Manley MONO & DUAL MONO 40 dB MICROPHONE PREAMPLIFIERS - Manual (Page 14)

HUM - Let's assume it knows the words. Once again - several possibilities - several cures. Most likely it is a ground loop.
The two most common procedures are: try a 3 pin to 2 pin AC adapter (about a dollar at the hardware store) which is better
than messing up the power cable by bending the ground pin until it breaks off. Method two - cutting the shield on one side
of the cable (PIN 1). This is done by some studios at every female XLR to "break" all loops. You may get a loop simply from
the rack. All the other gear in the rack is "dumping" ground noise onto the rack rails. Try removing the Limiter from the rack
so that it is not touching any metal. You may have cured a non-loop hum. Some gear radiates a magnetic field and some gear
(especially if it has transformers) might receive that hum. A little distance was all it took. A cool method of reducing all sorts
of hum and noise is to use the new 60-0-60 balanced AC power transformers available from Equi=Tech and Furman. It costs
more but works best. Hum is more likely with the unbalanced inputs and outputs because these signals are ground referenced.
IT MAKES NOISES WHEN THE FRONT PANEL IS TAPPED - An easy one. Some tubes become microphonic over
time. That means they start acting like a bad microphone. Vibration has caused the supports for the little parts in the tube to
loosen and now the tube is sensitive to vibration. Easy - Replace the tube. Which one? The one that makes the most noise
when you tap it. It will have to be on , connected and speakers up but not too loud for the sake of your speakers.
IT GOT HISSY - Also easy. This is again a common tube symptom. You could swap tubes to find the culprit but an educated
guess is OK too. Generally the first tube in the path is the one with the most gain and dealing with the softest signals. The
usual suspect is the tube that is usually located closest to the front panel volume pot or the 6072 (or 12AX7, 12AT7, 12AU7,
5814 on other models). You may find that you need to choose the quietest tube out of several of that type.
DISTORTION - This might be a tube. Swapping is a good way to find out. It may be a wiring thing or mismatch as well.
Wiring problems usually accompany the distortion with a major loss of signal. Mismatches are a bit tougher. The input and
output impedances of this preamp are very reasonable. Without even explaining the term "impedance" it is enough to say that
a lot of gear is simply not capable of driving pro levels and low impedances. It will sound like lost headroom, early clipping,
distortion on peaks.We do not expect these problems with this unit, but you may find this further down the chain. Often
changing the order of processors will do the trick. Another not so rare place to look is the patchbay, your settings, the meter
levels - it happens.
SNAPS, CRACKLES, POPS - Along with hiss, microphonics, a dead channel, distortion and a few other wierd symptoms
is probably due to a tube. Try a channel swap or a tube swap. Not the tube? The next most likely possibility is a bad tube socket
(caused by bent tube pins or roughly inserting a tube. A technician may have to replace the socket. Another possibility is the
famous "broken wire". This is usually obvious if one just looks. Vibration may eventually weaken a wire or solder joint. Again
this is an easy repair for any tech or someone handy with a soldering iron.
IT GAVE ME A SHOCK ! - If you got a little shock while handling a mic cable while phantom was turned on you probably
deserve it. Go back and read the section on Phantom Power. That was a 48 volt, almost no current, little surprise. Any and
every MIC PRE can give you this minor jolt. The solution? Turn Phantom off when patching. If you got the shock while the
top is open and you are changing tubes - well....again we warned you on page 2, page 12 and once again. Tubes use high voltage
(300-400 VDC) but very little current. One should be very aware of this before opening the top. Again - let the power supply
capacitors discharge, don't grab the circuit board, use one hand, use gloves, etc. For what it is worth, the voltage in the unit
can cause pain, a pin-hole burn and a big surprise but the current is so small that there is almost no chance it might be lethal.
A bigger concern is the current from the 120V or 220V AC mains is when you plug something in the wall. Be careful, be aware.
CAN'T GET ENOUGH LEVEL -This may happen. This design has a maximum gain of 50 dB. This may be short of what
is needed for some situations. Do not be afraid to turn up the INPUT ATTENUATOR pot. In other words it reduces the signal
as one turns counter-clockwise. If you turn up the INPUT, you are using less attenuation with the same gain. It is a good thing
to use both less attenuation and less gain. Not a conventional modern concept, but one rooted in old school recording and one
of the biggest reasons why this MIC PRE sounds so damn good. Sometimes we like to design where "all controls at 12:00"
is a good starting point but this is not so cool when it affects the sonic quality. Some designs just sound best at full tilt and
this MIC PRE leans that way. BTW, we built a 60 dB pre and almost nobody wanted it - now discontinued.
If this is the problem when using the INSTRUMENT (or DIRECT) IN - sorry. It is optimised for synths, drum machines and
electronic sources. You may find that a typical good direct box used normally into the MIC INPUT does the trick. The
Instrument Input has about 25 dB of gain and most guitars and basses seem to need in the neighborhood of 20 dB but some
guitars are not as hot as others and 25 dB has not been enough for some instruments. Another good reason for using a direct
box is the input impedance of the MIC PRE DIRECT INPUT is 100K ohms. This is OK, but with a direct box it is more likely
1meg (1000K) to 10 meg which is bound to give more highs. On the other hand, most owners report rather remarkable results
using the DIRECT IN for all sorts of instruments. Maybe that stomp box might help........
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