About the LDA do-it-yourself kits

The LDA kits are based on the Euraudio LDA17 (mk2/mk3) mono, transistor power amplifier and ultra low distortion, low noise power amplifiers can be built from them. They are primarily for home use, suited for audiophile or high-end loudspeakers. They can be used as reference amplifier or measurement amplifier, too, as their distortion is much lower than what even the most experienced music listener can tell. If you want to be 100% sure that the weakest link in your audio chain is not your power amplifier, then choose one of the LDA kits. The LDA17 board, depending on the supplied component set, can be built for max. 80 Watts power (LDA17mc), for max. 120 Watts power (LDA17hc), or for max. 200 Watts power (LDA17xc).
The LDA MIN kit is suitable for building a max. 2x80 Watts / 4 ohm stereo power amplifier, with turn-on thump muting via a mechanical relay, but without protections. This is the cheapest kit.
The LDA MID kit is suitable for building a max. 2x120 Watts / 4 ohm stereo power amplifier, with full, independent protection via ultra low distortion solid state relays for the two channels. The protections include turn-on/turn-off muting, loudspeaker protection, heatsink overtemperature protection, and amplifier overcurrent protection.
The LDA DM1 kit is suitable for building a max. 2x120 Watts / 4 ohm dual mono power amplifier, with full, independent protection via ultra low distortion solid state relays for the two channels. The protections include turn-on/turn-off muting, loudspeaker protection, heatsink overtemperature protection, and amplifier overcurrent protection.
The LDA DM2 kit is suitable for building a max. 2x200 Watts / 4 ohm dual mono power amplifier, with full, independent protection via ultra low distortion solid state relays for the two channels. The protections include turn-on/turn-off muting, loudspeaker protection, heatsink overtemperature protection, and amplifier overcurrent protection.
The LDA MON kit is suitable for building a max. 200 Watts / 4 ohm mono power amplifier, with full protection via an ultra low distortion solid state relay. The protections include turn-on/turn-off muting, loudspeaker protection, heatsink overtemperature protection, and amplifier overcurrent protection.

The output power of the amplifier(s) in the LDA kits depend(s) on the supply voltage. The amplifiers can be built for output powers lower than what is specified as the maximum power for the kit, without component changes, all you need to do is decrease the supply voltage. The assembly instructions of the Power supply board or Power supply & relay board found in the kit contains the recommended transformer and filter capacitor parameters for a few amplifier power ratings. None of the configurations were tested at unloaded supply voltages below +/- 35 V DC. If the supply voltage is so low that it falls below +/-25V DC at the maximum sinusoidal loading, then changing two resistor values in the amplifier is recommended.

The kits, especially the more expensive ones, are complicated and contain many components. Besides the workmanlike assembling, it is worthwhile to be aware of the principles regarding the grounding and wiring issues of audio amplifiers, as long as you wish to attain the achievable very low distortion. Therefore the kits are recommended for hobbyists, who have already completed a few simpler electronic projects, who have deeper experience in building audio amplifiers and have successfully built simpler audio amplifier(s).

This information is covered in the description of the LDA kits.

The kits contain all the PCB's described at the LDA kits documentation, furthermore they contain the components specified in the assembly instruction of each kit component; these usually mean all the electronic and mechanical components to be mounted onto the PCB's. The kits don't contain those components that can be arbitrarily chosen by the builder: it chiefly means the chassis of the amplifier, components fitted to the chassis, the main transformer, the heatsink, most of the wires and cables, the electrolytic filtering capacitors and fuses of the power supply(ies). The LDA MIN kit doesn't contain the AC power supply necessary for the operation of the turn-on thump muting circuit, but suggestions are provided in the form of a circuit schematic. The LDA MID, LDA DMx, and LDA MON kits contain the auxiliary power supply (MID2 or MID2A) necessary for the operation of the protection circuit(s), but don't contain the components for the MID2(A). Detailed information about what parts the kit components contain is given in the part list (BOM) found in the assembly instructions of each kit component.

For each kit component, detailed online assembly instructions are available in English and in Hungarian. Besides, where appropriate, I'm available via e-mail, if any issue arises due to incompleteness of the assembly instructions. I can't help in troubleshooting assembly faults, I can only resend the whole kit component or the full component set of the kit component on request.

In January of 2024, the price of the kits (purchased directly from me) is: LDA MIN: 106 EUR. LDA MID: 193 EUR. LDA DM1: 232 EUR. LDA DM2: 285 EUR. LDA MON: 145 EUR. If you are interested, write me an e-mail.

Yes, but the kit components are sold only together with their component set, you can't buy just the PCB's.
The solid state relay of the Euraudio Power supplies & relay boards (PSSR2S, PSS701) are controlled by the APBF/APBM/DPB protection circuit(s), so it doesn't make sense to buy them independently of each other.


