#variable impedance

Audient's rackmount preamps, for example the newly announced ASP880, offer switchable input impedance on every channel, effectively giving you three different sonic variations for every mic you plug in. However, my experience is that few recordists really have a handle on what kinds of changes to expect from this facility, or indeed how big the audible differences will be.

One reason for this is that different mics respond in different ways to alterations in mic-input impedance, and it's also because certain sounds (particularly percussive things) tend to show up the changes more than others.

As with any recording or processing option in the studio, you need to have some idea what to expect from that mic-impedance switch if you're going to work out when to call upon it in the heat of a session, so if you're still a bit in the dark about it, here are some audio files to help. I recorded them a while back with the band: Brushes Held Like Hammers, and regular readers of this blog may already have read my previous post about how the high-pass filtering and polarity switching on Audient's preamps helped me with my multi-miking setups for this session.

What I did was ask the band to perform a section of one of their songs three times in quick succession so that I could record comparison files with each of the three input-impedance settings (5000 Ohms, 1200 Ohms, and 200 Ohms) across 12 simultaneous microphones that were all running through Audient preamps.

Here's a list of all the mics, along with links to the different audio files, both in MP3 and WAV formats -- note that all these files are completely without any processing (beyond any variable high-pass filtering I applied on the preamp itself):

Kick drum though an ADK S7 large-diaphragm condenser mic placed roughly a foot away from the drum's resonant head. The mic preamp's high-pass filter was set to a 20Hz 'rumble removal' setting.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Kick drum though a Blue Kickball phantom-powered dynamic mic with its LF emphasis switch at its minimum setting. The high-pass filter was bypassed on this channel.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Snare drum through a Shure KSM137 small-diaphragm cardioid condenser microphone positioned around nine inches away from the edge of the drum, angled down diagonally towards the edge of the batter head and surrounded with acoustic foam around the sides. The high-pass filter was bypassed on this channel.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Snare drum through a Shure SM57 dynamic microphone positioned around nine inches away from the edge of the drum, angled down diagonally towards the edge of the batter head and surrounded with acoustic foam around the sides. The high-pass filter was bypassed on this channel.

5000 Ohms (MP3 - WAV)

1200 Ohms MP3 - WAV)

200 Ohms (MP3 - WAV)

Ride cymbal through an Avantone CK1 small-diaphragm condenser microphone using its cardioid capsule. This was protected from spill using an experimental improvised baffle made from acoustic foam and half a plastic bucket! The high-pass filter was bypassed on this channel. (NB. The ride cymbals only begins playing at around 0:28 in these files.)

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

The drum overheads were a stereo pair of KSM141s set to their cardioid mode and positioned above the kit roughly at the drummer's (seated) head height. Although these mics were on different sides fo the kit, they were both pointed back towards the snare, and equidistant from it too. No high-pass filtering was used on these channels.

5000 Ohms (MP3 - WAV

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

For my room mics, I had a pair of AKG CBL99 electret boundary mics (with a hemispherical polar pattern) on the floor a couple of feet behind the drummer, spaced about six feet apart. No high-pass filtering was used on these channels.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

The floor tom had an AKG C414B-XLS mic on it, with its figure-eight polar pattern and 18dB pad engaged. The preamp's high-pass filter was set to around 60Hz.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Here's Guitar 1 miked up with a Groove Tubes GT55 large-diaphragm cardioid condenser microphone placed close up to the Fender combo's grille cloth, miking the outer edge of its single speaker cone. The mic's 10dB pad switch is enabled, but not its high-pass filter, and the high-pass filter on the mic preamp was also bypassed.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Guitar 1 was also miked up with a vintage Sennheiser MD421 dynamic mic, with its filter switch set to 'M' (ie. with no low-end roll-off). Again, this was positioned close up to the Fender combo's grille cloth, miking a little wayin from the outer edge of its single speaker cone. The high-pass filter on the mic preamp was bypassed.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

I had another GT55 on Guitar 2, again with its 10dB pad switch in and its high-pass filter switch off. This was placed close to the grille cloth of an Orange combo miking the edge of the speaker cone. I applied a 80Hz high-pass filter on this preamp channel.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

Guitar 2 was also miked with a Shure SM57 dynamic mic close to the grille cloth of the combo almost at the centre of the cone. I applied a 50Hz high-pass filter on this preamp channel.

5000 Ohms (MP3 - WAV)

1200 Ohms (MP3 - WAV)

200 Ohms (MP3 - WAV)

(Click here if you'd like to download all the WAV files as a single 380MB ZIP archive.)

About The Author: Mike Senior is an award-winning audio engineer who's worked with artists such as The Charlatans, Reef, Therapy, Nigel Kennedy, and Wet Wet Wet. For six years he was Reviews Editor for Sound On Sound, the world's best-selling music recording magazine, and writes regularly for their popular Mix Rescue, Session Notes, and The Mix Review columns. He is also the author of the best-selling Focal Press book 'Mixing Secrets For The Small Studio', a complete mixing method based on the techniques of the world's most famous producers.

For more information on Variable Impedance check out this blog written by our Chief Technical Officer Tom Waterman or for more info on ASP880 visit our website here.

Thanks for reading,

Andy @ Audient 

Hey!

It's been a while since I've been on a blog - everything has been super busy with the all-new ASP880 production and exciting new toys on the way!

I'm sure some of you have already used a microphone preamplifier with variable impedance loading - but let me recap...

Variable Impedance - What is it?

Humour me for a minute - I have an analogy that works here... Imagine a train crossing a bridge that is suspended high above ground between two mountain peaks.

