Euraudio CTA12, CTA14 and CTA14A assembly instructions
Welcome to the Euraudio CTA12, CTA14 and CTA14A assembly instructions page.
The CTA12 and CTA14(A) are low distortion, mono DiY audio power amplifier kits. You need two kits for dual mono or stereo configurations. Please find the CTA12 and CTA14(A) specifications here.
Please read through these instructions before doing anything with the DiY kit.
Differences between the CTA12 and CTA14(A)
The CTA12 should be built to deliver not more than 50 Watts output power, while the CTA14(A) to deliver not more than 100 Watts output power.
Important: The absolute maximum unloaded supply voltage for the CTA12 is +/-35V, for the CTA14(A) it is +/-42V, but we recommend +/-29V unloaded supply voltage for the CTA12, and +/-37V unloaded supply voltage for the CTA14(A).
The recommended transformer and filter capacitor parameters for two CTA12's in a stereo configuration are as follows:
|
Amplifier power |
Transformer |
Recommended fuse (2 pcs) |
Recomm. filter caps (2 pcs) |
|
2x50 W / 4 ohms |
2x21
VAC, 180 VA |
T5A |
10,000uF
/ 40V |
|
2x50 W / 6 ohms* |
2x23 VAC, 180 VA |
T4A |
8,200uF
/ 50V |
* Do not exceed the +/-35V unloaded supply voltage.
The recommended transformer and filter capacitor parameters for two CTA14(A)'s in a stereo configuration with a power supply that has place for 2 filter capacitors (e.g. Euraudio PS1S) are as follows:
|
Amplifier power |
Transformer |
Recommended fuse (2 pcs) |
Recomm. filter caps (2 pcs) |
|
2x100 W / 4 ohms |
2x26 VAC, 320 VA |
T6.3A |
12,000uF
/ 50V |
|
2x90 W / 6 ohms* |
2x29
VAC, 280 VA |
T5A |
12,000uF
/ 50V |
* Do not exceed the +/-42V unloaded supply voltage.
** If the 12.000 uF / 50V value is difficult to obtain, you may use 10.000uF / 63V or 15.000uF / 50V electrolytic capacitors.
The recommended transformer and filter capacitor parameters for two CTA14(A)'s in a stereo configuration with a power supply that has place for 4 filter capacitors (e.g. Euraudio PSSR2S, PSS2S) are as follows:
|
Amplifier power |
Transformer |
Recommended fuse (2 pcs)** |
Recomm. filter caps (4 pcs) |
|
2x100 W / 4 ohms |
2x26 VAC, 320 VA |
T6.3A |
6,800uF
/ 50V |
|
2x90 W / 6 ohms* |
2x29
VAC, 280 VA |
T5A |
6,800uF
/ 50V |
* Do not exceed the +/-42V unloaded supply voltage.
** There are no fuses in the Euraudio PSS2S power supply.
What the CTA12 and CTA14(A) DiY kits contain and what they contain not
The kits contain the CTA12 or the CTA14(A) double-sided, fiberglass-reinforced epoxy PCB and all the electronic and mechanical components to be mounted on it, except the wires and the main heat sink.
Component list
Please download the CTA14A component list (BOM) from here.
Please download the CTA12 and CTA14 component list (BOM) from here. Note that a single spreadsheet file contains the BOM's for the CTA12 and the CTA14A.
General assembly advice
What you'll need and is not in the DiY kit
Solder iron, solder wire, wire cutter, shielded cable for the input signal, suitable wires, Phillips 1 (PH1) screwdriver, thermal grease, drilling machine, 2.4 mm and 3.2 mm metal drill bits, digital multimeter (DMM). An AAA battery and a utility knife for making the L1 coil. A heavier object with solderable surface (e.g. a tin can). A PCB holder jig and/or a model-making vice may come handy. Cable insulation stripper is recommended for thick wires. A solder suction pump and/ or desoldering wick may be needed if you make soldering mistakes.
Soldering
Please click this link, if you need soldering tips.
Where polarity matters
There are electronic components which are polarized, these have to be soldered in the right orientation. The polarity is shown in the PCB with appropriate marking. If you install any of the polarized components not in the correct orientation, that causes trouble when you power up the circuit; either the circuit will malfunction or even some of the components may be damaged.
Recommended assembly order
(1) For the CTA14(A), start with soldering the "collector wire links", see below. (2) Continue with soldering the smaller components, i.e. small resistors and small diodes first. (3) Solder bigger components. (4) Mount output transistors and bias setting transistor onto the main heat sink and solder them. (5) Solder the wires to the PCB.
Note: I can't help in troubleshooting assembly faults, I can only resend the whole component set on request.
