For sale is a hand built Class A, Single Ended, Zero Negative Feedback, all discrete, current source loaded 12AU7/MOSFET Headphone Amp/Pre-amp (wow that's a lot of buzz words!).HAND BUILT IN THE USA! Sadly, you don't hear the much these days. I always loved my old Yamaha Orthodynamic headphones and felt they had something special but frankly they can sound very unimpressive on most amps (no dynamics, muddy bass, sort of closed in sounding) But find the right amp and what a different beast! (they're open and detailed yet warm with a wide expansive soundstage and nice dynamics). So I set out to find the right amp. After owning a few commercial ones like the Musical Fidelity X-cans and Zero Dac etc... which sounded nice, but were somehow missing something I liked in my main system, I decided to build one similar to the kind of tube amps that I've built in the past and love and use regularly... a single ended design. But I didn't want to deal with the high voltages or size or heat from a large output tube so that's where the MOSFET came in. In this case the well reputed IRF510. The design is basically as simple as any single ended tube amp but uses a MOSFET for current gain and let's the tube provide the voltage gain. The beauty of this headphone amp is the current and the ability to deliver power to greedy headphones while still preserving the sound of the input tube. With 105ma on tap biased at around 16v on the MOSFET and a B+ of 32V you're getting tons of power to those headphone drivers and doing so with a low output impedance. I didn't reinvent the wheel. This amp is very similar to the one by Rogers Gomez on the DIY Audio Projects site which was based on a design by Greg J. Szekeres. However, all the operating points and thus component values are different (approaching 3 times operating voltage) and the layout is done all point to point like the vintage tube amps (hey why not the circuits simple right!?). I built a couple over the Summer. One for a buddy as an upcoming Christmas gift and for something to do as a hobby in the hot Arizona Summer since most other activities are curtailed for a few months ;>). The last image shown are the the 3 I built. The enclosure is all steel with real American Red Oak side panels stained a walnut color and the unit has a headphone output and a separate set of RCA's for use as a pre-amp. You can use it as a headphone amp and pre-amp at the same time in case you wanted to run a subwoofer or something similar to get some bass "feel" with the headphone experience (I admit I've done it). The outputs are of the same polarity so depending on the subwoofer amp you can adjust the polarities to coincide with what you're headphones are putting out. Okay, finally, here are some details of the amp: -- As mentioned above, the design is Class A single ended with the devices heatsinked to the chassis with mica insulators. This little fella runs hot, but it sounds best that way (I tried close to a dozen operating points before settling on 105ma). Class A with a rich bias really opens up the soundstage and improves dynamics. -- There's Zero Negative feedback and the higher operating voltage runs the tube and MOSFETs in a considerably more advantageous operating point over the 12-20V designs. -- Panasonic FC Gold Output Capacitors. The signal is directly coupled from the plate of the tube to the gate of the Mosfet and there's essentially a single capacitor in the main signal stage (ignoring the cathode bypass caps on the tube which are essential for the right gain). The caps are 2200uf which gives you great performance for even low impedance headphones. I tried a variety of capacitors and decided on the FC's since they're just so neutral! -- 14dbs of gain which is enough for most devices with on output down to about .5 volts. Yet it's not overdone so the SNR is nice. The operating points where chosen using both normal effiency Grado SR60s and Yamaha Orthodynamics with enough gain to play louder than is reasonable with either! -- SMPS with a dual voltage rail plus an additional CLC filter for the signal stage/s. The tube filament is run from a separate output than the signal stage enhancing decoupling those functions. -- Constant current source loaded output stage. A CCS output stage not only almost doubles output efficiency (much needed on a single ended design), it seems to really improve dynamics since the source ground load is much lower in resistance than a resistor loaded design. -- Current source filament regulation. The filaments are fed from it's own CCS which has an additional filter cap for super clean filament DC. I chose a current source even though it's a bit more difficult to implement simply because it's been demonstrated by a variety of DIY folks that a CCS on the filaments sounds as good as AC! -- Long Tube Life. The two main things that kill a tube over time are breakdown of the plate and cathode and filament erosion usually from current inrush. Well, the CCS on the filaments takes care of current inrush, so that's covered and the tubes are operated at a nice operating point (low enough for uber long life yet high enough to prevent what they call tube "sleeping sickness") that should make them last for years if not decades. -- Vintage look! I wanted these units to have that sort of vintage look. They're not steam punk but do have a combination of retro/postmodern aesthetic that I like (the clear chickenhead knob is quite funky), and hope others will appreciate as well. --Enough Already!!!! Okay I'm just about done. I'll cover shipping to the lower 48 on the asking price (shipped via USPS flat rate only). The tube included will be a vintage 12AU7 or 5963 (a military/industrial 12AU7 which sound great!). I'll provide a lifetime free labor warranty on the amp unit for the original buyer. Simply pay for shipping both ways and I'll fix it if something breaks and only charge for the parts. The tube included will be from a lot I bought off of ebay and tested them myself. But given that the power supply and tubes are used they'll get a standard 1 month type of warranty and as it should be the norm of reciprocity will rule any unanticipated situations.
I apologize for the images, it makes any dust or fingerprints show up like crazy. I'll try to get some images during the day with natural light.
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