So in preparation for a set of HiFiMan HE-500 cans I picked up an Emotiva mini-X a-100 amplifier during Emotiva's holiday sale. They were $170 shipped down from the normal $219. Then of course shortly after that they had their refurbed to like new secret warehouse sale and had them for $150 shipped with the same 5 year warranty the new ones come with. So if you're thinking about picking one of these up you know how the pricing works now. You can order or check the latest price on their product page: LINK
For those that don't already know the Emotiva mini-X a-100 is one of the most popular amps for the ever famous HiFiMan HE-500 headphones. It is inexpensive, very powerful, a great match sonically and fairly compact in size. Many have compared it to some of the heavy hitters out there like the Schiit Lyr and it has withstood the test and come out on top quite often. There is a rather large thread on using this amp on some very popular cans including the HE-500, LCD2 and AH-D5000 here: LINK. I figured for the price how could you go wrong?! Even if you dont like it or "upgrade" soon after you can use the mini-X a-100 as a 2 channel speaker amp because, well thats what it is!
Now, if you're going to use a speaker amp as a headphone amp there a few challenges that must be overcome to do so. First off is the physical connection between the amp and the headphones. Speaker amps typically have banana plugs or wire terminals of some sort to plug the speaker wire into that runs to the speakers. Obviously headphones do not come with either of those so you have to figure out how to connect a pair to the amp. And second there is the issue of dealing with what we'll call for layman's terms "power output" which is in this case more than what a set of headphones needs to the volume knob doesn't move much before the audio is at ear blasting levels and dangerous for numerous reasons to listen to. So some modifying somewhere has to be done to get a setup like this to work. It is for this reason that I have created this thread. I will cover both an electrical as well as a cosmetic modification in this thread.
First of all I'll start by saying I am not a very big fan of non-black electronics. For the last 25+ years every piece of technological gadgetry I have owned has always been black because, well that's the way I like it :) That being said the Emotiva mini-X a-100 has some IMO ugly silver accents that I knew if I was going to be taking apart and messing with the amp anyway I'd need to fix. So I think we'll start there:
This is what we start with for reference:
Now, it's not terrible I know, but like I mentioned above I gotta have my electronics all black, especially so that they match everything else :) So the first step was to remove the silver accent panels, the bolts holding them and the knob and figure out how to get those pieces black. After a quick email to Emotiva I learned a couple things. They do offer the silver panels in black for some of their products, but not for the mini-X a-100. They did say its something they are looking to offer in the future, but as yet they do not. They also do not offer the knob in silver and I confirmed with them that both the knob and the accent panels are indeed aluminum. They informed me that the knob has a plastic insert that allows it to mount to the volume potentiometer. You can do it how you like but I used an old school IC chip puller to safely and evenly pull the knob off. One like this: LINK
The bolts that hold the accent panels on were removed with an 1/8" Allen wrench. Once all of that was removed it looked like this:
And for reference the back of the aluminum knob looks like this:
These were the pieces I needed in black and since I knew they were all aluminum (except for the bolts which are steel) I knew they could be anodized and dyed black. For those that don't know about the process of anodizing aluminum do a search on YouTube and you can learn all about it. You can even do it yourself if you have the room, time, and desire. Its not an expensive project, just a little dangerous as you're dealing with high current electricity and battery acid and it's easier to just pay someone else to do it if you're not going to be doing it a lot. I called a few metal finishing companies in my area and all had a minimum purchase price with the cheapest I could find being $65 plus tax and a 48 hour turn around time. Well as chance would have it I happened to be floating around another thread here on the good old time and money wasting Head-Fi and happened to notice this guy who posted this bit of design style improvement:
Joe is his name and you can reach him here: LINK. For $30 plus shipping Joe had my three pieces perfectly anodized and all black just how I wanted them and back to me in a week! If you're looking to have stuff done send him a PM as he said hes looking to do more work. Once I got them back they looked like this:
The bolts on the other hand were a bit of a different story. I ended up having to buy a black accent kit for a different amp from Emotiva that came with the black screws. Luckily it happens to be for an amp I intend to get from them someday (the UPA-500) for a higher end computer surround system, but for now I used the black bolts from it and would have to find more to replace those when I do get the UPA-500. I was able to order those from Emotiva here: LINK. $20 is pricy if you're just going to use the bolts I know, but I'm sure you can find these bolts in black for much cheaper I just haven't looked yet as I knew I needed the black accent panels for the UPA-500 anyway.
