Properly loading the tubesįor good distortion and power performance, we usually want our transformer to reflect an impedance of at least 3x the plate resistance of the tube. Luckily, headphones need very little power, so we have some leeway in selecting a transformer when it comes to reflected impedance. Headphone impedances are much higher than speaker impedances, which is good, but finding an output transformer with secondary taps for a large range of headphone impedances can be difficult. This is what you see in speaker amps that have to contend with 4 and 8 ohm loads. The other approach for lowering output impedance is to use an output transformer. For reference, the damping factor of a typical 300B amplifier playing through 8 ohm speakers is around 4 or 5. That gives us a damping factor of about 2 with 300 ohm headphones (not terrible), but with low impedance headphones the damping factor is way lower.
In a standard cathode follower configuration, the output impedance of this tube would be around 140 ohms. The de facto tube of choice for this kind of design has been the 6AS7. This has been used in many headphone amp designs to good effect. Luckily, there are a couple of common methods to lower the output impedance of a tube.Ĭathode followers are able to achieve an output impedance of roughly 1/Gm. And who doesn’t love a tight bass? Due to their intrinsic plate resistancethrough, tubes generally have a high output impedance that we need to overcome to achieve a good damping factor. A higher number generally equates to tighter bass. It is calculated by dividing the speaker or headphone’s impedance by the output impedance of the amplifier. So if we’re going to design and build a tube headphone amplifier that will work reasonably well with a variety of models, what should we consider?ĭamping factor is a measure of how well an amplifier controls a transducer (speaker or headphone driver).
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