Sectember 25, 2005

The very positive experience using the DAC 5.0 and the many articles about the headphone amplifiers with operational amplifiers have bring me to this project.

Off course I will apply the same philosophy of my other projects with a "little" exception: the feedback.

The target was to create a compact headphone for my Beyerdynamics DT880 including the DAC stage.

Yes, an SPDIF to Headphone converter in a very compact box with the  hi-end performancs!

The starting point is, like the DAC 5.0, the DAC-AH complete by http://eshop.diyclub.biz sell at the  incredible price of only 135$.

Before to start to see the few modifications to do see the differences in the photos below.

The internal layout has been redesign to create the space for the 10K+10K DACT stepper attenuator and the Auricap interstage capacitors. The power switch has been replaced with a miniature type.

Of the original schematic of the DAC-AH has been modified the output stage with big differences from the original.

follows the new output stage

The current generated by the 8 x DAC TDA1543 is converted to a voltage with a new  250ohm resistor VISHAY (cod. 201-9854 of http://www.rs-components.com/), follows an Auricap interstage capacitor to skip the DC voltage (about 3.5V) to preserve the max dynamic range in the operational amplifier, a 10K DACT stepper attenuator is used like volume control.

The feedback resistors of the op amp. has been changed to get enough gain to drive the headphone.

370 ohm / 510 ohm + 1 = 1.72x

and now

10 Kohm / 1 Kohm + 1 = 11x => 20.8dB

To complete the modification is necessary cut two wire on the PCB to insulate the DAC output from the OP amps. 

These 2 resistors are responsibles, like the 250 ohm, of the sonic performance so must be only MK132 Caddock (see http://www.welbornelabs.com/cad.htm) or Vishay.

The output capacitor on headphone output has been delete because the DC offset of the operational amplifier is very low (max 3mV) and could be force to 0 using a simple trimmer (see OP602A datasheet).

The same values of resistances are used in the Pocket Headphone Amplifier by Chu Moy with OPA134.

Someone could think to change the OPA602 with a OPA134 or other type but I don't know if these will sound better.

About wire forget the teflon ! use only stranded tinned copper wire with pvc insulator and for loudspeakers the same.

A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

As support for the interstage capacitors has been used a Twist Eye Terminal by Grennan Audio.

Now we have a system with very few components on the signal path where every of these has been selected very well and the result is great!

As told before this project is born to drive my Beyerdynamics DT880 with a nominal impedance of 250 ohm and off cource can be used with any headphone with an impedance range 200 - 600 ohm. To drive low impedance headphone like Grado is necessary a modification of the output stage. In detail a BUF634 should follows the OPA602A to have enough output current. Follows the project PIMETA for more info.

One suggestion: to get the best sound wait 15-30 minutes before ear this amp.; like the vacuum tube and class A amplifiers.

Follows the calculation of the parameters:

DT880

Nominal impedance acc. to IEC 60268-7	250 Ω
Nominal SPL acc. to IEC 60268-7	96 dB
Nominal THD acc. to IEC 60268-7	< 0.2%
Power handling capacity acc. to IEC 60268-7	100 mW

Power = Vrms * Irms

Irms = Vrms / R

Power = Vrms * Vrms / R

Vrms = sqrt(Power * R) => sqrt(0.1 mW * 250 ohm) = 5 Vrms

Vp = 1.41 * 5 Vrms = 7 Vp

Irms = 5 Vrms / 250 ohm = 20 mA

with a 600 ohm headphone Vp is about 11V so the power supply at 15V is enough.

For the current we are near the limits of the operational device.

Follows the test results with Clio System by Audiomatica (see the description and the considerations of the image on the right).
	Here the result at the DAC chip output terminal (on 250 ohm). the distortion is very low (about 0.15%), note that the lower range is set to -80db the armonic decay has been optimized using the bias trimmer  to get the best single ended spectrum
	Here the result at the operational amplifier output (on headphone jack) without any load and at the maxim acceptable distortion level. the distortion is very low (about 0.15%) the decay spectrum is perfect
	Here the result at the operational amplifier output (on headphone jack) with a 250 ohm load and at the maxim acceptable distortion level. at the maxim level allowed for my Beyerdynamic DT880 (5Vrms see above) the distortion is very low (about 0.15%) the decay spectrum is still perfect
	Here the result at the operational amplifier output (on headphone jack) with a 250 ohm load and over than the maxim acceptable level  for my Beydynamic DT880 distortion is very low (about 0.17%) the decay spectrum is still perfect
	Here the result at the operational amplifier output (on headphone jack) with a 250 ohm load and at a higher distortion level. at this higher output level the distortion is still acceptable but not very low (about 1.7%) in this case the decay spectrum is not good but never this output level will be necessary to drive the headphone
	Frequency response at the DAC chip output low cut off -1db at 10Hz high cut off -3db at 20KHz probably I will modify the value of the capacitor C36 to extend the high frequency range
	Frequency response at the OP Amp. output like the previous

Follows the photo of my compact hi-end system with the TR3000 HP used like juke box to store 300 CD in TTA format (lossless) and Optoplay USB audio interface (not visible) to create an optical output to connect with a fiber cable to this amp.(on left under the headphone).

The little keyboard is a very good product sell by Precision Squared Tech