Now that the aesthetics were taken care of it was time to focus on the electrical. As I mentioned before there were two issues with using a speaker amp as a headphone amp. The first was the physical interface (how you connect a set of headphones to a speaker amp) and the second was how to you handle the fact that a speaker amp outputs too much power and doesnt give you any volume play in the knob when listening? Both problems can be handled at the same time, which is nice, with a little pre-planning. First off the connectivity issue.
Pretty much every headphone amp on the market comes with a standard 1/8" (3.5mm) or 1/4" (5.3mm) TRS jack unless its a balanced setup or some other weird design. Obviously a speaker amp would not. So in order to plug their headphones into this amplifier people usually make a female TRS to four banana plugs adapter which I did here:
Or people make some sort of converter box that takes in an interconnect terminated with banana jacks and converts it to a female TRS jack. I really didn't like either of those options as I thought they were too bulky and not clean looking. I had seen a few people who had installed a panel mount female TRS jack directly into the front chassis plate of the mini-X a-100 and thought yes! that has to be how its done! So we begin disassembling the amp:
First on either side of the amp there are two screws that have to come out:
Then there are four in the back that need to be removed:
Next on the bottom there are two towards the front that hold the front chassis plate on:
And lastly on either side of the chassis plate there is a side screw that needs to be removed:
I forgot to snap a pic of these side screws before I had installed the attenuation resistors so ignore those in this pic as we'll get to that in a bit.
With the panel removed it looks like this:
Then it came time to pic a panel mount female TRS jack. This was actually easier than I thought it might be as there are VERY few black panel mount TRS jacks out there. The only issue was that of course the one that was available was not cheap. What I ended up choosing was the Switchcraft 155 (LINK) They are however a bit tricky to find. You can check most resellers with a simple Google search: (LINK) but the only place at the time of this writing I could find them in stock was here: LINK and like I said, these arent cheap with respect to some of the other options out there.
So now we need a hole in the front aluminum chassis plate to mount the TRS jack into. This wasnt nearly as hard or scary as some might think. It was done with a drill press, something soft under the plate so the front face didnt get scratched, a center punch to mark where the center of the hole would be from the back side of the plate and two drill bits, one to create a starter hole and then the actual bit used to drill the hole and a hole reamer to clean up the edges of the hole after drilling. An 1/8" bit was used for the starter hole and then from the datasheet (LINK) for the TRS jack I used a 5/16" drill bit was used for the hole.
Now in picking a place to put the hole, you are pretty much limited to the left side of the front of the face plate as the circuit board that is mounted to the back is offset to the right a bit which leaves just enough room to fit the longer panel mount TRS jack on the left. I found that centering the hole above the "a-100" label ended up being perfect both functionally to avoid the circuit board and aesthetically.
Here is what the chassis plate looked like after the hole was drilled:
And then with the TRS jack mounted in it, which ended up being a perfect fit:
Now that the TRS jack was mounted the second issue of dealing with lowering the output through it to be better suited for headphone use and give more play in the volume knob had to be dealt with. As it turns out there is also a number of owners of this amp who also hear a tad bit of noise floor at listening levels with this amp when using it with headphones and thankfully how we deal with the output level and volume knob play issue at the same time fixes the noise floor issue as well.
