Follow this procedure to bring up your EHHA. You will need a digitial multimeter. If something goes wrong, disconnect the power connections immediately. You need to be reasonably skilled at using a DVM and accessing test points on a PCB.
Do not power up the EHHA boards until the power supplies are working properly!
- Verify the 6.3V AC (or DC) heater supply
- Verify the + 30V DC supply
- Verify the - 30V DC supply
A good power supply for the EHHA is the excellent σ22 from AMB Labs. For help and information on this PS see the σ22 web page. If that is still not enough, please post questions in the Headwize support thread for the σ22 / σ11 power supplies.
Each EHHA board needs to be verified independently. For simplicity, do them one at a time. Do not connect your headphones yet. Post questions and problems to the EHHA Headwize support thread.
Initial Checks and Powerup
- With the digital multimeter, check for shorts between ground, the + 30V supply, the -30V supply, the input, the output, and the heater connections on the EHHA amp board. If there is a short, do not continue until it is resolved.
- The servo opamp should not be installed yet.
- Connect the 6.3V AC (or DC) heater supply. Do not turn it on yet.
- Connect the +/- 30V DC supplies. Do not turn it on yet.
- Ensure that the R6 potentiometer is at it's midpoint.
- Ensure that R21 is completely counterclockwise.
- Double check that the connections are made correctly.
- Turn the heater power on. The 6GM8 tube should glow.
- Turn the +/-30V DC power on. The LED should now glow also.
Bias Point Check
- Measure the voltage accross R3 and R4. There should be 1.5V accross each. This tells us that that the bias current in the tubes is the desired 1.5mA per triode.
- Measure the voltage accross R10 and R14. There should be 1.5V accross each. This tells us if the current mirror is working.
- Measure the voltage accross R15 and R17. There should be 0.85V accross each, telling us that the current in the VAS stage is 8.5mA.
- Measure the voltage at pin 6 of the opamp socket. It should be close to 0V. If not, there is an imbalance in the sevro current feedback resistors or current mirror BJTs.
- Measure the voltage accross R16 & R18, R26 & R29. There should be 0.6V accross each, indicating that the servo current feedback is biased at 6mA.
- Measure the voltage at the output. Ideally, it will be close to 0V. If it more than 1.5V away, there should have been an clear imbalance in the previous measurements.
If one or more of these voltages is off, disconnect power and check all component orientations, resistor values, etc.
Output Quiescent Current
- Measure the voltage accross R32 and R33. For the Mosfet version, there should be 0.22V accross each 2.2Ω resistor. For the BJT version, there should be 1.0V accross each 10Ω resistor.
- Adjust potentiometer R21 for the desired voltage accross R32 and R33.
- If the voltage across R32 and R33 cannot be set properly, something is wrong. Double check that the correct resitor values are used in the Vbe multiplier.
Output DC Offset
- Meause the voltage at pins 4 and 7 of the opamp DIP socket. There should be +15V and -15V. If these are incorrect, ensure that the 7815 and 7915 regulators are properly installed.
- Turn the power off.
- Put the servo opamp in the DIP socket.
- Turn the power on.
- Let the servo settle. It should be done in about half a minute.
- Measure the voltage at the opamp output (pin 6). It should be within + /- 10V. If it is beyond this range, there is either an excessive imbalance in the tube or an incorrect component somewhere.
- Measure the voltage at the output. A value less than +/- 5mV is good.
- If the cause of the trouble is an imbalanced tube, you can partially correct this by adjusting R6. Rotate R6 both directions to see if the offset comes to its correct limits. If it doesn't then the tube is too far out of balance or some other component is wrong or defective.
Repeat this entire procedure for the other board(s).
The boards should now be ready for listening.