Grimm Audio MP1, Руководство пользователя

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Lower in fig.1 two rows of headers, and two trimpots can be seen. The right-hand
row sets the voltages, the left-hand row changes the according crowbar protection
levels. Again, the jumpers in both rows must be moved in unison, failure to do so
may shut the MP1 down, or incriminate correct protection against over-voltage. Both
jumpers are set to the upper position if 6,0 V heater voltage is desired. In the lower
position the MP1 delivers 3.9 V heater voltage; the value is intentionally chosen as
deviating from the nominal 4.0 V found in the datasheet of the AC701 (+/- 0.2 V is
allowed), as many manufacturers specify a slightly lower value for longer tube life
or for the subtle change in mic sound reported by some users. However, if long mic
cables with thin wires are being used, the voltage at the mic end of the cable may
be lower than desired, so if the exact value is considered critical the actual result-
ing value should be measured at the microphone itself. Normal tube microphone
cables of 10 meter typically show a 0.1 V loss at the microphone. At the factory, the
voltages are therefore set to slightly above 6.1 and 4.0 V respectively, which can be
measured at the multimeter sockets on the back of the unit.
With the jumpers in the mid position both trimpots become active. This allows to
set any desired heater voltage between 3.6 and 7.0 V. One can use this for tube mi-
crophones with unusual heater voltage requirements, or in case of very long cables
to adjust the heater voltage at the microphone to an exact value (e.g. 5.8 V or 3.8
V). The best procedure is as follows. First disconnect the mic cable. Decide what
the voltage Vx should be which would set off the crowbar protection (as a rule 10%
higher than the desired working voltage). Turn the protection trim (left-hand one)
totally anti-clockwise until soft clicks are heard. Switch the MP1 on (no mic connect-
ed!). Adjust the right-hand trim until Vx + 10% is reached at the blue test socket;
anti-clockwise will increase the voltage. Now slowly turn the protection trim clock-
wise until the crowbar steps in: a red led will glow. Back to the voltage trim (R-H)
again: do 10 clockwise half-turns (5 full rotations) to lower the setting of Vout. Re-
set the MP1 by switching it off, and after a few seconds on again. Now trim Vout to
the wanted Vx (do NOT touch the protection trim anymore) and make sure you have
set the right HT as well. Switch the MP1 off again, connect the microphone, switch
on. If desired measure the heater voltage at the microphone and make further small
adustments if necessary. This makes sense for instance if one uses longer cables
or cables without a thicker heater wiring. The heater voltage can be adjusted to an
exact target value by measuring the voltage in the tube microphone itself. Needless
to say this adjustment is better left to a trained technician.