Something A Little Different
I have never been fully satisfied with the designs of most vacuum tube headphone amplifiers. The reason for this is because so many of the typical designs have some serious shortcomings.
The typical cathode follower designs tend to be high distortion and sensitive to headphone impedance. The more exotic types, including the White cathode follower and the popular but misnamed SRPP, tend to be highly sensitive to component and tube variation. And both types of designs require large output capacitors that distort the sound and suffer high leakage currents; resulting in DC offsets which stress, and can even destroy, headphones. Most designs that use output transformers tend to be noisy and are required to be closely matched to the intended headphones. Hence, these designs tend to be only medium fidelity and lack versatility.
So imagine my surprise when I discovered a design that does not suffer any of these shortcomings. This is a transformer coupled design that is high fidelity, low,noise, works well with a wide range of headphone impedances. The minute I examined the schematic and realized its secret, I knew I had to build it!
The Electrical Design
The design for this particular amplifier comes from Bruce Heran over at OddWatt audio. This is a design that Bruce shared on the DIYAudioProjects website back in 2012. At the time I didn’t fully appreciate the unique feature of this amplifier. If I had realized how this circuit really operates when I first saw it, I would have built it far sooner. As it is, it took me several years to finally complete my build. Here is the deceptively simple schematic Bruce shared on the project page.
This design uses one 6DJ8 dual triode for each channel. The triode is wired in a push-pull configuration with a small line matching transformer. In the common (between the triodes) cathode circuit is a solid state three terminal voltage regulator wired as a constant current source. And that’s all there is to it. Add a few resistors, a couple of decoupling caps, and a volume control and the amp is complete.
Now this begs the question as to how so simple an amplifier can be as good as it is. And this is where the “not so” secret feature of this architecture comes into play. Those with sharp eyes will recognize this as nothing more than a simple, symmetrically loaded, differential pair with a current sink in the common tail. This is actually the exact same architecture used in the input section of virtually every solid state operational amplifier made today.
Those who have designed differential pair circuits with current sinks in the tail, whether with transistors or tubes, will know that this circuit has some very nice characteristics. It is inherently low noise with high common mode ripple rejection. It is very well behaved so long as the load is balanced, which this one is by definition. And here is the most important feature, it is inherently insensitive to load impedance variation. So this simple circuit fixes all the problems with those other headphone amplifier architectures. It is low noise and well behaved and hence high fidelity. It is well behaved electrically hence it is not sensitive to variation in tube parameters. And it is insensitive to load variation, so long as load symmetry is maintained, and hence it works well with a wide range of headphone impedances. This circuit is not simple, it’s elegant!
So I decided that I would build this amp and put my own twist on it. I decided to use vacuum tube rectification but I wanted to keep the cost down. So I chose an open frame power transformer from Edcor that perfectly fit the bill; the XPWR024. It has a 350vct (175-0-175) @ 60mA coil and a 6.3vct (3.15-0-3.15) @ 2A coil. Each amp channel draws 25mA and the total filament draw is 1.6A leaving some margin for an indicator light. Here is the schematic for my power supply.
The amp circuit only needs between 130v and 150v B+, so there’s a fair amount of dropping resistance to get the output voltage into an acceptable range. Bruce provided his own power supply design on the DIYAudioProjects project page for those looking for a solid-state solution.
The build for this amp needed to have a relatively small footprint so that I can use it on my desk at work. So I decided that I would mimic the very successful 6EM7 vertical map build. The total footprint on this amp is only 12 inches wide and 6 inches deep. The exposed front panel is 10 inches by 10 inches. This is a very nice size overall. Here are the complete chassis dimensions.
I decided to try something a little different in my material choice. The amp is made of a single plank of purpleheart (Peltogyne spp.) that had been sitting in my lumber rack for over 20 years. The wood was all planed, cut, and sized at the same time to ensure perfect color match. When the amp chassis was assembled, it got five coats of rubbed oil giving it a deep sheen. For the metal, instead of going with something like painted aluminum, I decided to go with buffed copper plate clear coated for all the exposed metal plates. This combination, oiled purpleheart and buffed copper, yielded a striking result that I really like.
As with the 6EM7 amp, the tubes all fit in cubby holes made of purpleheart with copper plates above and below. These can then be prewired prior to installation into the chassis. Here they are before I started wiring.
The small circuit board containing the two LM317T regulators can be seen mounted on the bak of the signal amplifier assembly. I like to do most of the work on these before they are installed in the chassis. Here is the amplifier module all wired and ready for installation.
In this picture you really get a feeling for how simple this circuit is to construct. Each channel has only two resistors and one capacitor for discrete components. The red leads are the B+ for each channel and the green lead is the signal ground. The twists at the bottom are the input signals for each channel (blue/black and purple/black) and the filament feed (green/black) for the two tubes. This amplifier module is completely wired and ready for installation into the chassis.
The chassis was sized so that everything sits neatly inside. Here is a view from the back with the rear cover removed.
Here the main power transformer are to the lower right and the power supply rectifier and filter circuitry are on the lower left. The amplifier section is all wired in on top. You’ll notice that I have all three leads off the transformer secondaries wired into the chassis. This is for the small switch on the right. This is a DPDT switch to use either the high or low impedance taps off the transformer.
I should note that there is a slight error on the schematic. The output tap labeled 75Ω should really be labeled 37.5Ω as this is half the number of turns as the total and hence one quarter the impedance. However, the design is so insensitive to load impedance that treating these two positions as just “Low” and “High” is acceptable. Headphones lower than about 100Ω should use the “Low” setting and headphone higher than 100Ω should use the “High” setting. Edcor also has a 10kΩ:600Ω matching transformer that would present 150Ω and 600Ω outputs hor those using higher impedance headphones.
The power and signal inputs are all on the left side of the amp (looking from the front). Here is a shot from the back showing the side panel and the controls on the back.
The ICE power connector and the fuse holder on in the lower cutout and the signal inputs are in the upper one. In this picture the High/Low impedance switch and the power indicator intensity control are also visible on the back.
So the only question that remains to be discussed is “How does it sound?” The answer is very good. This amplifier has none of the drawbacks of the others I have built. It works well with several different headphones I’ve tried, the frequency response is excellent, and there is plenty of power to drive all of my headphones to a very high output level.
This was a fun project to build and very satisfying. If you are in the market for a solid and very high performance headphone amplifier, I suggest you give this one a try. You won’t be disappointed!