https://theultimatetone.com/ Mon, 11 May 2026 12:48:01 +0000 MyBB https://theultimatetone.com/Thread-Headphone-amps-for-dynamic-standards Thu, 31 Aug 2023 20:28:39 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Headphone-amps-for-dynamic-standards
The every-day working man's headphone has always been the dynamic type, where the transducers are simply small dynamic loud speakers. These will be 8R up to 600R.

Traditional headphone outputs from power amplifiers were simply a set of 100R-1W dropping resistors feeding a stereo jack that had closed contacts to disable the main speakers. This meant that whatever the big PA was, it would be idling however hot it normally would, providing the usual voltage swing at much reduced current.

Preamps might have a headphone output that was fed from the main output, again with closed switches on the headphone jack, or from an auxiliary output dedicated to the task. If the latter, there would generally be a headphone Level control.

Dedicated headphone circuits were often just a couple of transistors before ICs became dominant, in which case a generic opamp circuit would drive the headphones. In all cases, there would be a low-value resistor added in series with each cup. This actually reduces THD and stabilises the driver from the cable impedance. At the lowest impedance, parallel opamps or a buffer help for high-SPL listening. 1mW at 8R is only 90mV at 9mA. 1mW at 600R is 770mV at 1m3A. These are RMS; peak values are 40% higher. Nested feedback circuits or discretes can push THD to inaudibility.

Most opamps can drive 1k loads, with more now capable and rated for 600R. The ubiquitous 5532 is rated for 600R loads yet it can drive 30R cups through 50R resistors quite well. For most people who only casually use headphones, this is adequate. The nice thing about opamps and most similar discrete circuits is the high power supply rejection ratio (PSRR), which is the ability to ignore supply noise. As a headphone driver with modest or unity gain, the PSU can be quite "cheezy" and still be hum-free.

Of course, with the low power requirement of headphones, we can be inventive and creative  and use tubes, transformers, or hybrid circuits as we see fit. Whatever your aesthetic is, just go for it.]]>
The every-day working man's headphone has always been the dynamic type, where the transducers are simply small dynamic loud speakers. These will be 8R up to 600R.

Traditional headphone outputs from power amplifiers were simply a set of 100R-1W dropping resistors feeding a stereo jack that had closed contacts to disable the main speakers. This meant that whatever the big PA was, it would be idling however hot it normally would, providing the usual voltage swing at much reduced current.

Preamps might have a headphone output that was fed from the main output, again with closed switches on the headphone jack, or from an auxiliary output dedicated to the task. If the latter, there would generally be a headphone Level control.

Dedicated headphone circuits were often just a couple of transistors before ICs became dominant, in which case a generic opamp circuit would drive the headphones. In all cases, there would be a low-value resistor added in series with each cup. This actually reduces THD and stabilises the driver from the cable impedance. At the lowest impedance, parallel opamps or a buffer help for high-SPL listening. 1mW at 8R is only 90mV at 9mA. 1mW at 600R is 770mV at 1m3A. These are RMS; peak values are 40% higher. Nested feedback circuits or discretes can push THD to inaudibility.

Most opamps can drive 1k loads, with more now capable and rated for 600R. The ubiquitous 5532 is rated for 600R loads yet it can drive 30R cups through 50R resistors quite well. For most people who only casually use headphones, this is adequate. The nice thing about opamps and most similar discrete circuits is the high power supply rejection ratio (PSRR), which is the ability to ignore supply noise. As a headphone driver with modest or unity gain, the PSU can be quite "cheezy" and still be hum-free.

Of course, with the low power requirement of headphones, we can be inventive and creative  and use tubes, transformers, or hybrid circuits as we see fit. Whatever your aesthetic is, just go for it.]]> https://theultimatetone.com/Thread-Headphone-amps-for-planar-magnetics Thu, 31 Aug 2023 20:10:28 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Headphone-amps-for-planar-magnetics
Planar magnetic headphones offer very high SPLs at pretty low distortion. The load is almost purely resistive, therefore it is easy to drive. The voltage and power required is also quite low.

Remember: The sensitivity rating  for headphones is at 1mW input, and this often produces 100dB of sound.

The leader in the planar magnetic headphone camp is Audeze. Their models have become rather diffuse inasmuch as every model is "the best" but they are styled for specific applications and music types. The element to be driven is a resistive track laid down on thin film. The resistance is usually 20R to 110R. At 1mW, the drive required is 140mV up to 330mV. at 7mA to 3mA, respectively. These are RMS values. You can see that driving the magnetic planar headphones is not very difficult.

If you want to push the drivers to their claimed 130dB limit, a bit more drive is needed, at 4V5 to 10V5 respectively for 20R and 110R. These voltage are why you see a recommendation to use a 5W amp, or a 10W amp. It is not that the headphones need the power; rather, that they need the voltage if driven single-ended. Differential drive cuts the voltage swing in half as far as each driver is concerned, except now you need two drivers per cup with symmetric signals.

