SP400 Passive Preamplifier

The SP400 is not strictly a preamplifier in that it does not provide any amplification. It would be more accurately described as an Input selection unit with adjustable attenuation.

The SP400 was designed to provide source and recording selection, together with level adjustment for the SA400 Semi Digital Amplifier. The design objective was to provide source selection and level control that would introduce as low a distortion and noise level as possible. To reduce distortion and noise to its lowest possible level, it was necessary to eliminate any electronics or reactive components in the signal path together with any power supplies that would be needed. By removing the mains transformer there is also one less system component, which could contribute to hum loops.

The SP400 uses just three basic components: -
    Input and Output Sockets
    Level Control
    Rotary Switches

All of the above components are the highest quality available to ensure that no additional noise or distortion is added to the input signal when applied to the SA400 Amplifier.

Equality of channel level is most Important for high quality audio equipment, which has to be better than +/-1dB. The manufacturers of level controls in general will only guarantee this level of matching from approximately 30% to 70% or 10 o/c to 2 o/c. The gain structure of the SP400/SA400 combination ensures that the level control for normal listening will be in this range.

A disadvantage of the passive design approach is that the output impedance is somewhat higher than that of an active (electronically powered) unit. Consequently, to avoid high frequency attenuation, attention to output cable length and capacity must be observed. The frequencyresponse table in the specification below shows various cable capacities and lengths at the worst-case position of the level control. Ideally of course, the SP400 would be mounted beneath the SA400, and, in this position; even the highest cable capacities will still ensure a frequency response to over 70kHz.

High quality screened cables must be used to interconnect the units.

As the unit has no powering of any kind, remote control is not possible. The SP400 has nevertheless, a Standby switch that will put the SA400 into Standby Mode.



Gain (Maximum): 1 (0dB)
Separation (2Hz - 20kHz):  >50dB
Frequency Response:
    Low frequency limit: D.C.
    High Frequency Limit (-3dB): Decided by output cable

Length of output cable versus capacity per metre:

The above figures were measured with a load of 20kOhms.

Output Impedance (Worst Case):
(Assuming Source Impedance <150 Ohms): 5kOhms
Channel matching: ± 0.2dB
Attenuation at 12o/c:
(Assuming Source Impedance <150 Ohms):
16dB Open Circuit Load
18dB 10k Ohms Load
Inputs: 5
Outputs (Main): 2
Outputs (Record): 2
Tape Loops: 1
Dimensions (Including Knobs): D 340, H 95, W 445 mm
Weight: Net 6kg; Packed 8kg

E. & O.E. The policy of Alner Hamblin Electronics is that of continuous design and development. We reserve the right to change specifications without prior notice.

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