can a dynamic mic's mid-boost circuit be added to a condensor mic or ribbon mic?

dkelley · May 28, 2009

can a dynamic mic's mid-boost circuit be added to a condensor mic or ribbon mic? one thing that I would love to have in my condensor mics is the ability to boost the mids some, give a presence peak that works well for certain vocalists, but to have it switchable since there are more times than not that I prefer not to have such a mid boost.

Attached is the schematic from one of my mics (from the manual). Considering how commonplace and public domain the circuit is I really don't think there could be an issue from my posting it here. If the company has an issue with my posting of a simple inductor-capacitor filter then they're misunderstanding basic electronics I think. So I'm attaching the file.

As noted it's a very standard filter network that works in a very popular dynamic microphone.

in case this image doesn't work or does need to be taken down, the circuit is simply the dynamic capsule's output being fed back through an 11mH inductor and a .22 mfd 35v capacitor. It looks to me from the photo (but it's hard to say how the switches actually work in the diagram to me) that the filter is actually running in parallel with the capsule itself, not in series with it, but I'm sure someone can confirm that for me.

My question relates to this: some chinese mics have, for example, a bit of a scooped sound, while others have too bright of a sound (the too bright ones are often a bright capsule that should have a compensating filter in the circuit but don't).

I've been wondering for years why people don't put something like the attached filter network on these mics (or something different but I'm just posting a known working example from a known working dynamic mic). The mxl603 (apex 180 and so on, same mic) have too much of a 6-8k peak for my tastes. They're nothing like the original km84 that I love. I'm thinking that with a simple passive filter or two like I'm talking about one could customize the tone to their tastes more and end up with a real killer mic that can mimick something else much more effectively, rather than the typical mic mods on here which seem to focus more on bringing out the best in the existing tone of the capsule (which is a good thing too of course).

I don't have any cheap mics currently that I'd want to hack apart, and before I start spending time and money on this I'd rather ask smarter people if a circuit like this could work on a condensor mic? I would imagine it would have to come after the active circuit since of course the capsule in a capacitor mic is just that, a capacitor. Basically I believe it would be great to find some cheapo way to pleasantly alter the frequency response (reduce or increase midrange or reduce the mic's natural resonance at 6k or whatever).

I haven't put much thought into this since I'm at work right now, but it was worth asking quickly since someone else I work with asked me about a similar concept just a few minutes ago.

cheers,
Don

dkelley · May 28, 2009

by the way, I'm in the middle of reading "the microphone book" online which contains all of the info I need but it's a slow read with a lot of content and I'm quite certain someone here can answer it far more quickly without having to use reference material and research the circuits like I do

cheers,
Don

rodabod · May 28, 2009

There's nothing to stop you adding a network, but you have to consider the source impedance which in the case of a condenser is quite different from a dynamic.

MagnetoSound · May 28, 2009

Also perhaps worth mentioning, if you are not already aware, is that there is no such thing as a passive boost circuit.

What you have there is a tuned shunt, or notch filter, and a bass cut, both separately switchable.

In combination they might sound like a mid-boost if you increase the gain, but that is not really what is happening.

dkelley · May 28, 2009

MagnetoSound said:
Also perhaps worth mentioning, if you are not already aware, is that there is no such thing as a passive boost circuit.

What you have there is a tuned shunt, or notch filter, and a bass cut, both separately switchable.

In combination they might sound like a mid-boost if you increase the gain, but that is not really what is happening.

Of course you're right, I didn't say that accurately at all. But the net result is what I was talking about, it is intended by the manufacturer to appear to the end user to work like a mid boost circuit and is described as such on the stock mic so that is how I described it, but of course it's really a way of lowering the highs and lows relative to the mids.

cheers,
Don

dkelley · May 28, 2009

rodabod said:
There's nothing to stop you adding a network, but you have to consider the source impedance which in the case of a condenser is quite different from a dynamic.

yes it definitely will be different. That's where I'm at a loss currently, how to calculate the impedance (considering that the capsule is a capacitor of unknown value to me) at various parts of the circuit and where to place the network within the circuit (eg: at the output before the xlr connector, before the output transformer (if applicable) or if transformerless then where exactly).

So I guess I'll have to do some more investigating, research and calculating. But at least neither of you laughed at me

so I'll take it as a positive sign that it's worth investigating further.

thanks for your input guys, much appreciated.

cheers,
Don

rodabod · May 28, 2009

Well, you're asking the right questions.

