jeffreyj
100+ Head-Fier
- Joined
- May 4, 2003
- Posts
- 465
- Likes
- 10
I tested 8 different dual op-amps that I had in stock for what might are arguably the most important parameters in a battery powered headphone amp: Maximum undistorted output voltage into a realistic load; maximum current drain at that load; quiescent current drain.
The tests were performed using a 2kHz sine wave from my function generator. The load for the amplifier was 100 ohms for each channel. The circuit incorporates 10 ohm series resistors to prevent instability from driving overly capacitive cables. The power was supplied from a three-terminal regulated supply (a 7808 I had laying around with 2 diodes in series with its ground pin to bring the output voltage up to 9.18V). The current measured was the total drain of the circuit, including 0.6mA/ch. of Class-A biasing and 0.8mA for the power-on LED. Distortion was "measured" by looking for flat-topping of the output with only one cycle at maximum vertical resolution possible showing on my trusty THS-720A 100 MHz digital storage oscilloscope. The output level of the function generator was adjusted until severe flat-topping occured, then backed off until it went away. Results, then, are most useful for the relative comparison of each op-amp, not as absolute results, per se. The op-amps are listed in decreasing order of peak-to-peak output voltage.
LMH6672M: Iq = 30.1mA; Imax = 58.1mA; Vpp = 6.08V
LMH6655M: Iq = 21.5mA; Imax = 50.8mA; Vpp = 5.88V
AD8620AR: Iq = 13.5mA; Imax = 44.6mA; Vpp = 5.84V
NE5532AP: Iq = 16.3mA; Imax = 42.5mA; Vpp = 4.84V
LM6172AC: Iq = 12.1mA; Imax = 37.2mA; Vpp = 4.48V
TSH22IN: Iq = 12.8mA; Imax = 37.1mA; Vpp = 4.32V
LF412ACN: Iq = 11.4mA; Imax = 33.6mA; Vpp = 3.80V
MC1458CP: Iq = 8.3mA; Imax = 27.4mA; Vpp = 3.12V
Of course, the last two aren't indicated for the application, especially not the last one?!?!, but I included them to show what performance a generic FET-input amp, the LF412, and a generic bipolar-input amp, the infamous 1458, would give.
The real shocker for many will probably be the NE5532's results. Lots of people turn their noses up at this op-amp because it's been around for so long and used in so many devices (hell, it's used for every gain stage in my M-Audio Audiophile sound card), but it delivers high drive capability without the high price.
Eventually I'll get around to running RMAA 5.1 on all of them, as well as posting some of the notes I took while A/B testing them (as well as A/B testing the Class-A bias circuit which seemed to obliterate the sonic differences between the op-amps, as long as the output level was such that all of them could drive the load without problem!)
The tests were performed using a 2kHz sine wave from my function generator. The load for the amplifier was 100 ohms for each channel. The circuit incorporates 10 ohm series resistors to prevent instability from driving overly capacitive cables. The power was supplied from a three-terminal regulated supply (a 7808 I had laying around with 2 diodes in series with its ground pin to bring the output voltage up to 9.18V). The current measured was the total drain of the circuit, including 0.6mA/ch. of Class-A biasing and 0.8mA for the power-on LED. Distortion was "measured" by looking for flat-topping of the output with only one cycle at maximum vertical resolution possible showing on my trusty THS-720A 100 MHz digital storage oscilloscope. The output level of the function generator was adjusted until severe flat-topping occured, then backed off until it went away. Results, then, are most useful for the relative comparison of each op-amp, not as absolute results, per se. The op-amps are listed in decreasing order of peak-to-peak output voltage.
LMH6672M: Iq = 30.1mA; Imax = 58.1mA; Vpp = 6.08V
LMH6655M: Iq = 21.5mA; Imax = 50.8mA; Vpp = 5.88V
AD8620AR: Iq = 13.5mA; Imax = 44.6mA; Vpp = 5.84V
NE5532AP: Iq = 16.3mA; Imax = 42.5mA; Vpp = 4.84V
LM6172AC: Iq = 12.1mA; Imax = 37.2mA; Vpp = 4.48V
TSH22IN: Iq = 12.8mA; Imax = 37.1mA; Vpp = 4.32V
LF412ACN: Iq = 11.4mA; Imax = 33.6mA; Vpp = 3.80V
MC1458CP: Iq = 8.3mA; Imax = 27.4mA; Vpp = 3.12V
Of course, the last two aren't indicated for the application, especially not the last one?!?!, but I included them to show what performance a generic FET-input amp, the LF412, and a generic bipolar-input amp, the infamous 1458, would give.
The real shocker for many will probably be the NE5532's results. Lots of people turn their noses up at this op-amp because it's been around for so long and used in so many devices (hell, it's used for every gain stage in my M-Audio Audiophile sound card), but it delivers high drive capability without the high price.
Eventually I'll get around to running RMAA 5.1 on all of them, as well as posting some of the notes I took while A/B testing them (as well as A/B testing the Class-A bias circuit which seemed to obliterate the sonic differences between the op-amps, as long as the output level was such that all of them could drive the load without problem!)
