Headphones and amplifiers: down the hill of impedance mismatches
Headphones are very sensitive devices (they require little power) but are actually very demanding due to the their ability to reveal every single detail of the records so low noise and precise amplifiers are mandatory. To make things more complex, the headphones do not have an easy and flat impedance across the audio spectrum and the matching with the amplifier may be unfortunate. Low and very low output impedance amplifiers can drive the headphones with accuracy and this is outmost important with the latest high sensitivity planars whose typical impedance is in the range of tens of ohm. The old rule of choosing an amplifier whose output impedance is as 1/10 or lower than the headphone impedance holds true.
Please note that the schematics in this paper are for example only and will require additional parts to work properly as an actual headphone amplifier.
Our take on headphone amplifiers
It is no mistery I like vacuum tubes and jFETs but the benefits and advantages of MOSFETs can play a very important role too so it would come with no surprise if I used them all in an headphone amplifers. The SMA uses a differential gain stage based on tubes that sports very low loise and distortion and is coupled to an intermediate jFETs buffer capable of swinging the signal at the gates of the output mosfets overcoming their capacitance with ease. This solution blends the best of the available devices securing top performances and great sound quality.
In the next weeks we will also unvheil details of a newly designed headphone amplifier for the BeCubes lines. Needless to say, reality is more complex and the headphone-amplifier matching is affected by other phenomena too and the design of an amplifier requires other tricks and tuning to perfect the end results.
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Author: Gianluca Sperti
Audiodinamica co-founder and product development