We've broken the explanations down into two articles: Input devices
such as instruments and preamps, and output devices
, usually speaker cabinets or headphones. This article is about the INPUT side, what you plug INTO an amp. There is another FAQ that talks about impedance from the speaker, or output perspective.
What the heck are impedance and ohms?
When you plug a microphone, instrument, or preamplifer into the input of an amplifier, that device becomes part of the circuit. When your amp was designed, all the parts inside were chosen carefully, and their values and characteristics are critical to getting the results intended. It's only logical that if you plug something in that doesn't match up to what the amp or preamp was designed to expect, strange and/or damaging things can happen. This article is a brief overview that should help you understand what works with what, what doesn't, and why.
Why should I care?
Because it can make you sound BAD, or mean that you might buy something that won't work properly with gear you already own.
Impedance is expressed in ohms,
which is typically a measure of resistance, but impedance involves more than just resistance. You'll see the values expressed as "ohms" or represented by the omega symbol: "
". You'll also see "K" (representing thousand) and "M" (representing million) with the numeric value. So 100K
expresses 100,000 ohms, 1M
expresses one million ohms, or 1 megohm.
Stuff you plug into an amp:
We can group input device impedance values into three general categories. These are identified not by the device's specific impedance, but the impedance value that is compatible with the input jack. Specific connectors do not always identify impedance; the incredibly common ¼ inch jack could be almost anything, including speaker connections! Check the product documentation.
, such as many Microphones (some can be high impedance, check the specs) and outputs from DI (direct injection, direct) boxes. These are typically devices that have XLR plug outputs. The approximate impedance is 600 ohms but can vary in that range. The inputs on devices are usually XLR jacks, although in some instances can be a TRS (stereo, three conductor) ¼ inch jack (though TRS connections can also be high or ultra-high impedance).
Low Impedance devices are usually (but not only) found plugged into PA boards, instrument amplifiers, and preamps with 150-600
XLR input jacks. However, you will also find some cases where ¼ inch TRS and mono jacks are used, but usually in conjunction with a power supply (phantom power
, such as most electric guitar and bass pickups and instruments, some (usually lower priced) Microphones (some can be low impedance, check the specs), most effects boxes, most preamps and instrument "processors" (stomp boxes), some of which may have both low and high impedance output jacks. "High" is probably the most common input impedance value in our musical input world, and the generic term "high impedance" is used quite often.
High Impedance devices are most often (but not only) found plugged into high impedance ¼ inch phone jack inputs in instrument amplifiers, PA boards, effects boxes, preamplifiers, etc. Input impedance values vary, but some typical values are 44K
(usually active) and 800K
(typically passive). It's ok to plug high impedance instruments and devices into ultrahigh impedance (1MegOhm and higher) inputs.
is the least common in general use, but the most encountered with piezo transducers (pickups) that sense vibration, such as most Upright Bass pickups and undersaddle transducers in acoustic-electric guitars and bass guitars. If the instrument has a preamp (with a battery, not just a passive control) it is likely to be high impedance (¼ inch jack output) or low impedance if there is an XLR jack. If passive, the amp or preamp should have a one megohm (one million ohms, also expressed as 1M
or 1Mohm) or higher input impedance.
Ultra High impedance pickups are most often found plugged into instrument amplifiers and preamps that have a 1 MegOhm or higher input impedance. While you can plug them into high impedance inputs, you will often hear undesirable characteristics from the difference (see below).
What happens if there is a mismatch?
That depends on the input device and the amp jack's characteristics. In many cases you'll get nothing, a very low signal level, distortion, or possibly a hum or buzz. In the case of an Ultra-high piezo pickup (like most upright bass pickups) and into a high impedance "bass guitar" amp (which expects high impedances in the bass guitar range), you'll ordinarily hear artifacts of the mismatch in the form of a thin, sort of "quacky" bass response, something that will make we bass players very unhappy - the impedance discrepancy actually creates a sort of "filter" that cuts certain frequencies. It's not as noticeable with higher pitched instruments, these artifacts most audibly affect lower frequencies. FYI: It's ok to plug high impedance instruments and devices into ultrahigh impedance inputs.
How can I avoid a mismatch?
The most common solution is to use an impedance buffering preamp, one that is usually aimed at acoustic instruments and has a 1 megohm (1meg
or 1 million ohms) or higher input impedance. There are a variety of devices out there, from simple, basic preamps that offer little to those that have a wide range of features to control your sound. There are also many amplifiers that offer an input impedance suited for piezo type transducers, so an external preamp isn't necessary.
Common Questions regarding amplifier inputs:
Can I just use an adaptor that changes the XLR cable connection to a ¼ inch plug?
Nope. Well, you can, but that's not enough, the only way to make them truly compatible is to use an adaptor/transformer. It will otherwise probably be craptastic.
My amp has two inputs, one is labeled high, the other is low, is that impedance?
Probably not. The most common meaning for those labels is the level of the signal, such as for an active (such as a bass guitar with a preamp on board) or passive instrument (no on-board battery or preamp). Unfortunately, this is not universal in meaning, so experience will tell you which is for which. You will have to consult the Specifications section of your amplifier's operating manual to discover the actual impedance rating — if
it is documented.
Can I plug the output of one amplifier into the input of another to make it louder?
The cables might let you, but please don't - you will likely blow up one or both amps. Don't even think about it.
My amplifier doesn't get loud enough. If I get a preamp will that make it louder?
It may, if whatever you are amplifying has a low signal level, but it is more likely that it will not. For example, if you have an amplifier that is rated at 100 watts, that's about all that you will get out of it no matter what you do. Most amps have two volume controls, one for the input gain, and then a master volume. As you may have already found out, if you turn them both up to maximum (or the only volume control on the amp if that's what it has) it will sound pretty awful. Adding a preamplifier will only increase the input volume level, which will only increase its level of awfulness.
I am using a preamp, but it is distorting the sound; or making the sound muddy; or...
A typical musical instrument amplifier head (amplifier only, no speaker) or combo (amplifier and speaker) has a preamp inside. A preamplifier's function is to increase the level of the signal from instrument level (out of your bass) so it is high enough to drive the amplifier. When we add an "outboard" preamplifier it is possible to overload the amp's preamp as well as possible to overload the amplifier section with too much signal from the amp's preamplifier. That usually results in distortion or otherwise oversaturated sound. Back off the volume on either or both the outboard and onboard preamps until the sound improves.