Before installing the RV1 trimmer potentiometer
Adjust the RV1 preset, if necessary, turning it counter clockwise until its resistance between the two legs shown below is 1.8 kohms or more.
Soldering the "collector wire links" (CTA14 and CTA14A)
Solder first the wire links marked JW1 and JW2. Do not use zero ohm (0R) resistors here. Use the supplied 0.95 mm diameter solid copper wire to make the links.
Polarity issues
In the CTA12 and CTA14(A) PCB you have to observe the polarity of the following components: diodes, transistors, polarized electrolytic capacitors.
Note: The orientation of transistors in TO-126 package (Q8, Q9) is not easy to identify on the PCB. The leads on a TO-126 package are not in the center-line of the plastic case of the transistor, they are displaced toward the front of the case. This is how you can determine the proper orientation.
Paralleled emitter resistors (CTA12)
Two 2-Watt resistors are paralleled in the CTA12 to make up a 4-Watt emitter resistor. This is why there are twin holes in the PCB, and the two resistors are stacked on each other, as shown below.
Winding the L1 coil
Please use the supplied enameled copper wire and wind 15 turns close to each other on an AAA size battery. Scrape off the insulation with a utility knife and spread solder over the wire ends. The result looks like this figure.
Heat sink
Heat sinks can be external on the chassis sides or can be internal. In any case it should be assured that air move freely around the heat sink. If you use an internal heat sink, ventilation holes must be drilled or cut in the chassis top and bottom: that is below and above the heat sink(s), and the chassis should have tall enough feet. In case you want to use fan cooling, then the main heat sink is allowed to be much smaller than specified below. How small it can be depends on many factors including the fan parameters, so I can't provide heat sink data for fan cooled versions of the CTA's. The Euraudio APBF and DPB protection boards have heat sink overtemperature protection as well as fan control capability.
Sizing the heat sink
The heat sink parameter that will be specified below is the thermal resistance to ambient air in K/W, where 25 °C air temperature is assumed. Refer to manufacturer data of the actual heat sink profile to determine or calculate the K/W value.
CTA12
For a mono CTA12, the main heat sink should have 2.0 K/W or somewhat lower thermal resistance to allow exploiting the full 50W power. For dual mono or stereo, use 2 of this heat sink or use a single heat sink with 1.0 K/W or lower thermal resistance.
The Dissipante 2U can be a suitable chassis that includes big enough side facing heat sinks for a dual mono or stereo amplifier. It can be ordered from diyaudiostore.com or modushop.biz.
CTA14(A)
Since the 4 output transistors are quite close to each other, the heat sink base should be at least 5 mm thick. This is to facilitate better heat conduction toward the remote parts of the heat sink. For a mono CTA14(A), the main heat sink should have 1 K/W or somewhat lower thermal resistance to allow exploiting the full 100W power. For dual mono or stereo, use 2 of this heat sink or use a single heat sink with 0.5 K/W or lower thermal resistance.
The Dissipante 3U can be a suitable chassis that include big enough side facing heat sinks for a dual mono or stereo amplifier. It can be ordered from diyaudiostore.com or modushop.biz.
Mounting output transistors and bias setting transistor to main heat sink
The output transistors must be tightened onto the main heat sink(s) with screws. We supply one DIN7981F ST2,9x13 and one ST2,9x16 self-tapping sheet metal screws for CTA12. We supply three DIN7981F ST2,9x13 and one ST2,9x16 self-tapping sheet metal screws for CTA14(A). These screws don't need a pre-made thread, they cut the thread for themselves in an aluminum heat sink. The important thing is to drill just the right diameter hole (it is 2.4 mm for the ST2,9 screws) so that the output transistors can be screwed tightly and securely to the heat sink. Alternatively, you may use M3 screws, but in this case you should cut an M3 thread into the heat sink material, so you'll need a thread cutter. Please use the supplied flat washer and spring lock washer to prevent the screw getting loose by the repeated warming and cooling of the transistor. If the power transistor gets loose, then it will be damaged, causing risk to the amplifier and loudspeaker.
Use the supplied mica insulator by applying thermal paste on both sides.
The bias setting transistor Q10 should be mounted on top of one of the output transistor cases applying some thermal grease between the two transistors. For the CTA14(A), mount the biasing transistor on one of the inside transistors (Q12 or Q13). Please use the tiny 6-hole PCB for the biasing transistor, and solder the supplied 3-wire colored ribbon cable onto the tiny PCB then solder the other end to the three Q10 solder pads in the CTA12/CTA14(A) PCB.