Whether single-ended drive is used or differential, standard opamp circuits with or without buffers added will work reasonably well. The ultimate performance is attained using discrete fully complementary circuits. Wherever possible, we use inverting gain stages to eliminate common-mode distortion, but we have to be careful to preserve overall phase coherence from input to output. Because this is all low-voltage +/-24V solid-state circuitry, distortions of all types can be pushed to -140dB.]]>
Planar magnetic headphones offer very high SPLs at pretty low distortion. The load is almost purely resistive, therefore it is easy to drive. The voltage and power required is also quite low.

Remember: The sensitivity rating  for headphones is at 1mW input, and this often produces 100dB of sound.

The leader in the planar magnetic headphone camp is Audeze. Their models have become rather diffuse inasmuch as every model is "the best" but they are styled for specific applications and music types. The element to be driven is a resistive track laid down on thin film. The resistance is usually 20R to 110R. At 1mW, the drive required is 140mV up to 330mV. at 7mA to 3mA, respectively. These are RMS values. You can see that driving the magnetic planar headphones is not very difficult.

If you want to push the drivers to their claimed 130dB limit, a bit more drive is needed, at 4V5 to 10V5 respectively for 20R and 110R. These voltage are why you see a recommendation to use a 5W amp, or a 10W amp. It is not that the headphones need the power; rather, that they need the voltage if driven single-ended. Differential drive cuts the voltage swing in half as far as each driver is concerned, except now you need two drivers per cup with symmetric signals.

Whether single-ended drive is used or differential, standard opamp circuits with or without buffers added will work reasonably well. The ultimate performance is attained using discrete fully complementary circuits. Wherever possible, we use inverting gain stages to eliminate common-mode distortion, but we have to be careful to preserve overall phase coherence from input to output. Because this is all low-voltage +/-24V solid-state circuitry, distortions of all types can be pushed to -140dB.]]> https://theultimatetone.com/Thread-Headphone-amp-for-electrostatics Thu, 31 Aug 2023 19:50:35 +0000 K O'Connor]]> https://theultimatetone.com/Thread-Headphone-amp-for-electrostatics
The lowest-distortion headphones available are electrostatic types. The most famous brand offering these, Stax, from Japan, lovingly calls the headphones "ear speakers".

Electrostatic headphones require a high driving voltage, plus a high fixed bias voltage, none of which can be provided by a standard audio power amplifier. The bias voltage is between 200V and 600V and is current-limited by 5-10Megohms. The reference drive is 50Vrms + 50Vrms for 100dB of acoustic output. The listing of two voltages means there is differential drive, two inputs of opposite phase, which could also be listed as 100Vrms differential drive.

The headphone element has two fixed elements to which the audio signals are applied. Between these is a metalised film of thin plastic that has the fixed voltage applied to it. As the signal voltages move positive and negative, the positive-charged middle element is pulled towards one stator while being pushed by the other, then drawn and pushed to the opposite stator over the audio signal cycle. Since there is electrostatic force applied evenly over the movable film, it can move very quickly and as a rigid plane with very low distortion.

The stators appear capacitive to the amplifier, typically around 100pF or so each. This is not a particularly difficult load except that the charging and discharging currents can be high if there is no current limiting during fast transitions.

Stax offered many types of ear speaker drivers ranging from passive to exotic, setting the standard for others like Koch to follow. The passive driver uses a step-up transformer to raise the voltage needed for 8R speakers up to the requirement for the electrostatic panels. The secondary of the transformer is center-tapped, with the CT grounded, creating the symmetric anti-phase drive signals. A small transformer provides mains isolation and feeds a voltage multiplier to generate the bias voltage. Compared to the distortion room loudspeakers add, the distortion of the transformer and the very-low-THD of the ear speakers combined is much lower, and usually lower than a conventional dynamic headphone driven through the usual 100R dropping resistor from the amplifier output.

Dedicated headphone amplifiers for ES were introduced which allowed the user to turn off his large amp and not waste all that power, driven by the system preamplifier / source selector. The dedicated driver is always made as a differential amplifier from input to output, with dual feedback loops. Internal voltages are typically +/-400Vdc, providing an incredible signal swing approaching 280Vrms x2. However, considering that a combined 100Vrms produces 100dB of output, these rails would allow 5.6 times the total voltage swing, assuming no losses, which corresponds to only 15dB more sound. We are already way above the Human Scale of Loudness, so all this headroom gets us is freedom from clipping on signal peaks. We would also expect there to be better linearity when not using so much of the available swing.

