The β22 Stereo Amplifier

Before you start

Use the step-by-step procedure below to set up your amplifier after you have completed assembling all parts on the circuit board and wired up all its input and output connections. Read through all steps first and familiarize yourself with what you're about to do before proceeding. Do each step in order, and don't proceed to the next step until you have successfully completed each step.

Do not connect the power supply or turn on the power until called for.

You will need a multimeter for this procedure (a digital multimeter is preferable). If at any point your measured voltages do not match what is described, immediately remove power and check the board and wiring for errors. Be careful not to short circuit any component with your test probe leads. Also, if you have sharp meter probes, take care not to pierce through the circuit board soldermask when making measurements to prevent a short circuit to the ground plane.

Several steps of this procedure requires you to be measuring the voltage across a resistor while adjusting a trimpot at the same time. Unless you have three hands this is tricky to do, and a slipped meter probe could cause a short circuit and lead to damage. Thus, please use mini-grabber adapters for your multimeter's probes (e.g., Radio Shack 270-334) so that they can be securely clipped to the resistor. This also prevents a sharp meter probe from scratching through the board's soldermask and shorting to the ground plane.

Initial setup & adjustments

The following procedure describes the steps to set up one β22 board. You should repeat this for each board in your amplifier.
  1. Pre-set the VR1 and VR2 trimpots to their minimum positions (fully counter-clockwise). You should feel a slight click when the stop is reached. Pre-set the VR3 trimpot to approximately the center of their control range. For example, if you use a 20-turn trimpot then rotate counter-clockwise to its stop, then rotate 10 turns clockwise.

  2. Measure the resistance between the V+ and G, as well as V- and G. They should not be short-circuited. Similarly, measure the resistance between the Output pad and ground. There should be no shorts.

  3. Make sure your power supply outputs the correct DC voltage on each rail. See the Power supply section.

  4. Connect the power supply to the amplifier. Be absolutely certain that you did not mix up the V+, G and V- wires. Set the volume control to its minimum position (fully counter-clockwise).

  5. Turn on the power. The power LED(s) should glow. Measure the DC voltage from V+ to G and also from V- to G. These should be the expected PSU rail voltages.

  6. Measure the DC voltage across the R9 resistor. Adjust the VR1 trimpot until you get 4.5V. This corresponds to 2mA through the input stage. Check the voltage across the R10, R11 and R12 resistors. They should all be approximately 4.5V.

  7. Set your multimeter to DC mV range. Measure the DC voltage across the R34 resistor. Adjust the VR2 trimpot until you achieve the desired voltage drop across it. This voltage is calculated according to the following Ohm's Law formula:
    V = I * R
    where,
    V is the voltage drop across R34 you're trying to achieve (Volts)
    I is the desired output stage quiescent current (Amperes)
    R is the resistance value of R34 (Ω)

    The recommended quiescent current is between 120mA-160mA.

    For example, if your R34 is 0.47Ω, and you're aiming for 120mA quiescent current, then:

    V = 0.12A * 0.47Ω = 0.056V = 56mV

    Check the voltage across the R35 resistor, it should be about the same as what you had set for R34.

  8. Set your multimeter to DC volts range. Measure the voltage between the Output pad and signal ground (you may use the ground pad next to either one of the input terminals). This is the output DC offset. Adjust VR3 until you achieve 0mV. As you adjust the trimpot, switch your multimeter to a more sensitive range (DC mV) to get better display resolution near 0mV. The DC offset may drift a little when the amplifier is warming up. A few mVs is OK.

  9. The heatsink should begin to feel warm. Let the amplifier warm up for about 10 minutes and recheck the voltage across R34. Re-adjust VR2 to achieve 55mV-75mV. Due to the MOSFETs' negative temperature coefficient, the current will drop and stabilize at a lower value as it warms up, so you will have to repeat the adjustment again in order to get the amplifier to idle at 120mA-160mA when it reaches operating temperature.