The crude way to add the filter network is a little bit like above in your drawing where the generator is loaded at a particular frequency. We'd do that after the amp in a condenser though. A neater (and different sounding) way to achieve this would be to use feedback.

In the example above, all you need to know are component values and Z=1/(2*Pi*f*C) and Z=2*Pi*f*L. The generator impedance I'm guessing will keep around 150 or 200 Ohms-ish, depending on how (Shure?) designed the element.

dkelley · May 28, 2009

rodabod said:
Well, you're asking the right questions.

The crude way to add the filter network is a little bit like above in your drawing where the generator is loaded at a particular frequency. We'd do that after the amp in a condenser though. A neater (and different sounding) way to achieve this would be to use feedback.

In the example above, all you need to know are component values and Z=1/(2*Pi*f*C) and Z=2*Pi*f*L. The generator impedance I'm guessing will keep around 150 or 200 Ohms-ish, depending on how (Shure?) designed the element.

that makes sense superficially, I'll re-read it later when I can concentrate more to make sure I fully understand before I start breaking things.

I don't know why you'd guess the circuit was from Shure ;-) (has any other major mic builder added a "presense-boost" circuit to their popular mic?)

thanks again, very helpful.

Always open to more ideas from other minds.

cheers,
Don

dkelley · May 28, 2009

as for feedback, I've looked into it (it's what all the mic designers talk about) but I'm very familiar with filters so am more confident about how I'd get my result this way.

It opens up several options - a notch filter at a certain frequency to counteract capsule resonance, the worst culprit for issues in chinese mics imho (there always seems to be one strong peak in a region that isn't very attractive for the listener). Or of course the "presence-boost" circuit being discussed presently. Ideally I'd rather load the capsule differently to move the resonant peak around but I'm not certain how possible that is, my knowledge of how a condensor mic's capsule is created and tuned is theoretical compared to my knowledge of the other end of the chain (loudspeaker impedance relating to resonance and how it can be manipulated).

So I'm really feeling uncertain about a lot of this but I figured I'd start with some very basic fundamental stuff, fixing frequency issues with filters. It certainly isn't elegant or efficient, but if it works I honestly don't care about the elegance of it at least for version 1.0

cheers,
Don

abbey road d enfer · May 28, 2009

To me, this is a rather strange undertaking. This is not flexible at all. Basically, you'll end up with one or two switchable characteristics, which may or may not be usable. If you feel that you have to change the frequency response of your mic, you want to have control over the amount and frequency. Why not use an EQ?

dkelley · May 28, 2009

because this is a simple, cheap, cost effective way to give me low budget bulk useful condensor mics. several of them in use at the same time (without needing an eq for each one). I prefer to use eq as a mixing tool for choice and get the best tone possible while tracking without eq needed at that point. obviously I'm talking about working with either crap mics for really really cheap and basically fixing their worst characteristics this way or using very versatile multi-use relatively flat mics (3 micron 797 capsule types) for more specialty uses such as rock vocals for example without having to spend more on other relatively pricey specialty mics.

Trying to cover all the bases possible with a low budget. if I can put $20 of parts into $60 mics and make them as useful as $200 mics (ie fix things like mxl 2001s and make them close to as useful as cad m177s, just for example) and I can take things like cad m177s and make them as useful as SM7s or V77s (again, just for example) with that sort of price outlay then obviously I'll do it.

But mostly the reason is that I love DIY and if I stumble across something that really does actually work as well as I hope it could then it's a double winner. if it doesn't work out well then it was still fun along the way.

I do build my own pro-audio gear but mics have always been something I haven't modded too much (although I'm credited briefly in the apex 460 mod pdf that's rolling around the internet). So this is an experiment to see what I can come up with.

By the way, if it's such a bad idea then I suppose Shure isn't laughing all the way to the bank every time they sell another SM7 (which has exactly what I showed in my schematic at the top).

anyway, it's all about the fun and learning. if it works out, great, we'll make a sticky. if not then it can be a fun expeirment!

cheers,
don

PRR · May 28, 2009

An 11mH choke may not be easy to find, nor cheap.

dkelley · May 28, 2009

PRR said:
An 11mH choke may not be easy to find, nor cheap.

true, true. not without ripping apart a certain dynamic mic anyway which would defeat the purpose. what about an rc filter?

abbey road d enfer · May 28, 2009

dkelley said:
because this is a simple, cheap, cost effective way to give me low budget bulk useful condensor mics. several of them in use at the same time (without needing an eq for each one). I prefer to use eq as a mixing tool for choice and get the best tone possible while tracking without eq needed at that point. obviously I'm talking about working with either crap mics for really really cheap and basically fixing their worst characteristics this way or using very versatile multi-use relatively flat mics (3 micron 797 capsule types) for more specialty uses such as rock vocals for example without having to spend more on other relatively pricey specialty mics.

