Manley SHRIMP - Manual (Page 7)

FEATURES AND APPLICATIONS
7
1. VERY SHORT SIGNAL PATH WITH ABUNDANT POWER GAIN.
Each of the SHRIMP’s five sets of input jacks subjects the incoming signal source to loading of
no less than 100,000 ohms, while the output can comfortably provide over 3 volts into a test
load of 600 ohms. This implies a POWER gain of better than 22 deciBels, even though the
VOLTAGE gain is held to no more than 12 dB when the volume attenuator is at maximum. In
this case, for example, with 1 volt at the selected input and 1 volt present at the output under
test, the power gain would be more than 22 dB, or about 166 times more power out than in,
even though the VOLTAGE gain between input and output is zero! When this amount of gain is
present in circuitry containing very little negative feedback, the tendency is to reveal the
inherent nature and personality of the devices used in the circuit, along with the circuit itself.
Here, for example, the choice of input tube has a bearing on what the final sound character will
be.
The SHRIMP’s two pair of output connectors have been hard-wired in parallel for convenience
in routing output signals about for special applications such as bi/tri amplification, or for driving
satellite speakers plus subwoofer. For best performance, however, keep the combined load on
the outputs to a minimum of 1500 ohms or higher, and preferably above 5000 ohms total. When
connecting two loads to the outputs, the total load in ohms may be figured as follows:
(Zin of amp 1) x (Zin of amp 2) = Ztotal, Total load impedance
(Zin of amp 1) + (Zin of amp 2)
Where (Zin of amp 1) and (Zin of amp 2) are the rated input impedances of the two amplifiers
intended for use with the SHRIMP outputs. Example: Amp 1 has an input impedance of
15Kohms (“K” designates thousands of ohms here, hence Zin =15,000 ohms) and amp 2 has an
input impedance of 100Kohms (Zin=100,000 ohms). Then:
(15,000) x (100,000) = Ztotal = 13043 ohms, or 13kilo-ohms thus
(15,000) + (100,000)
13 Kohms
The choices of 15K and 100K above reflect typical input impedance ratings of transistorized
and vacuum-tube based power amplifiers respectively. Notice that no matter what the two
power amplifiers’ input load impedances are, the combining of the two in parallel will always
be smaller than either one standing alone. In the case of two power amps with identical input Z
ratings, the total load impedance would simply be half the value of either, i.e.: two 15Kohm
amps in parallel would present a 7.5Kohm load.
That is your math lesson for the week. There will be a quiz next Tuesday.