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 edu-
cated 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 (12AU7). 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 ELOP
®
has a high input impedance (HI Z) but some of our gear has a reasonably low input impedance. 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. 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.
DC OR SOMETHING AT THE OUTPUT THAT IS INAUDIBLE
- This might happen and it might be accompanied with
a strange hum or little whistles as it warms up. It only seems to occur with long cables. It happens on an older unit but not
on the newer one. Years ago we found a problem with our line driver section when it fed very long or cheap wire. The out-
put would tend to oscillate in the inaudible ultrasonic frequencies ( 200 kHz to 500 kHz) into high capacitance like a long
cable. We cured it by adding a simple 47 ohm resistor at the XLR. Newer units should not have a problem but if you have
an old one, we can FAX you a 25¢ fix. Real DC at the output would be such a rare find that we would be pretty surprised.
We use a $30 output cap to block DC. Most manufactures use a 30¢ electrolytic cap but we don’t like the sound or reli-
ability of these. We invest more in these two output caps than most limiters spend on the entire parts list. A little very low
frequency noise might be seen at the output if you use a scope. This is generally very low level and caused by AC power
fluxuations.
THE METERS ARE OUT OF CALIBRATION
- If the problem only seems to be when the unit is just turned on it’s nor-
mal. It should warm up. It might be a half dB out for 15 minutes - relax. If they drift a tenth of a dB over the course of a day
it is because of bad AC power fluxuations - chances are other gear is doing worse, you just haven’t found out yet. Your unit
will have been factory calibrated and tested twice before you received it. Sometimes parts drift a bit in value over the years,
or you have repaced tubes and want the unit calibrated at the same time, or you just want it as perfect as it can be. These are
good reasons to turn the page and go through the calibration procedure or sent it to a technician or the factory for a tweak. If
you send the unit to a tech, you should include this manual because they will need it. If you do it yourself, you will need an
Oscillator and a few screwdrivers and it would be nice to have a VOM meter and Scope but not necessary.
Once in a while we get a call from a client with a “digital studio” with confusion about levels. They usually start out by
using the digital oscillator from their workstation and finding pegged VU meters the first place they look and they know it
can’t be the workstation. Even a -6 level from their system pegs the meters. Some of you know already what ‘s going on.
That -6 level is referenced to “digital full scale” and the computer might have 18 or 18.5 or 20 dB of headroom built in.
That -6 level on the oscillator is actually a real world 12 or +14 and those VU meters don’t really go much further
than +3. There are a few standards and plenty of exceptions. One standard is that pro music (non-broadcast) VU meters
are calibrated for 0VU = +4 dBm =1.228 volts into 600 ohms. Another standard is that CDs have a zero analog reference
that is -14 dB from digital full scale or maximum. This allows sufficient peak headroom for mixed material but would be a
bad standard for individual tracks because they would likely distort frequently. This is why digital workstations use higher
references like 18 and 20 -to allow for peaks on individual sounds. It may be too much in some cases and too little in oth-
ers. Add two other sources of confusion. Peak meters and VU meters will almost never agree - they are not supposed to.
A peak meter is intended to show the maximum level that can be recorded to a given medium. VU meters were designed
to show how loud we will likely hear a sound and help set record levels to analog tape. By help, I mean that they can be
only used as a guide combined with experience. They are kinda slow. Bright percussion may want to be recorded at - 10
on a VU for analog tape to be clean but a digital recording using a good peak meter should make the meter read as high as
possible without an “over”. Here is the second confusion: There aren’t many good peak meters. Almost all DATs have poor
peak meters that do not agree with each other. One cannot trust them to truly indicate peaks or overs. Outboard digital peak
meters (with switchable peak hold) that indicate overs as 3 (or 4) consecutive samples at either Full Scale Digital (FSD) are
the best. They won’t agree with VU meters or Average meters or BBC Peak meters either. Each is a different animal for
different uses. The Limiter should help digital and analog achieve consistent levels but use each meter for it’s own strength.
The Reduction mode is useful with everything.
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