BDP-95 and BDP-83 with High-Sensitivity Headphones

Oct 4, 2011

The OPPO BDP-95, BDP-93, and BDP-83 Blu-ray players don't include a headphone output jack. A headphone output capability is included with many players and in stereo amplifiers and receivers. Owners of high-performance players which don't include a headphone output will often use a separate headphone amplifier unit. However, for very high sensitivity headphones (>110 dB SPL) in the 30 - 60 ohm impedance range, the headphones can in fact be driven directly from the player line-out, using a very simple passive voltage divider network, preserving the exceptional audio performance that these OPPO players provide. The divider circuit and the range of resistor values which are suitable for this usage are discussed. A convenient script calculator is included which facilitates converting headphone specifications (sensitivity and efficiency) and displaying dBSPL listening level achievable with any player, headphones and circuit component values.

The OPPO Analog Output

The OPPO players above have a maximum analog audio output signal level of 3.1Vpeak (2.2Vrms) for both the dedicated stereo and multi-channel analog outputs. The effective output impedance of these particular players are 100 ohm (measured), which provides output capacitance isolation and is probably intended as a current limit resistor in case of an output short circuit. This design, although not intended for the application discussed here, makes it possible to have the analog circuit output opamps (NE5532 for the BDP-83 and LM4562 for the BDP-93 and 95) drive their maximum specified load current to a connected load of lower impedance. Although the player is designed to be connected to an amplifier or receiver with a typical input impedance of over 10 kohm, a much lower output impedance load can in fact be used with negligible deterioration of the superb analog audio performance offered by these players.

The Circuit

The simple passive adapter voltage divider circuit shown in the schematic diagram below, showing one for each channel, uses only two resistors. The output of the BDP-83 or BDP-95 player is modelled as an ideal voltage source in series with the (measured) effective internal source resistance Rs of 100 ohm. Resistor R2 (150-300 ohm), in parallel with the headphones, is not technically required, but provides a fixed load when the headphones are disconnected. The specific example discussed here uses the Sennheiser HD 598 headphones with an impedance of 50 ohm and sensitivity of 112 dB SPL(1kHz/1Vrms). The network shown below uses R1 = 1 kohm. With these headphones, and typical audio content, the audio level (with full digital volume set at the Blu-ray player) with the HD 598 headphones is a very comfortable listening level (about 89 dBSPL). The jacks can be easily assembled in a very small shielded box. For best performance, no adjustable pots are used. Either a 1/8" stereo minijack (shown below) or a stereo 1/4" phone jack (or both) could be used as the headphone jack.




Component Range

With lower values of R1, the headphone output level increases. With the HD 598 headphones, some listeners may prefer a higher audio level. How much can R1 be reduced? This limit depends on the maximum opamp output drive current capability and the power dissipation limits of the output opamps of the player. For the players discussed here, the maximum (peak) opamp current is over 20 mA. Considering the maximum internal ambient temperature of the player (above which the fan kicks in) and the thermal resistance of the opamps used in these players, R1 should be no lower than 200 ohm (which means a total opamp load of 350 ohm, which includes the player internal 100 ohm resistance and the 50 ohm headphones). At full signal level from the player, the output opamps in this case will deliver ~ 9mA, well below the maximum specification. For the HD 598, the audio level will be high at about 100 dBSPL, and the player digital volume control could be used to lower the audio level. For these headphones, I recommend an R1 value of 500 ohm - 1kohm. Listening tests demonstrate the outstanding audio performance of these players, uncolored by additional active electronics.

Audio Listening Setup

With a compact portable LCD monitor, CDDA, SACD and DVD-A headphone playback with convenient track/title navigation can be achieved without a large connected monitor, as shown below:




Technical: Use of Headphones with Line-Out without Amplifier

Generally speaking, CD and DVD players with an analog line-out connection are typically intended to be used with amplifiers or receivers that have an input impedance greater than 10kohm. Many of these players have an OUTPUT impedance on the order of 1 kohm or more and will not be able to provide sufficient current to drive headphones directly. The internal design of the output circuit of these players will determine if headphones can be directly connected (without any resistive divider network) to the lineout without degradation of audio performance. The OPPO players discussed below have internal out impedances of 100ohm and can therefore, within the limitations described above, source considerable current or power to headphones with a proper voltage divider circuit.

In order to facilitate comparison with other headphones, it is important to know how to convert headphone sensitivity in units of dB SPL (1kHz/1V) to headphone efficiency specified as dB SPL (1kHz/1mW). The conversion factor involves the impedance of the headphones. The sensitivity specifies the measured sound pressure level (in a controlled testing device with microphones) with the phones driven at 1 Vrms. This corresponds to an electrical drive power level of P(1Vrms) = Vrms^2/R = 1/R Watts or = 1000/R mW where R is in ohms. This specification at fixed drive voltage provides an amount of drive power which clearly depends on the headphone impedance. To convert the sensitivity rating (for example, 112 dB SPL for the HD 598) to an efficiency rating (for 1mW electrical drive power), we simply subtract the power difference in dB: 10*log(1000/R) or:
       dBSPL(1mW) =  dBSPL (1Vrms) - 10*log(1000/R)   =   dBSPL(1Vrms) - 30  + 10log(R) 
or, for the HD 598 headphones:
       dBSPL(1mW) =  112 - 10*log(1000/50)   =   112 - 30  + 17 = 99 dB 
Therefore any headphones with an impedance of 50 ohm and an efficiency of ~ 100 dBSPL(mW) should perform similarly to the Sennheiser HD 598 when used in the voltage divider circuit discussed above.

As another example, the Sennheiser HD 650 and HD 800 headphones both have an impedance of 300 ohm and a sensitivity of 102 dBSPL (1Vrms) or an efficienty of 97 dBSPL (1mW). Using the divider circuit above with modified component values of R1 = 300 ohm and R2 = 1 kohm, these headphones will provide a sound level of 100 dBSPL when used with the OPPO players at full signal analog output level. (the maximum internal voltage of the OPPO players are 2.2Vrms, and the voltage divider circuit, including the OPPO internal 100 ohm resistance, provides 0.81Vrms at these headphones which is about 2dB lower level than for a 1Vrms signal).

The calculator below converts headphone efficiency to sensitivity or vice versa. In the bottom row, the calculator also displays the actual audio level dBSPL (Circuit) for the specified headphones, using the voltage divider circuit values and the player source voltage and internal resistance values specified in the 3rd row. (The default values correspond to the example given above for the OPPO players) In addition the RMS headphone voltage with this divider circuit and the headphone electrical power dissipation are displayed. The source voltage and resistance, and divider resistor values can be changed. A warning is displayed if the total load resistance (Rs + R1 + R2||Rphones) is less than 500 ohm. The Clear button resets the source and circuit component values to the default circuit values shown above. The schematic diagram below the calculator shows the component names (for one channel):

Headphone Sensitivity / Efficiency Converter and dB SPL Calculator
  Rphones (ohms):      Sensitivity (dBSPL/1Vrms):     Efficiency (dBSPL/1mW) :   
         
  Vs(Vrms):    Rs(ohms):      R1 (ohms):      R2 (ohms):   
  dBSPL (Circuit):      Phones Voltage (Vrms):      Phones Power (mW):