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July 7, 2017

Fred's Amplifiers Portable 12AU7 Tube Headphone Amp

I was browsing around the net looking for a quick and fun little project when I discovered Fred's Amplifiers. Fred has a small site with a number of PCBs and kits, most of which are portable headphone amplifiers, along with a portable guitar amp kit and a couple of crossfeed units.



I decided to order the 12AU7 Valve headphone amp PCB. It's a single sided PCB and was only $10, with the BoM (bill of materials) posted on the site. I went with the PCB versus the kit as I have tons of parts lying about, but the kit is only $45 if you want to go that route, just note you need to buy the chassis (Hammond 1593) and A/C adapter (12v 1A Regulated DC supply, center pin positive) on your own. The unit only requires around 25 parts to complete, so you'll be done in no time. The Op-amp in the kit is the venerable JRC 4556 used in most CMOYs / RA1 clones and similar portable low-power headphone amps.

I went with the translucent blue version of the Hammond case (1593KTBU). Trimming into the plastic Hammond case requires you to go slowly on the drill press or the plastic will crack. You may want to buy two cases (they're $4 each) just in case your holes don't line up perfectly.





My unit features Takman carbon film resistors, a diffused 5mm amber LED, a milled aluminum knob from Kilo, and 470uf 16v Elna Silmic II capacitors. Elna Silmic II are widely accepted as the best electrolytic available for the signal path. They are a tough fit, and will require you melt down the PCB standoffs on the bottom of the Hammond case to get everything to fit nicely. You could also drill holes at the top of the chassis for clearance, but that reduces the portability in my opinion. The AC adapter I went with was a $10 Meanwell unit - SGA12U12-P1J. I'm sure there are cheaper, I just plugged the specs into Mouser and bought the first one on the list. 

Here are a few more photos of the amp:    










There's nothing challenging or unusual about this build, so if you'd like a fun and affordable little portable tube headphone amp, put this one on your short list. 


Please remember that building/modifying circuits can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. 

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