PREVENT INPUT DAMAGE
From several sources, a pattern of damaged audio equipment inputs has emerged. Here are some hints to avoid these
disappointing and costly events:
- Watch what you connect to that input. The following should be OK, provided there is no malfunction:
- All passive devices, like mics, guitar pickups, and other unpowered equipment
- Guitar pedals and stomp boxes
- Most line level outputs -- but watch low impedance ones with +4 dBu professional outputs, which may be too much
for some inputs
- Home stereo equipment line level signals -- but see below
- Always use a protective attenuator pad for recording from any of the following:
- Speaker and headphone outputs
- 50-ohm line drivers
- 25-volt and 70-volt audio lines
- +4 dBu line outputs with low impedance (under 200 ohms -- I found it necessary to pad the outputs of our Mackie
and Samson mixers.)
- Other low impedance outputs
- Outputs with a DC voltage superimposed on the signal (some computer connections have this problem)
Build a headphone output attenuator for protecting line inputs.
Fix your headphone overload noises with this cable.
- NEVER connect any of the following directly to a sensitive input:
- Guitar amps with ground reverse switches (unless modified for safe 3-wire grounded-cord operation): Even the guitar
cord plugged into this is dangerous. Use a direct box, with the ground lift switch taped down in the "lift" position.
BUT watch for accidental contact between a mic connected to your input and metal on the guitar, cables and amp. Such
contact can do the same damage to the mic channel.
- Radio or TV equipment that originally had no connections to tap off signals: Many of these have circuitry that is
internally connected to one wire of the power cord. It is dangerous to humans and equipment to try to use them as
- Old tube-type consumer audio equipment: Many of these have internal connections between chassis ground and one wire
of the power cord through a capacitor. An upgrade to a 3-wire power cord and plug (and removal of the capacitor) can
- Proprietary connections (such as a spring reverb transducer connection on a guitar amp): These are not intended
for use by other equipment, and may contain damaging levels or voltages.
- Unisolated condenser mics: These can put 48 volts on your input.
- Anything connected to power through a 3-wire to 2-wire adaptor
- NEVER defeat the equipment safety ground on equipment to remove a pesky ground loop. Instead, make sure all
equipment grounds are intact, connect all equipment to the same power strip (or strips plugged into that strip), and
use ground-loop breaker devices on the signal cables if hum persists.
- If the safety ground is defeated (or absent, as in the case of the guitar amp with the ground switch), and a
ground fault happens, the input of your equipment can take the entire brunt of the ground fault.
- So can a musician. Defeating a safety ground is dangerous.
- Watch interconnecting equipment tied to different grounds. Most home recordists won't see this problem, but
performing bands do, all too often:
- One common way equipment is destroyed occurs when the venue has an insufficient electrical supply. A portable
generator is brought in to supply enough power for a large band, or a second electrical service is ordered. Trouble
occurs when patch cords or snakes are run between devices powered by these different supplies. Bonding is supposed
to be provided between the supplies if interconnection is possible, but too often, this is not done, or is done
insufficiently. That leaves the patch cord carrying the bonding current too. A little resistance in the shield,
combined with a surge, can fry an input.
- In some stage-lighting systems, 120 volts is derived by an autotransformer from two legs of a 240 V 3-phase
delta system. Neither wire of this 120 volts is at ground potential. It must not be used for anything but the
incandescent lighting it was intended for.
- The ground at an outlet at one end of a long snake can be very different than the ground at an outlet at the
other end. It is better to run a power cord to the mixer alongside the snake, than to risk carrying differential
ground currents through the snake and the inputs it is connected to.
Plug every piece of sound equipment into the same outlet if possible, using enough power strips to supply
everything. The power source used should be at the end where the power amplifiers are, usually at the stage, not
at the mixer.
If all of the equipment draws more than 15 amps, then put just the power amps on any other power circuits
used, and use isolation transformers (direct boxes with the ground lifted, placed at the amp end of the cable)
to feed signals to them.
- Watch static electricity: Winter, sweaters, jackets, and carpets are usually the culprits here. A good dose
of anti-static spray should be applied in October, and after carpets or clothing are cleaned.
- NEVER plug a mic into a headphone or loudspeaker output. That can reduce an expensive mic to uselessness
in a millisecond. On some equipment, it can also fry the right channel headphone amp.
- Do I need to remind people of the hazards of spilled drinks and wet weather? Water entering a guitar amp at the
other end of the snake can send voltages up your direct connection and fry your input long before the guitar player
- Remember that there are widely differing levels of signal in audio equipment. Be sure you know which one you are
dealing with in each case:
- Microphones put out very tiny signals, on the order of 1 millivolt.
- Phono pickups also put out tiny signals, but need completely different treatment (an RIAA compensated preamp).
- Guitars put out signals somewhere between mic and line levels.
- Line level devices put out approximately 1 volt, or about 1000 times what a mic puts out. Most electronic devices
interconnect at line level.
- Headphone outputs put out from about 2 to 5 volts - enough to damage sensitive inputs.
- Speaker outputs can put out from 2 to 150 volts, depending on wattage.
- Transmission lines can put out over 75 volts.
Be sure to use the proper matching device to convert the voltage to a safe level for the input.
Don't put the job ahead of safety.
- Make sure there are no hazards to the equipment.
- Make sure there are no hazards to anyone operating the equipment.
- Never defeat safety grounds.
- Use the above list to determine the proper methods to use.
Note that you can get a speaker (or headphone) output to mic input attenuator cord at Radio Shack. But you will
probably have to adapt it - it has 1/8" plugs.
Mixing your own sound