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:
- Rail voltage = between 7V and 12V to allow use of a 9V internal battery and an external 12V supply.
- Total current drain = <3mA.
- Frequency range = 200Hz - 3000Hz.
- Maximum input signal amplitude = 100mVp-p.
- Output signal amplitude = 1V.
- 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).
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| 250mV Output for 22mV Input. Gain =11.4. No Distortion |
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| 800mV Output for 100mV Input. Voltage gain = 8. Distorted. |
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| Prototype 1 |
As I used to be an active pilot I had a DC H10.13.4 headset. When I operated QRO my ICOM 746 would drive the audio sufficiently but I needed an adapter for the mic on the headset. I wired up an adapter for the mic and experienced my first real RF bite as the mic touched my lips. I tried all manner of things to shield that connection to no avail.
ReplyDeleteWhen you work up a clean amp circuit I'd be interested in using it to drive my external Yaesu PA speaker. Ive tried choking the power supply to my little radio shack speaker amplifier but it still picks up RF on 30m.
As I used to be an active pilot I had a DC H10.13.4 headset. When I operated QRO my ICOM 746 would drive the audio sufficiently but I needed an adapter for the mic on the headset. I wired up an adapter for the mic and experienced my first real RF bite as the mic touched my lips. I tried all manner of things to shield that connection to no avail.
ReplyDeleteWhen you work up a clean amp circuit I'd be interested in using it to drive my external Yaesu PA speaker. Ive tried choking the power supply to my little radio shack speaker amplifier but it still picks up RF on 30m.
Will do Richard and many thanks for the comments and all noted. It took me sometime to find the correct jack for the mic. Eventually I sourced one from Aircraft Spruce. So my plan is to first get the headset correctly matched. Thereafter some time down the road I will tackle the mic. That will also need a pre-amp I think and of course a voltage source as you indicate. I think the radio shack speaker issue needs a different solution to this high impedance headphone kind of amp since I am assuming it is a low impedance.
DeleteRichard, you have made the same mistake I used to make when I started design. Your amplifier can not supply enough current for your load. AT 1V from Ohms law your 150 Ohm headphones need about 7 mA, or more likely 10mA to stay linear. Change your R3 to 270 Ohms and scale your other values and you should be okay. 73's Mark VK6WV
ReplyDeleteHi Mark,
Deletemany many thanks for your reply. You will be amused to hear that I figured it out about 30 minutes after my experiment to increase the voltage across the emitter resistor. We then went on a long sunday evening to take our dog for a walk. That is when the penny dropped hi! Of course I was starving the transistor of current since it was being consumed by the load and thus turning off. In fact the calculations show that the transistor would indeed turn off at about 250 - 300mV load voltage.
Now I thought that R3 (collector resistor) should be changed to 150ohms and that about 14mA (or more) would be needed. With a collector load of 150ohms I think I would get maximum power transfer to the load. Please kindly let me know why you recommend 270 Ohms? Thank you Mark and your comments are exactly what I was hoping for by posting the write up. 73. Dick
Hi Mark
DeleteOk I think I know why you recommend 270 ohms as opposed to 150 ohms for the collector resistor. This is because I only need 1V p-p output. I could get more power into the headphones than 1V if I used 150 Ohms. However this would require higher quiescent current.
Richard Yes that is the reason. Class A is very inefficient unfortunately. If current draw is an issue maybe an audio output transformer and a slightly different design could reduce current drain to 1/2 or for a good design 1/3 of what a resistor based class A amp will draw. Of course the trade off is the cost of the transformer and the fact that the range available these days is greatly reduced. If you can find an old driver transformer which used to drive a push pull transistor output stage it should do the job.
ReplyDelete73 Mark VK6WV