Now available is a dual-to-single op-amp adapter which allows a dual op-amp to be used in a socket for a single-channel op-amp. The adapter uses a parallel configuration which combines the two outputs of the dual op-amp into one. This circuit technique is commonly used for increasing the effective output current of an op-amp, resulting in a higher maximum output power and lower overall distortion when driving headphones.
AD8620 offers higher output power and lower distortion
Leckerton Audio is now offering the Analog Devices AD8620 as an op-amp option for the UHA-6S.MKII amplifier. The AD8620 is the dual version of the standard op-amp, the AD8610. Compared to the AD8610, the AD8620 gives better distortion performance and higher output power with headphones that have an impedance lower than 100 ohms. Above 100 ohms, there is no benefit over the AD8610 in terms of output power, but there is still an improvement in distortion at higher output levels. For a full performance comparison, see the table below.
The AD8620 requires twice as much power supply current as the AD8610, and this means the tradeoff for higher performance is lower battery life. When using the analog input, the UHA-6S.MKII battery life will drop to about 20 hours, compared to about 32 hours with the AD8610. Using the USB input (with charging disabled), battery life is about 9 hours, compared to 10 hours with the AD8610.
The AD8620 is a great option for those wanting to maximize the performance potential of the UHA-6S.MKII. The level of audible improvement it provides over the standard AD8610 is subjective, of course, and it also depends on things like the quality of your headphones, the type of music, and your listening level. With the AD8620, there is more available output current, and that means you are less likely to hear distortion which might occur as the op-amp runs out of headroom. Having more headroom means the op-amp can do a better job of maintaining signal linearity during peaks in the music.
The dual-to-single adapter is compatible with many different dual op-amps, with a few caveats: the op-amp must be unity-gain stable and available in a SOIC package with standard pinout. The adapter fits in a DIP socket for a single-channel op-amp with standard pinout. If you have something other than the AD8620 you’d like to try, please feel free to request it.
The technical nitty-gritty
At the interface pins, the dual-to-single adapter has the pinout of a standard single-channel op-amp. The circuit board is wired as shown in the figure below. (The power supply connections to the dual op-amp are not shown.)
Figure: Dual-to-single op-amp adapter pinout
The output of one op-amp channel feeds the input to the other. The second stage is configured for unity gain. The outputs are summed together with high-linearity, 1/4W metal film resistors. These resistors provide a little bit of isolation to prevent the outputs from driving each other directly. The dual op-amp outputs have nearly identical audio signals, but the DC offset and output noise can be different between the two channels. This can cause a small AC or DC voltage drop across the summing resistors, but even assuming the the worst-case specs for the AD8620, the calculated power dissipation is negligible (a few nW).
For some single-channel op-amps, pins 1, 5, and 8 have functions related to DC offset nulling. In this case, these pins are not connected to anything on the adapter board. (DC offset removal in the UHA-6S.MKII is accomplished in a different circuit.)
Technically, the signal through the second stage is delayed relative to the first due to the additional signal propagation path, but this delay is negligible in the audio frequency range.
The dual-to-single adapter can be used in most common circuit topologies, including inverting and non-inverting stages. With external feedback applied, the summing resistors appear inside the feedback loop, and therefore they typically have a negligible effect on the apparent output impedance of the circuit.
The table below compares UHA-6S.MKII performance with the AD8610 and AD8620 op-amps.
|Table: UHA-6S.MKII performance, AD8610 vs AD8620|
|16 ohm headphones||Max output power:||25 mW||80 mW|
|THD+N at 1 mW:||0.0033%||0.0017%|
|THD+N at 10 mW:||0.0045%||0.0017%|
|THD+N at 50 mW:||n/a||0.0049%|
|32 ohm headphones||Max output power:||55 mW||140 mW|
|THD+N at 1 mW:||0.0018%||0.0013%|
|THD+N at 10 mW:||0.0022%||0.0010%|
|THD+N at 50 mW:||n/a||0.0098%|
|THD+N at 100 mW:||n/a||0.023%|
|62 ohm headphones||Max output power:||90 mW||120 mW|
|THD+N at 1 mW:||0.0012%||0.0011%|
|THD+N at 10 mW:||0.0014%||0.0008%|
|THD+N at 50 mW:||0.022%||0.0049%|
|THD+N at 100 mW:||n/a||0.011%|
|100 ohm headphones||Max output power:||90 mW||90 mW|
|THD+N at 1 mW:||0.0010%||0.0010%|
|THD+N at 10 mW:||0.0011%||0.0008%|
|THD+N at 50 mW:||0.012%||0.0033%|
|300 ohm headphones||Max output power:||40 mW||40 mW|
|THD+N at 1 mW:||0.0008%||0.0007%|
|THD+N at 10 mW:||0.0023%||0.0011%|
|Output impedance:||< 0.2 Ω||< 0.2 Ω|
|Battery life, analog mode:||~32 hrs||~20 hrs|
|Battery life, USB mode:||~10 hrs||~9 hrs|