DIY Impedance Control; Condenser Mic

Axemaster G

New member
Hi there, new to the forums, how's it going? I was hoping somebody might be able to answer this basic audio question for me:

I've seen some simple designs for a DIY variable impedance control for your microphones which is comprised of some XLR sockets, a potentiometer and some connecting wires. I like the thought of making one myself, just for fun, but I want something that will work with a condenser mic which obviously requires phantom power. I currently have a mixer powering my mic, but would this same design work if I were to use an external phantom power supply before it such that the signal chain goes as follows:

Mic > Phantom Power Supply > Impedance Control > Mixer

Thank you.
 

ecc83

Well-known member
Hi there, new to the forums, how's it going? I was hoping somebody might be able to answer this basic audio question for me:

I've seen some simple designs for a DIY variable impedance control for your microphones which is comprised of some XLR sockets, a potentiometer and some connecting wires. I like the thought of making one myself, just for fun, but I want something that will work with a condenser mic which obviously requires phantom power. I currently have a mixer powering my mic, but would this same design work if I were to use an external phantom power supply before it such that the signal chain goes as follows:

Mic > Phantom Power Supply > Impedance Control > Mixer

Thank you.
Hi Axe' and welcome. Such an impedance 'modder' is simple in the extreme to make* but do bear two things in mind.
1) as you shunt the mic's output you will attenuate the signal and, since capacitor mics are active, a load much below 500 Ohms could cause distortion, especially on peak signals.

2) The output of a capacitor mic is essentially a pure resistance and so, apart from signal loss there is unlikely to be any tonal changes. Most cap' mics these days do not use output transformers but if you have one that does you MIGHT hear a change in sound. Whether you will like that change? Lap of the gods.

The 'fashion' for "Vari-Z" pre amps was really designed for dynamic mics and especially ribbons. Furthermore the 'tweaks' reckon going much higher in Zin is when the magic happens...5k even 20k in some cases. It is generally agreed that the effect is subtle (to bugger all?)

* XLR in, XLR out, pot' wired between pins 2 &3 with a 'stop' resistor. Put it all in a tin. If you want a schematic and circuit values give me a day.

Dave.
 

Axemaster G

New member
Hi Axe' and welcome. Such an impedance 'modder' is simple in the extreme to make* but do bear two things in mind.
1) as you shunt the mic's output you will attenuate the signal and, since capacitor mics are active, a load much below 500 Ohms could cause distortion, especially on peak signals.

2) The output of a capacitor mic is essentially a pure resistance and so, apart from signal loss there is unlikely to be any tonal changes. Most cap' mics these days do not use output transformers but if you have one that does you MIGHT hear a change in sound. Whether you will like that change? Lap of the gods.

The 'fashion' for "Vari-Z" pre amps was really designed for dynamic mics and especially ribbons. Furthermore the 'tweaks' reckon going much higher in Zin is when the magic happens...5k even 20k in some cases. It is generally agreed that the effect is subtle (to bugger all?)

* XLR in, XLR out, pot' wired between pins 2 &3 with a 'stop' resistor. Put it all in a tin. If you want a schematic and circuit values give me a day.

Dave.

Thanks for the info. So you don't think there would be much benefit to using something like this on a condenser? What's this 'stop' resistor that you mentioned? The design that I saw didn't feature one of those. :unsure:
 

ecc83

Well-known member
"Stop resistor" if you simple connect a variable resistor across the mic's output you could completely short it out! Bazinga, no signal.
You have to decide on the minimum resistance you might use, 150R say (for a dynamic, that would bork a capacitor) and then the pot takes the value up from there. Since most commercial AI and mixer pre amps are rarely much above 2k if that, not a lot of variation?

I am of the opinion that you could duplicate the effect in your DAW. Of course you would need to know WHAT the effect is first of all!

I am suspicious of the whole concept since the major effect of shunting the mic is a loss of signal. How does the listener separate that effect from any mild tonal change? For any specific microphone it would be possible to design a circuit that kept the level fairly constant with change of load but it would be a pretty clever and expensive bit of 'tronics! I know of no one that does it?

Then of course a different microphone with a different source impedance would not play ball.

Dave.
 

rob aylestone

Well-known member
I'm so sceptical of these ideas. Every mic sounds different from it's neighbour by a lot or a little and the science that goes into the design of a microphone (especially those made by the very expensive makes) is considerable. If there was a magical improvement to sound by changing the impedance, you'd expect to find it as a switchable option in good quality mics and available as an after market product badged by Shure, Neumann, Sennheiser, AKG etc - the fact they don't make them is enough evidence for me that this is another Barnum product, that does indeed change the sound, in a similar manner to features we already have in our DAW. If Neumann decide the final circuit impedance is 254Ohms, why would we wish to change it? I also suspect that these things also take a perfect sounding room and wonderful monitors to be heard at the subtle end. For most folk, turning the knob rolls off the top end, and that is it!
 

TalismanRich

Well-known member
There are, of course, mic preamps that incorporate adjustable impedance loading on the mic input. Something like the ART Pro preamps have variable impedance, from something like 150 to 3000 ohms. I haven't tried any of them, but it's been said that the loading effects the sound of some microphones.

I don't know how the device the OP was looking to build would work. If the input impedance of the device is already 2K or more, you don't really have a way to go below that. My Tascam interface has an input of 2.4K Ohms. The Motu M4 is 2.65K. The Scarlett 18i20 is 3K. A Clarett 8 has two inputs, 6.2K without "Air", and 2.2K with "Air".

Anything you insert is only going to increase the impedance from there.
 

jamesperrett

Active member
My Audient ASP008 has switchable input impedance on all the mic inputs of 200, 1.2k and 5k ohms. This is very handy for me when doing live recordings using a split from the PA as I can use the 5k setting to reduce the additional loading on the mic. When I first bought it I did a quick test using different impedances and, as expected, dynamic mics changed their sound while condenser mics showed no change in sound at different impedance settings.
 

ashcat_lt

Well-known member
Anything you insert is only going to increase the impedance from there.
Well, anything you try to put in series will increase the Z, but will probably cause more harm than good. Like ecc83 said above, you can wire a variable resistance parallel to the input to reduce the impedance pretty easily, and I’d expect it to work a bit better. Either way, you get some attenuation, but series resistance will probably be worse for noise.
 
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