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. Usually this will be one of the smaller (gain stage) tubes closest to the front. The VOXBOX
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 nd the culprit but an
educated guess is OK too. Generally the rst tube in the path is the one with the most gain and dealing with the softest
signals. The usual suspect is the shorter tubes - the 12AX7 in the preamp or EQ. You may nd that you need to choose
the quietest tube out of several of that type - like we do at the factory.
DISTORTION - This might be a tube. Swapping is a good way to nd 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
VOXBOX has a high input impedance and low output impedance that can drive 600 ohm inputs of vintage “style” gear.
Best place to start is check your settings and meters. It may not be your rst guess.
DC OR SOMETHING AT THE OUTPUT THAT IS INAUDIBLE - The 1/4” unbalanced outputs have a frequency
response that goes way down to below 1 Hz. A little very low frequency noise may be seen as speaker movement when
monitors are pushed to extreme levels. The XLRs do not exhibit this because the transformers lter below 8 Hz. Also the
unbalanced outputs do not like long cheap high capacitance cable. Occasionally a very high frequency oscillation (200
kHz to 400 kHz) may occur in these conditions. Once again use the XLR outputs. Problem solved.
THE METERS ARE OUT OF CALIBRATION - If the problem only seems to be when the unit is just turned on
it’s normal. It should warm up. It might be a half dB out for 15 minutes - relax. Everything should stabilize within 15
minutes as the temperature reaches “normal”. Your unit will have been factory calibrated and tested at least 4 times
before you received it. Sometimes parts drift a bit in value over the years, or you have replaced 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 go through the calibra-
tion 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 audio oscillator and good voltmeter and a few
screwdrivers and it would be nice to have a Scope but not necessary. 4 out of 5 of the trims are just for metering and the
audio trim just sets unity gain for the EQ. A little advice: if its not broke - don’t x it. We get a number of units back
because someone thought they should tweak and couldn’t.
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 nding pegged VU meters the rst 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 analog +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 sufcient peak headroom for mixed material but
would be a bad standard for individual tracks because they would likely distort frequently. This is why digital worksta-
tions 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 others. 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, we
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 strange peak meters that do not agree with another company’s DAT. 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 consecu-
tive samples at either Full Scale Digital (FSD) are the best. They won’t agree with VU meters or Average meters or BBC
Peak Programme (PPM) meters either. Each is a different animal for different uses. The Compressor and Limiter in the
VOXBOX should help digital and analog achieve consistent levels but if you are recording to digital, use your machines
meters for the recording level. The VU meter on the VOXBOX is great for analog tape, establishing “loudness” decisions
and for calibrating levels using an oscillator but not for digital peaks. Some engineers can interpret a VU and get near
ideal digital levels but it takes years of experience because it depends on the sound. We sometimes think about adding an
accurate and useful peak LED but that question of “accurate to what” comes up. If it were just an LED indicating when
our units reached clipping you’d probably nd the machines’s meters would be pegged.
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