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grace design m201
owner’s manual
15
INPUT OFFSET ADJUSTMENT
The input amplifiers in the m201 are ultra-precision laser trimmed devices which have
a very low input offset. However, even the smallest differential offset at the input can
result in an audible “click” when the gain control is turned. It should be noted that this
type of click will be audible when no signal is present. With no signal present there is
no signal amplitude to be changed unless input offset is present. This procedure nulls
the input offset and minimizes the clicking sound with no signal present. In contrast,
there will almost always be an audible click if turning the gain control while a signal is
present. This is because the gain control, being a stepped control, will make an instan-
taneous change in amplitude when turned, which creates a transient signal.
EQUIPMENT NEEDED
• DC Volt meter with at least 1mV sensitivity
• Plastic alignment tool or a small screwdriver
• #2 phillips head screwdriver
PROCEDURE
1) Remove the 8 phillips head screws from the top lid but leave the lid in place.
2) Set all of the preamplier gain controls to maximum.
3) Turn on the m201 preamplier and allow to warm up for at least 45 minutes. (go
get a nice cup of coffee and do some stretches)
4) Remove the top lid and locate the DC offset test points TP1A and TP2A
(see figure 4
next page)
. Place the Volt meter probes into these holes.
5) Locate DC offset potentiometer VR1A (VR1B for channel 2) and rotate it for a read-
ing of 0.000VDC +/- 0.003V.
6) The input offset does drift with temperature and having the lid off will allow the
circuitry to cool. It is advisable to replace the lid and let the preamplifier warm up
again (about 20 minutes) and then re-check your adjustments.
Cleaning
Your m201 preamplifier chassis is constructed out of high quality stainless steel. Under
normal circumstances, virtually no maintenance is required to keep the unit looking
shiny and new. However, if your unit becomes smudged or dirty, here are some clean-
ing tips: We recommend using either Pledge furniture polish or Zep brand stainless
steel cleaner (available at the hardware store). Apply cleaner to a clean, dry, lint free
cloth and gently wipe all stainless surfaces, taking care not to allow the cleaning prod-
uct to build up around the panel switches or knobs.