The Revelator (Another Nice Tube Phono Preamp)

@Salectric just mentioned my phono design on his A Very Nice Tube Phono Preamp thread, so I thought I would start a new thread with details on my design.

In keeping with the theme of @Salectric's post, I offer no claims of supremacy, affordability, or innovation. There is nothing new under the sun, as the saying goes. Yet since building my prototype into its final home this summer (the rebuild took less than a weekend), I have been wearing out my record collection - my love for vinyl restored after a decade of listening almost exclusively to digital.

In subsequent posts I will describe the circuit and provide some context, but since many folks seem to just care about the pictures...here they are!

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Background

Its hard to pinpoint when I stopped playing records, but relocating my record collection three times in a span of 8 years made me seriously consider throwing in the towel. Many issues were due to the 'table, and perhaps just a lack of synergy between the table, arm, cart, etc. Not since my first few years of playing records -- with a Music Hall table / Grado and modified Dyna PAS phonostage -- had I preferred the sound (from a hi-fi perspective) of records to CDs on a cheap mass market TEAC player. A lot of gear came and went in the hopes of restoring my early love of vinyl - including names like Rega, Shure, Dynavector, VPI, Galibier, Schick, and Soundsmith.

Fast forward - my current setup is a Garrard 401 on a DIY baltic birch plinth, an Audio Note Arm Three/II tonearm, and a rebuilt Denon 103R with MR/boron cantilever.

Phono preamps

Throughout this time my phono stages were all DIY, beginning with the modified Dynaco. The Dyna was a strong starting point, and was modded by my dad (a/k/a @Salectric) who has some street cred in this arena. He's not one to boast but it seems the articles he published in the earliest days of DIY hi-fi helped inspire a number of professionals/DIYers. One of them was about mods to the Dyna PAS.

Skipping over several DIY designs, for the last few years my phono pre was a version of @Salectric's d3a/5687 phono. The audio circuit was pretty much the same, but I used active loading on the output stage and a shunt regulated power supply with a CCS and voltage regulator tubes. I liked this unit, but it was never quite as resolving or coherent as @Salectric's build. Plus I lost favor with the 5687. It and its cousins (mainly 7044) used to be a favorite tube of mine, but as all the different brands sound great in different ways -- Tung-Sol 5687 has awesome bass and dynamics, RCA 7044 is balanced and detailed -- no one brand did everything well.

The nail in the coffin for my old phono - let's just say the chassis was not conducive to experimenting.

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So...last year I started from scratch. Hoping that prototyping various phono stage designs would tell me once and for all whether it was time to throw in the towel on vinyl.
 
Design Objectives

Starting from a blank canvas and budget not being a limiting factor, you can get lost in the endless possibilities. It can help to define your priorities:
  1. No HF nasties. Even new vinyl can have annoying ticks, pops and groove noise; the rack/table/arm/cart interface works so hard to not emphasize those high freq artifacts that I wanted the phono stage to do its part, and just let the music come through. Hence, "the Revelator."
  2. Tone
  3. Small footprint and minimalist signal path. At the time, I had a Box Furniture Company rack and my rectifier had about 3/16" clearance to the shelf above. Not wise. So the chassis had to be 3" height or less. Also the signal path had to be short and simple to preserve the tiny voltages - read: two triode stages - and umbilicals are out of the question, so the power supply and audio circuitry had to be in one chassis.
  4. Highish gain. With no linestage, and even though my power amp is fairly sensitive, I did not want to limit future amp possibilities (i.e., having to build a 3-stage amp) so I wanted to err on the side of too much gain rather than too little.
  5. Tolerable noise. It had to be quiet enough in my room, in my listening chair, to my ears.
  6. Long-term reliability. With conservative tube operating points and no unobtainium tubes, one could leave the thing on all day (hey, you never know when a pandemic will strike) and not worry about burning up the tubes.
Other factors specific to my system. On the input/source side, my 103R cart is a LOMC so I use a step up transformer - a 1:20 copper and 80% nickel from Dave Slagle. The cartridge and SUTs are given. The relatively low signal level out of the SUT increased the demands on V1 for a good signal-to-noise ratio.

On the output/load side of the phono stage was an autoformer volume control (AVC), another of Dave Slagle's products. This can present a difficult load so I needed some drive from V2.
 
Tying together several of my design priorities - one of the key steps in my prototyping was to try mount the SUTs inside the chassis. This proved to be the most time consuming part of the build, but probably the most worthwhile as the ability to mount the SUTs in the chassis would eliminate a set of interconnects - but I could not determine the chassis size and layout until this was proved.

