Headphone Hybrid Tube Amp (SSMH)

For audiophiles and DIY hobbyists, this amp is the mecca of affordable bang for buck audio awesomeness. This amp can be built for dirt-cheap with great results. Don’t get me wrong, there are better amps out there that are more fine tuned to you and your listening style, but it comes with a very hefty price tag. For those of us still wanting great sounding audio and a low price tag, this is it. Let me introduce you to the "Starving Student Millet Hybrid" tube amp, or abbreviated as the SSMH.

Instructables has very limited information on headphone tube amplifiers. So I have decided to help fill the gap and hopefully inspire some other builders and audiophiles. The build I have chosen is the “Millett Starving Student Hybrid Tube Amp”. Reasons being, it’s cheap (can be easily done for less than $125 for all materials except tools), sounds great, has a simple circuit and has an almost cult like following. Online support for this amp is huge, which makes questions and troubleshooting a lot easier.

There is a surplus of information about the schematic and how to wire the circuit online. So I will attempt to give you the resources about the building process excluding the schematic.

I would like to thank Pete Millett for his development of this amp and schematic. He is a brilliant and generous man and deserves credit.

Important Links:

Pete Millett’s original post:
http://www.pmillett.com/starving.htm

Head-Fi thread:
http://www.head-fi.org/t/319231/millett-starving-student-hybrid-amp

DIY Forums:
http://www.diyforums.org/SSMH/SSMHoverview.php

Bill of Materials:
http://www.diyforums.org/SSMH/SSMHbom.php

Schematic:
http://www.diyforums.org/SSMH/variants/SSMH-12AU7.gif

Pete Millett’s design uses a pair of 19J6 tubes for amplification, however they have since almost disappeared from every retailer and auction site in existence. Don’t even bother trying to find them, people have been trying for years without luck, there’s an easier way. With credit given to Dsavitsk – he slightly modified the schematic to use a pair of 12AU7 tubes that are still readily available all over the place. Here is the schematic: http://www.diyforums.org/SSMH/variants/SSMH-12AU7.gif Also, note the highlighted optional components. They are not essential for a functioning amplifier but will add enhanced audio output and help buffer the pot (volume control).

Now lets look at materials. Here is the standard Bill Of Materials (BOM).

http://www.diyforums.org/SSMH/SSMHbom.php

My amplifier uses the exact same electrical components as above. However, I wanted to go above and beyond for sourcing an enclosure, heat sink, tubes and other materials not involved in the electronics. It was important for me to achieve an overall look that fit my tastes.

While looking at pictures, I became increasingly unimpressed and decided that there had to be a better enclosure option out there. After much distress I found what I finally was looking for. Hammond makes an enclosure designed for DIY guitar pedals that’s already polyester powder painted! It’s perfect. I then moved onto the heat sink and found one that fit the bill. Finally some awesome classic switches, aluminum milled knob and some sweet input/out connectors.

Enclosure:
http://www.hammondmfg.com/dwg_SBVer.htm
Knob:
http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMuiwDVLTMm01SnXt%252bxMNAYRiE4hqsuOv%2fk%3d
Heat Sink:
http://www.mouser.com/ProductDetail/Wakefield/641K/?qs=sGAEpiMZZMsv9efUTL1hjoRaZH8LLHQ7
Heat Sink Mounting Grommet:
http://www.mouser.com/ProductDetail/Keystone-Electronics/730/?qs=sGAEpiMZZMsQtlBhqKq43fwJXtsOdb9G
This site has some cool connectors and tube sockets:
http://beezar.com/catalog/index.php

We will now begin to drill our holes in the enclosure. My best advice is to wrap the entire enclosure in blue painters tape. You can now use a pencil to lightly mark measurements on your enclosure where holes will line up and need to be drilled. The blue painters tape also helps to prevent scuffs and scratches during drilling.  I also recommend using a caliper to measure the size of all your connectors, switches, tube sockets and other various mounting holes. Use that same caliper to judge the size of your drill bit. It’s the straightest forward/hardest to screw up method that I have found. It would also be wise to invest in a stepping drill bit if you don’t already have one for the larger holes (ie tube sockets). Good luck and take your time.

Time to wire. I am assuming that you are capable of reading a schematic and then knowing how to wire it. This circuit is easily made possible point-to-point. I debated with the idea of printing a circuit board or using proto board. However, after I built the circuit on breadboard, I realized how finicky the circuit could be. For instance, if a specific wire were too long or in close proximity with another wire weird stuff would happen. For instance I was able to pickup a radio station! Thankfully it was nice smooth jazz. But more commonly I would get humming in one or both channels. It took some time but I finally found the perfect layout for total silence from the circuit in my enclosure.

Some tips include:
• Braid or twist your input cables off of the RCA inputs. Braiding/twisting helps to eliminate any RF interference.
• Braid your LED runs into the tube sockets. For some reason the negative lead picked up some RF interference when near the tubes. Braiding the leads solved this issue.
• Check for shorts. A couple of times I fired up the amp in the preliminary phases and I would get clicking or dead silence. It was a short every time. One loose strand of braided cooper wire touching a resistor or something stupid. More than likely the error is human, the chances of a failed resistor or MOSFET are incredibly low.
• Use the optional resistors from the Dsavitsk schematic; they help eliminate humming and background noise.
• Lastly, keep your runs as short as possible. Audio circuits are finicky.

So, is the heat sink really necessary? Yes, the MOSFETs melted my plastic breadboard after less than a minute of use. Those guys get hot. You don’t want to melt your MOSFETs and risk destroying your circuit. 

The added LED’s were a must for me. I love the way that they look. The tubes do glow, however they’re hardly noticeable unless you’re looking for the filament. Plus the LED’s allow seeing if the circuit is on/off. I also added an additional switch just to turn the LED’s off and on. If you are wiring in the LED’s, make sure to use a resistor rated for the voltage drop. The power supply is 48v, that’s a big drop down to 3v to power the LED’s. The resistor needs to be able to dissipate that wattage. Use a resistor calculator to calculate the wattage rating needed for your resistor (http://led.linear1.org/led.wiz). I used a resistor rated for 2 watts, well above what should be required.

I hope this instructable has been fun and possibly inspired you to build one yourself. Here are some more pictures! Happy building and listening.