Download building the chu moy headphone amplifier

BUILDING THE CHU MOY HEADPHONE AMPLIFIER
Chu Moy is an extremely popular headphone
amplifier designed by a person of the same name
that’s easy to build. The design is extremely simple
yet performs exceptionally. In spite of being such a
simple design the amplifier is very powerful, it drives
even very inefficient headphones to very loud levels
with ease. The amplifier has sufficient gain and
works wonderfully from even weak sources.
There could not be a better project to get started
with diy in audio. The amplifier requires very little
effort in terms of soldering to finish. A through hole
board has been designed keeping in mind ease of assembly even for novices. It’s quite a small PCB in spite of
being a through hole design.
THE AMPLIFIER
The amplifier is really a simple design but there are a few areas that need a bit of explanation.
Power supply
Most high quality opamp that are available from different
manufacturers are really designed to work from bipolar power
supplies i.e. a +ve, –ve and ground. Here comes the challenge,
for a headphone amplifier that you want to be portable the only
source of power would be batteries. Batteries only have +ve and
ground.
To obtain a split supply from a source like a battery novel scheme
is used here(Figure1). Using two resistors of the same value
across the power source and then using the midpoint between
the resistors as a virtual ground. The two resistors act as a
voltage divider each having approximately half the total voltage
over it. For example if you use two 9V batteries in series the total
Figure 1
voltage is 18V, but the voltage across each resistor is 9V. If you refer the
midpoint between the resistors as ground you have a split supply with + 9V, -9V and ground. Note that we
need to measure the voltages from virtual ground.
The capacitors are needed to actually deliver the current that is required to power the amp. The resistive
voltage divider cannot really deliver current; its sole purpose is to create a virtual ground.
This configuration however might become unbalanced with lower impedance loads. A better commonly used
solution would be to use a rail splitter like TLE2426 or build one using a buffer. Further discussion on this topic
is beyond the scope of this document.
Adjusting Gain
Gain simply put is the factor by which the amplifier multiplies its input voltage i.e. Vout / Vin. The amount by
which the opamp multiplies the in input voltage is determined by 1 + Rf /Rg,
how this formula is arrived is beyond the scope of this document.
In the case of our amplifier the gain is set at 11, the amplifier is perfectly happy
at this gain. In fact with most headphones you will not be able to turn the
volume knob beyond the halfway mark. You can set the gain lower by altering
the values of Rf and Rg. It is however recommended not to set very low gain
values < 2. If an opamp is not unity gain stable it will oscillate and play havoc
with your amplifier.
PARTS SELECTION
This section goes through the list of parts required to assemble the amplifier only.
PCB
A lot of builds on the web are done on prototyping board with jumpers and solder bridges; the amplifier is too
simple to become adversely affected by a prototyping board build. A pcb
however is highly recommended, not only it simplifies the build but it
benefits in terms for sound quality and might also stop unwanted
oscillations that might result from poorly done prototyping board builds.
A pcb also looks nicer compared to a prototyping board, we’re sure you
would agree.
We have designed our pcb to make assembly a piece of cake, it is a
doubled sided fibre glass board. The board is fully star grounded
although it might seem over the top for a simple amp like this, but a
good grounding design never hurts.
Capacitors
Very few capacitors are required; however some capacitors are more important than others, as always any
capacitor on the signal path needs to be a film capacitor to maintain the sound quality. A 1uF capacitor is used
as DC blocking capacitor. The original Chu Moy schematic shows a 0.1uf capacitor, however in the interests of
better low frequency reproduction the largest capacitor that you can afford in terms of size and cost should be
used here. We have used 1uF metalized polyester cap here. The 1uf metalized polyester is very reasonably
priced and is quite compact. You can choose not use any DC blocking capacitor at all, but if your source has
any amount of DC offset, it would appear across your headphones also, possibly damaging them.
A pair of large electrolytic caps is used just after the virtual ground resistors; these capacitors actually deliver
all the current. You can go with as little as 47uF but we would recommend slightly larger caps e.g. 220uf would
be a good choice.
The schematic as well as the board show two optional de-coupling capacitors, these are not really required if
you are running your amp off 9V batteries. However if you are going to use a wall adapter for power supply
the two 0.1uf caps are essential.
Resistors
All the resistors in the amplifier are 1% metal film type resistors, although these were considered exotic some
time back but have become de-facto standard for electronics especially in audio video industry. You can
definitely use carbon film resistor in place of metal film ones; there may be a slight degradation in sound
quality. Frankly we haven’t tested with carbon film resistors. There would be hardly any price difference if you
are buying small quantities like the numbers required for assembling this amplifier. If you are not able to buy
metal films locally you can always buy from us. If you chose to you buy the complete kit it will contain metal
film resistors.