There’s another good thread on this forum showing the process, but here is what it looked like for me. I purchased the nickel cans from Dave along with a brick of beeswax, and managed to source some attractive and vintage looking steel housings that the potted transformers slid right into.

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Thanks to the long leads, I was then able to carefully hold the canned and shielded transformers - with the volume cranked up - to see what hum resulted from various locations on the prototype chassis.

This was really interesting. To my amazement, not only did this prove that SUTs can be mounted in the same chassis as the power supply, but they could be within a few inches of the Hammond power transformer before any hum started to emerge from my speakers.

Encouraging!
 
Designing/Prototyping the Phono Circuit

My initial interest was in building an LCR or LR eq design, which I have heard on a couple occasions and thought had great promise. However, I then heard a head-to-head comparison among an LCR, LR, and conventional RC toplogies keeping all else constant. It was surprisingly close, and although I preferred the LR overall, the differences between the RC and LR seemed more qualitative (i.e., the LR was better in some ways, the RC better in others). So I tabled the LCR / LR for another day.

Tube auditions

Having decided on a conventional eq using caps and resistors and a 2-stage topology, I started trying different configurations of tubes, loading and coupling. For V1, the primary challenges are to achieve high gain, low output impedance (to drive the eq) and low noise - since the V1's signal-to-noise ratio basically sets the floor for the preamp. I started with active loading, based on DN2540 cascode constant current sources (CCS) loading high-gm tubes 417a and 6DJ8, and LEDs for bias. This arrangement maximizes their gain, which is around that of the plate-resistor-loaded d3a. The two tubes had a familiar sound - balanced and both more revealing in the highs than the d3a (to my ears).

But the first time I tried the low-gm 420a, the sound started to click.

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The 420a is electrically pretty much a miniature 6SL7 but with different biasing. I tried it on a whim before moving to (or so I thought) a 6SL7, since I could use the same eq components for either tube type. And, I had had success with the 6SL7 for V1 in a prior phonostage - a sort of modern take on the classic RCA Phono circuit using the 6SL7 (the 12-volt 12SL7 in my case). I was so impressed by the tone and open, airy quality of the 420a - not to mention it being dead quiet and microphonic-free - that I never tried anything else for V1.

The 420a has a mu of 70 (good), and a plate resistance around 60k (not good) at around 1ma and zero bias (more on that). Low current is not ideal for CCS loading, but taking the output from the top of the CCS current set resistor dramatically improves the output impedance and ability to drive the eq.

As noted, the 420a has different biasing than the 6SL7. It can run hot with very little bias, and can even be run with zero bias. This means no cathode resistor, no bias supply, just a wire from the cathode to ground, which appealed to my minimalist tendencies. But would it sound any good? The downsides of zero bias (or even low bias; less than -1v for similar common tubes) are linearity (debatable whether that matters at such low signal levels) and the tendency for electrons to hit the grid on their way to the plate, known as grid current. It's easier for me to just try it rather than predict the outcome, so that's what I did. Frankly, the sound did not change that much when replacing the 10R cathode resistor with a piece of silver wire, but I felt it sounded a bit more open and relaxed. There was no perceptible distortion in loud passages or upon ticks and pops, so I left it at zero bias.

Tubes for V2

Selecting a tube for the second stage is an afterthought compared to the front-end of a phono stage. I settled on the 396a, an old standby I had used on several occasions. To my ears, it has the "western electric" sound, however you want to describe it, and has a balanced sound tone-wise. Nothing exaggerated, nothing missing.

During my prototyping, I tried various loading schemes, mainly different plate chokes and a quality output capacitor. When I built the final version, I decided to use active loading to keep the output impedance low and out of laziness - plate chokes take room and introduce noise issues of their own. For biasing, I initially used a red LED in the cathode. Thinking there would be poetic symmetry of having no cathode biasing in BOTH tubes, I decided to try contact bias for V2. Perhaps more importantly, the large grid resistor that makes contact bias possible presents a lighter load to the 420a. This was a significant improvement to my ears - better bass actually, and a nice warm midrange.
 
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The Power Supply

If anyone is still reading... I won't say too much about the power supply as my intention was to continue experimenting here. But as my preference is for choke input supplies, there is no excuse to not implement this in a phonostage where the current demands are low.
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The tube rectifier is an RCA/Cunningham 280/380 globe tube. These were made in the late 1930's, and they seem to last a long time. Oh, and they don't sound too bad either...maybe a little laid back but with beautiful tone.