  10. Re-check the output DC offset. Adjust VR3 to zero it if necessary.

  11. If you have a digital multimeter (DMM), set it to the AC mV range and measure the voltage at the output. The reading should be less than 0.1mV. Note that many DMMs cannot measure such low voltages accurately. Also, if your amp isn't yet mounted in an enclosure and grounded properly with good wire routing, then you may not get the expected measurement result.

  12. Turn off the power and wait at least 30 seconds for voltages to drop.

  13. Repeat all the above steps for the other channels.

  14. Optional: If you have an oscilloscope, you may wish to connect it to the amplfier outputs to check for oscillation.

  15. The initial setup is done. Before you connect any input source to the amplifier, measure the source to make sure it has no DC offset at its output. Any such DC offset will be multiplied by the gain of the β22 amplifier.

  16. If all is well, you can now connect the input source as well as headphones, and turn the volume up for a listen.

Notes

  • Never connect headphones to the output if the DC offset voltage is not lower than a few millivolts. Otherwise damage to your headphones may occur.

  • The 120mA-160mA MOSFET quiescent current recommendation is not a rigid requirement. It is an aggressive bias setting, deeply in class A, for any headphone loads. Depending on your heatsink choice and enclosure ventilation, this level of bias may cause the MOSFETs to run too hot. It is OK to reduce the bias and run the MOSFETs at 100mA or even somewhat lower to keep the temperature reasonable. If you have a DMM with a thermocouple, you could measure the heatsink and case internal temperature (with cover closed).

  • Do not use an unventilated case, the MOSFETs may overheat, and the other parts (in particular the electrolytic capacitors) may also suffer reduced lifespan while being enclosed in a hot enclosure.

  • If you are going to be moving the board, desolder parts or otherwise work on the board after turning off the power, be sure to wait enough time to let all capacitors discharge completely. Stored charge in the capacitors could damage parts if board traces are short-circuited to each other by contacting other objects or the soldering iron.

  • The TRS headphone plug creates momentary short circuits while being inserted or removed. Since β22 does not have output current limiting, such short circuits could potentially damage the output MOSFETs. If your R34 and R35 resistors are the default 0.47Ω, it is recommended that you turn off the amplifier before inserting/removing the TRS plug. Or, at the very least, turn the volume to minimum and insert/remove the TRS plug very quickly. These notes do not apply to XLR plugs. Please see the notes about R34 and R35 in the Parts list section.

Troubleshooting

The DC offset trimpot adjustment range was intentionally designed to be relatively small for high resolution of adjustment. If your β22 passes all setup procedures except that you cannot set the DC offset to within ±5mV at the limit of the trimpot travel, here are some things you can do to remedy the situation:
  • The input JFETs are matched within acceptable limits, but the N-channel and P-channel devices are installed in a manner such that their deviation from perfect match becomes additive. In this event you can try swapping the order of Q1 and Q2, but keep Q3 and Q4 where they are. Alternatively swap Q3 and Q4, but leave Q1 and Q2 as-is. Then power-up and re-check the DC offset and see if it could be trimmed down to near zero.

  • The input JFETs may not be matched closely enough. If this is the case, you can try hand-matching additional JFETs for better Idss match and replace Q1 through Q4 with the closest quad you could find.

  • You may increase the DC offset adjustment trimpot range. One way to do this is to reduce the value of R13. You can try going as low as 5KΩ. Note that if this is being done to a left or right channel board, then the other channel should also be similarly modified because this modification will also cause a small change in overall voltage gain.

  • You can also increase the DC offset adjustment range by modifying the D3 and D4 diodes such that they are each two 1N4148 diodes in series.
In case you encounter other difficulties in the initial setup, here are two PDF files showing the β22 schematic diagram and the operating points. All voltages are shown relative to ground. Your actual measured voltages will vary somewhat from the shown values (due to parts tolerances and bias adjustment setting differences). Check your amplifier against these voltages to see if there are undue large deviations, which may help localize problem areas.
Use the β22 forum for help.

Share your joy

When you are all done and the boards are installed in the case, show your pride by submitting photos of your β22 for inclusion in the β22 gallery!

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