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the two terminals on Card ZE 0067 (Fig.3.11 ). Typically, an output imped-
ance increase of 10 dB may be obtained for frequencies greater than
100Hz. The link must be replaced for operation in the "Low" impedance
mode or the amplifier may oscillate.
Fig.3. 11.
High Z Terminals. Card ZE 0067
3.7. REDUCTION OF BACKGROUND NOISE AT THE EXCITER TABLE
The field supply current is normally derived from a full-wave rectifier,
and because of ripple some residual motion will still exist at the exciter
table, even with no input applied to the amplifier. The ripple is at twice
mains frequency ( 100 Hz or 120 Hz).
The residual motion of B & K systems with the Power Amplifier Type
2707, the Exciter Body Type 4801, and a matching Exciter Head is typical-
ly 70 dB below the motion corresponding to full force. This low level of
background motion is insignificant for most applications.
An additional 10 to 20 dB reduction in the level of the residual motion
(10 to 20 dB increase in dynamic range) can be achieved by connecting a
100,000 pF, 30 V electrolytic capacitor, by thick, short leads to the Exciter
Body terminal block. This type of capacitor is polarized and must be con-
nected positive to terminal 1 and negative to terminal 4 (Fig.3.12). Larger
capacitors will further reduce the residual motion level.
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