22 August 2015

At Last: The Return of the
Aikido Phono Preamp

Aikido Phono preamp, Rev. B
The Aikido PH-1 returns—and yes, it has been a long, long time. What is an Aikido PH-1? The Aikido PH-1 is an excellent tube-based phono stage. It holds two Aikido gain stages per channel and uses a passive RIAA equalization network in between the two. In other words, four Aikido gain stages reside on the PCB. One Aikido PH-1board is all that is needed for stereo phono pre-amplification.

Thus, eight tubes are needed, as each AIkido gain stage requires two tubes. In addition, each Aikido gain stage gets its own RC filter to isolate it from the B+ connection, thereby furthering enhancing the already fine PSRR. Great, John, but who would want a tube phono preamplifier, in a world filled with iPods and streaming digital audio? I would, for one.

Despite predictions to the contrary, spinning vinyl to the warm glow of vacuum tubes persists. In spite of the popularly held belief that both LPs and tubes are dead, long dead— most believe that the vacuum tube died about 50 years ago along with black & white TVs and that the long-playing record died 35 years ago at the birth of the CD—both tubes and LPs grow more popular with each coming day. Tubes simply refuse to fade away and solid-state audio gear continues to be—embarrassingly—advertised as sounding tube-like, just as high-resolution digital music files are said to sound more like analog; and turntables and phono cartridges continue to be developed and sold and new records continue be pressed daily. Both tubes and LPs, or as the Britts say V & V (valves and vinyl), are here to stay.

If you were born in the last 30 years, your might wonder: Why do we need a phono preamp? The answer is simple. The phono cartridge's minuscule output signal, being only millivolts, is too weak to drive a power amplifier, even with a line-stage amplifier's help; in addition, its output signal is not flat, as the signal coming off the LP's surface is not flat, having been equalized with boosted highs and attenuated lows. Thus, a phono preamp both amplifies and flattens the frequency response.

Regardless of the underling technology used, tube or solid-state, a phono preamp must undo the RIAA (Recording Institute Association of America) equalization curve or the CCIR (Comité Consultatif International des Radiocommunications) or IEC (International Electrotechnical Commission) the European versions of the RIAA) curve used in making the record. Why was this curve imposed on the records? Using it improved the signal-to-noise ratio of the record by boosting the highs going to the cutting head, while greatly extending the play-back time by cutting the lows. The end result was a fairly even grove cut, regardless of the frequency. The inverse of the RIAA curve returns the signal to flat by cutting the highs and boosting the bass. (Bear in mind that most records made before 1950 may not have followed the RIAA curve, but held some other proprietary-to-the-record-label equalization curve.)

In the Aikido PH-1, the first Aikido gain stage amplifies the cartridge's tiny output voltage and then passes the signal to the passive inverse RIAA equalization network, which boosts the lows and attenuates the highs, restoring a flat signal, but at the cost of an insertion loss of -20dB. The second Aikido gain stage then further amplifies the flat signal, bringing it up to a useable level. The final gain of the preamp can vary from 30dB to 48dB, depending on the tubes used. If a low-output moving-coil cartridge is used, then either a step-up transformer or a pre-preamp will be needed.

The Aikido PH-1 phono preamp is an extremely flexible design, as a nearly infinite number of different Aikido Phono Stage configurations can be built on the PCB. Nonetheless, our goals always remain constant: high-gain, low-noise, and low output impedance. The input tubes (V1, V5, V3, and V7) provide the voltage gain in this preamp, while the output tubes (V2, V6, V4, and V8) isolate the input tubes from both the equalization network and the external load, and they deliver a low output impedance.

Due to its symmetrical design and its forgoing of negative feedback, the Aikido gain stage is a great candidate for tube rolling. For example, a 12AX7 input tube can be replaced by a 12AT7, 12AY7, 5751, 5965, 6072, and even a 12AU7, without having to change any of the part values.

