DIY Impedance Control; Condenser Mic

[Axemaster G]

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]

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]

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.

 

[ecc83]

"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]

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]

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]

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]

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.

 

[Axemaster G]

The plan is to wire a variable resistor in parallel with the path of the mic going into the pre. I got the idea from a DIY Recording Equipment video. Maybe it's not such a great idea, but I've committed to buying the components now so it'll make an interesting experiment if nothing else! Might be that all it does is to leave me having to crank up the preamp gain more or there's loads more noise in the signal or something, but screw it!

 

[ecc83]

The plan is to wire a variable resistor in parallel with the path of the mic going into the pre. I got the idea from a DIY Recording Equipment video. Maybe it's not such a great idea, but I've committed to buying the components now so it'll make an interesting experiment if nothing else! Might be that all it does is to leave me having to crank up the preamp gain more or there's loads more noise in the signal or something, but screw it!

Fair enough. You can do no harm, even if you short out the phantom supply (you should not. Love to see a schematic) you will do the pre amp no harm.

When (pretty sure "when" not if!) you find this circuit makes no audible difference other than stealing signal, assuming it all comes in a tin, chuck the internals away and buy the smallest, cheapest 1:1 or close to that ratio, audio transformer you can find and fit that in the signal path (you would have to 'hop' phantom power over the traff)

The mild distortion produced by such a component might (I repeat, "might") add a bit of 'interest' to the sound?

Dave.

 

[rob aylestone]

If you ordered a tiny preset pot, it would probably fit in an XLR connector, or maybe one of the male to female barrel type connectors - and you could drill a little hole for the screwdriver to get in and adjust it. I can't help but think that as this 'mod' can be done for less than a pound, if it worked, everyone would be doing it. They are not, which probably means people are left totally underwhelmed. Reducing the output and making it up with gain seems a good way to make it sound worse to me - but I'm an eternal sceptic on this magic tricks and tweaks. I often wonder if preamp likes and dislikes are often just these difference in matching working for you or against you.

 

[Axemaster G]

Fair enough. You can do no harm, even if you short out the phantom supply (you should not. Love to see a schematic) you will do the pre amp no harm.

When (pretty sure "when" not if!) you find this circuit makes no audible difference other than stealing signal, assuming it all comes in a tin, chuck the internals away and buy the smallest, cheapest 1:1 or close to that ratio, audio transformer you can find and fit that in the signal path (you would have to 'hop' phantom power over the traff)

The mild distortion produced by such a component might (I repeat, "might") add a bit of 'interest' to the sound?

Dave.

Thanks for the advice, I'll give the transformer a shot if this doesn't do anything worthwhile (which seems likely)!

If you ordered a tiny preset pot, it would probably fit in an XLR connector, or maybe one of the male to female barrel type connectors - and you could drill a little hole for the screwdriver to get in and adjust it. I can't help but think that as this 'mod' can be done for less than a pound, if it worked, everyone would be doing it. They are not, which probably means people are left totally underwhelmed. Reducing the output and making it up with gain seems a good way to make it sound worse to me - but I'm an eternal sceptic on this magic tricks and tweaks. I often wonder if preamp likes and dislikes are often just these difference in matching working for you or against you.

Do you mean one of those trimmer pots? Sounds intriguing, might be a good idea in future...

 

[ecc83]

Thanks for the advice, I'll give the transformer a shot if this doesn't do anything worthwhile (which seems likely)!



Do you mean one of those trimmer pots? Sounds intriguing, might be a good idea in future...

Push the boat out a bit and by an XLR 'phase switch' then you will have loads more room and I suggest a wee multi-turn Cermet trimmer.
If you were really clever you could retain the phase switch.

Dave.

 

[Axemaster G]

Push the boat out a bit and by an XLR 'phase switch' then you will have loads more room and I suggest a wee multi-turn Cermet trimmer.
If you were really clever you could retain the phase switch.

Dave.

Thanks Dave, I'll look into it. ?

 

[Robert Tielke]

The plan is to wire a variable resistor in parallel with the path of the mic going into the pre.

Doesn't that change the impedance related to the output resitor in the MIC? ... and causes deviation of the frequency curve? According to my knowledge it is in fac the low impedance of the MIC which pulls down the resitance of the whole line - even if the mixing console comes with a higher input impedance.

 

[Axemaster G]

Doesn't that change the impedance related to the output resitor in the MIC? ... and causes deviation of the frequency curve? According to my knowledge it is in fac the low impedance of the MIC which pulls down the resitance of the whole line - even if the mixing console comes with a higher input impedance.

Not sure, it's just what I was told.

 

[ecc83]

Doesn't that change the impedance related to the output resitor in the MIC? ... and causes deviation of the frequency curve? According to my knowledge it is in fac the low impedance of the MIC which pulls down the resitance of the whole line - even if the mixing console comes with a higher input impedance.

Yes...sort of. The impedance 'fashion' started AFAICT with ribbons where you necessarily have a step up transformer and that will have been designed originally to deliver the best, flat response into the common 1.2k to 2k that mic pres have largely been for 50+years.

Feeding the transformer into a significantly higher resistance WILL change the frequency response, generally giving a slight extension to the HF response (aka "air" by the cork sniffers)
I would expect loading a ribbon mic with a LOWER resistance would make it slightly 'dimmer'.

But, capacitor mics rarely have transformers and even those that do use one of a low, often 1:1 ratio and so will not show any of these response changes. All that will happen is a signal loss and, if the load gets too low, distortion.

Dave.

 

[jamesperrett]

Doesn't that change the impedance related to the output resitor in the MIC? ... and causes deviation of the frequency curve?

Yes, that's the whole idea behind using a variable impedance input. If the mic has a frequency dependent output impedance it will sound different feeding a different input impedance. As Dave says, this will mainly affect dynamic mics because capacitor mics generally have a frequency independent output impedance.

 

[ashcat_lt]

We never care what the load thinks.

From the perspective of the source, the bottom of the voltage divider gets smaller. If that source is capacitive (easy to assume with active sources), this divider is a highpass filter and the corner frequency gets higher as that bottom Z gets smaller. It was probably reeeaalllly low anyway, so there'd be a good amount of wiggle room for most things we're trying to record.

BUT if this is a condenser mic, then the pre is also a source because phantom power, and making the bottom of that divider smaller will drag down that supply voltage, and how that affects things really depends on the mic itself.

 

[ecc83]

Phantom power will not be affected because the extra load is being imposed between pins 2 and 3 and each pin has an identical voltage on it of the same polarity therefore no current can flow and no V drop. For sure there will be an extra load on the mic signal and that could cause distortion.

Dave.