is a superior tube amplifier kit, which has many enhancements that allow you to enjoy exquisite tube audio in just a few nights.  For those who love headphone music, you are in for a big treat. DreamONE has everything you need which includes a chassis, power supply, PCB, Vacuum Tube (JJ Gold Pin Preamp), Capacitors (WIMA, TDK, Panasonic, and Nichicon), Transistor (On Semi) and all parts. Believe or not, C-100HA is capable of drive the Sennheiser HD-800 up to 110dB easily. 

It all began when I spent a ton of money on a ‘Great’ headphone amplifier. What was included in this supposedly hi-fi headphone amplifier was a single class-AB IC with 2 to 3 coupling capacitors, an OPamp with four transistors working as a buffer. Now its manual said that the headphone amplifier was capable to driving Sennheiser HD800 or equivalent headphone. That’s totally absurd! For example, HD800’s maximum long lasting input power is 500mW. To reach that output (of 500mW), your amplifier needed to deliver 35 Vpp (peak to peak voltage) for HD800! That means you needed an amplifier with at least ±18V working voltage (or a single DC voltage above 36V). Unfortunately, this is a common problem with high impedance headphones (Sennheiser HD800, or Beyerdynamic T1... etc.).

My dream headphone amplifier should consist of the most simplistic circuit as necessary (simple meaning using as few components as possible, and therefore creating less distortion), high out-put voltage and low output impedance running concurrently. Good harmonic characteristic is necessary for long-periods of listening and music enjoyment. The most important part is a wide frequency response that meets modern high resolution streaming music.

To meet these requirements, a class A, single-ended (SE) triode gain stage is a good start. Followed later by a PowerMOS source follower, which can deliver low output impedance to headphones.

The reason for choosing PowerMOS as an output device is because PowerMOS is a  “transconductance” device - a voltage controlled current device just like a tube.

This circuit (shows one channel only) passes many comprehensive tests. The input stage has a triode single-ended structure with constant current load, which provides good linearity and enough amplification gain. The constant current source is formed by a low noise, 1.7V red LED and Q5 transistor. A blue LED is series connected in order to light up the bottom of the tube. The output of this single-end stage is directly coupled to the Q6 transistor base, an emitter follower stage, then directly connected to the PowerMOS Q2. This provides enough driving capability to headphones. By using direct coupling, we not only reduced our cost, but also eliminate quality issues (for example: leakage current, ESL, ESR, etc.) from coupling capacitors.

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Headphones have a wide range of impedance from 20 to 600 ohms. A low output impedance headphone amplifier is essential for driving low impedance headphones. A good amplifier should keep low impedance in all frequency range to ensure the same driving power is sent to all music spectrums. Even up to 100 KHz, it should keep the output impedance as low as possible. So, we need a high frequency response as well as low total harmonic distortion (THD). An output transformer simply won’t do the job!

Here is how the circuit designed. The power MOSFET (IRF610) has characteristics of a tube, which has high bandwidth with over 300 MHz and high input impedance.  Adding an emitter follower in front of the MOSFET can decrease the PowerMOS input capacitance loading-effects, and also increases its bandwidth. The IRF610 has input capacitance of 140 pF. By inserting this emitter follower, it can improve the bandwidth significantly. 

By the experiments, show that the bandwidth was tremendously increased with the implementation. With high bandwidth and low harmonic distortion, the amplifier was able to reproduce full range melodies and recreate music coming out from the headphones.

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Adding a resistor on the emitter side of transistor (Q1) solves the current imbalance problem from the transistors. The two transistors (left and right channel) share one 13V Zener diode. This Zener stabilizes the voltage to 13V, which is necessary for the tube filament (12AU7 filament is 12.6V). Adding a parallel capacitor next to the Zener diode allows the capability of reducing breakdown noises from the Zener diode.

Why use transistors with Zener diodes rather than regulator ICs? It is simply, better than using complex voltage regulator ICs! I also prefer using US made components like On-Semi, old Motorola, etc.

A high level of experience and expertise is vital to designing the perfect amplifier circuitry. A good measure of results may not truly reflect a great amplifier. However, good measuring results are still essential for developmental purposes. I believe an amplifier with good performance on top of excellent measured data is achievable.

Taking a look behind the scenes, what is behind the circuit structure of building the perfect headphone amplifier? It all comes down to selecting the right components!

For my headphone amplifier, I choose the best linearity transistors from On-Semi. Specifically, we decided to use from On-Semi, the MJE15030, the 2N5087, and even the Zener diode. In addition, we chose PowerMOS IRF610 from Visay, audio grade volume controller from ALPS, very low high-frequency impedance and long life capacitors from Panasonic, high performance capacitors from WIMA, and 6.3mm Neutrik headphone connector.

Typically, it’s rare to see so many high-grade and high-cost components in commercial products. 

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As I mentioned previously, headphones show different impedance under varies frequencies and/or voltages. Unless you have a very low impedance circuit, your amplifier may experience varies gains under different frequencies.

In order to solve this problem, I added some reverse feedback that can further lower the distortion, increase the bandwidth, and lower our output impedance. The R18 and R19 form the reverse feedback network and the overall gain is controlled about 10dB. Actual lab measurement shows that it can drive loading over 30 Ohms and none of the performance is lost.

Why did we choose to use just 10dB? This portion is where you can get the best fine tune over the volume. It will not run above the destination gain when tuning up and will not cause too little gain while tuning the volume down.

The trouble of using the dual triode tube, however, is that each half of the tube may not perfectly match the other. In other words, it may generate different gains from each half of the tube output. Question is, can you tolerate the 1dB volume difference between your left and right ear?

Our enhancement of our feedback network has successfully reduced the gain gap down to less than 0.1dB! This is important since the use of headphones placed right above the ears. Any minor gain differences can result in poor performance. This is why the designing a speaker amplifier versus building a headphone amplifier is different.

Test results show that has a gain error less 0.1 dB. Its bandwidth can achieve more than 200 kHz and the two output channels are identical.  

You don’t want to buy cheap online headphone amplifiers just because the circuitries are not carefully designed. In reality, the same design may not have same quality from the components those cheap online headphones used.

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Base on calculations, a 42V power source should be able to deliver 7Vrms to the headphones and drive the Sennheiser HD800 up to 120 dB sound level. In order to adjust the plate voltage and maintain the maximum swing at this amplifier, I added a potentiometer at the cathode of 12AU7. As you know tubes are handmade (I think tube makers are artists!). Each tube has some different characteristics. By adding this potentiometer, it will help fine tune the operating points.

Some people try to adjust the resistor at the tube plate to get better results. However, it may end up with a large distortion. Like the old saying - the devil is within the details.

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The first figure is an example of a frequency response. It only drops 1dB when the frequency reaches 200KHz and will not be affected by the changes from the headphone’s impedance. This is very important when designing a high performance headphone amplifier. In reality, headphone impedance is not at a constant value. It changes along with different frequencies (just google the HD800 impedance graph!). A poorly designed headphone amplifier will result in a non-constant amplification when the headphone’s impedance is changing. Imagine the sound - seriously twisted. Add News Story here