Mic impedence tricks and tips

[chanceoperation]

Hi All

I am trying out an ART Voice Channel https://artproaudio.com/product/voicech ... ital-outs/, which has some interesting features.

One is a dial to vary the microphone impedance continuously from 3k all the way down to 120 ohms. So I have been testing this against a bunch of dynamic and condenser mics, having output impedances ranging from 100 to 350 ohms. I also have tried this with a Triton Fethead.

I have also read Hugh's "Using Impedance" article https://www.soundonsound.com/techniques ... -impedance, and some other sources on the topic. I know about the 10-1 rule. And I understand that dropping below that ratio can cause a HF roll off but it's typically very high so not really audible.

What I have learned so far is that any frequency response change I cannot hear. But the sensitivity changes a lot when you lower the input impedance. So naturally you turn up the preamp gain, and then the sound can change a lot (the AVC has a tube, but lots of mic pres do nice things when driven).

There is lots to discover here, but initially I have two questions:

1. Am I right that if I can't hear any HF roll off I should not worry about anything?

2. If the impedance lowering also lowers the sensitivity, then I use the gain on the preamp to bring it back up, am I right that I am engaging more of the preamp's natural tonal breakup colour -- or -- am I just introducing more noise because the preamp is not being overloaded in this scenario?

Maybe others have stories to tell about playing with this variable mic impedance business??

Andrew Culver

[Mike Stranks]

Simple question: Why do you want to alter the impedance?

Is it just because there's a control on the piece of kit, so you thought you'd give it a twiddle to see what happens?

If it ain't broke...

[Arpangel]

Is it just because there's a control on the piece of kit, so you thought you'd give it a twiddle to see what happens?

If it ain't broke...

Oh dear oh dear, I was going to ad a blush emoji but there isn’t one big enough.

[Kwackman]

Simple question: Why do you want to alter the impedance?

Is it just because there's a control on the piece of kit, so you thought you'd give it a twiddle to see what happens?

If it ain't broke...

Really?
Surely there's nothing wrong with being curious about "what does that knob do". As I read it, the OP is curious about what changing the mic impedance control is doing, he's taken the time to read Hugh's article and having experimented is just asking a couple of questions.

[ef37a]

Simple question: Why do you want to alter the impedance?

Is it just because there's a control on the piece of kit, so you thought you'd give it a twiddle to see what happens?

If it ain't broke...

Ha ha! Nice one Mike! First off, loading a capacitor mic as low as 150 Ohms is likely to cause distortion and possibly clipping of the internal electronics. That is not likely to be pleasant and the other problem with the whole exercise is that any effects, nice or nasty are 'baked in' and cannot be undone.

Then, if you cannot hear any sonic changes other than level, I don't see the point? This idea of variable input impedance on mic pres is a quite recent 'fashion' and not one I suspect many of the 'top bods' here give much time to?

Counter to that last is that some have commented that a higher than normal load, 5k+ rather than the commoner 1k5 can change the quality of some classy ribbon mics in a subjectively pleasant way but I think the effect is subtle?

Maybe get out more Andy?

Dave.

[Mike Stranks]

Simple question: Why do you want to alter the impedance?

Is it just because there's a control on the piece of kit, so you thought you'd give it a twiddle to see what happens?

If it ain't broke...

Really?
Surely there's nothing wrong with being curious about "what does that knob do". As I read it, the OP is curious about what changing the mic impedance control is doing, he's taken the time to read Hugh's article and having experimented is just asking a couple of questions.

Whoa!

I never said there was anything wrong with experimenting or wondering what effect a control produces...

I merely asked why the OP wanted to do it...

... and relax...

[Hugh Robjohns]

I raised an eyebrow too. I'm sure it wasn't intended, but it did come across as a bit of a dismissive put down. Sorry. Unintended consequences and all that...

The variable input impedance thing has become a bit of a fashionable gimmick, but it has very limited applications in real life.

To address the OPs main question, lower input impedances will definitely reduce the mic's signal level significantly because a greater portion of the mic's output voltage develops across the mic's output impedance and less across the preamp's input impedance.

So lower input impedances will need more preamp gain to restore the preamp output level. That will inevitably increase noise and, in some cases, the amount of distortion too — not because you are 'driving' the preamp with signal level (you obviously aren't!), but just because some preamps introduce more self distortion at higher gains (this depends on active device linearity and amount of negative feedback). The preamp's own frequency response may also change at high gain, typically rolling off the high end.

There may also be tonal effects on the mic itself when driving a low impedance. Not just a change to brightness, but also a change to fast transients and dynamic range, potentially resulting in a thicker, more solid, denser sound. This is obviously dependent on the source as well as the mic.