I tried different chokes for the input choke, starting with one from Audio Note, but settled on a vintage "filter reactor" from Chicago which I found NOS. I'll confess I also liked how it looked on top of the chassis. These are readily available, less so with higher-inductance values for whatever reason. The second choke is a modern nickel choke from Dave Slagle at Intact Audio. It's a nice balance sonically with the Chicago choke, and the low current draw means a small unit would fit neatly inside the chassis.
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Finally, I used vintage paper-in-oil capacitors. Given their age, I went to a lower-voltage power transformer in order to keep the voltages down - especially at turn-on. Such that between the time the directly-heated rectifier starts conducting and before the tubes begin the draw current, the caps are still well below their rated DC voltage.
 
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Tying together several of my design priorities - one of the key steps in my prototyping was to try mount the SUTs inside the chassis. This proved to be the most time consuming part of the build, but probably the most worthwhile as the ability to mount the SUTs in the chassis would eliminate a set of interconnects - but I could not determine the chassis size and layout until this was proved.

There’s another good thread on this forum showing the process, but here is what it looked like for me. I purchased the nickel cans from Dave along with a brick of beeswax, and managed to source some attractive and vintage looking steel housings that the potted transformers slid right into.

View attachment 30103View attachment 30104
Thanks to the long leads, I was then able to carefully hold the canned and shielded transformers - with the volume cranked up - to see what hum resulted from various locations on the prototype chassis.

This was really interesting. To my amazement, not only did this prove that SUTs can be mounted in the same chassis as the power supply, but they could be within a few inches of the Hammond power transformer before any hum started to emerge from my speakers.

Encouraging!
I very much like this idea. Where did you buy the nickel cans and round can?
 
I very much like this idea. Where did you buy the nickel cans and round can?
Dave sells the nickel cans - they are cylinders with one end open.

I found the steel enclosures on the 'bay - presumably the people's republic of china. Oh well, they fit beautifully, and the threaded corners made installation easy.
 
Sorry for my ignance but who is Dave??? I just tried looking up Steel Enclosures on the bay and got millions of hit for big ass stuff. What search terms did you use?
 

Redboy

Knobophobe
Very nice! You had me at the first post with the Chicago choke and the globe 80... now I want to build one!
 
Thank You, Michael for your build presentation. I too want to build one and the level of detail in your pictures makes it almost doable, for me, a novice diyer. So, could you add a few specifics on voltages and parts used? What power transformer did you use, tube operating voltages and current draws and the schematics of your two ccs circuits. I really like the look of that globe 80. I have a bunch of globe 81s I'd like to use. If you can find the time to provide more details I'd appreciate it
Regards,
DVid
 
Nice write up. Always fascinated how folk arrive at the phono stage destination. May I ask how you did a head to head comparison with LR, LCR and CR EQ methods keeping all else constant?
 
Thank You, Michael for your build presentation. I too want to build one and the level of detail in your pictures makes it almost doable, for me, a novice diyer. So, could you add a few specifics on voltages and parts used? What power transformer did you use, tube operating voltages and current draws and the schematics of your two ccs circuits. I really like the look of that globe 80. I have a bunch of globe 81s I'd like to use. If you can find the time to provide more details I'd appreciate it
Regards,
DVid
Hi - I’ll send you a message with some more details. The PT is 275-0-275 @ 50ma. The total B+ current draw is around 25ma. It’s just a L-C-L-C with a ~30k bleeder resistor to help control turn-on voltage.

Never tried the 81 rectifiers, but they are appealing other than the filament supply and the need for two (since they are a single diode). The bottles look massive too - like type 50s.

By the way, the most challenging aspect for a DIY builder are wiring up the current sources. I bought a bunch of blank circuit boards back in the day from - wait for it - Radio Shack. An easier alternative is to buy some kits from Kevin Carter at K&K Audio, if he still sells them.
 

MrEd

Senior Nobody
Thanks for sharing.
Reminds me of the old saying "the acorn doesn't fall far from the tree".
It is really great that you share this audio passion with your dad, he must be a pretty cool mentor.
Nice build and good insight into your thoughts on design and function.
 
Nice write up. Always fascinated how folk arrive at the phono stage destination. May I ask how you did a head to head comparison with LR, LCR and CR EQ methods keeping all else constant?
Hi Stephen. I wish I knew how he did this too, but it was in the emia room one year at capital audio fest.

Not certain but I think my dad was present for that comparison too. @Salectric, do you recall any specifics about the demonstration?
 
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