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June 11, 2015

Speaker Amp to Headphone Converter Box

With the recent proliferation of low and mid sensitivity planar magnetic headphones from vendors such as Audeze and HiFiMan, it's easy to see why headphone enthusiasts are craving more power. Whether it's get that extra bass oomph and ability to listen at higher volumes with less distortion, having headphone amplifiers with between 1-6 watts is becoming more commonplace.

Since high wattage headphone amps are becoming more and more the norm, some have decided to hook up their headphones directly to the speaker taps. That's totally doable in a number of cases (see the speaker to headphone amp cable here) however there are other situations when this is not recommended, such as with tube amplifiers with output transformers. The output transformers can short if they don't have enough load, causing damage to your equipment.

Headphones also have a higher impedance than speakers, so while common speaker amps have outputs for 4, 8 and 16 ohms, headphones are often somewhere in between 32 and 600 ohms. You'll possibly hear noise on headphones from a speaker amp as they are more sensitive than speakers, which creates an unwanted noise floor. So to get rid of these problems, a small resistor network is necessary to modify the load for your amplifier, and reduce the audible noise.

A resistor for each channel is needed to attenuate the volume so that it is more compatible with headphones and help reduce noise, and additional resistor is needed to reference ground (again for each channel). These resistor values will vary depending on the impedance of your cans.

The project starts with a small Hammond Aluminum case. Hammond is the standard when it comes to small project boxes like this. You can get this in anodized red, blue, black or plain silver. the usual suspects have it: Mouser, Digikey and Angela.

Hammond Black Anodized Case

The first task is to punch the hole for the headphone jack. I'm using a Neutrik locking in this case. This particular build is a single ended (e.g. not a balanced headphone jack with isolated grounds). This is only possible IF AND ONLY IF the amp's left and right ground is shared. This is simple to check with a multimeter.

To punch the hole I'll be using a Greenlee punch. This makes a nice clean opening to install the Neutrik locking plug. These are expensive, so only get one if you plan to get good use out of it.

Pilot hole

Fixing the Greenlee in place


And the punched hole

Next is the task of drilling out and installing the binding posts on the back. I found that a vertical arrangement allowed the panel space to be used most effectively.

Binding post installed on back panel


The panel holding the binding posts is fastened to the case with four screws, and two teflon solder posts are mounted on the base of the unit. These can be found at VT4C. The resistors for this application need to dissipate power, and therefore heat, and they need to be large for that purpose. There are various power resistors out there that fit the bill, sandcast, metal oxide, wirewound, etc., but they can't degrade the sound, so I am using Mills 12 watt non-inductive wirewound resistors as we don't want any induction screwing with the impedance. The Mills are well thought of for good sound quality, so it was an easy choice. I've also used the larger Kiwame 5W carbon film resistors in parallel.


Resistors wired up

For the wiring I chose to use some choice Kimber TCSS copper in Teflon. 


Kimber wiring




The front panel is secured with four screws, and the top aluminum panel is slid back into place. I adorned it with a nice little branding element. 


Speaker Amp to Headphone Converter Box Front

Speaker Amp to Headphone Converter Box Back

I hope you enjoyed this post, please contact Zynsonix if you're in need of a speaker amp to headphone converter box.


The Fine Print:
The above steps detailing the building of a speaker amp to converter box are for entertainment purposes only and not to be performed under any circumstances. The owner of this blog and all associated parties can not / will not be held responsible if you attempt the process posted and cause physical harm to yourself, your surroundings or your property. Please keep this in mind.