LJM HA-PRO Amp

Discussion in 'DIY' started by Cspirou, Oct 7, 2015.

Tags:
  1. Cspirou

    Cspirou They call me Sparky

    Pyrate
    Joined:
    Sep 27, 2015
    Likes Received:
    8,200
    Trophy Points:
    113
    Location:
    Northwest France
    My collection has a bunch of high impedance headphones including the Koss MV1(250 ohms), Sennheiser HD250 Linear I (600 ohm) and the recently acquired Sennheiser HD600 (300 ohms). These need a dedicated headphone amp to power and all I had were speaker amps. I just wanted a solution that worked and was cheap. I already had a 24VAC transformer that was itching to be used so I looked around for a DIY solution that used the same voltage. I found this Chinese made class A headphone amp for 24 Euros at audiophonics that fit my requirements just fine.

    http://www.diyaudio.com/forums/headphone-systems/197601-ha-pro-single-ended-mosfet.html
    http://www.audiophonics.fr/hapro-amplificateur-casque-stereo-class-mosfet-irf610-p-7029.html

    Now some of you might be scratching your head as to how you can have a class A amp without any heatsinks and in a small form factor. Well with a gain of 11x and low current availability, this little amp is basically purpose built for high impedance headphones. The need for a heatsink is to dissipate energy from a high bias current for running in class A. However the current requirements for high impedance headphones are a tiny fraction of low impedance headphones and if you just completely ignore the needs of low impedance headphones you can make a class A amp without the need for a ton of heat management.

    I currently have it wired pretty simply without a casing just so I can get something working. I think it sounds great compared to the headphone out of my Topping TP-22 and laptop output. Since it's my first (and only) real headphone amp I don't really have anything to compare to but I wouldn't hesitate recommending this as a cheap DIY solution for someone that needs an amp for hi-Z headphones.

    Next I need to work out a casing for this board. I have already decided to make the transformer separate so it can be powered by a 24VAC wall wart and have a Schiit Modi sized footprint. I am thinking of doing something laser cut with either plywood or acrylic. I am leaning toward making this enclosed rather than a Garage1217 type enclosure because I still need to add the RCA connections and a DC barrel. After I get it worked out I will share the DXF file and BOM so that anyone here can make there own casing for minimal cost. Since it's so cheap I might just build another and send it to one of you guys for measurements and a much more informed opinion.

    Sorry for the lack of pics too.
     
    Last edited: Oct 7, 2015
  2. firev1

    firev1 Friend

    Pyrate
    Joined:
    Sep 27, 2015
    Likes Received:
    44
    Trophy Points:
    18
    Location:
    Singapore
    Nice getting an LJM amp to work LOL. Been looking at a bunch of his stuff and sometimes they look iffy or for some reason he can't put proper schematics up for diagnostics and repair.
     
  3. Cspirou

    Cspirou They call me Sparky

    Pyrate
    Joined:
    Sep 27, 2015
    Likes Received:
    8,200
    Trophy Points:
    113
    Location:
    Northwest France
    I didn't know that LJM had a reputation of being difficult to work with. The schematic for this amp is actually at the diyaudio link I posted.

    One of the annoying things about the amp is the inputs are right up front by the 6.3mm jack and its hard to get wires in those terminals. The preamp output has the same problem. That being said I am impressed with the features he packed in there. Here is a list.

    *built in regulated power supply-just add a transformer.
    *relay for protection
    *preamp out
    *accmodates rk27 ALPS potentiometers
    *led power indicator
    *runs in class A with negative feedback.
     

Share This Page