Important: the Q10 (BD139-16 transistor) pinout from left to right is E-C-B. Note "B C E" indicated in the CTA12 / CTA14(A) PCB, so the 3-wire ribbon cable should be twisted half a turn for proper orientation.
For appropriate heat transfer, if possible, mount the power transistors directly to the main heat sink (not through an L-profile).
Soldering the OPTs is usually the last step of the installing of components. The recommended procedure: (1) Drill the mounting holes into heat sink. The transistor midpoints are 48 mm apart for the CTA12 and are 24 mm apart for the CTA14(A). Trim any burr around the holes and clean off all metal chips. (2) Screw the OPTs to the heat sink, but do not tighten the screws yet. (3) Insert all transistor leads into the holes in the PCB. (4) Tighten the screws first and then solder the transistors. (5) Check with a DMM that there is no short circuit between the heat sink and the transistor collectors (collector is the middle pin).
Ensuring the two OPTs are from the same manufacturing batch (CTA14 and CTA14A)
For the CTA14(A), output transistor pairs are installed. To ensure better current sharing between OPTs, I supply the two NPN and the two PNP OPTs from the same manufacturing batch. If you order more than one amplifier kit, then it is not sure that all of the NPN and PNP output transistors will be from the same batch. Therefore always pick two NPN transistors with the same batch code and two PNP transistors with the same batch code for use within a CTA14(A) amplifier channel. The batch code is etched beside (below, above) the type of the transistor.
Ground points and ground wires
There are 2 ground points in the CTA12 and CTA14(A) PCB: "GND" and "PGND". These should be connected to the star ground point in the power supply via independent wires. It's recommended to hold the 2 ground wires together with cable ties. If you use one of Euraudio's power supply PCB's, then there are grounding pads marked with the same name on the power supply PCB. The recommended wire cross sections are as follows: GND=0.35 mm2, PGND=0.5 mm2. Wires longer than 30 cm may require thicker cross sections than these.
Power supply wiring
The +VCC and -VCC points connect to the +/- power supply. Use wires of at least 0.5 mm2 cross section. Important: Twist the +VCC and -VCC wires together.
High current connections
The current carrying copper foils of the +VCC, -VCC, and AOUT high current connections all run on the bottom of the PCB. Therefore, if you push the wires of +VCC, -VCC, or AOUT into the PCB from the top downwards, then everything will be alright. However, if you mount any of them from the bottom upwards (which may be more esthetic or more to the purpose in some cases), then you must ensure that the solder around the wire spills over to the bottom side of the PCB. It's because the galvanic plating of through-holes is not as sturdy and does not conduct as well as the copper foil on the PCB surface; at high currents, this may lead to overheating and the plating of the hole may crack and open.
Input connection
Shielded cable should be used to connect the input signal to the input connector J1. Do not connect the cable shield to the chassis or to system ground separately! It will be grounded via the CTA12 and CTA14(A) PCB.
If you build a stereo amplifier using two CTA12 or CTA14(A) PCB's, do not connect the shields (grounds) of the 2 input channels at the input connector or at any other point, they both will be grounded via the two CTA12 and CTA14(A) PCB's.
Assembling the input connector
If you don't have a crimp tool suitable for the contacts of the input connector, you can always solder them, please click this link. You need 2 contacts, I supply 3, just in case you spoil one of them.
Input live and ground point
The pinout of J1 is the following ("live" is the internal core and "gnd" is the shield of the input cable):
Volume pot
Ideally, the CTA12 and CTA14(A) amplifiers should be driven from a low impedance source (e.g. active preamplifier), so it has no volume pot connection. However, if need be, you may fit a logarithmic volume control pot on your own, through which you can feed the input of the amplifier. For more information, see the "Volume control" section later in this page.
R3 resistor
R3=short is recommended in mono or dual mono amplifier; the short should be realized not with a 0R resistor, but with a piece of wire. R3=10 ohm is recommended in stereo amplifier; the 10R is damping the hum introduced by the ground loop formed around the shields of the 2 shielded cables running into the stereo amplifier.
Connecting to the APBF/APBM/DPB overcurrent protection
If you use the Euraudio APBF, APBM, or DPB protection board, they need the voltage across the emitter resistors for their overcurrent protection. There are 2 emitter resistor (ER) test points in the CTA12 and there are 4 ER points in the CTA14(A) PCB. You'll have to connect two of these points (one + and one - point) to the ERV connector in the APBF/APBM/DPB board with correct polarity. Caution: Reverse polarity (+ and - points exchanged by mistake) renders the overcurrent protection ineffective and may damage the protection board as soon as the output current of the CTA12/CTA14(A) gets high enough.
How to avoid increased distortion?
