The 6DJ8 Headphone Amplifier
While surfing the net one evening, I stumbled across this interesting post about a vacuum tube headphone amplifier designed by Bruce Heran and posted to the DIY Audio Projects Forum. After spending some time reading through the post and checking out the schematic, I thought I'd build one!
Quoted from Bruce:
The amplifier will deliver over 100 mW of output into pretty much anything from about 32 ohms to 300 ohms. With the Edcor XSM10K/150 audio output transformers the optimum match is at 75 and 150 ohms. Using headphones with an impedance below 32 ohms will result in decreased performance as the load on the tubes will be below the best range. Above 300 ohms it will also be less than the best match, but many phones in that range seem to be more sensitive than those with lower impedance and it may not matter. The frequency response at the 50 mW level is from 20 Hz to 28 kHz within 0.5 dB and the wide band signal-to-noise is at -84 dB. I didn't run a full distortion series on the amplifier as the output levels are rather low (voltage wise) and the ambient EMI in my shop would tend to confuse the results. On a spectrum analyzer I was able to get as much as 2 volts output before there were obvious harmonics appearing on the display. At more typical listening levels (about 0.1 to 0.2 volts into 100 ohms) there was no sign of any harmonics above the noise floor. I would conclude from this that the actual distortion levels at such levels are well below 1%. Power consumption of the headphone amplifier is approximately 20 Watts (W).
What I liked about this headphone amp was the simplicity in the amplifying circuit. No coupling capacitors to "color" the sound and no negative feedback used. I also liked the ideas he used in the power supply, adding voltage regulators in the cathode supplies, and using filtered DC on the tube heaters. All great ideas for clean audio.
The only thing I didn't like was the solid-state power supply. While there is nothing wrong with using diodes in the B+ supply, I've always liked using full tube rectification in the audio chain. So, I kept Bruce's project bookmarked and in the back of my mind.
Then I ran across Matt over at cascadetubes.com. Matt built his own version of this amplifier using a 6CA4 (EZ81) tube for B+ rectification! Almost perfect for me. I liked the idea of using filtered DC on the tube heaters. This is a headphone amplifier where listening through headphones is a more intimate experience than listening from speakers across the room. Any extra filtering added, even in the tube heater supply, could only lead to a quieter noise floor. While I'm not sure I could hear the difference between AC and DC powered tube heaters, why take the chance?
So, I took the best of both designs and built this:
The tube filaments are powered from filtered DC provided by an upgraded Edcor XPWR024 power transformer and a 10 amp bridge rectifier. Adding the extra EZ81 rectifier tube increased the heater supply load by 900mA or so, so the upgrade in heater power (6.3VDC) was needed. I also omitted the voltage regulator.
The B+ side of the power supply is pretty much Matt's design. I used a 6H choke instead of an 8H choke simply because I couldn't find one. The 6H works fine.
The rest of the circuit is Bruce's design. I only modified the power supply to suit. After reading through the forums, I found a few posts talking about the placement of parts, especially the power transformer, could affect the performance (noise floor, hum, etc.) of the amplifier. So, I tried to make good use of placement and space. Still, this is a big box headphone amplifier!
The chassis is 1/8" stainless steel, laser cut to my design and simply folded down at the ends for a front and rear apron. All nuts and screws used are stainless steel. All filter capacitors are Sprague Atoms. The volume potentiometer is an Alps Blue Velvet. The switch on the front panel is to select high or low impedance from the output transformers to the headphones.
I asked my friend, Mike Amos to craft me a nice wood base for the amp chassis and he did not disappoint.
So, how does it sound? Well, really good with a few considerations. With no input, I do get some hiss and a slight hum above about 2/3 of the way through the volume control. However, with decent input signals, the volume level at this point is so loud that I don't think anyone would hear it. I can easily say it doesn't bother me at all and doesn't distract from the audible performance of the amp.
It's simple, single gain stage works quite well but it's really not enough to properly amplify input signals from low-powered devices like phones and tablets, a common use for a headphone amplifier. Keep this in mind! Connected to proper line-level devices (~2V signal) like CD players, radio tuners, DVD players, etc. the amp sings nicely!
Conclusion: To me and several others, the elegance and cleanliness of the amplifying circuit and the care in designing a proper power supply makes this amp worth building. With no coupling capacitors in the signal path and no feedback used in the circuit, what you hear is 100% your vacuum tubes! The sound quality depends on the tubes you use so you can "tube roll" all you like and really tailor the amp's sound character to suit your listening preference. And that's just cool!
The amplifier doesn't have enough gain to work well with low-powered devices like the headphone output from cell phones, tablets, and the like. These devices are designed for maximum battery life using little earbuds, not driving power amplifiers. Most headphone amps out there have enough gain to compensate for the low signal drive these devices produce, this amplifier does not. Yes, it will work and produce the tubey goodness we're looking for, but it will not "crank up" like one would expect. Using proper RCA (line-level) devices that drive the amp right will produce plenty of power and get loud enough to drive just about any headphones you plug in!
If you are thinking of building one of these amps, make sure you read through the forums first. There are several tips and considerations posted from others whom have built one that are good to know. If you have any questions or have built one of these amps, drop me a line! I'd like to know how it went and your impressions on the amp.