Creating Headphone Amplifier

tanihhiavlt

I've got a tiny pedal board with a stereo out - both the mini vent and the eventide and I wanted a quick way to test the patches and figured a headphone amp was the quick solution. 

I soon found the CMOY amp 

https://web.archive.org/web/20150415003947/http://headwize.com/?page_id=707
https://tangentsoft.net/audio/cmoy/

Not sure if I benefit from a headphone amp the rest of the time - the impedance from the the ATH-m50x is 38 ohms and the CMOY recommends using an extra load resistor for headphones below 33 ohms.

So I'm going to be making one or more of these - will photos some stuff and record the layouts I try  

Danny1969

The Cmoy is basically about driving higher impedance headphones with a higher voltage to get a better level. Most generic headphone outputs are designed for lower imedance, so can drive more current at a lower voltage to get the same power level. Better phones like Grado's won't really drive well for this reason. 
This is essentially the same deal as 16 Ohm guitar speaker drawing 32V at 1.5A to get 50 watts ish or an 8 Ohm guitar speaker drawing 16V at 3 amps  .... same power it's just the distribution of power that's different. However on a valve amp the secondary winding of the output transformer is switched to compensate for this. On a simple opamp headphone amp like the Cmoy it's not.
The Cmoy also needs a dual supply which is a bit of a pain. Although making one from 2 equal resistors across the rails and choosing the middle point as 0V it's a bit primitive and wasteful. 

For everyday run of the mill cans headphone stage I either use a design using the NE5532 with parallel amp for extra drive or buy one of the many pre build headphone boards for £5 which will happily run from a single 5V supply 

tanihhiavlt

I used a 1044s to invert the voltage on a prototype and used a 712 I had kicking around - I'm really pleased with the improved  response - I got my son to do a blind test to see if he thought there was a difference - he seems to like the headphone amp too  

tanihhiavlt

So the mk1 (above) used a voltage invertor to give +9v and -9v which according to my calculations makes for around 100mAh and not a good look for a battery powered headphone amp, so I used the LT1054 in a different configuration to provide +4.5v and -4.5v which is supposed to use about 6mAh which I'm calling the mk2 



or more simply:



The mk2 is currently solely battery powered and without the input volume control. Input are switchable - left-right/stereo - like that pedal mod trick Dan Steinhardt shares for eventide stereo pedals. 

I stuffed it in a repurposed DIY patchbox.



Plans are something like:

• ream the remaining hole for a 2.1mm plug
• fit an on/off switch and a baleful LED
• fit a dual gang pot for volume control (it was too crunchy with any drive)

After that, it'll be used on my desk driving my headphones and to test the stereo signal from the small board I have wot is stereo. (till I buy a second zt lunchbox or something).

I am planning mk3 - which will fit in a Hammond 1455 extruded case I've got, I'm having to sculpt end pieces for this from some offcut darkwood as the replacement ends are more expensive than a brand new hammond case!



The new board will has the ICs side by side so that it fits next to the 9v battery to drive it. This one I'll take on the train and such when I'm commuting