Yes, I understand that; imo, don't get overexcited about it. That's exactly what Audix have done with their kick-drum mic (D6 iirc); it has a unique sound, so unique that many people just can't use it (I'm one).

But mostly the reason is that I love DIY and if I stumble across something that really does actually work as well as I hope it could then it's a double winner. if it doesn't work out well then it was still fun along the way.
I do build my own pro-audio gear but mics have always been something I haven't modded too much (although I'm credited briefly in the apex 460 mod pdf that's rolling around the internet). So this is an experiment to see what I can come up with.

By the way, if it's such a bad idea then I suppose Shure isn't laughing all the way to the bank every time they sell another SM7 (which has exactly what I showed in my schematic at the top).

anyway, it's all about the fun and learning. if it works out, great, we'll make a sticky. if not then it can be a fun expeirment!

cheers,
don

That is an excellent reason to do it, and, as PRR mentions, coils are not cheap, so you may enjoy winding your own coils, making different taps to experiment. Unfortunately, I see that the Siemens ferrite cores that I used in equalizers not so long ago (well into the 90's) are not available anymore.

dkelley · May 28, 2009

I'm sure that I could rip apart some old gear and find ferrite cores etc should the need arise. I'd rather go off the shelf so others can get a benefit from this in the off chance it actually pays off.

By the way, I understand what you're saying about the audix mic, but again that's because you don't like the settings they chose for the filters don't you think? I know that mic has worked out really well for some other engineers, I don't happen to love it either though.

But the fact that I'm doing this myself kind of negates your point I think, just throwing this out there I mean. Since I can set the eq points how I like in each mic, I can in theory adjust them how I want, so I can build them exactly to my personal liking and needs. Kind of better than the audix in quesiton, that's all I mean.

anyway, it will be fun either way so what the hell eh? PS - if you don't want your audix send it my way LoL, I'll play with the filter points and maybe I can turn it into my dream vocal mic heh heh

cheers again, thanks for your input!

Don

abbey road d enfer · May 28, 2009

dkelley said:
I'm sure that I could rip apart some old gear and find ferrite cores etc should the need arise. I'd rather go off the shelf so others can get a benefit from this in the off chance it actually pays off.

By the way, I understand what you're saying about the audix mic, but again that's because you don't like the settings they chose for the filters don't you think? I know that mic has worked out really well for some other engineers, I don't happen to love it either though.

Yes, that's exactly it. Some people love it, I don't.

But the fact that I'm doing this myself kind of negates your point I think, just throwing this out there I mean. Since I can set the eq points how I like in each mic, I can in theory adjust them how I want, so I can build them exactly to my personal liking and needs. Kind of better than the audix in question, that's all I mean.

I understand your point, but, basically what I was saying is that you may find an EQ that you like in a certain application, and find out that it's utterly unuseable in other circumstances. About 30 years ago, I was called to do a session in a studio where the owners had no clue at all (they just had a hit record, made a lot of dough, and bought a studio...). Right after I finished the session, they made notes of all the settings on the mixer; I asked them why, they said they were going to use these settings because they sounded good!

anyway, it will be fun either way so what the hell eh? PS - if you don't want your audix send it my way LoL, I'll play with the filter points and maybe I can turn it into my dream vocal mic heh heh

cheers again, thanks for your input!

Don

I never owned a D6. I did a survey for a magazine 4 years ago and that's what I found with the D6. OTOH, there was also a Samson Q-kick, that has the same type of circuit in it, but it sounds much better to me. Today, in my mic cupboard, for kick drum, I have an ATM25, an EV RE20, a Beyer M88,...and a Q-kick!

PRR · May 28, 2009

> true, true. not without ripping apart a certain dynamic mic

You didn't even search, did you?

DigiKey stocks a dozen parts with inductance 10mH to 22mH, DCR<150. Try parts M10084-ND M10089-ND PLK1296-ND

> what about an rc filter?

Generally much higher loss.

And... can you design and trim it?

Really: IMHO, flavoring while tracking can be a mistake. What sounded great at 3am's musical madness may sound like mud in the light of the next day. Use a small-fault mike with an appropriate pattern, capture clean. Leave the flavoring for another day, when you have most of the project in hand.