About the sonic fidelity of the LDA amplifier

There are many factors that play a role in sensing distortion. Just a few: (1) On what signal do we want to hear distortion? It's much easier to hear distortion on clean, sinus-like signals than it is on complex signals, e.g. human speech. (2) What harmonic content does the distortion have? The low order, e.g. 2nd, 3rd harmonics are much more difficult to tell than higher order harmonics. (3) What frequency band do the distortion product fall into? Those harmonics that fall into the 2...6 kHz range, where the human hearing is most sensitive, are more audible. (4) How experienced one's hearing is. The ability to sense distortion can be learned and there are individual differences. If you put all this together, you come to the conclusion that no single percentage can be defined as to where the audibility of amplifier distortion starts: there is a from...to domain in which it can be audible depending on the test sound and the listener. It is also dependent on how low the distortion of your loudspeaker or headphones is.

The distortion of the LDA17 (and its predecessors, the LDA172, LDA162) amplifier is very surely too low for anyone to hear; it absolutely faithfully reproduces the signal fed into the input at the output. This was affirmed by several ABX tests and a nulltest. If the sound of an amplifier without sound signature of its own is to be characterized, transparent and neutral are the words that come to mind.

On the prototypes of the LDA17 (and on the parameter-wise same LDA172 and LDA162) amplifier, we measured 0.0007%...0.0013% for the 1 kHz total harmonic distortion (THD) in the range spanning from 1 Watt to the maximal output power and loaded by 3.5 ohms. The intermodulation distortion is very low up to 20 kHz frequency, and the transient intermodulation distortion is also remarkably low. In case of workmanlike assembly, you can expect similar results.

Good question because I'm sure not everyone understands the same thing on high end. By this I mean that the amplifier has excellent measurable parameters and adds almost no distortion or noise to the signal applied to its input, i.e. its sonic fidelity is very high. On the other hand, there are those who understand on high end that the amplifier adds something plus to the sound, distorts it in a way that tickles the ear, sounds interesting or pleasant. The distortion of the LDA17 is so low that it is difficult to find a commercial amplifier on the market that could compete with it. Due to its ultra low distortion, the LDA17 can be used for speaker measurement or as a reference amplifier when comparing amplifiers. But whether a sound that reproduces the input signal at the output completely faithfully is high end for someone or is enjoyable for someone, that is determined by their subjective taste. In its original sense, HiFi (High Fidelity) meant high sonic fidelity, and high end was the pinnacle of HiFi, in this sense the LDA17 is high end. I’m sure who really cares about high sonic fidelity will appreciate the LDA17 as excellent, and if someone uses the LDA17, it certainly won’t be the weak link in the audio chain. If you hear the distortion of something, it will not originate from the amplifier, but from the speaker, room acoustics, or the source of the sound material (preamplifier, or sound file if it is of poor quality).

The nulltest performed on the amplifier was very convincing. In the nulltest, the output signal of the amplifier down-scaled to mach the amplitude of the input signal exactly is subtracted from the input signal (e.g. music), so the difference of the two is composed. If the amplifier was perfect, then there would be no difference between these two signals except for the amplitude, so the result would be perfect silence (null signal). Of course, there is always some distortion, noise and sometimes hum added by the amplifier, but if the magnitude of these is low enough, then the listener will not be able hear anything at normal volume level and at a usual distance from the loudspeaker when the subtracted result, the so called nullfile is played back and listened to. During the playback of the nullfile of the LDA17 (LDA172,  LDA162) amplifiers we couldn't hear anything, not even when we almost stuck our ears to the loudspeaker.

I didn't experience increase in the distortion with the mechanical relay, but I didn't do elaborate testing on that issue. The distortion of the LDA17 is specified for the amplifier alone, without the output relay. Generally speaking, mechanical relays are inclined to develop a thin oxide layer on their contacts, the resistance of which can degrade the excellent distortion of the LDA17, especially at low volume level. It is, however, unlikely that the distortion would worsen to a degree which is audible. As the mechanical relays are somewhat erratic regarding distortion and their contacts may corrode over time in certain conditions, I suggest you choose a kit with solid state relay(s) (LDA MID, LDA DM1, LDA DM2, LDA MON) if reproducible and long term ultra low distortion is important.

Many people warn about more complicated compensations, but I have not experienced any sign that the two-pole compensation makes this amplifier prone to oscillations. Between 2 kHz and 20 kHz, TPC reduces distortion measurably, but even with simple Miller compensation, distortion is so low that it is inaudible to the human ear. Measurements were done with the TPC jumper installed.