The Aikido PH-1 phono stage PCB requires an external power source for the B+ voltage and the heater voltage. The best choice is a well-regulated power supply, but even a well-designed non-regulated power supply will work well, as the PCB holds two cascading RC filters per channel to filter away ripple from the B+ connection. Ideally, the heaters will get a regulated DC power supply, as AC heater voltages will introduce too much hum. A good choice for the power supply is the GlassWare PS-1, which offers both regulated high-voltage and low-voltage outputs. Below is my actual build, with PS-1 power supply. The switch on the left is a GlassWare stereo/mono switch and the switch on the right is an GlassWare AC power switch, which allows a staggered turn-on for the heater regulator and the B+ regulator.

Viewed from the back, you can see the AC power connector jack and GlassWare House Ground PCB.

The boards are six inches by eleven inches, with ten mounting holes that prevent excessive PCB bending, while inserting and pulling tubes from their sockets. In addition, the FR-4 PCB is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through 2oz copper traces, and the boards are expressively and lovingly made in the USA.

The GlassWare PH-1 phono stage kit includes all that is needed to populate, including the capacitors (31), including four CDE 1kV polypropylene coupling caps and hand-matched RIAA equalization capacitors, PCB, resistors (with many redundant values), standoff assemblies with O-rings and screws (10), and ceramic tube sockets (8), except for the tubes; sets of eight high-quality tubes, however, will also available once I refuel my supplies. The Kit includes a 16-page user guide. The GlassWare PH-1 phono stage kit is available now at the GlassWare-Yahoo store.

 

Many PCBs Back in Stock
Many of the most popular PCBs and kits are back in stock, such as the BCF, PS-1, PS-7, AC Switch, mono octal Aikido, Tetra phono stage, Select-4, and Rectifier-1 socket & PCB kit. The UPS truck arrived last night. Yes, at night, as my street is probably the UPS driver's last drop-off. I also have some new PCBs that I am super eager to test. Check out the GlassWare-Yahoo store to see what's there.

 

Where Did July Go?
"Blogfade" along with "podfade" are new words to describe when a blog or a podcast that had been posted regularly begins to post sporadically and then ceases altogether without neither warning nor explanation. Do not worry, I don't plan on letting my posts fade away. (Interesting, does anyone plan on fading away? or does it just slowly happen?) Ideally, as I have often said, I would love to post every week of the year; and if I were a trust-fund baby or independently rich or retired, that is what I would do. My reality, in strong contrast, is that there is only me, no staff, no employees, and I only have so much time available and everything takes gobs of time, from taking care of my kids, keeping my wife happy, emptying my dogs, to answering mountains of e-mails, so my posts get put on hold, as they require the biggest chunk of time. (By the way, I have received several e-mails over the last 17 years, filled with anger and impatience, that demand just where the hell is my new post. My answer is that the reader should be outraged and that he—it's always a he—should demand a full refund of his entire $100 subscription fee. Usually, a mild-mannered apology follows, but not always.) Well, summer is always a tough time for me, as my kids are home and they must be driven everywhere.

The problem I now face is that, even when I don't post, I never cease from generating new blog material, which continues to fill my hard drive with schematics and SPICE simulations. In addition, I have also held several long e-mail exchanges with readers on varied topics that could easily have created three or four new blog posts. In fact, just my exchanges with Ziggy of hiend-audio could fill a new web site. So where is the problem, as most could not generate enough new material in a lifetime to fill more than one or two posts? The problem is that most of this work now feels well-chewed and well-digested. Imagine having to read last year's newspaper or having to watch last year's talent-search TV show and you will see what I mean.

Wait a minute, I have just decided that since I started this post on the Aikido phono stage, I should continue with phono stages.

