Appendix 1: Tweaking the Amp for Low Impedance Headphones
The OPA134 opamp produces a small DC offset voltage, which does not affect the amp’s performance when driving medium to high impedance headphones (over 100 ohms). Low impedance headphones (32 ohms or less) can cause the power supply to become unbalanced, because a small current flows though the load, even when the amp is at idle. This table compares the power supply voltages with the Sennheiser HD600 (300 ohms) and Sony MDR-G52LP (24 ohms) headphones connected to the amp.
Amplifier Load V+ V-
No headphone 3.9V -3.9V
HD600 (300 ohms) 3.9V -3.9V
MDR-G52LP (24 ohms) 4.2V -3.7V
Note: the battery by itself measured 8VDC.
There is disagreement about whether this almost negligible offset is worth the trouble to fix. With opamps other than the OPA134 series, the offset might be higher and the power supply imbalance could be greater. The offset has not damaged any of my headphones, but it might impact performance slightly by reducing the amp’s power output, injecting noise and/or draining the battery. To determine whether a certain headphone unbalances the power supply, measure the V+ and V- values with and without the headphones plugged in (and no music playing).
For those who want to reduce or block the offset current, figure A1 shows two ways to modify the amp for optimal performance with low impedance headphones: a) add a load resistor or b) AC-couple the amp’s output. A third way is to rebuild the power supply with an active virtual ground device like the TLE2426 or an opamp-based equivalent. Active virtual ground circuits are described in the addendum.
Modifying for Low Impedance Headphones.
Figure A1
Solution A is the simplest and allows the output to remain DC coupled. The load resistor (figure A1a) will help stabilize the virtual ground and reduce any hiss or noise in the system. The load resistor does create a voltage divider effect with low impedance headphones, and so may lower the amp’s gain and maximum output power and possibly alter the frequency response. Some say that the pocket amp’s gain of 11 is too high for low impedance headphones, so the small drop in gain due to R5 might be desirable anyway. Choose a R5 value just large enough to stabilize the power supply without too much volume loss. I recommend a 1/4 watt, metal film resistor in the 20-50 ohm range.
Solution B avoids a voltage divider effect because although the capacitor blocks DC current, it is largely invisible to audio frequencies. The circuit in figure A1b shows how to switch between AC-coupled and DC-coupled outputs for the highest fidelity with medium and high impedance headphones (the load resistor in solution A could be switched too). Choose the largest value electrolytic capacitor that will fit in the enclosure. A 220uF capacitor will give a flat response down to about 22Hz in 32-ohm headphones.
Use a high quality, low impedance electrolytic capacitor to minimize any sonic coloration. High quality electrolytic caps don’t have to be expensive. The Nichicon Muse KZ series 470uF, 25V sells for less than $1.00 at the time of this writing. The Panasonic FC and FM series caps are also less than $1.00 each. The exotic Elna Silmic II series (which feature a silk fiber dielectric instead of paper) has a 470uF, 25V unit for less than $2.00 each. By comparison, an ultra high-end type like the Black Gate 470uF, 16V typically sells for around $12.00 each and is not recommended for this amp.
