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 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.
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.
Initial setup & adjustments
- Set the volume control to its minimum (fully counter-clockwise)
position. If you do not have a volume control potentiometer wired
up, then temporarily short circuit the audio input pads while
performing the initial setup.
- Pre-set the R19 and R43 bias adjustment trimpots to their minimum
positions (fully counter-clockwise). You should feel a slight click
when the stop is reached.
- Measure the resistance between the +15V test pad and the star ground.
Similarly, measure the resistance between the -15V test pad and the
star ground. They should not be short circuited. Also, measure
the resistance between each of the OL and OR output pads to the
OG pad. There should also not be any shorts.
- Make sure the DC servo opamp is installed in its socket.
- Install a 250mA slow-blow fuse in your IEC power entry module,
and then plug in the AC power cord.
- Turn on the power. The power LED should glow. If not, check the
LED polarity and the fuse.
- With your multimeter in DC V mode, check the
voltage at the +15V test pad relative to star ground. Similarly,
check the voltage at the -15V test pad. They should both read
close to +15V and -15V, respectively.
- Set the multimeter to DC mV mode. Measure the DC offset voltage
between OL and OG. Similarly, check the voltage between OR and
OG. They should both be under 1mV.
- With your multimeter in DC mV mode, measure the voltage across
R23. Slowly turn the R19 trimpot clockwise until you read
about 8mV. This corresponds to about 17mA of quiescent current
through the output stage. Repeat this while measuring R47 and
adjusting R43.
- Let the amplifier warm up for about 15 minutes. The output
transistors' quiescent current will increase due to the
positive temperature coefficient of BJTs.
- Measure the voltage across the R23 and R47 resistors again.
This time adjust the R19 and R47 trimpots, respectively, until
you read about 14mV. This corresponds to about 30mA quiescent
current. You may need to re-check and re-adjust this several
more times to get it to settle consistently at about 14mV.
The output transistors should feel a bit warm.
- Check each channel's output DC offset again to make sure it remains
below 1mV.
- Optional: If you have an oscilloscope, you may wish to connect it to
the amplfier outputs to check for oscillation.
- If you had the inputs shorted for this procedure, remove the
short and wire up the audio inputs appropriately.
- 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.
- 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 within the specified limit. Otherwise damage to your headphones
may occur.
- 30mA should be adequate quiescent current for most applications.
If your output transistors have heatsinks, then you may increase
the current up to around 60mA, keeping in mind that the additional
heat must also escape the enclosure. Since the Hammond 1455Q case
dpesn't have any ventilation slots, it is recommended that you keep
the quiescent current at the default setting.
- 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.
Troubleshooting
In case you encounter difficulties in the initial setup, here is
a PDF file showing the CK²III 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.
Note that only the left channel is shown. The right channel (with
different part IDs) is otherwise identical. The D1-D8 rectifier
diodes (and their associated snubber capacitors C25-C32) are not shown.
Lastly, the DC servo opamp in the diagram is a OPA134 single-channel opamp,
while the actual one on the board is OPA2134 dual-channel. They are
electricaly identical except for pin number assignments.
Use the
CK²III discussion thread on headwize.com
for help.
Share your joy
When you are all done and the amp is installed in its case, show your
pride by submitting photos of your amp for inclusion in the
CK²III gallery!
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