Euraudio LDA172mc and LDA172hc assembly instructions

Welcome to the Euraudio LDA172mc and LDA172hc assembly instructions page. The LDA172mc and LDA172hc are high performance, low distortion and low noise, mono DiY audio power amplifiers. You need two of them for dual mono or stereo configurations. They are part of the Euraudio LDA  53DM, LDA 53M, LDA43S, and LDA MIN DiY kits. Please find the LDA172 specifications here.

Please read through these instructions before doing anything with the DiY kit.

Differences between LDA172mc and LDA172hc

The PCB is the same for LDA172mc and LDA172hc, only the component set differs. The main difference is that the LDA172hc is the "high power" (max. 200 Watts / 4 ohms) version utilizing both output transistor pairs in the PCB, while the mc version is the "medium power" (max. 100 Watts / 4 ohms) version that needs only a single output transistor pair installed. The other difference is that in the hc version some components are more heavy-duty and some component values slightly differ.

The recommended transformer and filter capacitor parameters for two LDA172mc's in a stereo configuration can be found in the assembly instructions of the PS1S Power Supply board.

The recommended transformer and filter capacitor parameters for one LDA172hc in a mono configuration can be found in the assembly instructions of the PSS701/PSS702 Power Supply & Relay board. Two of this power supply are needed for dual mono configuration.

Both the mc and hc versions can drive a 4-ohm load.

What the LDA172mc and LDA172hc DiY kits contain and what they contain not

The kits contain the LDA172 double-sided, fiberglass-reinforced epoxy PCB and all the electronic and mechanical components to be mounted on it, except the wires and the main heatsink.

Component list

Please download the LDA172hc and LDA172mc component list (BOM) from here. Note that a this single spreadsheet file file contains both the hc and mc BOMs.

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, Phillips 2 (PH2) 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. 


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 solder 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) Start with soldering the smaller components, i.e. small resistors and small diodes first. (2) Solder bigger components. (3) Mount output transistors and tempco transistor onto the main heatsink and solder them. (4) Assemble the driver heatsink and solder its transistors. (5) Solder wires to the PCB.

Note: I can't help in troubleshooting assembly faults, I can only resend the whole component set on request.

Polarity issues

In the LDA172 PCB you have to observe the polarity of the following components: diodes (including LEDs), transistors, polarized electrolytic capacitors. C3 is different, its polarity doesn't matter.

Note: The orientation of transistors in TO-126 package (Q8, Q9, Q11) 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.

D3 and D6 are LEDs. The longer leg of the LED is the anode, this should be connected to the round pad (marked "A" in the image below). The shorter leg is the cathode, that connects to the rectangular pad (marked "C" in the image below).

Hard to read capacitor values

There are a few capacitors that may be supplied with the LDA172hc or LDA172mc kit, on which the marking is hardly visible. See image below for identification.

Paralleled emitter resistors

Two 3-Watt resistors are paralleled to make up a 6-Watt emitter resistor for each installed output transistor. 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 16...17 turns close to each other on an AAA size battery. Scrape off the insulation with a utility knife and solder over the wire ends. The result looks something like this (fig. 1). Or you can make the coil like in fig. 2, and solder it with its axis upright.


Main heatsink

Heatsinks can be external on the chassis sides or can be internal. In any case it should be assured that air move freely around the heatsink. Suitable chassis that include side facing heatsinks can be ordered from (I have not tested these chassis, but customer satisfaction seems to be quite high.) If you use an internal heatsink, ventilation holes must be drilled or cut in the chassis top and bottom: that is below and above the heatsink(s), and the chassis should have tall enough feet. In case you want to use fan cooling, then the main heatsink 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 heatsink data for fan cooled versions of the LDA. The Euraudio APBF and APB protection boards have heatsink overtemperature protection as well as fan control capability.

Sizing the main heatsink

The heatsink 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 heatsink profile to determine or calculate the K/W value.


Mount the 2 output transistors so that the transistors are evenly distributed on the heasink(s) and have plenty of space between them. I recommend installing Q14 and Q17 because my LDA172mc tests and measurements were done with these ones installed. For a mono LDA172mc, the main heatsink should have 0.9 K/W or somewhat lower thermal resistance to allow exploiting the full 110W power. For dual mono or stereo, use 2 of this heatsink or use a single heatsink with 0.45 K/W or lower thermal resistance. The Dissipante 2U or 3U (e.g. with deluxe-style faceplate) can be a suitable chassis that includes big enough side facing heatsinks for a dual mono or stereo amplifier. They can be ordered from


Since the 4 output transistors are quite close to each other, the heatsink base should be at least 5 mm thick. This is to facilitate better heat conduction toward the remote parts of the heatsink. For a mono LDA172hc, the main heatsink should have 0.5 K/W or somewhat lower thermal resistance to allow exploiting the full 200W power. For dual mono or stereo, use 2 of this heatsink or use a single heatsink with 0.25 K/W or lower thermal resistance. The Dissipante 4U (e.g. with deluxe-style faceplate) or Deluxe 4U  can be a suitable chassis that include big enough side facing heatsinks for a dual mono or stereo amplifier. It can be ordered from

