transistor headphone amp
I must have repaired hundreds of amps over the last 30 years and I have collected a box full of 'japanese' transistors as a result of this. I also could do with a battery powered headphone amp to complete my mobile recording set.
Now, for the best result, one could use an opamp and two 9 volt batteries and bob's your uncle.
However, it's more fun to cook something up with a few transistors.
Time for some design goals:
I used a topology similar to my transistor preamp.
There is no particular reason for the transistor selection other than that I had them at hand; The A970 and C1818 are low noise types and the C2229 and A948 are in fact high voltage types. I have no spice models for these transistors, I tweaked everything on a breadboard (the fun part of the project).
Power and low idle current do not go together very well in this design but with the given values I ended up with 2 mA of idle current.
I cannot compare with other cans but the DT100 is already loud enough with 0.3V . Time will tell but 10 dB of gain should be enough. The gain can easily be set by changing R5.
Using a toggle switch to power the box is not a good idea I'm afraid. When you throw this in a bag you can be certain that the battery will be flat when you take it out of the bag...
I decided to use a cheap 3X4 rotary switch to switch the power and use the 3 remaining positions to set the volume in roughly 10 dB steps.
As usually I matched the resistors for the attenuator.
A quick check with rmaa
Ruler flat as expected, The top roll-off is due to the sound card. The -3 dB point is around 150 kHz.
Distortion is around 0.01 %
Crosstalk
battery management
The NIMH battery needs to be protected against undervoltage. As I understand it is not very good to drain it below 6.5V so I had to find a way to cut off the power. I could have used the little micro as in my battery box but since I had the cute battery meter I looked for something simpler. Also a micro defeats a bit the vintage spirit of the project.
I have a bag of BS250 P-mosfets and was about to breadboard something with zenerdiodes and what not.
when I discovered that it works fine with just a resistor divider. Using a trimmer I could easily find a suitable setting where the BS250 starts to cut out around 7V. Under 6.5 V it totally cuts off.
The battery meter kicks in at 6.8 V and I chose a resistor value to have the midpoint at 8.4 V.
Charging the battery with a fast charger (the recommended method for NiMH) was not an option as the case has no battery compartment. After some googling I chose to slow charge the battery at 0.05 C, while not ideal it is a safe method. The only drawback is that you better disconnect the power supply after a day of charging.
Max cell voltage for NiMH is 1.8 V, There are 7 cells so by using a 12 V power supply this upper limit cannot be reached.
The vintage style pointer knob completes the project.
Elsewhere on the net
Now, for the best result, one could use an opamp and two 9 volt batteries and bob's your uncle.
However, it's more fun to cook something up with a few transistors.
Time for some design goals:
- 9V NiMH battery power as I have a few laying around.
- low idle current
- must drive my 400 Ohm DT100
- must accept -10 dBV levels
I used a topology similar to my transistor preamp.
There is no particular reason for the transistor selection other than that I had them at hand; The A970 and C1818 are low noise types and the C2229 and A948 are in fact high voltage types. I have no spice models for these transistors, I tweaked everything on a breadboard (the fun part of the project).
Power and low idle current do not go together very well in this design but with the given values I ended up with 2 mA of idle current.
I cannot compare with other cans but the DT100 is already loud enough with 0.3V . Time will tell but 10 dB of gain should be enough. The gain can easily be set by changing R5.
Using a toggle switch to power the box is not a good idea I'm afraid. When you throw this in a bag you can be certain that the battery will be flat when you take it out of the bag...
I decided to use a cheap 3X4 rotary switch to switch the power and use the 3 remaining positions to set the volume in roughly 10 dB steps.
As usually I matched the resistors for the attenuator.
A quick check with rmaa
Ruler flat as expected, The top roll-off is due to the sound card. The -3 dB point is around 150 kHz.
Distortion is around 0.01 %
Crosstalk
battery management
The NIMH battery needs to be protected against undervoltage. As I understand it is not very good to drain it below 6.5V so I had to find a way to cut off the power. I could have used the little micro as in my battery box but since I had the cute battery meter I looked for something simpler. Also a micro defeats a bit the vintage spirit of the project.
I have a bag of BS250 P-mosfets and was about to breadboard something with zenerdiodes and what not.
when I discovered that it works fine with just a resistor divider. Using a trimmer I could easily find a suitable setting where the BS250 starts to cut out around 7V. Under 6.5 V it totally cuts off.
The battery meter kicks in at 6.8 V and I chose a resistor value to have the midpoint at 8.4 V.
Charging the battery with a fast charger (the recommended method for NiMH) was not an option as the case has no battery compartment. After some googling I chose to slow charge the battery at 0.05 C, while not ideal it is a safe method. The only drawback is that you better disconnect the power supply after a day of charging.
Max cell voltage for NiMH is 1.8 V, There are 7 cells so by using a 12 V power supply this upper limit cannot be reached.
The vintage style pointer knob completes the project.
Elsewhere on the net
cadac mixer schematics
amp design
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