- Category: Headphone amplifiers
- Published on 23 July 2013
Before I started doing DIY audio the DIY headphone amplifiers always caught my attention. These seemed quite easy to build, making them ideal devices for a first DIY audio project. However my first project became my CD-Pro transport. After having build a CD-transport, amplifier and DAC I decided that the time had come to start building a headphone amplifier. The amp had to contain some tubes. Not that I prefer tube amps over other amps, but it seemed fun to me to build a tube based amplifier. The Millett Hybrid MiniMAX, sold at Beezar, seemed like the best choice for a first tube amp. The MiniMAX is a descendant of Pete Millett's Low-voltage hybrid/line amp. The amp has a tube stage followed by a BJT-input/output totally discrete Diamond Buffer output stage. It's save to build and well documented. Lots of information can be found at the diyforums MiniMAX pages, so I'm not going too much into the details here. For support one can visit the Head-Fi - New Millett Hybrid MiniMAX thread. The seller (tomb) gives very good support to those who build the MiniMAX. Now you're probably still wondering why it's safe to build? That's because unlike most tubes the typical "Millett" tube is generally rated for 12V on the heater, and from 12V to 30V on the plate. Most other tubes operate at much higher and lethal voltages!
The MiniMAX can be bought as a kit or single PCB. I choose the second option. I had a chassis lying around and sourcing the parts myself turned out to be a lot cheaper. The BOM is available on the diyforums MiniMAX pages complete with Mouser and Digikey part numbers. I ordered most parts at Mouser, except for the Alps potentiometer, tubes, tube sockets, output coupling capacitors and matched transistors. These where bought at Beezar.
There are generally three tubes referenced for the MiniMax as can be read here. I choose to go for the 12FM6 tubes, which have a basic amplification factor of 10. There are also the 12AE6 and 12FK6 tubes that can be used with the MiniMAX. These have a basic amplification of respectively 14 and 7. The next three quotes come from the Beezar webshop:
It's been said that the 12AE6 has the most punch because of that gain and can really drive the bass frequencies, but still with good highs and details. This contrasts well with the 12FK6, which is known for "airiness" and extended highs, but may lack the low end punch of the 12AE6.
It's been said that the 12FK6 has the most "air" and extended highs of Millett-style tubes. However, it may be a little light on bass, depending on your particular setup. It and the 12AE6 are a good contrast to one another.
Typically speaking, the 12FM6 has been described as a combination of the 12AE6 and 12FK6 tubes - good highs, good punch. They also happen to be the rarest of Millet-type tubes.
The 12FM6 seemed the best choice to me. Unfortunately I was not aware of the tube matching service at Beezar when I ordered the tubes, but luckily I got tubes from the same manufacturer. The tubes are rated 90 and 85 MicroMhos, which is within the 10% range, so they should be OK. Later on I did order some matched 12FK6 tubes. I will try these after some time to see which tubes I prefer. At the time of writing this article I still have a pending order for two sets of matched 12FM6's. Using matched 12FM6's will make a better comparison with the matched 12FK6's.
Different BJT output transistors can be used for the Diamond buffer. Each witch their own sound characteristics. I choose to use the 2SC2238's and 2SA968's from Toshiba. Some reviews on the sound characteristic from all output transistors can be found here. This is what the review says about the 2SC2238's and 2SA968's:
Neutral and balanced sound, tight bass. detailed and clear highs without being bright, easy to listen to. Excellent choice, but note reversed pinout!
These Toshiba transistors can be a little bit hard to source. When searching on ebay lot's of fakes are found. That's why I ordered these at Beezar. Beezar does also match the transistors, so enough reasons for me to pay a bit more.
For the coupling capacitors at the output I used some 470uf Nichicon Muse ES's bypassed with Spraque 196P 0.22uf Vitamin Q oil in paper caps. The Muse ES is a very good capacitor for audio, just like the Blackgates. The Blackgates are hard to source and very expensive. The Muse ES bypassed with the Vitamin Q's is said to be equally good performing in the signal path as the Blackgates. Like the Blackgate capacitors the Vitamine Q's are not manufactured anymore. I bought mine at Beezar, but currently they have only a small stock left of the 0.22uf Vitamin Q's. Some can still be found on ebay at the time of writing this article.
Instead of using a so called wall wart power supply I choose to use a toroidal transformer. I had one lying in a drawer waiting to be used. These transformer also outperform most or all wall wart supplies. It's a 2x24Vac (225VA) toroidal transformer from Velleman. I know that it's overkill using a 225VA transformer, but hey... at least I did not had to buy a new one.
The chassis is made out of a steel bottom and cover plate and aluminium front and back plates. It can be bought at Conrad Electronics. It's a nice chassis for such projects, but it's not very sturdy. The back and front panels are a bit wobbly when plugging or unplugging the power or RCA plugs. I reinforced those panels by gluing some metal strips to the inside of the back plate. The chassis has some venting holes on the bottom and cover plates. I drilled and filed two holes in the top cover for the tubes. The tubes would not fit in otherwise. They also need to dissipate quite some heat, so it's better to air the tubes. It also looks quite nice to have the tubes sticking of the chassis.
After a ran the amp I noticed that the air temperature inside the chassis raised way too much. The venting holes where obviously not letting in enough cool air. The original MiniMAX chassis has lot's of large venting holes and for a good reason. The LM317 voltage regulator from the power supply, tubes and output transistors at the diamond buffer stage all produce a lot of heat. Opening up the existing venting holes and drilling in a row of 6mm holes in the bottom did the trick. It still warms up, but certainly not as much as before adding the extra ventilation. Two isolated RCA jacks, three tip jack for measuring tube bias and a FM1275 flange socket (normally used as speaker connector, but now used as power connector) are mounted to the back panel. The headphone jack connector and potentiometer (both from the amp's PCB), power switch and LED socked are mounted to the front panel.
The large toroidal transformer was certainly not going to fit in the same chassis as the amp, so I bought the same chassis for the transformer. The 24Vac output of the transformer is connected to a same FM1275 flange socket as used in the headphone amp chassis. The connection between the transformer and amplifier is made using the FM1255 cable connector. Although these are nice connectors they are a bit large for a headphone amp. If I was to do it all over again I'd certainly choose a smaller type of connector and a smaller transformer that would fit inside the amp's chassis.
How does it sound?
Not going in to much details here as the perception of sound differs from one person to another, but I can say that I'm very pleased with the amp. The only headphone I own is a Sennheiser HD595 and no other headphone amps to make a descent comparison, so this is only based on my experience with the MiniMAX with the HD595. At the time of writing I had made a good comparison between the 12FM6 tubes to the 12FK6 tubes, so this small review applies to the 12FM6 tubes.
To my ears the amp sounds very sweet and detailed. Especially women's voices do sound very nice. It has a nice and tight bass, certainly not boomy. It's performing at it's best at the mid and high frequencies, meaning that at these frequency ranges the amp sounds very rich and sparkling. I noticed that switching to other tubes can make quite some difference in sound. I'm currently using it with two unmatched 12FM6 tubes resulting is some imbalance between the channels. One channel is a tiny bit louder than the other. This problem will most probably be solved when I receive some new matched tubes.
At after power-up the signal is switched to the headphone using the relay. When the relay actuates it causes a soft click through the headphone. When the power is switched off the click of the opening relay is a bit louder. It certainly won't kill you headphones or ears, but it's probably best to plug in the headphone after the relay has actuated and unplug it before switching the amp off.