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Old 16th August 2004, 11:48 PM   #1
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Default Cheap and Easy Tube Phono Preamp

Well, I guess its not technically DIY since its a kit, but I have my Bottlehead Paramour 2A3s and Foreplay preamp together and I think they sound really nice through my Fostex 206E backhorns.

I would like to try my hand at a tube phono preamplifier, but I was going to try and break away from the kits for this experiment. I have read and read and read and read the forums and I've seen some phono preamps that look pretty good, but I wanted to see if anyone has encountered a cheap and easy design that they would recommend for a moving magnet cartridge.

Right now I'm running a Hagerman Technologies ultra-super-Cheap-O Bugle Op Amp for my phono pre. I actually think this sounds pretty good, but I really wanted to go all tube.

Any suggestions? Remember-- cheap and easy. Like Mrs. Annan used to be. Hey-- you know I love ya, baby!

Kofi
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Old 17th August 2004, 04:23 PM   #2
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I have built the RCA phono circuit several times with good success. There is a good write up on it on the JE Labs website. On that site Joseph recommends 6SL7s, but I found them to be too microphonic to work with. I've had better success with 12AX7's. One of the things I like about this phono stage is that the RIAA equilization is between two stages and is not in a feedback loop. Other DIY phono stages I've tried are the Curcio replacement for the PAS phono stage, okay, but sounds a bit congested to me; and the phono stage from the Audio Note M7. This is a nice little circuit and easy to build. Overall I think the RCA version is more musical. But YMMV.

Now I will add that I've recently been thinking of taking another run at a phono stage myself. And that is likely going to be the Marantz 7C (you can see it on the Fi Primer site) or a clone of the Fisher X-100 series style. I've heard the Fisher and really liked the sound. The only trouble with using clones of these old stages is that you have to "backwards engineer" it from a schematic that generally includes pretty intricate input selector switch configurations. Once you can trace it out, however, it should be pretty easy.

By the way, layout of this stuff is much more important than a line stage. I'd highly suggest you get a look into the bottom of something like a Fisher integrated and see how they layout the parts and grounding scheme. It is pretty important stuff. This also means you can borrow what others have learned in terms of layout, rather than be disappointed after stuffing in a bunch of expensive parts.
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Old 17th August 2004, 05:18 PM   #3
SY is offline SY  United States
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I'd clone the old SP-6B circuit. It's based on a classic technology, uses easy-to-source tubes (12AX7), is quite simple, and has a cathode follower to drive the RIAA network.
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Old 17th August 2004, 06:32 PM   #4
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Hi Guys,

Please be kind enough to post the links to the sites hosting the circuits of the pre-amps described above. I am in the market for a tube pre-amp which preferably used 12AX7s, EF86s or maybe 12AT7, as I have these valves available. I would like a variety of inputs, from phono to CD player.

TIA!
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Old 17th August 2004, 06:35 PM   #5
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My buddy built the RCA circuit (using the Handmade boards -- i don't believe these are available any longer) and it works pretty well.

dave
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Old 17th August 2004, 06:37 PM   #6
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And the circuit

dave
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Old 17th August 2004, 07:04 PM   #7
smbrown is offline smbrown  United States
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The Fi Primer site is: http://tinpan.fortunecity.com/saints...mer/index.html

You should be able to find JE Labs from a Google.

Audio Note M7 here: http://www.drtube.com/audioamp.htm#AN

(also others like Conrad Johnson and HK Citation).

Scott here: http://hhscott.com/database/vhhs_00011.html

Fisher here: http://oldtech.net/Fisher.html
The X-101 C is the one I'm most interested in right now. Though you'll find a couple of errors there - a 22m resistor should be 2.2m and a 22k resistor should be 2.2k (cathod of the V1 vs. V2, and grid of V1 vs V2).

As for internal layouts, do some searching on ebay and you'll often find photos of the insides of old equipment. With a bit of luck you'll find one with enough clarity that you can figure out the general idea. Another source of info on layouts is to get the manuals for some kits, like a Heath AA-151, which has a pretty neat and orderly chassis layout including a buss bar grounding scheme.
Happy building!
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Old 17th August 2004, 07:32 PM   #8
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Thanks guys, I have some interesting reading to do!
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Old 17th August 2004, 07:45 PM   #9
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Look for post #10 in the thread "dc phono", similar topology to the RCA circuit with 2nd stage using 6DJ8/ECC88 which has lower plate impedance for better output drive and grid leak bias for better LF response, also has 2nd stage input coupling capacitor after the RIAA eq network for DC on the eq caps which may improve sonic character. I want to build the "dc phono" myself but need to find time to do it.

I built the RCA circuit in the early 80's and the high output impedance didn't work well with my SS line stage at that time, if I were to try it again I would use 6072/12AY7 instead of 12AX7, lower gain but also lower noise, lower plate impedance, and less competition for quality NOS tubes, but the values of plate resistors and the 470k eq network resistor would have to be changed.

Another option is to buy the Bottlehead Seduction preamp kit for US$279 but special arrangements are needed for orders outside US and Canada.

The SP-6 clone which uses active RIAA eq would probably sound great (to me, anyway) but is more complicated than the "dc phono" thread or the RCA. Active RIAA eq seems to be out of fashion today anyway due to feedback affecting sonic character, and is more difficult to design for due to the eq accuracy being affected by the limited open loop gain available from two resistive-loaded triodes, one or both stages can be cascoded with more complexity..... Passive eq is simpler to design but eq network source and load impedances must be considered for best eq accuracy. I have tinkered with the Dyna PAS phono which is an active EQ design without output follower and HF extension is weak even into a high impedance load, OK if you like a "soft" sound.
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Old 17th August 2004, 08:22 PM   #10
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I'd clone the old SP-6B circuit.

Good idea. Cloning AR you can't go too wrong. At the time i cloned SP3, Sp6 and Sp10 and although none provided the exact sound i was looking for i could easily live with any of them. The Sp10 in particular impressed me with tremendous macro dynamics, great bass and amazing soundstage. Admittedly it had some weaknesses but the redeeming features were too many.
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