The first diff-amp offerings had open-collector outputs, where there is active pull-down and passive pull-up. This was because the highest-voltage semiconductors were NPN. Even with this, cascoding was used throughout to distribute voltage stress and to improve high-frequency response. The next iteration added buffering with current-source loading for active up/down load control. This reduced THD slightly. A further variant was like the first with the high-voltage output transistors replaced with a dual-triode tube.

Stax offered a lower-output battery-powered driver that unfortunately used a switch-mode power supply. The hash from the SMPS was all over the output and the THD was poor and fidelity low. On a good analogue scope or any DSO, the SMPS noise at the output is terrifyingly ugly.

Personally, I have built drivers for Stax headphones using linear +/-100V rails (same as the Stax unit above) and have zero issue with clipping. THD is unmeasurable. The topology is a typical form using only inverting gain blocks and fully-symmetric circuits. Now that we have highly complementary semiconductor pairs, circuits like this are very common. As an experiment, I switched to 40V rails and still had no evidence of clipping when listening at sane loudness levels.

Many headphone enthusiasts have designed all-tube, hybrid and solid-state drive circuits for ES cups and report their findings. Some of the listening tests seem a bit scary to me, when they have super-high-voltage drive circuits and can hear clipping !]]>
The lowest-distortion headphones available are electrostatic types. The most famous brand offering these, Stax, from Japan, lovingly calls the headphones "ear speakers".

Electrostatic headphones require a high driving voltage, plus a high fixed bias voltage, none of which can be provided by a standard audio power amplifier. The bias voltage is between 200V and 600V and is current-limited by 5-10Megohms. The reference drive is 50Vrms + 50Vrms for 100dB of acoustic output. The listing of two voltages means there is differential drive, two inputs of opposite phase, which could also be listed as 100Vrms differential drive.

The headphone element has two fixed elements to which the audio signals are applied. Between these is a metalised film of thin plastic that has the fixed voltage applied to it. As the signal voltages move positive and negative, the positive-charged middle element is pulled towards one stator while being pushed by the other, then drawn and pushed to the opposite stator over the audio signal cycle. Since there is electrostatic force applied evenly over the movable film, it can move very quickly and as a rigid plane with very low distortion.

The stators appear capacitive to the amplifier, typically around 100pF or so each. This is not a particularly difficult load except that the charging and discharging currents can be high if there is no current limiting during fast transitions.

Stax offered many types of ear speaker drivers ranging from passive to exotic, setting the standard for others like Koch to follow. The passive driver uses a step-up transformer to raise the voltage needed for 8R speakers up to the requirement for the electrostatic panels. The secondary of the transformer is center-tapped, with the CT grounded, creating the symmetric anti-phase drive signals. A small transformer provides mains isolation and feeds a voltage multiplier to generate the bias voltage. Compared to the distortion room loudspeakers add, the distortion of the transformer and the very-low-THD of the ear speakers combined is much lower, and usually lower than a conventional dynamic headphone driven through the usual 100R dropping resistor from the amplifier output.

Dedicated headphone amplifiers for ES were introduced which allowed the user to turn off his large amp and not waste all that power, driven by the system preamplifier / source selector. The dedicated driver is always made as a differential amplifier from input to output, with dual feedback loops. Internal voltages are typically +/-400Vdc, providing an incredible signal swing approaching 280Vrms x2. However, considering that a combined 100Vrms produces 100dB of output, these rails would allow 5.6 times the total voltage swing, assuming no losses, which corresponds to only 15dB more sound. We are already way above the Human Scale of Loudness, so all this headroom gets us is freedom from clipping on signal peaks. We would also expect there to be better linearity when not using so much of the available swing.

The first diff-amp offerings had open-collector outputs, where there is active pull-down and passive pull-up. This was because the highest-voltage semiconductors were NPN. Even with this, cascoding was used throughout to distribute voltage stress and to improve high-frequency response. The next iteration added buffering with current-source loading for active up/down load control. This reduced THD slightly. A further variant was like the first with the high-voltage output transistors replaced with a dual-triode tube.

Stax offered a lower-output battery-powered driver that unfortunately used a switch-mode power supply. The hash from the SMPS was all over the output and the THD was poor and fidelity low. On a good analogue scope or any DSO, the SMPS noise at the output is terrifyingly ugly.

Personally, I have built drivers for Stax headphones using linear +/-100V rails (same as the Stax unit above) and have zero issue with clipping. THD is unmeasurable. The topology is a typical form using only inverting gain blocks and fully-symmetric circuits. Now that we have highly complementary semiconductor pairs, circuits like this are very common. As an experiment, I switched to 40V rails and still had no evidence of clipping when listening at sane loudness levels.

Many headphone enthusiasts have designed all-tube, hybrid and solid-state drive circuits for ES cups and report their findings. Some of the listening tests seem a bit scary to me, when they have super-high-voltage drive circuits and can hear clipping !]]>