SparkoS Aries Headphone Amplifier

Interior Circuits

The main audio path is on the right hand side of the unit and consists of two pairs of our SS2590 discrete op amps that make up the input and output stages with the relay switched stepped attenuator in-between them. Our SS3601 and SS3602 discrete op amps handle the XLR inputs and drive the preamp outputs. There are no capacitors in the signal path and potential DC offsets are handled by a DC servo. The power supply is regulated by a pair of our discrete voltage regulators, and the toroidal power transformer is shielded with Mu Metal for extremely low magnetic field pick up and hum. The AC power input is universal and can work with either 120V or 240V. Both the signal and ground side of all inputs are switched with high quality latching relays. Switching the ground side of the inputs (rather than just tying them all together) will prevent ground loops from forming between interconnected pieces of equipment. The micro-controller that controls the attenuator and runs the display will go into sleep mode when the unit is idly playing music to further reduce noise.

Stepped Attenuator

The volume and balance controls are implemented with a reed relay switched stepped attenuator. Using reed relays eliminates much of the annoying "click and clack" that relays have when switching, and instead has a much quieter "ting and tick" that can be heard in the demo video.

"The large knobs have an excellent tactile feel and play, and the relays have a soft, subtly satisfying ‘tick’ rather than an obnoxiously loud ‘clicking’ noise that plagues some stepped attenuators. It’s an incredibly precise system that allows for delicate adjustment and has perfect channel matching even at low levels."

Matty Graham, Headfonia | 2 December 2019

The attenuator is 64 steps, 1dB per step, and will mute when the volume is turned all the way down. The resistors inside the attenuator (and in the audio path as a whole) are all precision 0.1% tolerance thin film resistors. The attenuator has a constant output impedance of 180 ohms for extremely low Johnson noise that is consistent across all volume settings.

Stepped attenuators give superior right to left volume level matching that is critical when listening to headphones. Pot or Rheostat based volume controls, even the good ones like the Alps Blue Velvet typically have something like a +/-20% tolerance so the left to right channel matching can easily be off by a couple dB. Their output impedance (and associated Johnson noise) also varies by where the volume control is set. For example, a 50K volume control set to the middle will have 12 times the noise (22dB) of the Aries volume control, and about twice the noise as the entire Aries amplifier.