The single output transistor pair for LDA172mc

Since only 1 of 2 output transistor (OPT) pairs is fitted for the LDA172mc, you decide which transistors you wish to mount onto the PCB. You should install 1 of the positive side OPTs, and 1 of the negative side OPTs:

Base and emitter resistors that belong to output transistors (LDA172mc)

Since there are only 2 output transistors installed in the LDA172mc, only 2 sets of emitter and base resistors are supplied for them. With the OPTs you also need to mount the base and emitter resistors that belong to them. So you should install

either  ( Q14 + R28 + R30 )  or  ( Q16 + R32 + R34 )


either  ( Q15 + R29 + R31 )  or  ( Q17 + R33 + R35 ).

Note: I recommend installing Q14 (green) and Q17 (red) and their resistors because the LDA172mc tests and measurements were done with these ones installed.

Mounting output transistors and tempco transistor to main heatsink

The output transistors must be tightened onto the main heatsink(s) with screws. We supply one DIN7981F ST2,9x13 and one ST2,9x16 self-tapping sheet metal screws for LDA172mc. We supply three DIN7981F ST2,9x13 and one ST2,9x16 self-tapping sheet metal screws for LDA172hc. These screws don't need a pre-made thread, they cut the thread for themselves in an aluminum heatsink. 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 heatsink. Alternatively, you may use M3 screws, but in this case you should cut an M3 thread into the heatsink 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.

Between the output transistors and the heatsink, the supplied high-efficiency thermal pads should be placed. This is a phase-change thermal interface, its surface material will melt when the transistor heats up the first time, so it fills the surface imperfections and drives out air voids. Therefore no thermal compound or grease is needed, and you mustn't use thermal compound or grease with this product! If the output transistor ever needs to be replaced or removed from the heatsink, then its thermal pad also needs to be changed. I order this product from Mouser, and here is the order code: CD-02-05-264. If you can't purchase this Wakefield-Vette product, you may replace it with a mica washer plus a thermal compound or grease, but the thermal conductivity will be inferior with the latter.

The tempco transistor Q10 should be mounted on top of one of the output transistor cases applying some thermal grease between the two transistors. If all 4 output transistors are in use, then mount the tempco transistor on one of the inside transistors (Q14 or Q15). Please use the tiny 6-hole PCB for the tempco 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 LDA172 PCB.


Important: the Q10 (BD139-16 transistor) pinout from left to right is E-C-B. Note "B C E" indicated in the LDA172 PCB, so the 3-wire ribbon cable should be twisted half a turn for proper orientation. Note: Make sure the ribbon cable is kept away from the immediate vicinity of the driver heatsink that can get very hot and might melt off the cable insulation.

Soldering the OPTs is usually the last but one step of the mounting of components. The recommended procedure: (1) Drill the mounting holes into heatsink. The transistor midpoints are 26 mm apart. Trim any burr around the holes and clean off all metal chips. (2) Bend the transistor leads, as shown below.

(3) Screw the OPTs to the heatsink, but do not tighten the screws yet. (4) Insert all transistor leads into the holes in the PCB. (5) Tighten the screws first and then solder the transistors. (6) Check with a DMM that there is no short circuit between the heatsink and the transistor collectors (collector is the middle pin).

Ensuring the two OPTs are from the same manufacturing batch (LDA172hc)

For the LDA172hc, both 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, 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 an LDA172hc amplifier channel. The batch code is usually etched below the type of the transistor.

Auxiliary heatsinks

Q8 and Q9 heatsinks

When the supply voltage is higher than +/-50V, small heatsinks are recommended on Q8 and Q9. These 2 small heatsinks are not supplied with the LDA172mc kit, only with the LDA172hc. You must drill the hole yourself, as shown here:

Apply a bit of thermal grease between the transistor case and the heatsink, and screw the transistor to the heatsink using an M3x10mm screw, flat washer, spring lock washer and nut.

Driver heatsink

The 2 output driver transistors and a diode-connected temperature sensing transistor sits on a separate aluminum heatsink in the middle of the PCB. Installing these 3 transistors and mounting the small heatsink to the PCB is usually the last step, comes just after mounting the output transistors. You must drill the holes yourself according to the following pattern:

Trim any burr around the holes and clean off all metal chips.

Before soldering in the three transistors, first mount the transistors onto the heatsink, but do not tighten the screws yet. You'll need three M3x10mm screws with flat washer, spring lock washer and nut. Put silicon pads between the transistors and heatsink to isolate their exposed metal part from the heatsink. You may get the middle transistor (Q11) in a TO-126 package totally encapsulated in plastic. In this case, Q11 doesn't need a silicon pad, you just have to put some heatsink compound beneath the transistor. The two transistors at the side need the plastic ring that isolates the M3 screw from the metal tab of the TO-220 package.