The distortion of this amplifier is very low, and it can easily be compromised by incorrect grounding scheme, incorrectly routing cables/wires or incorrectly positioning the amplifier board within the chassis. For the advises below, I assume a conventional power supply with rectifier bridge and filter capacitors. The amplifier wasn't tested with switch mode power supply.
Regarding grounding: I recommend you use one of the power supply boards that I designed. If you use your own power supply board, the grounding reference should be a star-point, and route the critical ground points of every subassembly into the star point with separate wires.
Regarding cable/wire routing: (1) The most important is to avoid running any wire or cable, including wire from the amplifier output (AOUT) near the L1 output coil. (2) Avoid running the input cable (a) near the output wire, (b) near the +/-VCC wires, (c) near the cable/wires going to the bridge rectifier or (d) in the vicinity of the main transformer. (3) Avoid running the wire from the amplifier output (AOUT) (a) near the +/-VCC wire, (b) near the cable/wires going to the bridge rectifier or (c) near the main transformer. If you can't altogether ensure a large distance between these cables/wires, then at least don't run them parallel, rather just let them cross at a single point at right angle.
Regarding board positioning: Avoid placing the input connector side of the CTA12/CTA14(A) board near the transformer(s), near the area of the bridge rectifier on the power supply board, or near the cable running to the bridge rectifier.
Assembled PCB look
This is how an assembled CTA12 looks like without the bias setting transistor, the power transistors, and the wiring (CTA14 and CTA14A are similar).
Powering up the first time and setting idling current
1. CAUTION! Before powering up for the first time, the resistance between the two legs of the RV1 preset shown below should be 1.8 kohms or more. On an assembled PCB, this resistance can also be measured between the two extreme legs (Emitter-Base) of the Q10 bias setting transistor.
2. Check that every wire is connected.
3. Do not connect anything to the input and output. Turn on the power supplies to the amplifier.
4. Check the DC voltage at the output, it should be less than 30mV, if it is not, then you've made a mistake.
5. Adjust the voltage between an +ER and -ER point. On the CTA14 and CTA14A, measure between one of the +ER and one of the -ER points. On the CTA12, the +ER point is very close to the heat sink; be careful not to short-circuit the grounded heat sink to the +ER point by accident, or else the amplifier will be damaged. If you've already connected the wires to the APBF/APBM/DPB overcurrent protection and so the +ER and -ER points are not freely accessible, then you can measure the +ER and -ER voltage at the leads of the emitter resistors, as shown below (between a plus and a minus):
When the output transistors are cool (20...25 °C) and immediately after the power is switched on, set potentiometer RV1 (idling current increases clockwise) until the measured DC voltage between the +ER and -ER points reaches 20mV ±2mV. Initially, several turns may be needed until the voltage moves away from near zero. With the amplifier as it will be used (e.g. close the top of the chassis), wait 20 minutes for the transistors to warm up. Adjust the potentiometer again if necessary so that the voltage is 30mV ±2mV. Don't turn the pot back and forth a lot, as it may fail or loose accuracy.
Volume control
The CTA12 and CTA14(A) are best used with an active preamplifier that has low output resistance (e.g. 50...300 ohms). High serial resistances at the input of the CTA12 and CTA14(A), such that are presented by passive preamplifiers or volume pots have their disadvantages.
Disadvantages of passive preamplifiers and volume pots
High resistance seen at the input of the CTA12/CTA14(A) will impair (1) the noise and (2) the PSRR of the amplifier, and (3) will create a wiper position dependent high frequency roll-off, because the resistance forms a low-pass filter with the RF input filter capacitor of the CTA12/CTA14(A).
Using a passive preamplifier
If you use a passive preamplifier with the CTA12/CTA14(A), then consult the specifications or the schematic of the preamplifier to ensure that the output impedance of the preamplifier is lower than 2.5k ohms at all positions of the preamlifier controls. (If the passive preamplifier is a single volume pot, then the resistance of the pot should be 10k ohms or lower.)
Volume pot
If you don't have a preamplifier and want volume control on the CTA12/CTA14(A), then you may install a logarithmic volume pot between the input connector on the chassis (e.g. RCA) and the J1 input connector on the CTA12/CTA14(A) PCB. You must make the little PCB yourself for the volume pot. In a stereo (or dual mono) system, do not short the cable shields of the left and right channels at the volume pot PCB or at the input connector (e.g. RCA), because the shields will be grounded through the CTA12/CTA14(A) boards. The volume pot should be 10k ohms or lower.
The connection of the potentiometer:
Never use a pot higher than 10k ohms, because it will create a low pass cut-off with C1 and may affect audible frequencies. Use only quality potentiometer.
Now only one thing remains: enjoy the music!