The Delta
Amplifiers inherit some of the well accepted characteristics of previous Leak
models, e.g. plug-in printed circuit boards. Uprating circuitry, heat sinking
and fusing has led to significant performance improvements, particularly under
high duty conditions.
There are
four plug-in boards:
Two
identical preamplifiers (left and right);
Two
identical power amplifiers (left and right0
The power
transistors, transformer and other power supply components are mounted on the
chassis.
Due to the
similarity of construction and circuitry between the Delta 30 and Delta 70,
both units are incorporated any differences.
Circuit
description
Power
Amplifier - The output transistors and
temperature compensation diodes are mounted on the heat sink, the remainder of
the circuitry being located on the power amplifier P.C. boards. Power
transistors are insulated electrically from the chassis by mica washers.
Quiescent
current is measured by removing the appropriate HT fuse, and inserting a
current meter. The current is adjusted by P5 and should be approximately 40 mA.
The
amplifier D.C. output voltage (pin D on power amp board) should be +23 V with
an HT supply of +48 V. This voltage is set by fixed circuit components, and
significant deviation of this "midpoint" voltage indicates component
failure,not necessarily an output transistor.
Full output
for each channel is realised with 11 V RMS across an 8 ohm load.
Pre-Amplifiers
The P.C.
contains tone control circuitry (excluding potentiometers) R1AA equalisation for disc input, filter
components, and appropriate amplification for all inputs.
With all
inputs short circuit, volume control maximum, other controls level, noise
output voltages should be less than the following: 7-5 mV on Tuner and
Replay, 24 mV on other inputs
The
sensitivities for each input to provide full output are indicated in the
specifications (1 kHz sine wave). The corresponding output from the
preamplifier,measured at the tape sockets is: phono socket output 400 mV
approx. DIN output 40 mV approx.
Specification
Rated Power
Output (both channels sine-wave driven at 1 kHz):
20 +
20 watts into 4 Ω
15 +
15 watts into 8 Ω
10 +
10 watts into 15 Ω
Music Power
Output (IHF):
60
watts into 4 Ω loudspeakers,
40
watts into 8 Ω loudspeakres,
24
watts into 15 Ω loudspeakers.
Total
Harmonic Distortion:
0-1%
at ALL power levels up to 10 watts at 1 kHz each channel into 8 Ω
loudspeakers.
Total
Intermodulation Distortion:
Input - 70 Hz and 7 kHz in ratio of 4:1,
Less
than 0-2% for ALL power levels up to 15 watts into 8 Ω loudspeakers
Less
than 0-2% for ALL power levels up to 10 watts into 15 Ω loudspeakers
Overload
Distortion:
Less than
0-1% for input signals up to 20 dB above stated sensitivity from 30 Hz to 15
kHz
Hum and
Noise:
63 dB below
30 watts into 8 ohm load on "Tuner" and "Replay"
53 dB below
30 watts on other inputs.
Cross Talk
(between L & R channels): -50 dB up to 1kHz and -30 dB at 10 kHz.
Damping
Factor:
20
measured at 1 kHz for 8 Ω
loudspeakers
40
measured at 1 kHz for 15 Ω
loudspeakers
Input
Sensitivities (the sensitivities shown below give 30 watts output at 1 kHz into
8 Ω loudspeakers with tone controls
"zeroed" and volume control on maximum):
Disc 1
(RIAA Characteristic) : 2 mV at 1 kHz
Input Impedance 47 k Ω
Disc 2
(RIAA Characteristic) : 10 mV or 30
mV at 1 kHz Input Impedance 33 k Ω , 100 k Ω
Tuner
1 / Mic : 25 mV or 2 mV at 1 kHz / 47 k Ω 47 k Ω
Tuner
2 : 60 mV 250 mV / 50 k Ω 50 k ohm
(Selected by
switched input attenuator marked "HI" and "LO" on rear
panel).
Replay
: 400 mV / 47 kohm
Frequency
Response: 30 Hz to 20,000 Hz (±1 dB)
Tone Control
Range: 50 Hz to 14,000 Hz (±16 dB)
Filter: 12 dB per octave roll off above 6 kHz
Dimensions: 344 x 119 x 254 mm (13,5" x 4,7" x
10")
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