Now get the steel corner bracket, and screw it to heatsink using the M3x8mm countersunk head type screw, flat washer, spring lock washer and nut. Then align and push the transistor leads into their corresponding holes in the PCB, and screw the steel bracket to the PCB using the M4 screw with M4 tooth lock washer and M4 nut. Tighten all screws first on the steel bracket then on the transistors, and when all is tightened, solder the 3 transistors to the PCB. Check with a DMM that there is no short circuit between the heatsink and the transistor collectors (collector is the mid pin on all of these transistors).

Ground points and ground wires

There are 3 ground points in the LDA172 PCB: "GND", "ZGND", and "PGND". These all should be connected to the star ground point in the power supply via independent wires. It's recommended to hold the 3 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, ZGND=0.35 mm2, PGND=0.5 mm2. Long wires 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 0.75 mm2 cross section. Important: Twist the +VCC and -VCC wires together.

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 LDA172 PCB.

If you build a stereo amplifier using two LDA172 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 LDA172 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 LDA172 amplifier 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.

Connecting to the APBM/APBF/APB overcurrent protection

If you use the Euraudio APBM, APBF, or APB protection board, they need the voltage across the emitter resistors for their overcurrent protection. There are 4 emitter resistor (ER) test points in the LDA172 PCB. You'll have to connect two of these points (one + and one - point) to the ERV connector in the APB(F/M) 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 LDA172 gets high enough, so watch out.

All the 4 ER test points (2 +ER and 2 -ER) are active in the LDA172hc, and you should select one of the + and one of the - points for connection (I recommend the J5, J6 points around the middle that are close to each other).

Only 2 ER test points are active in the LDA172mc, where you should select the points that belong to the installed output transistors. The following listing and image helps you identify them.

J5: +ER, belongs to Q14;        J6: -ER, belongs to Q15;        J7: +ER, belongs to Q16;        J8: -ER, belongs to Q17.

How to avoid increased distortion?

The distortion of this amplifier is remarkably 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) 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. (2) Avoid running the output wire (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 LDA172 board near the transformer(s) or near the area of the bridge rectifier on the power supply board.

Assembled PCB look

This is how an assembled and wired LDA162mc (which is very similar to the LDA172mc) looks like without the overcurrent protection connection. In this instance the Q14 and Q17 output transistors were mounted.

Before powering up the first time (setting idling current)

Before powering up the first time, check that every wire is connected, then turn the RV1 preset counter-clockwise, until its resistance is about 1.5 kohms or more. Power up the amplifier. Check the DC voltage at the output, it should be less than 30mV, if it's not, then you've made a mistake.

Measure the voltage between the +ER and -ER points that belong to installed OPTs. If you've already connected the overcurrent protection wires 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 installed emitter resistors, as shown below:

Turn the pot clockwise until the voltage reaches 15mV ±2mV. Wait for 7-8 minutes, until the amplifier heatsink warms up. Adjust the pot again, if necessary, until the voltage is 18mV ±2mV.

Volume control

It's challenging to find an active preamplifier that meets the low distortion and low noise of the LDA172, yet this amplifier is best used with an active preamplifier that has low output resistance (e.g. 50...300 ohms). High serial resistances at the input of the LDA172, 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 LDA172 will impair (1) the very low noise and (2) the excellent PSRR of this 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 LDA172.

Using a passive preamplifier

If you use a passive preamplifier with the LDA172, 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 LDA172, 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 LDA172 PCB. You must make the PCB yourself for the volume pot. In a stereo (or dual mono) system, do not connect the two input cable shields at the volume pot PCB, let the shields be grounded through the LDA172 boards individually. The volume pot should be 10k ohms or lower. Never use a pot higher than 10k ohms, because it will create a low pass cut-off with C1 and may affect audible frequencies. For lower noise and higher PSRR, I recommend a 5k ohm volume pot, as long as your source equipment can drive a 5k ohm load without increase in distortion. Use only quality potentiometer.

Selecting the frequency compensation

The J4 jumper lets you select between TPC and Miller frequency compensation. Miller compensation results in somewhat increased distortion above 1kHz. I think the difference is not audible, but you can try for yourself. You may need a tweezer to remove or install the jumper.

The tube-like distortion tweak

It is possible to spoil the very low distortion of the LDA amplifier in a way that the spectrum of distortion harmonics will look similar to those of quality tube amplifiers, that is, stronger low order, mostly 2nd and 3rd order harmonic distortion will be generated. With this modification the total harmonic distortion will become about 20 times higher in the low and mid frequency range. To achieve this "tube-like distortion profile", remove R13 (originally 4k3 or 4k7), and also change the value of R14 (originally 910R) to something between 2.2k...3.0k ohms (this resistor is not supplied with the kit). I'm interested whether you can hear any difference in how the amplifier sounds with this modification.


Now only one thing remains: enjoy the music!



Main page