About the assembly of the LDA kits

The max. 2x200 Watts LDA DM2 dual mono amplifier requires two separate mains transformers. To take advantage of the full 2x200 Watt power, two pieces, each of approx. A 300VA transformer is required. If there was a single on/off button, then for two transformer with a power rating greater than approx. 200VA each, their combined inrush current could trip a small circuit breaker in many households, unless soft-start electronics are built into the amplifier. Examining the soft-start electronics, I came to the conclusion that a soft-start electronics, which would not fail even if the amplifier is switched on and off several times in a quick succession, especially when it is hot, would take up a lot of space on the power supply. Therefore, I decided to design a separate power button for the two amplifier channels, so there is no need for a soft start, just the two switches need to be turned on one after the other, with a break of at least half a second.

As a general rule, mount it on the output transistor that is heated the most. For musical signals approx. the same power is dissipated by each output transistor. If they are in similar position on the heatsink, then they warm up equally. But if one of them has a smaller heatsink portion, then it is advisable to fasten the tempco transistor onto that one, because it can heat up more. In the case of LDA17xc, the tempco transistor usually needs to be placed on one of the two inside power transistors because they heat up more.

Have you considered the possible incompatibilities mentioned in the assembly instructions? The Power supply & relay PCB's are not fully compatible with all protection circuit PCBs. The PSS701 and PSSR2S are fully compatible with DPB. However, if you use the PSS701 or PSSR2S with the APBF/APBM, then the D91, D92 (BAV21) diodes on the Power Supply & Relay PCBs must be installed backwards relative to the marking on the PCB.

R3 = short circuit is recommended in mono or dual mono amplifier; it is recommended to implement the short circuit not with a 0R resistor but with a wire. R3 = 10 ohms is recommended in stereo amplifiers; the 10R reduces the hum due to the ground loop between the shields of the 2 shielded cables running into the stereo amplifier.

Checking the DC voltage at the output as described in the LDA17 (LDA172) assembly instructions is the most important. If you may have a professional (ultra low distortion and low noise) sound card, you can measure the distortion with a 4 ohm load. Free software suitable for distortion measurement is e.g. the ARTA.
If you have an oscilloscope and a dummy load, you can see what a 20 kHz frequency sine signal driven into slight clipping looks like at a 4 Ohm load. Such a 20 kHz full amplitude or clipping test should only be done with a room temperature, cool amplifier, even so it is only recommended for 2-3 seconds and then wait a while for the amplifier to cool down. Dummy load is necessary, 50-100 Watts. Check and make sure there is no self-sustaining excitation.

It does look ugly measured with an oscilloscope at the input of the rectifier bridge when the leakage inductance of the transformer oscillates with the capacitance of the rectifier bridge and the transformer, but it is usually around 30-300kHz and is no longer present in the filtered supply voltage. I have tried to reduce this phenomenon with an RC snubber in one of my prototypes equipped with traditional rectifier diodes. The snubber, if well sized, will largely take out this oscillation even with slow diodes. The distortion measurement results, however, have not improved with this, I think 30-300kHz is too low to interfere with anything as radio frequency interference. I have doubts about whether it’s worth sizing a snubber or using fast diodes in the rectifier bridge. But if someone puts fast (and expensive!) diodes in the power supply, then they should also deal with snubber sizing, because it is not only the capacitance of the rectifier diodes that causes the oscillation at the input of the power supply, but also the interwinding capacitance of the transformer, which is not affected by the fast diodes.



No, you need to turn off the amplifier first and wait a few seconds.
When connecting a speaker to the output, especially when tightening a bare cable end, make sure that you do not accidentally make an output short circuit (for example, the conductor of the wire into a red speaker connector also touches the metal housing).

The green LED on the Power supply and relay PCB is the indicator LED of solid state relay operation; when the relay is on (passes signal), the green LED lights up. The red LED on the APBF/APBM is the overcurrent protection operation (OCP) LED. The DPB contains indicator LEDs for all protections, not only for OCP. If the OCP LED is lit, see the next question.
The green LED (PSS701, PSSR2S) does not illuminate when (1) the power on/off muting is active or (2) there is a fault that has caused the protection circuit to intervene. The fault can be of three types: DC offset (i.e. dangerous DC voltage at the output that can damage the speaker), heatsink overtemperature, or overcurrent.

The overcurrent protection latches itself, so the amplifier will have to be turned off and on to work again, but never turn on the amplifier again until the cause of the overcurrent has been determined! If the OCP LED was lit (APBF/APBM) or is blinking (DPB) due to an output short circuit and nothing was damaged at first, something is likely to be broken as a result of the repeated on and off switching, because again and again a huge current load is applied to the amplifier when the solid state relay is turned on.
I recommend the following procedure if the red overcurrent indicator (OCP) LED is lit or blinking: (1) Check for a short circuit at the output. (2) If there was short circuit and it has been cleared, then disconnect the speaker from the output before turning the amplifier on again, and measure whether there is dangerous DC voltage at the output with a multimeter after turning on the amplifier. This is a check in case both the amplifier and the solid state relay are faulty and the solid state relay is short circuited.
The protection circuit immediately detect an excessive DC voltage at the output, and in such a case it does not turn on the solid state relay (the green solid state relay LED does not light up), but if the solid state relay itself became defective due to the output short circuit, then it probably went short-circuited and it passes everything, even the dangerous DC voltage, which can damage the loudspeaker.