 

Balanced Phono Preamplifiers
Recently, I have been getting e-mail about balanced phono stages, so I put my mind to work again. Mind you, I did a lot of experimenting with balanced phono stages 20 years ago. See tubecad.com/july99/page10 and blog0120. What must be achieved is a non-gratuitous balanced design, which means that balanced portion is not just duplicated (doubled-up) existing unbalanced phono stage, such as the Tetra or Aikido PH-1, but a design that uses a balanced approach to produce greater gain and CMRR—if only in enhanced PSRR. Just doubling up on a Tetra or PH-1 would be a gratuitous balanced design (and a great waste of many tubes). So what would a non-gratuitous balanced design look like? Here is an example from blog post 120:

I just love the look of the thing. Just one gain stage and one buffer stage. Passive equalization. Step-up transformers, front and back. One reader told me that he plans on building this design, but I haven't received any updates yet. The only thing I would change is the ground connection on the phono cartridge, as it undermines the magic that balance creates. (The secret to balanced signal transmission is equal impedances from both signal wires relative the the balanced amplifier's ground.)

Or, if we do not feel so bold, the following balanced design makes use of only an output transformer, which would allow either single-ended or balanced output.

Click on schematic to see closeup

The two resistors across the cartridge create two phases of signal, but of half amplitude of the single-ended signal. Only four tubes per channel are needed and the output transformer adds +12dB of gain. In addition, it adds the missing CMRR (common-mode rejection ratio), which is an essential aspect of balanced designs. Common-mode noise from the pickup cartridge and power-supply noise, which should be common-mode, are ignored by the transformer. Note the large 10µF coupling capacitor to the output transformer. Why so big? The output transformer's winding ratio of 1:4 not only results in a gain of 4, but also an impedance reduction of 16:1. So, a 100k line stage input resistance would look like a 6,250-ohm load to the coupling capacitor; a 20l volume control would appear five times smaller in impedance; thus, the big capacitor. Is a coupling capacitor actually needed, since both cathode follower outputs should be at the same DC voltage? Sure it is, as the cathode voltage are not likely to be exactly the same and even a 1Vdc difference can saturate some signal transformers.

We can even add a negative feedback loop or, rather, two negative feedback loops.

Note the -6.3Vdc power-supply rail, which is needed to give the constant-current source (CCS) more voltage headroom within to operate. Moreover, it could power the tube heaters. Also note the 10µF capacitor that effectively ties the two cathodes together (for audio frequencies, but not DC voltages) and thus improves the CMRR of the second gain stage.

 

 

Aikido Cartridge Configuration
Think about it: the great thing about the phono cartridge is that it is naturally a balanced signal source. In addition, the cable is short and no power source is needed. In other words, we already get most of the benefit from the phono cartridge's own intrinsically balanced nature, even with our unbalanced phono stages. Since the cartridge floats, i.e. it is not tied to ground, however, we can do something sneaky with it, such as the following.

The MC phono cartridge is no longer grounded directly; instead, it floats atop the 1k resistor. The 48.7k resistor allows a predetermined amount of power-supply noise to leak through, which will be both amplified and inverted at the plates, so a noise null obtains, as the amplified anti-phase power-supply noise will meet the in-phase power-supply noise and cancel; 1 -1 = 0.

What would Ziggy say? is a always a good question for me to ask myself. He would probably say, "Sure, it is clever, John, but you know that the free-floating aspect of a phono cartridge coil is not limited to use with balanced input stages, as a single-ended stage could also benefit from its floating feature." And he would be right.

The above MC pre-preamplifier employs the same Aikido noise-null technique. How simple can a circuit get? Just one 6DJ8 and a handful of parts and we have lift off. Of course, we are not limited to MC pre-preamps, as this technique could be used in typical +40dB phono preamp, as long as the cartridge is truly free floating and not grounded at the tonearm.

Okay, I believe that I have my contractual obligations for a full post, so I will sign off now, as at my back I hear the sound of time's winged chariot approaching and my kids must be driven somewhere fun.

 

Next Time
Many new developments.

    

User Guides for GlassWare Software
Just click on any of the above images to download a PDF of the user guides. By the way, all the links for the PCB user guides shown at the right now work.

Since I am still getting e-mail asking how to buy these GlassWare software programs: