Euraudio LDA172 (LDA162) specifications

Welcome to the Euraudio LDA172 specifications, in which you can read about the system requirements (application criteria), operation and technical data of the LDA172. The information applies to the LDA162, too.

The LDA172 is a low distortion, low noise transistor audio amplifier, intended for use with high performance home audio loudspeakers. LDA172hc is the high power version of the amplifier, capable of max. 200 Watts per 4 ohms output power. LDA172mc is the medium power version of the amplifier, capable of max. 110 Watts per 4 ohms output power. Please find the LDA172hc and LDA172mc assembly instructions here.

LDA172 system requirements

LDA172 is just an end stage, it doesn't contain power supply and protection circuits, however there are Euraudio DiY kits that do. LDA172hc is part of the dual mono Euraudio LDA 53DM DiY kit and is part of the mono Euraudio LDA 53M DiY kit. The LDA172mc is part of the stereo Euraudio LDA 43S and the stereo Euraudio LDA MIN DiY kits.

LDA172hc and LDA172mc can be used with your own power supply, the only requirement is that the supply should be dual (symmetric) and its voltage should be between +/-30...45 V for the LDA172mc and it should be between +/-30...55 V for the LDA172hc for 4 ohms load.

Although it's not recommended, and it wasn't tested, let's mention higher supply voltages. If you can ensure that the load is never lower than 8 ohms, then you may raise the supply voltage of the LDA172mc up to +/-54 V and the supply voltage of the LDA172hc up to +/-68 V. In the case of LDA172hc, you should also change the supplied 63 V electrolytic capacitors that are in the rail and rail filter lines to ones with higher withstanding voltage (e.g. 80 V or 100 V). These capacitors are C7, C10, C15, C17 in the LDA172 (and C9, C11, C15, C18 in the LDA162).


The LDA172 has a 3-stage transistor amplifier structure. It consists of the input, voltage amplifier (VAS) and output stages. It mostly follows the concept that was set forth in the books of Douglas Self and Robert Cordell, and what Douglas Self called a blameless amplifier. For now, I decided not to publish the exact circuit of the LDA172 on this home page, but here's a simplified blameless amplifier schematic from page 98 of Douglas Self's book, "Audio Power Amplifier Design Handbook", 5th edition.

A short overview of the simplified circuit

The input stage is a differential amplifier (TR2, TR3) fed by the current source (TR1) and loaded by the current mirror (TR10, TR11) as well as the input (TR12) of the voltage amplifier TR4. Degeneration resistors R2, R3 make the input stage much more linear. In the VAS, TR12 is often called a beta enhancer. It's purpose is to provide high impedance at the VAS input. The current mirror and TR12 together make the input stage see a very high impedance at its output, and consequently the input stage gain will be very high. The voltage amplifier also has a current source (TR5), and consequently has quite high gain, which is only degraded somewhat by the input impedance of the output stage. The high gains of the input and VA stages provide an open loop gain of about 120 dB (1,000,000) at low frequencies. Dividing by the closed loop gain of about 30 dB (LDA172hc), there is still plenty of signal available for the negative feedback to correct the nonlinear output stage. The double emitter follower ("EF2") output stage is biased in the low distortion region between class B and class AB. The driver transistors (TR6, TR8) always work in class A, due to the shared emitter resistor R15. The overall negative feedback is stabilized by dominant pole compensation via capacitor C3 (Miller capacitor).

TPC versus Miller compensation

In the actual LDA172 (LDA162) circuit, the J4 (TPC) jumper serves to select two-pole frequency compensation instead of the traditional Miller compensation. TPC results in somewhat lower THD above 1 kHz.

Technical data

At 25°C ambient temperature unless otherwise noted.

Input impedance @ 1kHz: 15.8 kohms (LDA172hc);  13.5 kohms (LDA172mc)

Input impedance @20 kHz: 7 kohms ± 15%

Voltage amplification: 29.5 dB (LDA172hc), 28 dB (LDA172mc)

Allowed max. sinusoidal output power: 200 Watts (LDA172hc);  110 Watts (LDA172mc), see derating curve for testing purpose.

Frequency response (-3dB): 5 Hz to 280 kHz (with 100 ohm source impedance)

Signal to noise ratio (without hum): > 114 dB @ 200 W/ 4 ohms (LDA172hc);  > 113 dB @ 110 W/ 4 ohms (LDA172mc)

Slew rate: > 15 V/us

Damping factor (1kHz, 8 Ohms): > 200

Allowed minimum load impedance versus unloaded supply voltage (with adequate heatsinking):

Typical distortion data*

Total harmonic distortion (1 kHz): THD < 0.001% (2 Watts into 4 ohms)*

Total harmonic distortion (10 kHz): THD < 0.006% (2 Watts into 4 ohms)*

Intermodulation distortion (19 kHz:20 kHz = 1:1): IMD < 0.006% (2 Watts into 4 ohms)*

Transient (dynamic) intermodulation distortion (Square wave 3.15 kHz, sine wave 15 kHz): DIM < 0.004% (2 Watts into 4 ohms)*


* With TPC jumper installed. Distortion depends on many factors that are in the hands of the DiY builder, such as cable routing, board positioning, construction and vicinity of the power transformer, etc.  2 Watts into 4 ohms were chosen because the crossover distortion tends to be the most significant in this output voltage range. Distortion is similarly low at all other power levels.


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