I do not recommend it. Wiring diagrams for LDA amplifiers are not currently published, but repairs to amplifiers built from LDA kits are undertaken by the designer.
If you are a trained electrician and want to repair the amplifier yourself, you also need to pay attention to electrical safety. The Euraudio LDA MIN kit does not include the "MIA" mains voltage input/distribution board, which must be designed and made by the users. The other LDA kits include the MID2(A) mains voltage input/distribution board, but you should use it at your own risk. If the mains voltage input/distribution board is damaged or loosened and the lower voltage parts are exposed to the 230 VAC mains, life-threatening voltage may appear on any metal surface of the amplifier that is not in strong, high-current connection to the protective earth of the device. Such larger-area, easily touchable metal parts are the smaller heatsinks: e.g. the driver transistor heatsink on the LDA17 PCB or the heatsink of the rectifier bridge on the PSS701 or PSSR2S PCB. Do not touch these surfaces until you have made sure that the mains voltage input/distribution board is undamaged and is secured in place.
After high-power usage, the internal heatsinks can be very hot, up to 80-90 °C. Until they have cooled down, care must be taken.

There can be many reasons for hum, one is that an output short circuit has destroyed the R3 resistor.
R3 is 10 ohms in the LDA17 supplied in the LDA MIN and LDA MID kits. If you measure a value well above 10 ohms, the R3 resistors are broken and this can be the cause of the big hum. In the LDA17 supplied in the LDA DMx and LDA MON kits, R3 is 0 ohms (short circuit). If you implemented this short circuit not with copper wire but with 0R resistor, it could also be damaged.



APBM, as well as its fan control capable versions, APBF and DPB, are mono protection circuits. There is no relay on them, but they are connected to a Power supply & relay board (the mono PSS701 or the stereo PSSR2S). The APBF/APBM/DPB detects many things: (1) the DC offset appearing at the amplifier output (for the purpose of loudspeaker protection), (2) the temperature of the heatsink using a thermistor (for the purpose of overtemperature protection), (3) the output current of the amplifier using the voltage across the emitter resistors of the amplifier (for the purpose of overcurrent protection), (4) and it provides a swich-on delay (turn-on thump muting), furthermore it disconnects the loudspeaker quickly at power off, thus preventing unwanted turn-off noises.
In a stereo system built with two APBF's/APBM's/DPB's, in case either channel fails, then only the defective channel switches off, as there is an independent protection circuit and an independent relay for each channel.
For dual mono amplifiers 2 pcs of PSS701 + 2 pcs of APBF/APBM/DPB are necessary.
The APBF/APBM/DPB needs a regulated supply voltage; +9V DC, max. 15 mA (if connected to PSSR2S) or +12V DC, max. 15mA (if connected to PSS701). The +9V (or +12V) is fed to the APBF/APBM/DPB via the connected Power supply & relay board.

There has been no testing for this, and there is no specification nor guarantee for this. Full protection includes overcurrent protection, which is designed to disconnect the speaker if a normal load, i.e. an impedance of not less than 3.5 Ohms, is connected to the amplifier and nevertheless overcurrent occurs, thus protecting the amplifier from overload. In the event of an output short circuit, the load is close to zero ohms, and suddenly a large current can start that can damage the amplifier and even the relay before the overcurrent protection can intervene. Whether the amplifier may survive an output short circuit depends on several factors, e.g. how quickly the relay turns off (the solid-state relay of the PSSR2S and PSS701 has a short turn-off time compared to mechanical relays) and how hot the output section of the amplifier, especially the output transistors and emitter resistors, were when the short circuit occurred. In addition, the current due to a short circuit may exceed the maximum allowable current of the solid state relay, and therefore the normal behavior of the relay is not guaranteed. The protection circuits of most factory-made amplifiers use mechanical relays, which are usually not able to protect against the damaging effects of the output short circuit. The complete protection of the solid state relay of the LDA kits is faster than these, but the protection against output short circuit is still not guaranteed.
As a note, one of the LDA MON prototype amplifiers with its APBM protection electronics and Power Supply & Relay board protected the LDA162xc amplifier from two accidental output short circuits during testing.
If the red overcurrent LED on the APBF/APBM/DPB protection electronics lights up, always check whether output short circuit has occurred. If so, do not connect a speaker until you check that there is no dangerous DC voltage at the output. This is especially important for solid state relay amplifiers because if the solid state relay fails, it will usually short circuit and therefore the protection will not be able to turn off the speaker if there is a dangerous DC offset at the output of the amplifier.


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