Euraudio LDA14 specifications
Welcome to the Euraudio LDA14 specifications, in which you can read about the system requirements (application criteria), operation and technical data of the LDA14.
The LDA14 is an ultra low distortion, mono, transistor audio amplifier, intended for use with high performance home audio loudspeakers, but it can be used as a reference or measurement amplifier as well. LDA14uc is the low power version of the amplifier, capable of max. 60 Watts output power. LDA14mc is the medium power version of the amplifier, capable of max. 90 Watts output power. LDA14hc is the high power version of the amplifier, capable of max. 120 Watts output power on 4 ohms or 6 ohms. Please find the LDA14uc, LDA14mc and LDA14hc assembly instructions here.
LDA14 system requirements
LDA14 is just a power amplifier circuit, it doesn't contain power supply and protection circuits, however there are Euraudio DiY kits that do. LDA14uc is part of the stereo Euraudio LDA MINI Do-it-Yourself kit. LDA14mc is part of the stereo Euraudio LDA MIN+ and the stereo Euraudio LDA MID-M Do-it-Yourself kits. LDA14hc is part of the stereo Euraudio LDA MID-H and the dual mono Euraudio LDA DM Do-it-Yourself kits.
The amplifier wasn't tested with switch-mode power supplies. LDA14uc, LDA14mc, and LDA14hc can be used with your own conventional power supply, if it is dual (symmetric) and its unloaded voltage is at least +/-29 V and not more than the"Absolute limits of unloaded supply voltage" given in the Maximum ratings section below. The maximum unloaded supply voltage may be further limited by the "Minimum permissible load impedance" specified in the Maximum ratings, and -- in some of the Euraudio do-it-yourself kits -- by the voltage tolerance of the solid-state relay (SSR) connected for protection.
Recommended unloaded supply voltage: +/-43V (LDA14hc), +/-37V (LDA14mc), +/-33V (LDA14uc), this ensures that the amplifier is not overloaded even with a 4 ohm speaker impedance.
Higher supply voltages are not recommended, and none of the LDA14 amplifier versions has been tested above +/-48V, so you use the LDA14hc with an unloaded supply voltage between +/-48 and +/-53V at your own risk.
Operation
The LDA14 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 LDA14 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 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 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 (LDA14mc), 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 frequency compensation (C3 Miller capacitor).
TPC versus Miller compensation
In the actual LDA14 circuit, the J3 (TPC) jumper serves to select two-pole frequency compensation instead of the traditional Miller compensation. With TPC compensation, lower distortion was measured at high frequencies.
Maximum ratings
Allowed max. sinusoidal output power: 120 Watts (LDA14hc); 90 Watts (LDA14mc); 60 Watts (LDA14uc) see derating curve for testing purpose.
Absolute limits of unloaded supply voltage (in some Euraudio kits, further limitations may apply due to SSR): +/-53V (LDA14hc), +/-48V (LDA14mc), +/-41V (LDA14uc)
Minimum permissible load impedance versus unloaded supply voltage (with adequate heatsinking):
Technical data
At 25°C ambient temperature unless otherwise noted.
PCB size: 91x61 mm
Input impedance @ 1kHz: 28.5 kohms (LDA14hc, LDA14mc); 23.5 kohms (LDA14uc)
Input impedance @20 kHz: 11 kohms ± 15%
Voltage amplification: 31.3 dB (LDA14hc, LDA14mc), 29 dB (LDA14uc)
Frequency response (-3dB):
6 Hz to 220 kHz (with 100 ohm source impedance)
Signal to noise ratio (without hum): > 107 dB @ 120 W/ 4 ohms (LDA14hc); > 106 dB @ 90 W/ 4 ohms (LDA14mc); > 105 dB @ 60 W/ 4 ohms (LDA14uc)
Damping factor (1kHz, 8 ohms): > 150
Typical distortion data*
Total harmonic distortion @1 kHz: THD < 0.001% (2 Watts into 4 ohms)*
Total harmonic distortion @10 kHz (48 kHz bandwidth): THD < 0.005% (2 Watts into 4 ohms)*
Intermodulation distortion (19 kHz:20 kHz = 1:1): IMD < 0.005% (2 Watts into 4 ohms)*
Transient (dynamic) intermodulation distortion (Square wave 3.15 kHz + sine wave 15 kHz): DIM < 0.005% (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, PCB 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.