David Clark Headphones amplifier design 2

The simple common emitter circuit was redesigned with an increased target collector current of 10mA. This allowed enough current to supply the 150 ohm load and ensure that the transistor was operating as a constant current source.   

Varying emitter degeneration and/or collector current(by increasing the rail voltage)  demonstrated the effect on amplifier voltage gain and dynamic range. As can be seen from the measurements, collector current increase results in amplifier dynamic range improvements (seen as reduced gain compression in the measurements). Also increasing emitter degeneration improves dynamic range but at the expense of amplifier voltage gain.

So you cannot 'have your cake and eat it'. Better dynamic range comes at the expense of increased battery consumption or reduced gain.  With a 7V supply I found the gain limit to be less than 6.5. Increasing the gain to the 10 - 13 range required a collector current of 18mA. Clearly either option was problematic for this application. 

After increasing the emitter degeneration resistor to 15 ohms I then tried the amplifier connected to my K2 and using a Vcc of 7.5 volts.  Listening to our club evening 80m net with high QRN I was very interested to note slight distortion in the headphones. The received CW note sounded undistorted to my ear at normal listening levels. However the background noise sounded slightly distorted. The measured 3dB points of the amplifier were 200Hz - 1800Hz. When the K2 audio gain was turned down to a low listening level no distortion was detected. So in spite of measuring an undistorted (as far as I could tell) 1kHz output sinewave approaching 600mV (p-p)  on my bench. In practice the noisy band sounded distorted. Could this be due to noise spikes from the QRN exceeding the dynamic range of the amplifier? Using a single signal input to simulate a real radio channel is simplistic. 

Next it would be interesting to try a 2 transistor amplifier using an emitter follower on the output. This would allow an increase in the load resistance of the common emitter input stage which would allow improved dynamic range and gain at a lower collector current. However at least 10mA would be needed to drive the emitter follower. Is this a zero sum game?

20mV input, 128mV output, Vcc=7V Gain (V) = 6.4 'undistorted'

100mV input, 500mV output, Vcc=7V. Gain (V) = 5 'distorted'

250mV input, 1.3V output, Vcc=7V. Gain(V) = 5.2 'distorted'

250mV input, 1.55 V output, Vcc = 12V. Gain(V) = 6.2 'undistorted'

David Clark Headphones amplifier design. 1

I was lucky enough to receive a pair of David Clark Headphones from my kids as a birthday gift. The model I have is the classic headset and mic designed for use in General Aviation cockpits and known as Type H10.13.4. This is the best selling headset in aviation. They fit tightly over the head and have a 33dB (@1000Hz) external attenuation.

For amateur radio use I found them to be a bit quiet and require me to turn up the AF Gain on my K2 Elecraft rig to near maximum volume before I could hear them.

I figured it would be a great project to try to build an audio booster amplifier from scratch.

The design in this blog post did not meet the objective.

The headset has an impedance of 150ohms. I determined through listening to a 1KHz tone that a peak-peak voltage of 1V maximum was required to drive the headset to a loud volume. However output distortion occurs when greater than 250mV. This distortion could be easily heard in the headphones. The reason has not been determined. Next I will try raising the emitter voltage to greater than 1V.

As a part of the design process a 150Ohm resistor was used. The results were the same when the headphones were actually connected to the output.

Design Input requirements:

1. Rail voltage = between 7V and 12V to allow use of a 9V internal battery and an external 12V supply.
2. Total current drain = <3mA.
3. Frequency range = 200Hz - 3000Hz.
4. Maximum input signal amplitude = 100mVp-p.
5. Output signal amplitude = 1V.
6. Load impedance = 150 Ohms.

The common emitter design is based on achieving a gain of 10. 

Calculated and installed component values are shown on the included schematic. 

Calculated and measured parameters are shown on the included schematic.

The 3dB filter roll-off points were measured as 200Hz and 1800Hz (approximately).

250mV Output for 22mV Input. Gain =11.4. No Distortion

800mV Output for 100mV Input. Voltage gain = 8. Distorted.

Prototype 1