Mic Input
Connector: XLR-3 female
|
Pin |
Signal |
|---|---|
|
1 |
Screen |
|
2 |
Hot (+ve) |
|
3 |
Cold (-ve) |
Output
Connector: XLR-3 male
Line Input / Insert Send / Return
Connector: Balanced (TRS) 1/4” Jack socket
|
Pin |
Signal |
|---|---|
|
1 - Tip |
Hot (+ve) |
|
2 - Ring |
Cold (-ve) |
|
3 - Sleeve |
Ground |
Insturment Input
Connector: Unbalanced (TS) 1/4” Jack socket
|
Pin |
Signal |
|---|---|
|
1 - Tip |
Hot (+ve) |
|
2 - Sleeve |
Ground |
A major element of the sound of a mic pre is related to the interaction between the specific microphone being used and the type of mic preamp interface technology it is connected to. The main area in which this interaction has an effect is the level and frequency response of the microphone, as follows:
Level
Professional microphones tend to have low output impedances and so more level can be achieved by selecting the higher impedance positions of the ISA Two mic preamp.
Frequency response
Microphones with defined presence peaks and tailored frequency responses can be further enhanced by choosing lower impedance settings. Choosing higher input impedance values will tend to emphasise the high-frequency response of the microphone connected, allowing you to get improved ambient information and high-end clarity – even from average-performance microphones. Various microphone/ISA Two preamp impedance combinations can be tried to achieve the desired amount of colouration for the instrument or voice being recorded. To understand how to use the impedance selection creatively, it may be useful to read the following section on how the microphone output impedance and the mic preamp input impedance interact.
Note
Impedance Setting – Quick Guide
In general, the following selections will yield the following results:
High mic preamp impedance settings:
-
Generate more overall level
-
Tend to make low- and mid-frequency responses of the microphone flatter
-
Improve the high-frequency response of the microphone.
Low preamp impedance settings:
-
Reduce the microphone output level
-
Tend to emphasise the low- and mid-frequency presence peaks and resonant points of the microphone.
Dynamic Moving Coil and Condenser Microphones
Almost all professional dynamic and condenser microphones are designed to have a relatively low nominal output impedance of between 150 Ω and 300 Ω when measured at 1 kHz. Microphones are designed to have such low output impedance because of the following advantages result:
-
They are less susceptible to noise pickup
-
They can drive long cables without high-frequency roll-off due to cable capacitance
The side-effect of having such low output impedance is that the mic preamp input impedance has a major effect on the output level of the microphone. Low preamp impedance loads down the microphone output voltage and emphasizes any frequency-related variation in microphone output impedance. Matching the mic preamp resistance to the microphone output impedance (eg., making a preamp input impedance 200 Ω to match a 200 Ω microphone) still reduces the microphone output and signal-to-noise ratio by 6 dB, which is undesirable.
To minimise microphone loading, and to maximise signal-to-noise ratio, preamps have traditionally been designed to have an input impedance about ten times greater than the average microphone, around 1.2 kΩ to 2 kΩ. (The original ISA 110 preamp design followed this convention and has an input impedance of 1.4 kΩ at 1 kHz.) Input impedance settings greater than 2 kΩ tend to make the frequency-related variations of microphone outputs less significant than at low impedance settings. Therefore high input impedance settings yield a microphone performance that is flatter in the low and mid-frequency areas and boosted in the high-frequency area when compared to low-impedance settings.
Ribbon Microphones
The impedance of a ribbon microphone is worthy of special mention, as this type of microphone is affected enormously by preamp impedance.
The ribbon impedance within this type of microphone is very low, around 0.2 Ω, and requires an output transformer to convert the low voltage it generates into a signal capable of being amplified by a preamp. The transformer uses a ratio of around 1:30 (primary:secondary) to increase the ribbon voltage to a useful level. This transformer ratio has the effect of increasing the output impedance of the mic to around 200 Ω at 1 kHz.
The transformer impedance, however, is very dependent upon frequency – it can almost double at some frequencies (known as the resonance point) and tends to roll off to very small values at low and high frequencies. Therefore, in common with dynamic and condenser microphones, the mic preamp input impedance has a significant effect on the signal level and frequency response of the ribbon microphone output transformer, and the associated ‘sound quality’ of the microphone. It is recommended that a mic preamp connected to a ribbon microphone should have an input impedance of at least 5 times the nominal microphone impedance.
For a ribbon microphone impedance of 30 Ω to 120 Ω, the input impedance of 600 Ω (Low) will work fine. For 120 Ω to 200 Ω ribbon microphones, the input impedance setting of 1.4 kΩ (ISA 110) is recommended.