Now essentially what we are going to do is called attenuating the signal and we are going to do it using high wattage resistors inline with the headphone output that was just installed. This will not affect the speaker outputs in the back of the amp in any way and those will still function just as they always did as long as they are used with no headphone plugged in the front. There are two common styles of tackling this issue and they are depicted here from Head-Fi user robrob's website where he has a calculator for such things:
Now there are arguments for both sides as to how the signal should be attenuated, but for solid state amps like the mini-X a-100 the attenuation only method works just fine. The preferred resistor network is meant to match the 8 Ohm load that a typical speaker amp would expect to see with 8 Ohm speakers attached to it, however from my research this is really only necessary when working with tube based amps. But you can choose whichever you like, and if you choose to go with the preferred resistor network it is recommended to use 5W 1% resistors for R2 and 3W 1% for R3, IMO a combo that gets you around 18dB attenuation and this diagram also from robrob's website might help make it a little clearer:
I opted to go with the attenuation only option. At first I tried 110 Ohm 5W 1% resistors and found that I still didnt have as much volume play as I would like so then I moved up to 220 Ohm 5W 1% resistors and was much happier with that. Listening level is around the 12 o'clock position on the volume knob with the 220 ohm resistors. These are the resistors I used: LINK
These 5W resistors are a little big, but I was able to figure out a way to get them to fit around the TRS jack and take up minimal space by bending the leads using some needle nose pliers and soldering them directly to the TRS jack terminals. I removed the circuit board to make working around in here a little easier.
Then from these resistors and the TRS jack ground/sleeve terminal a wire had to be run to the back of the amp and attached to the backs of the speaker terminal banana jacks. Remember this has all been to tap into the speaker terminals with inline attenuation (the resistors) to both lower the noise floor and lower the output volume giving us more play in the volume knob while listening through headphones attached all while keeping this install clean and minimal. For the run from the TRS jack to the speaker terminals I used a bit of Canare L-4E6S Quad Star wire which I bought here (LINK) in about a 3FT length just to be safe. You can of course use any wire you like, but I had some of this already and the star quad formation and wire braid shielding adds some noise protection as well, so I though this would be best. Plus it has four wires in it to start so it makes the other end of this run a bit easier which youll see in a bit.
Soldering to the TRS jack looks like this:
Youll notice two of the four wires are used for ground. When we get to the other side of this run youll have to use a digital multimeter or other continuity tester to determine which two of the four are you grounds. We'll use a blue wire for the right channel and a white wire for the left channel from the two remaining wires. You can see in this picture the back of the face plate after the wire was soldered to the TRS jack. The labels A1 and A2 were to keep track of which wire plugged into where from the rest of the amp to the front circuit board. On the board itself there are silkscreened labels "A1" and "A2" and I put some masking tape with the matching labels on the corresponding wires that those ports connect to. This picture shows the circuit board mounted back in and the wire soldered on.
After you measure you length needed to go around the transformer and back to the backs of the speaker terminals you have to terminal the other end of the wire. I used some circle connectors that I found at a local electronics store and soldered one to each wire and the heatshrinked them like so:
These are the connectors I used:
It can be very helpful is not required to remove the large heatsink in order to get under it and run the wire to the speaker taps. It can be removed by removing the four mount screws located here:
And then carefully removing the wires that are connected to the circuit board attached to the bottom of the heatsink:
The run I took from the TRS jack to the rear speaker terminals looked like this:
After that the rest is pretty easy. Plug the wires back into the circuit board underneath the heatsink. Put the heatsink back on being careful not to pinch any of the wires underneath it. Put the face plate/front chassis plate back in and the four screws holding it. Then put the top cover back on and put the four screws in the back in and then the two screws on either side. Once that was all buttoned up I just had to put the accent panels and knob back on:
You can see the plastic insert for the knob here and how it mounts on. You have to make sure that hole in the plastic insert matches up with the hole in the knob as that is how the blue light from the LED gets through to show the volume knob position in the dark. And the finished product:
I hope you guys found this write up useful. It was certainly a fun project and I am very happy with the results. The amp now looks and functions as great as it sounds! And before anyone PMs me about it, I would probably be willing to do this mod or the adapter cable for others however you have to know that none of these parts were the cheapest route and it did take quite a bit of time and waiting for parts to come in or get done so it wouldnt be quick or cheap to do as it wasnt for me. Just doing a quick tally in my head I think I spent about $85 just in the parts alone not to mention the additional parts I bought but didnt end up using just to try different configurations. Or you can of course do it yourself and have all the fun! Either way, Enjoy :)
Edited by ben_r_ - 3/2/14 at 2:28pm