variable input impedance - does it alter the sound??

[junplugged]

i'm seeing this on some mic preamps. is it only for efficient energy transfer, levels, and hum eliminination or also audio control?

 

[SRR]

variable input impedance - does it alter the sound??

Yes sir it does, but maybe not enough with some mics where you should get your panties in a bunch if shock and horror you bought a pre without variable input impedance. Think of it as a mild tone control for SOME mics.

 

[MOFO Pro]

Audio control for sure... Any mismatch in impedance creates reflections which when mixed with the forward signal either add or subtract energy in a given frequency(s) (Whether it boosts or cuts depends on the phase of the signals as they meet)...

A perfect impedance match will result in a fairly flat, accurate response... a significant impedance difference will provide a response with boosted and cut frequency bands that may sound warmer and fuller... or thin and brittle...

 

[rsolinski]

short answer.....yes

Ray

 

[MOFO Pro]

short answer.....yes

Ray

Then let me expound some more...

If your tranmission medium has an impedance of say 600 ohms, and it terminates on a connection rated at the exact same 600 ohms... all of the energy is absorbed by the termination (or input). If the termination doesn't match the impedance of the transmission medium, the energy that is not absorbed is reflected back in the reverse direction or the transmission medium. (The amount of reflected signal depends on the degree of mismatch). As this reverse signal energy meets the forward stream it mixes... if the signals meet in phase with each other... they are additive and you will see a boost in these frequencies... If they meet out of phase... the frequencies will be cut... If two equal signals are combined 180 degrees out of phase with each other, theoretically, they would completely cancel each other. These reflections though, are at a much lower amplitude to forward signal so the result is not so pronounced but the affected frequency band is attenuated.

 

[boingoman]

Then let me expound some more...

If your tranmission medium has an impedance of say 600 ohms, and it terminates on a connection rated at the exact same 600 ohms... all of the energy is absorbed by the termination (or input). If the termination doesn't match the impedance of the transmission medium, the energy that is not absorbed is reflected back in the reverse direction or the transmission medium. (The amount of reflected signal depends on the degree of mismatch). As this reverse signal energy meets the forward stream it mixes... if the signals meet in phase with each other... they are additive and you will see a boost in these frequencies... If they meet out of phase... the frequencies will be cut... If two equal signals are combined 180 degrees out of phase with each other, theoretically, they would completely cancel each other. These reflections though, are at a much lower amplitude to forward signal so the result is not so pronounced but the affected frequency band is attenuated.

You are talking about a transmission line. Mic lines don't act as transmission lines. They aren't long enough by far. For a mic cable to act as a transmission line, it would have to be close to half a mile long long just to affect signals at 20kHz.

In audio we usually look to bridge an impedance, not match. This means an input has higher impedance than the source. 10X is the usual standard. This gives good signal transfer and frequency response.

line out impedance=1000 ohms line in impedance= 10000ohms
mic output impedance= 250ohms mic pre input=2500ohms

Devices like mics and guitar pickups are sensitive to the impedance they are plugged into, but not because of reflections in the line. It's because they have varying output impedance by frequency all by themselves. When plugged into an input of sufficient impedance, the source's impedances become less relevant. Changing the input impedance allows the source's impedance/frequency curve to have more or less effect on the frequency response.

For instance, guitar pickups have an impedance peak at their high rolloff frequency. A 7000ohm guitar pickup may have an impedance peak that hits 40,000ohms or more. This is why guitar amps often have input impedance of 1megohm. 40,000X10= 400,0000ohms at a minumum to ensure good strength and response.

 

[MCI2424]

i'm seeing this on some mic preamps. is it only for efficient energy transfer, levels, and hum eliminination or also audio control?

The reason why impedance is so important
is the impact it has on the frequency
response and output of the microphone.
A key factor is the mic-pre input impedance
in relation to that of the microphone. Their
absolute values is less important than
their ratio. Impedances vary considerably
but some microphones have a nominal
value as low as 30Ω, some are around 150-
200Ω and some as high as 600Ω.
Microphones with higher output impedance will have
a greater voltage output but will show
more variability in frequency response.
A pre-amp with a low impedance “sees”
a lower voltage level from the mic and is also
likely to emphasize its frequency variations,
resonances and dips. However a relatively
high impedance yields a greater overall level and,
often, a flatter frequency response. It may also change
HF and transient response to give an
increase in clarity and brightness. Dynamic
microphones, without any form of amplification,
are sensitive to the impedance that
they see and may produce strange frequency
responses with pre-amp impedances dramatically
different from what it was designer for.
This is because the circuit impedances have a direct
effect on the damping of the voice coil.
Matching microphone and pre-amp impedances to
the same value reduces both the
level and the S/N ratio by 6dB and is not a used
for those reasons. For dynamic
and condenser microphones, the preferred preamp
input impedance is generally about six-ten times
that of the microphone output; normally around
1.2kΩ or 2kΩ. This figure is partly chosen because
of inherent noise considerations and also for additional
factors (like phantom power
resistors).
Ribbon microphones require careful preamp
selection since their output impedance
is very low (less than 2 ohms), as is their voltage
output. In order to bring this voltage up to a
useful level, a transformer is used which
also raises the output impedance.

 

[Robert D]

Spot on, MCI.

I have a Tampa with variable input impedance, and I really like having that control. At first blush you hear tonal shaping as you vary the impedance, but with more carefull listening you hear a kind of softer/crisper response change also. It's a good thing, kind of like getting 3 or 4 preamps for the price of one, though you can only use one at a time.

 

[boingoman]

Spot on, MCI.

Yup, good find. Let's give credit, though. That was written by Rob Jones at Focusrite a few years ago. (I though it sounded familiar)

 

[MOFO Pro]

Yup, good find. Let's give credit, though. That was written by Rob Jones at Focusrite a few years ago. (I though it sounded familiar)

Well I was talking off the top of my head... (might as well have been out of my ass though)

Thanks Boing... if there's anything I hate more than being wrong... it blindly misinforming... Thanks for the spanking

 

[junplugged]

very interesting. i thought it might, considering it's sometimes right up there on the front panel as big as the other controls. i'd like to hear it, i think the mpa gold is about the lowest price imp. adjustable pre i've seen.

this gets me thinking, wondering about how the extremely low frequency audio signals are picked up and converted into electrical signals by the mic electronics, i guess there is a higher hz electrical signal, that is modulated by the variations of the audio by the capsule. i'm probably thinking in radio terms....

 

[boingoman]

Thanks for the spanking

I try not to spank, but it doesn't always work out that way.

 

[boingoman]

this gets me thinking, wondering about how the extremely low frequency audio signals are picked up and converted into electrical signals by the mic electronics, i guess there is a higher hz electrical signal, that is modulated by the variations of the audio by the capsule. i'm probably thinking in radio terms....

It's just a one-to-one correlation between input frequency and output frequency. 20hz audio at the diaphragm makes a 20hz electrical signal at the output. It's a direct electrical analogy of the sound, hence the term "analog".