High Fidelity Pre-Amplifier

Mullard Wireless, London (see also Mullard Radio Valve)

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  • Year
  • 1955
  • Category
  • Kit (Parts plus instruction) or building instructions only
  • Radiomuseum.org ID
  • 283570

Click on the schematic thumbnail to request the schematic as a free document.

 Technical Specifications

  • Number of Tubes
  • 3
  • Main principle
  • Audio-Amplification
  • Wave bands
  • - without
  • Power type and voltage
  • Powered by external power supply or a main unit. / DC: 250 & AC: 6.3 Volt
  • Loudspeaker
  • - For headphones or amp.
  • Material
  • Metal case
  • from Radiomuseum.org
  • Model: High Fidelity Pre-Amplifier - Mullard Wireless, London see
  • Shape
  • Chassis only or for «building in»
  • Notes

  • 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

  • Author
  • Model page created by Gary Cowans. See "Data change" for further contributors.

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