CCS Tube Phono Stage

As my knowledge and skills in tube gear progresses, I have become confident enough to develop my own designs from scratch. Make no mistake: I am not an expert. I know just enough to be dangerous. I have a resolving system to determine what sounds good through trial and error, and I have enough test equipment to fine tune with measurements.

That being said I’m confident enough in the results to release my designs here, as “open source” free to use designs – including gerber files to print your own PCBs if so desired.

Both designs use constant current source plate loading, diode cathode biasing, and mu-follower outputs for the most linear response, the greatest fidelity, and the lowest possible output impedance – in the case of my line stage, measured at 30 ohms.

10M45S depletion mode MOSFETs are used for the CCSs as these are cheap, readily available, and don’t require any additional power or reference to ground – just stick em in where the plate resistor normally lives, set the current with the appropriate resistor, and you are done. Gate stopper resistors are included to prevent oscillation. To calculate the current set resistor: 3000/Ia = resistance in ohms (so 9ma is 3000/9 = 330R, 1ma is 3000/1 = 3K).

Before the purists start scoffing at the silicon bits I’d like to point out that Conrad Johnson has used diode bias for decades and Audio Research uses elaborate constant current source schemes on nearly all their products.

These designs lean towards the neutral side of tube sound – think Audio Research but a touch smoother with a bit more midrange and you’ll have an idea of what to expect. Bass is tight and well defined. Highs are extended but never harsh. Midrange is slightly forward. I voice my equipment to maximize the breadth and depth of soundstage while maintaining balanced tonality and a smooth top end – I hate harsh highs. It isn’t the last word in microdetail but you’d need to spend a lot more to get better. If you are looking for that old fashioned warm, syrupy sound like grandpa’s Dynaco don’t bother with these.

Recommended B+ is 180-280V; with CCS plate loads the precise voltage isn’t as important as having a dead quiet well regulated supply with sufficient current and as close to zero ripple as possible. Solid state or tube, your choice. I prefer remote external supplies to keep noise to a minimum. I only use 6.3VDC heaters. Heater ground is elevated with a voltage divider connected to B+ voltage: thus DO NOT CONNECT HEATER GROUND TO CHASSIS OR SIGNAL GROUND. I have provided my design for an all-tube series regulated power supply here.

I do not include provisions for local filtering caps on my boards because I use large film capacitors mounted remotely in my designs – I recommend you do the same. Solen makes affordable large value film caps that are ideal for tube power supplies.

---

Passive RIAA network moving-magnet zero-feedback constant-current-source mu-follower tube phono stage (whew) with 12AX7 input and 6922 anode follower. I say 12AX7 but this is just the “easy” answer – gain is high enough that a 5751 or 12AY7/6072 are drop in substitutes that still offer 40+db of gain. 

12AY7 is my personal choice as the lower plate resistance is a better match for the RIAA network, and it has lower miller capacitance which can mess with the cartridge loading. Gain with a 12AY7 installed is 42db, while a 12AX7 gives 50db.

12AX7 plate current is set to 1ma per triode, 6922 is 9ma per triode. First stage has 0.7V cathode bias with a single 1N4148, second stage 1.4V with a pair of 4148s in series. 

This is a shockingly quiet phono stage, quieter than most solid state designs – absurdly quiet for a tube design. I was honestly shocked at how low the noise floor is. It sounds big, dynamic and full. Soundstage is huge and deep. It’s a lot of fun to listen to and suits a variety of genres. Be warned it is very resolving and will not flatter poor recordings. Quality vintage tubes are recommended; I’ve tried it with modern production tubes and they are way too harsh and grainy in the treble for my taste.

I’ve included slots for loading capacitors but most of the time you are better off leaving them empty, unless you have a vintage cart that needs a ton of capacitance to shine. Keep in mind the miller capacitance of a 12AX7 can be as high as 200pf, that alone is more than enough for most applications. Default load resistance is 47K but may be modified as required.

Part II, making the CCS Phono More Better: Parallel Triodes

How do you make a good design even better? Add more tubes!

Ok more specifically we are paralleling the 2 triodes within each tube, splitting the L/R channels into separate tubes, and otherwise leaving the circuit as above.

The benefits: more gain, less noise, lower output impedance on both stages, and less crosstalk between channels. Everything is just a little bit better, though not dramatically so - if you have a really resolving system you'll appreciate the improvements, if you have an average system then the default design works just fine.

The downside: now you need twice as many tubes. But outside of that the parts count is the same, as you are just strapping the two internal triodes together (linking the grids, plates and cathodes) and running a single current and bias source for each tube.