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The four types of amplifier used in the broadcast chain were referred to as A, B, C, and
D amplifiers. The 'A' amplifier was the first in the chain and had a gain of around 50dB.
It raised the level incoming from the studio from -68dBu to -18dBu. There were 31 of these
amplifiers installed, 21 for use with studios, 5 for echo rooms, 2 for the Dramatic Control
Panels and 3 spares.
The outputs from the sources' A amplifiers at -18dBu were distributed to the control positions
in the control room where the operator (an AME, Assistant Maintenance Engineer) selected
a studio on his 2 or 4 channel 'fade unit'. The output of the fade unit then passed through
the 'Main Control Potentiometer'. This introduced a loss of about 14dB at its normal setting.
It was followed by a 'B' amplifier, sometimes referred to as 'the control amplifier', with
a gain of up to 40dB. This raised the level up to about 0dBu with maximum peaks of +10dBu.
This attenuation followed by amplification was to make it possible to increase the level
of low level sources which otherwise would have been impossible. There were 16 'B' amplifiers,
one for each control position plus two spares.
After the programme had been mixed and controlled so that the dynamic range was suitable
for transmission, a 'C' amplifier was used to feed the Post Office line to the transmitter.
This had a high input impedance (so that many 'C' amplifiers could be fed from one 'B' amplifier)
and an output impedance of 600 ohms to feed the Post Office line. The 'C' amplifier could
be adjusted to output the level specified by the Post Office for a particular line which
could in fact be a loss rather than a gain. The amplifier's function was to act as a buffer
as well as to feed the line from the correct source impedance.
The 'D' amplifiers with a gain of about 45dB were used to amplify incoming Post Office lines
and were somewhat similar to 'A' amplifiers. There were 6 of them, 2 for incoming programmes
from Leeds and Daventry and the other 4 for Outside Broadcast lines and as spares.
In addition to these 4 types of amplifiers there were also 'trap-valves' which were buffer
amplifiers used to feed the studio listening room loudspeakers and for various headphone
feeds. Each transmission position had a trap valve amplifier and there were 3 more for rehearsals
and spares.
Finally there were programme meter amplifiers which fed meters with a scale that looked
just like a PPM but wasn't. The meters were neither peak reading nor average reading. Line
up was to '5' and peaks were to '7'. The PPM did not arrive until 1938 and for a time retained
the '5' and '7' line up.
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