Volume control potentiometer
The cmoy as the Chu-moy is lovingly called has become very popular and a lot of international sites and
Chinese sellers offer the amplifier in kit or assembled form. Almost invariably the kit contains the “Alps Blue”
aka Rk097 series potentiometer. The speciality of this pot from Alps is the plastic conductive material used as
the resistive element compared to the carbon in ordinary pots. The popularity of this part has led to lot of
duplicates and astronomical prices for a relatively simple part.
We have after lot of consideration chosen to use a Panasonic potentiometer from the EJ-VC series which also
uses the plastic conductive material but is significantly cheaper and very compact. If you chose to build this
amplifier on our pcb you would have to use this potentiometer as the pcb is specifically designed to work with
this. If you chose to buy the kit, the kit will come with this potentiometer.
Choosing the correct Opamp
This is the single most critical part of this amplifier; the choice not only affects sound quality but power
consumption also. The amplifier may not work at all if you chose an opamp that is susceptible to oscillations.
Some opamps tend to oscillate at very little provocation whilst others are pretty tolerant.
Cost of the opamp is one more factor that needs to be considered before you chose a part. The most popular
choice for the amp is the brilliant OPA2134. This particular opamp although not really cheap is a good
compromise between price and exceptional performance. We wouldn’t say that the OPA2134 is the most
tolerant in terms of oscillations but it’s not too bad. If you construct your amplifier with a properly designed
pcb or proper layout on a prototyping board this opamp performs flawlessly.
A very expensive but high quality suggestion would be National semiconductor’s brilliant LME4562. You could
also use generic but audio grade omamps like NE5534. A good discussion on choosing an opamp can be found
here .
Batteries
Although a single battery is shown in the power supply (Figure1), we would recommend you use two 9V
batteries (in series). The pcb too is designed to accept two 9V batteries in series. One of the reasons behind
this is that most opamp perform well only when supplied with more than -/+ 5 volts.
Alkaline batteries would be the best choice as they last quite a bit longer. You could also use rechargeable
Nimh batteries, although the PCB doesn’t have provisions for charging them. Rechargeable batteries aren’t
exactly cheap but you can use them a lot
ASSEMBLING THE AMPLIFIER
We recommend you open the schematic before you read
this section.
Assembling the amplifier with a pcb should a very straight
forward job and for an experienced builder would hardly
take an hour so to get this amp running.
The first things you should assemble are the power supply
components, which would mean in this case R9, R10, R11,
LED and the two battery connectors. Make sure you
connect the Red wire of the battery connectors to the
points marked + on the pcb.
Once you have ensured the battery connectors are
attached correctly, connect two batteries check the voltages across R10 and R9, you should get +9 and -9V
volts respectively. The LED should also be glowing.
Once you have verified the working of the power supply
section go ahead and assemble the rest of the amplifier,
Figure 2
starting with all the resistors first. The IC socket and the output and input jacks should be soldered next. Input
output jacks both have 5 legs, it might be a bit tricky to get them fitted on the board. If you are not able to fit
in the first go, don’t panic straighten the legs with tweezers and try to fit them again.
The volume potentiometer might also take a bit fiddling before it fits correctly on the board. Do not try too
much force on the pot as you might end up damaging the pot. If you bend some pins in the process straighten
with a pair of tweezers and try to fit again. The side tabs of the potentiometer will be the culprit in most cases.
R7 and R8 should be fitted with wire jumpers; these are needed if your amp is oscillating or unstable. Bend
some resistor legs and fit them to act as jumpers.
SW1 and SW2 are points to connect a switch, if you do not wish to use a switch just solder a wire jumper in
their place. The images in the document show a wire jumper in place SW1 and SW2.
TESTING THE AMPLIFIER
Do not connect your head phones to the amplifier. If you followed the assembly instructions part of the
checks are already done i.e. checking the power supply.
Do not put the Opamp into the socket before you do this test.
Connect the ground lead of your multi meter to the small ground island
(Figure 3) on the bottom of the pcb, with the other lead check the
voltages on pin 8 and pin 4 of the IC. You should get +9V and -9V
respectively.
Once you confirm these voltages, take out the batteries and plug in the
IC into the socket. Be careful not to bend any pins, if the IC does not fit in
the first go align the pins with tweezers and try again. Make sure the IC is
seated firmly in the socket.
Figure
Figure43
Connect the batteries and power up the amplifier. Put your multi meter in
the mili volt test range. Connect one lead of the multi meter to the ground island and with the other lead
check the voltages on Pin1 and Pin7. The voltage should not exceed 20mV. This is the DC offset voltage of the
amplifier. It will never be zero and the offset will be different on each channel. If the DC on the output exceeds
20mV check the amplifier thoroughly for any solder bridges or short circuits. Change the batteries and check,
old batteries can sometimes cause the amplifier to behave erratically.
If you find the DC offset is within 20mV go ahead and connect headphones as well as input source, ensure the
volume pot is turned all the way anti clock wise. Slowly turn up the volume control and enjoy the music!