The train is your source resistance - or in this case, the microphone. It can be low impedance (a light train) or high impedance (a heavy train).

The bridge is your load impedance - or in this case, the preamplifier. The bridge can be stiff or saggy (I know, I know!). The stiffer the bridge, the easier it is for the train (passengers = signal) to cross the mountainous gap! The heavier the train, the stronger the bridge should be...

In a typical "bridging" audio connection - you have a low source impedance and approximately 5-10 times that impedance in the load (stiffer bridge), this way as much voltage signal as possible arrives at the destination. The goal is to keep the signal "above" ground.

Normally we assume 150 ohms for the microphone output impedance and 1500 ohms for the preamplifier input impedance (x10). However microphones vary their output (source) resistance with frequency as both resistive AND reactive (inductive/capacitive) components are at play when forming an "impedance".

Impedances & Bridging Loads - How They Work

The connection between mic and preamplifier can be seen as a typical potential divider - let's take a standard SM57 dynamic microphone as an example. It's output impedance is stated as a nominal 310 ohms.

The ASP880 has three input impedance options:

LO - 220 ohms (this is a saggy bridge)

MED - 1200 ohms (this is a classic 1970's style bridge)

HI - 2800 ohms (this is a modern, strong and well braced bridge)

Imagine when the two devices are connected together - a 310 ohm source with a variable load from 220-2800 ohms:

The SM57 connects to the ASP880 preamplifier and we want to get as much tone and output voltage from the mic as possible - the preamplifier then adds 40-60dB of gain to this signal for recording.

For anyone not familiar with a potential divider - this simple circuit is the building block of all electronics and allows us to figure out "voltage transfer" between two points.

The simplest way to look at it is that the stiffer the load (R2), the more voltage from the source (R1) is carried to the output.

Vin = 1 volt, R1 = 310 ohms, R2 = 220, 1200 or 2800 ohms, Vout changes:

R2 = 220 ohms, Vout = 0.41 volt

R2 = 1200 ohms, Vout = 0.80 volt

R2 = 2800 ohms, Vout = 0.90 volt

Of course, who cares about voltage when we're sound engineers?!

To quote Ray Charles "How's it sound baby?" - well, we'll get there soon...

If we take into account that all dynamic microphones such as SM57/58, SM7, D12, MD421 or ribbon mics such as a Coles 4038 or Royer R121 have substantial varying output resistances with frequency - they use coils, transformers and/or large magnets to create inductive fields that convert acoustic movement into electrical voltage... what we get is a dynamically changing frequency response that can be heavily affected by the loading - or stiffness of the bridge!

The typical output impedance of an SM57 vs frequency looks approximately like this, it might remind you of a small loudspeaker:

If we were to figure out a potential divider for every point on that purple curve, we could understand the signal transfer for each frequency from the mic to the preamplifier. What we actually end up with is not just a change in signal level BUT A CHANGE IN TONE!

What To Expect & Listen To

On top of level changes (which you should be aware of if you want to do a proper level matched A/B) we can experience the following changes in tone when varying the load that the microphone operates into:

Speed of delivery - transient content

Punch

Detail - in particular the "room" pickup from the off-axis parts of the mic

Frequency response & timbre

Overall output level (-5dB for 220 ohms, -1dB for 1200 ohms, referenced to 0dB attenuation at 2800 ohms with a 150 ohm source)

I like to display these on something I call the TRIANGLE OF TONE. Essentially a system that allows you to mark up the audible changes to triangulate the appararent sonic footprint of a mic/pre combination.

Any of the descriptors can be as you desire - just make sure that opposites face each other on a line...

To describe how I hear it - the ASP880 when set to HI provides detail, punch and a fast presentation with lots of low end weight.

On MED the sound becomes a bit tighter and mid focused with a good balance of everything - a classic kind of presentation.

Set to LO, microphones tend to take on a different personality with a more glued transient to the main body of the sound, an overall slower presentation with some loss of space/detail - great for making a bad room sound a bit better!

Of course - each microphone reacts differently and it is definitely a case of FLIP THE SWITCH, LISTEN & DECIDE.

However - some things to bear in mind:

Moving coil and ribbon mics display the greatest tonal change

Transformer output capacitor microphones can also provide tonal variation with variable impedance

Solid-state output (50 ohm typical) capacitor microphones are such a well designed, light and nimble train that they are often immune to loading effects and will display a much more subtle change, if at all

Contrary to popular belief, many ribbon microphones provide more generous low frequency performance (with less ringing) and greater sonic delivery when operated into higher impedances, so be sure to try the 3600 ohm HI setting!

GET ON WITH IT ALREADY - "How's It Sound Baby?!"

Drums taken from our ASP880 session with Swift 89.

ASP880 - Swift 89 - Keep It Moving

Kick: Sontronics DM1B

Snare: Audix i5

Punch: SM7B

Overheads: Sontronics Orpheus

Tom Toms: Sontronics DM1T

For the actual record, we combined them, Kick on HI (for meaty punch), snare and toms of MED for a tighter sound etc. However below are three clips on Soundcloud that feature all channels on each setting.

The differences can be subtle and vary greatly depending upon the microphone, so experiment!

REMEMBER TO LISTEN ON STUDIO MONITORS OR HIGH QUALITY HEADPHONES FOR MORE ACCURATE JUDGEMENT.

LO Z: Snare presentation in room affected in an interesting way!

MED Z: Smoooooth!

HI Z: Punchy and fast!

Thanks for reading,

Tom @ Audient