High Fidelity Pre-Amplifier

Mullard Wireless, London (see also Mullard Radio Valve)

Datos
Foro
Imágenes
Colecciones

  • Año
  • 1955
  • Categoría
  • Kit (Partes con instruciones) o Solo Instrucciones
  • Radiomuseum.org ID
  • 283570

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 Especificaciones técnicas

  • Numero de valvulas
  • 3
  • Principio principal
  • Amplificador de Audio
  • Gama de ondas
  • - no hay
  • Tensión de funcionamiento
  • Alimentado desde otro equipo o aparato principal. / DC: 250 & AC: 6.3 Volt
  • Altavoz
  • - Este modelo usa amplificador externo de B.F.
  • Material
  • Metálico
  • de Radiomuseum.org
  • Modelo: High Fidelity Pre-Amplifier - Mullard Wireless, London see
  • Forma
  • Chasis (tambien de autoradio)
  • Anotaciones

  • Three‑stage EF86 pre‑amplifier published by Mullard as the companion circuit to the Mullard 20‑watt high‑quality amplifier. The design provides the required gain, equalisation and filtering for magnetic and crystal pickups, radio/tape sources, and low‑level microphones. Output is approximately 200 mV at low impedance, matching the input sensitivity of the Mullard 20‑watt amplifier, including the later 1960  version.

    The pre‑amplifier is powered from the main amplifier’s HT rail via a 250 V / 3 mA feed obtained through a 56 kΩ dropping resistor and decoupling capacitor.

    Circuit Summary
    V1 provides high‑gain input amplification with anode‑to‑grid feedback to set sensitivities for each input. V2 restores gain lost in the passive tone network and maintains low source impedance for the tone controls. V3 incorporates the high‑pass and low‑pass filters and drives the output gain control. The design maintains stable overload behaviour, with distortion remaining low even at 20 dB overload.

    Performance

    • THD: < 0.1% at 200 mV output; < 0.2% at 2 V (20 dB overload)
    • Intermodulation: ≤ 1% at 20 W system output
    • Noise: –53 to –64 dB depending on input loading
    • Frequency Response: Flat to 20 kHz in filter position 3; response shaped by tone controls and selectable filters

    Technical Notes

    Inputs:

    • Microphone (1.5 mV, 1 MΩ)
    • Radio/Tape (100 mV, 100 kΩ)
    • PU1 LP (4 mV), PU1 78 (5 mV)
    • PU2 LP (50 mV), PU2 78 (60 mV)

    Tone Controls:

    • Passive Baxandall bass and treble network using logarithmic‑law potentiometers (10% law), with “flat” near mid‑rotation

    Playback Equalisation:

    • Microgroove (LP): RIAA‑based playback characteristic, with Mullard’s low‑frequency modification below 1 kHz to approximate earlier pre‑RIAA curves
    • Standard (78): Based on EMI recommended characteristic, with additional HF cut to reduce surface noise

    Filters:

    • Low‑pass at 5 kHz, 10 kHz, or 20 kHz (flat)
    • High‑pass (rumble) at 35 Hz, ≥12 dB/octave

    Output:

    • Low‑impedance feed via 100 kΩ log gain control; stable with up to 400 pF load (≈20 ft coax)

    Power:

    • 250 V at 3 mA HT; 6.3 V heater supply (0.6 A total)

    Examples of commercial Pre-amplifiers

    Stern

  • Autor
  • Modelo creado por Gary Cowans. Ver en "Modificar Ficha" los participantes posteriores.

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