Best Sounding Phono Input Tube

[tubewade]

Opinions on the best input tube for a phono preamp with passive RIAA? I've used several integrated amps that use 12AX7A/7025 or 6EU7. Lots of other preamps use 6DJ8 variants. Some use 6SL7. Interestingly, many people seem to feel that a 12AX7/7025 is really not good for a phono amp, that a 6DJ8 is a poor sounding tube altogether, and that a 6SL7 sounds "squawky", whatever that means. So what is really the best sounding tube for a MM phono preamp. I'm planning to build a stand alone unit and don't want to just duplicate something that I already own in another amplifier. At this point I'm leaning towards Bob D's phono amp at http://www.geocities.com/TimesSquare/1965/phono.html but with 6 volt tubes rather than 12 volt since that is what I already have. Any input will be much appreciated.

Merry Christmas

[SY]

There is no one "best." It depends on the cartridge, the topology, the presence (or not) of input transformers, and desired complexity or lack thereof. The sorts of generalizations you've heard are, by and large, wrong. Can you be a little more specific about what it is you want to do? That way, we can pass by the audio legends and zero in on a good solution for you.

[Miles Prower]

Originally posted by tubewade
Opinions on the best input tube for a phono preamp with passive RIAA?

That's a virtually unanswerable question. What one designer will swear by, another will swear at. It's that much a matter of personal preference.

FWIW: 6DJ8s are at their best when cascoded. Used as a straight triode, they ain't much.

Another consideration: build the phono preamp, and include gNFB. Adding gNFB makes the circuit performance less dependent on both active and passive components. Of course, you're gonna hear from those who will tell you that's "heresy". Are they right or wrong? Not necessarily.

[Jeb-D.]

I like the 6021. 6111's work well too, but you need more gain following it.

[andyjevans]

I'd like to hear an ECC40. sounds great in everything else.

[tubewade]

Thank you, SY. I value your opinion.

The cartridges to be used are a Shure M97xe on a Linn Axis, Shure M95ed and M75ed-t2 and Ortofon OM20 on a couple of lesser turntables. Topology I prefer would be probably classic two stages or two stage with follower like the RCA circuit in the back of the tube manual. I prefer simple to complex, where possible, in all things. I know that some things are too simple for their own good so I don't want that either.

No input transformer. Directly driven from the MM cartridge.

I want to build a (phono) preamp with the benefit of other peoples experience so I'm not just blindly copying the circuit and sound from an old integrated amp or receiver, unless, of course, there is some truly exceptional one that I need to know about.

Miles, your response is exactly the conclusion I had mostly come to. It's just a matter of preference. So what would you use?

Jeb and Andy, thanks for those suggestions. I'll study up on those valves.

[Eli Duttman]

Originally posted by tubewade
Opinions on the best input tube for a phono preamp with passive RIAA? I've used several integrated amps that use 12AX7A/7025 or 6EU7. Lots of other preamps use 6DJ8 variants. Some use 6SL7. Interestingly, many people seem to feel that a 12AX7/7025 is really not good for a phono amp, that a 6DJ8 is a poor sounding tube altogether, and that a 6SL7 sounds "squawky", whatever that means. So what is really the best sounding tube for a MM phono preamp. I'm planning to build a stand alone unit and don't want to just duplicate something that I already own in another amplifier. At this point I'm leaning towards Bob D's phono amp at http://www.geocities.com/TimesSquare/1965/phono.html but with 6 volt tubes rather than 12 volt since that is what I already have. Any input will be much appreciated.

Merry Christmas

Dude,

The other posters are correct. An absolute best simply does not exist.

An interesting complement is a 6ER5 in the 1st position and a 6GK5 in the 2nd position. Lots of gain is produced, which makes the combo an excellent mate for HOMCs.

The weaknesses of the "classic" RCA circuit are mediocre bass extension and miserable drive capability. Those problems can be addressed easily enough. A tweaked RCA circuit schematic is attached. A plus frequently overlooked about the 'X7 is the availability of a FINE tube, the Sovtek 12AX7LPS. Specimens of the Russian tube, with a superior noise factor, are available at modest cost. Add a nice, "uncolored", tone and the selection nears being a "no brainer".

[SY]

Coincidently, that's something I'm dealing with right now- after some years using MCs, I've ordered a promising MM, and need to design a new phono stage for it.

MMs in general, and Shures in particular, are fussy about their loading. If you use a high mu tube at the input, you'll have a very high input capacitance because of the Miller effect. For high mu tubes like 12AX7, trioded D3a, 6SL7, and the like, the input capacitance will run 300pF or more. With reasonably achievable values of cable capacitance, the cartridge will not be happy and the frequency response will look like a roller coaster, but you'll be able to tell everyone you know that 6SL7s sound squawky.

You then have three choices:

1. A lower mu input tube, but the need for two more gain stages.
2. A tube cascode
3. A hybrid cascode with a FET on the bottom

If I were to do one of these three today for a publishable design, I'd probably use an ECC88 variant, just because I know them so well and they're easy to find and inexpensive. Another choice, highly recommended by two guys I trust (and thanks for the tubes, fellas!) is the 6AQ4, and I'll probably end up using that in the one I'm starting next.

FWIW, my last phono preamp, which I used for over 20 years, went with approach 3, using an E88CC variant on top and an NTE458 on the bottom.

And purely IMO, but with some objective reasons, there is not a single "classic" phono stage that's worth a warm bucket of spit.

[korneluk]

Originally posted by tubewade

I want to build a (phono) preamp with the benefit of other peoples experience so I'm not just blindly copying the circuit and sound from an old integrated amp or receiver, unless, of course, there is some truly exceptional one that I need to know about.

You can trust Gary Pimm's ears and designs. Here's the info:

http://home.pacifier.com/~gpimm/phono.htm

Enjoy,

-- josé k.

[Miles Prower]

Originally posted by tubewade
Miles, your response is exactly the conclusion I had mostly come to. It's just a matter of preference. So what would you use?

First choice: KA5532. Yeah, I know: EWWWWW! SAND! But, seriously, what's another op-amp or two?

If I had to go hollow state, I'd go with a differential cascode up front. For a magnetic cartridge, you're gonna need gain, and lots of it. The usual solutions, a high gain triode or pentode present their own problems indeed. These high-u triodes (12AX7A, 6AV6, 6SF5, and so forth) have some horrendous Cmiller. That's been the big problem using these "up front", regardless. Pentodes have their own problems: noise.

So that leaves the cascode: lots of gain, and manageable Cmiller, and no partition noise. For a phono preamp, I'd go with cascoded 6SN7s. The 6SN7 is quite linear, and, at least the Sovtek version, has very low microphonics. I'd get two of them, and build a differential cascode with an active tail load composed of cascoded BJTs. The magnetic cartridge, being a coil, operates in a differential mode naturally, and a differential front end also eliminates common mode noise. That'll get you your gain. Output stage: Brosky Cathode Follower balanced-to-single-ended converter.

Need I mention well filtered DC for all filaments?