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A while ago I bought the following headphone amp on Ebay as it looked like a nice small project. The included case and battery are also quite nice and the price was very affordable. A few weeks later the parts arrived in a very recognizable china envelope. In the package I found, exactly as pictured on Ebay, A nice looking PCB, a bunch of parts, a battery and a case. The first downside is the lack of documentation. There are no build instructions, making it a bit less nice for beginners.

All the values of the parts are printed on the PCB silkscreen and written on the parts whenever needed. So building it takes a bit of figuring out but is very doable. 2 issues arise fairly quickly though. The diodes have no anode or cathode marked on the PCB and the IC that is surrounded with diodes has the marking removed. I think it’s clear where this is going, I soldered all three diodes incorrectly and after correcting the IC got insanely hot in less then a second. Lovely.

The rest of the PCB went OK to build, the parts are from a good quality brand and the PCB looks nice with the gold plating on it. The only issue where the diodes.

But now I’m stuck with a non working PCB as the IC that generates the voltages for the opamps is well, dead. After emailing the seller they couldn’t give me the schematic. So well, time to reverse engineer the schematic :)

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My current power amplifier is very simple. It has 1 input, 1 output and 1 LED to show it’s on. Which was fine until I got a record player. This required me switching cables and without a volume control between the record player and the power amplifier the only volume I had was well, deafening. Time to build a small preamp with the following features:

• 3 inputs, PC, record player and an extra one.
• Volume control
• Buffer build in so a headphone or low impedance amp can be connected.
• 1 rca output to go to my power amp and 1 3.5mm output to connect a headphone, switch able.

I had almost everything laying around, a pre-build PCB for a headphone amp based on an LME49710 opamp and a LME49600, an enclosure that should just fit the components and a 2*12V transformer for powering the thing. Sadly enough the enclosure was just too small, the pre-build amplifier oscillated and got way too hot to be usable.

After a some cursing and a look in my random parts bin I found 2 OP07 opamps, precision opamps with specs more then good enough for a simple audio application. It’s a bit old and a bit slow compared to newer opamps but it should do the trick. I also found 2 perhaps older buffers. The burr brown 3329/03, now replaced by the, still manufactured OPA633 buffer. The specs are overkill compared to the OP07 but hey, 100mA output current, short circuit protected and a full power bandwidth response of 1Mhz. And that in 1988. The datasheet can be found here.

First step, breadboard it:

Well, that worked, no oscillations but it does look like spaghetti, luckily I had a PCB breadboard from Adafruit laying around.

Well isn’t that much tidier :-)

To make this a pre amplifier some things still have to be added, an input selector, a volume potentiometer, an output switch and of course a power supply. Fast forward a weekend and it’s finished. And I again got reminded that I should order enclosures instead of doing it myself as everything is as straight as the gay parade :-(

Ah well, at least I did a half decent cable job I’d say. The schematic is the one from the datasheet of the buffer, a simple non inverting opamp, in my case with a gain of about 4. Power supply is a simple 7812/7912 build. I was missing a few RCA plugs and will probably make a new back for it as the RCA plugs are way too close to each other. I still need to drill mounting holes for the PCB. The amplifier sounds nice and very clean and it has more than enough power for my 250 ohm headphones.

And an action show:

Now time to finish my tube headphone amplifier and see what sound I prefer :-)

The amplifier can run on a single 300V power supply. A downside of most tube amplifiers, this one included is the poor power supply rejection rate, or how much noise on the power supply ends up in the output signal. The White cathode follower has a fairly good PSRR for a tube amplifier but the ECC83 input stage has a poor PSRR. To test this in simulation I used a 310V power supply with a 1V 100hz or 10Khz sine wave on it. With no input voltage applies a 10mV sine was visible on the output at both 100Hz and 10Khz. This is about 100x less then the power supply noise or an PSRR of 20db. Compared to the 100db PSRR opamps can reach 20db is fairly low.

To make sure there won’t be a audible 100Hz noise from the power supply on the headphone the power supply has to be well designed. With 32 ohm 100db/mW headphones an 1mV 100Hz signal still generates 0.3nW, 30nW sounds like nothing but with 100db/mW this equals almost 40db, enough that it is audible. With 20db PSRR this means that 100mV of noise on a 300V power supply is hearable in some headphones. A standard tube power supply with a few big caps and resistors is not going to cut it here.

