Kohm, not a 22 Kohm, part should be used in the RCA setup's EQ network. RCA called for 22 K, given wide availability. Way back when, the wide variety of 1% tolerance metal film resistors we take for granted were not available.
100% tubed also implies AC heating. The original RCA setup calls for 7025s, not 12AX7s, for exactly that reason. Low voltage DC, without SS, is highly problematic. The 7025 contains the same triodes as a 12AX7, but the heater is spiral wound and hum bucking. The current production Sovtek 12AX7LPS is a true 7025 equivalent. FWIW, LPS = long plate spiral.
5AR4s and (IIRC) EZ81s are currently being produced. The only current production 5U4 worth a damn is the ElectroHarmonix (EH) 5U4GB. All the rest are guano.
BTW, it is well established that a 24 Kohm, not a 22 Kohm, part should be used in the RCA setup's EQ network. RCA called for 22 K, given wide availability. Way back when, the wide variety of 1% tolerance metal film resistors we take for granted were not available.
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See https://www.diyaudio.com/forums/tub...-amp-build-recommendations-3.html#post6237480 for the amplifier part, and see the two posts below it for comments from someone who doesn't like it and for the phono amplifier's measured results.
This is the kind of thing I had in mind. I know there were books of schematics put out by tube manufacturers back in the day with common applications for their products (I even started to reference these in a draft to a previous response). Incorporating your suggested changes sounds doable (though I admittedly am a bit too naive still to fully appreciate what is and isn't easily doable), though there will be some research required on my part to figure out implementation barring something already extant available (is there, or is it easy to make as a favor to an absolute internet stranger starting down this path?). Is there an equivalent schematic for the requisite power supply?
I'm not married to any particular tube re: 5U4, or even 12AX7 (are 7025s still produced? I have a few here in my stash...). I simply don't want to spend significant time or money hunting obsolete tubes down when others are inexpensive and readily available. It's been my experience that these are readily available anywhere there's a guitar store, whereas hifi audio stores aren't on every corner. 6L6 and EL34 follow the same logic when I get around to the power amplifier side of the setup. My goal is "really good" not the best that ever was ever. I'm working on evolving my rig, and the limiting factor is going to be the 95 year old house in the mountains around it.