Dynamic (moving coil) mics (and mics with output transformers) tend to respond most to impedance changes because their output impedance changes significantly with frequency. Consequently, as the mic output impedance and preamp input impedance form a potential divider, the mic's frequency response changes depending on the preamp input impedance. Often subtly, sometimes dramatically. The closer the preamp impedance is to the average mic output impedance, the more pronounced the changes will be; and the higher the input impedance the more consistent and neutral.

Some vintage designs were intended to feed 30 or 50 Ohm inputs, but most most expected 200-300 Ohms rather than the 1k5 standard we have today So it could be argued that they only deliver the intended frequency response and thus sound 'right' when facing a low(ish) impedance. The sonic differences usually aren't huge, but are often noticeable. Whether actually better or worse at different impedances is a subjective and contextual thing, obviously.

Capacitor mics with output transformers can also respond noticeably to impedance variations (mainly vintage valve mics and vintage capacitor mics). However, more modern transformerless designs don't respond well to low impedance loads at all. Transient distortion and a reduced headroom are common indications of an unhappy mic. So best to keep the impedance above 1k5 Ohms for them. 2k5 to 5k is ideal.

The only preamp I have with variable impedance (ISA428) normally stays at its 5k setting — but most of mic mics are transformerless capacitor mics or active ribbons.

Most passive ribbons prefer much higher impedances — typically over 5k Ohms, and dedicated ribbon preamps typically offer inputs at 15-30k Ohms.

The idea of switchable or variable input impedance is not new, though. The classic Neve 1073 preamp from the 70s had provision for selecting either 300 or 1k2 Ohm input impedances, as did many preamps of that era — often by reconfiguring the input transformer windings between series and parallel. This was because some institutions were still working in a matched-impedance environment, and others in the modern matched-voltage format... but engineers/producers found that switching impedances was sometimes musically beneficial.... and later manufacturers seized on the idea as a USP.

[James Perrett]

The only preamp I have with variable impedance (ISA428) normally stays at its 5k setting — but most of mic mics are transformerless capacitor mics or active ribbons.

The same goes for my Audient preamp with switchable impedance. I tend to keep it on 5k because, when I'm recording live, it will be in parallel with another preamp so 5k places less additional load on the mic. I did notice a fairly obvious change in sound when I experimented with different impedances with a dynamic mic but I don't often get the opportunity to do much experimentation on most recording sessions that I do so I'll leave it on the one setting.

[chanceoperation]

The ART Voice Channel manual actually says to use the impedance dial to taste. I just wanted to know what the thinking behind the whole variable impedance idea for mics is.

Thank you all for contributing. I think Hugh summed it up best (no surprise there) by saying his only unit with that feature is set at the highest setting all the time.

I'll still muck around with it for a while, because, well, to paraphrase Sir Edmund, because it's there.

[chanceoperation]

Oh, and James too -- nice one.

[Hugh Robjohns]

The same goes for my Audient preamp with switchable impedance. I tend to keep it on 5k...

Oops! I have an Audient ASP880 and completely forgot that it even had variable impedance (220, 1200, or 2800 Ohm options). It's never moved from the switch-middle 2k8 option!

But it has reminded me that the user manual for the 880 included an interesting chart suggesting sonic differences for different input impedances for moving coil mics with transformer outputs (like the SM7B, SM57, M201, D224e etc)

Lo = 220 Ohms, Med = 1200 Ohms, Hi = 2800 Ohms.

[chanceoperation]

Now that's interesting.

Did you mean that your setting has never moved from the Middle 1200 position, or High 2.8k position?

Have you enjoyed the blissful benefits that this chart promises -- or is this Audient running with an idea to have some fun?

[Hugh Robjohns]

Did you mean that your setting has never moved from the Middle 1200 position, or High 2.8k position?

I've tweaked my post to make it clear — the switch middle position is the higher impedance setting (which the manual says is 2k8 and my review said was 3k6!)

Have you enjoyed the blissful benefits that this chart promises -- or is this Audient running with an idea to have some fun?

I think the chart's claims of sound characteristics are broadly true for most moving-coil mics, in my experience. The relevant impedance values aren't tgat critical but depend on the specific mic's own frequency-dependent impedance curve.

As a rule of thumb, low would count for anything around 100-300 Ohms-ish. Medium would be 1-2k-ish, and high above 3k Ohms... in broad numbers.

[Wonks]

The ASP880 impedance switch goes (from top to bottom)

Low
Hi
Med

So the high (2.8k) position is 'switch middle'.

[chanceoperation]

The ASP880 impedance switch goes (from top to bottom)

Low
Hi
Med

So the high (2.8k) position is 'switch middle'.

Got it. Thx.