The better option is a regulated power supply, a bit like an 7805 or LM317 power supply but then running at 300V. As a normal LM317 is not capable of going up to those volatages without some tricks it’s time for an MOSFET follower. There are more then enough cheap MOSFETs that have no problems with 300V or more, even at 100 or more mA’s. Add a few high voltage zener diodes and a simple regulator is born.

After rectification and the big 47uF capasitor there still is 15Vpp of 100Hz noise at the input of the MOSFET. At the output with an 100mA load, just 40mV. As the tube amplifier uses about 50mA’s this power supply is good enough for the job. With different zener diodes different output voltages can be used making it a versatile schematic.

Tubes need to get warm and cozy inside before they start working so an 30 second delay was added using an NE555 timer and a relay that run of the 6.3V filament power supply.

An LT1085 was added to regulate the 6.3V filament power supply. After all the effort for a clean 300V the last thing we want is the tube filaments ruining our amplifier.

PCB’s where made for the power supply and ordered at elecrow in china. A few weeks later they arrived and building and testing a power supply was a piece of cake.

Everything worked as expected, the only issue is that I forgot a resistor that empties the big caps over time. Luckily the tubes use a constant 20mA per channel so when the power supply is in use this shouldn’t be an issue. The Eagle files can be downloaded here and are CC-BY-NC-SA licensed.

As a hobby project I decided to build a tube headphone amplifier.

Why tubes you might ask, well, because they are awesome. Looking at efficiency, distortion etc transistors are vastly superior. But tubes are fairly simple to work with, look cool and it’s something different then all the opamp designs.

Requirements:

• Can drive 32 ohm headphones
  • Some tube designs without an output transformer has issues driving low ohm headphones, as I also use normal 32 ohm headphones I need to have an amp that can drive them
• Can drive 250/600 ohm headphones
  • As I also use a Beyerdynamic 990 pro with an 250 ohm impedance the amp also needs to be able to drive these headphones.
• Low(ish) distortion
  • As it will be a tube headphone it’s impossible to get the ghastly low distortion figures that are possible with an opamp or transistor design. Still the distortion has to be low
• Low noise
  • headphones tend to be extremely sensitive, 100db of sound with just 1mW of input power is nothing special. So the noise on the output has to be low, even 1mV of noise will be heard through some headphones.
• Low(ish) price
  • It’s still a hobby project and I’m still a student. I can’t really spend hundreds of euros on an amplifier.

Circuit topology:

I want a fully tube design, not a hybrid design. Getting tubes to drive low low impedances like 32 ohm is not a problem with an output transformer. The downsides of output transformers is that multiple tabs are needed when you want to drive 32 and 250 ohm headphones, meaning they will have to be custom made. This means they will be expensive and hard to get.

An output less tube amplifier is quite common for headphones, designs using an 6AS7 or 6080 tube as cathode follower with half a tube per output are common. The downside is that the 6AS7 needs quite a bit of juice to get glowing, 2.5 amps to be precise. The other downside is the distortion. Simulated it went over to 1% at a 1Vpp output.

A better approach is using a so called “White cathode follower”. This is a topology that uses 2 tubes per output in a push pull configuration. This topology can deliver more current then an cathode follower and has a lower output impedance. It is a bit complexer and it requires two tubes per output but it has no problems driving an 32 ohm headphone. After a bit of searching online I found this schematic using an ECC99 tube per channel:

They claimed it can deliver 2V rms into an 32 ohm load. This is more then enough to generate over 120db of music. The other nice thing is that I had a few 6N6P tubes laying around. These cheap Russian tubes are very similar to the more expensive ECC99 tubes. I didn’t have an ECC82, only an ECC83, a tube with a much higher amplification factor.

Luckily this design uses some negative feedback to lower the output resistance, meaning the amplification factor is adjustable. After messing around a bit in LTSpice I came up with the following schematic:

It can deliver 45mA’s or about 3Vpp into an 32 ohm load or a massive 20Vpp into an 250 ohm headphone. 600 ohm headphones can expect a silly 42Vpp, which will fry most headphones with ease. Simulated distortion levels are low for a tube amp. About 0.3% @ 2Vpp in 32 ohm or 0.1% @ 10Vpp in 250 ohm. That looks good enough to try in real life.

The ECC99 design is designed by Menno van der Veen and found on his website which has a really nice tube preamplifier: http://www.mennovanderveen.nl/eng/ts-vv-2006.html