Here's a reactance-chart layout of the SM7 impedances. I've Ass-umed 300 ohms capsule resistance.

The 240mH choke is much less than 300 ohms for all frequencies below 250Hz, it shorts-out the bass.

The 11mH choke alone shorts-out below 2,500Hz.

The 0.22uFd cap alone would short-out above 4,000Hz.

Taken simply, the 11mH and 0.22uFd would short-out only between 2,500 and 4,000Hz. And since the impedance of each is only 240 ohms in this zone, you expect a very slight notch. The exact depth of notch is not so simple.

That's not quite what the published curves show. The capsule impedance may not be 300 ohms. It may interact with the acoustic effects which cause the peak. It is not a Simple Situation.

What they are really doing: the mike diaphragm size and shape bring-up a bump near 3KHz, the "Shure peak". This could be considered a design fault. But it clarifies mumble-mouth vocals. It made the SM58 the Voice Of Rock And Roll. The SM-7 is a little more refined, but in the same family. SM7 has deep bass, but in close-mike work you may get too much, hence the low-roll. It has the Shure peak so mumble-mouth broadcast announcers sound clear, and an electric filter to kill that for your nightbird midnight DJ's mellow murmurings.

dkelley · May 29, 2009

PRR said:
> true, true. not without ripping apart a certain dynamic mic

You didn't even search, did you?
nope, like I said I was at work at the office, searching wouldn't have been a good idea.

DigiKey stocks a dozen parts with inductance 10mH to 22mH, DCR<150. Try parts M10084-ND M10089-ND PLK1296-ND
cool, thanks!

> what about an rc filter?

Generally much higher loss.
never considered that, was thinking only that the response curves/points would be less affected by the rest of the circuit/capsule than an inductor/capacitor filter would be. Good point, thanks.

And... can you design and trim it?
yes, but as noted I'm unaware of capsule specs especially for condensor mics and need to make some guesses and do some tests.

Really: IMHO, flavoring while tracking can be a mistake. What sounded great at 3am's musical madness may sound like mud in the light of the next day. Use a small-fault mike with an appropriate pattern, capture clean. Leave the flavoring for another day, when you have most of the project in hand.

If what you say is true then the sm7 would be a huge failure. although I do happen to dislike it's presence boost, many thousands of engineers have recorded lead vocals with that presence boost on and sold trillions of records that way, so I suppose there is the other school from your viewpoint. But while I agree with you about flavor, I disagree speaking as a recording engineer about tracking with mics that have some type of altered eq. EVERY mic other than maybe earthworks has an imperfect eq. we use different mics every single day that are better suited to the particular task and the target than a flat mic would be. Otherwise there would be no need for anything other than one perfect mic (or a collection of identical perfect mics) for every recording studio.

And I think I also explained clearly that I am hoping to be able to take some already uneven mics and do some taming of their peak(s) with such an eq circuit.

All in all I think I explained it ok earlier in this thread but I think you probably missed my description, or maybe I was really unclear. Sorry if I was!

Anyway, back to the task at hand, because your post is really informative and I'm excited about the chart you made coming up...

Here's a reactance-chart layout of the SM7 impedances. I've Ass-umed 300 ohms capsule resistance.

The 240mH choke is much less than 300 ohms for all frequencies below 250Hz, it shorts-out the bass.

The 11mH choke alone shorts-out below 2,500Hz.

The 0.22uFd cap alone would short-out above 4,000Hz.

Taken simply, the 11mH and 0.22uFd would short-out only between 2,500 and 4,000Hz. And since the impedance of each is only 240 ohms in this zone, you expect a very slight notch. The exact depth of notch is not so simple.

extremely useful info, I did come to this conclusion but haven't spent any time yet (no time to spend!) calculating things on paper so it's so appreciated, thanks again!

That's not quite what the published curves show. The capsule impedance may not be 300 ohms. It may interact with the acoustic effects which cause the peak. It is not a Simple Situation.

What they are really doing: the mike diaphragm size and shape bring-up a bump near 3KHz, the "Shure peak". This could be considered a design fault. But it clarifies mumble-mouth vocals. It made the SM58 the Voice Of Rock And Roll. The SM-7 is a little more refined, but in the same family. SM7 has deep bass, but in close-mike work you may get too much, hence the low-roll. It has the Shure peak so mumble-mouth broadcast announcers sound clear, and an electric filter to kill that for your nightbird midnight DJ's mellow murmurings.

yes, in reality the sm58 an sm57 have the same capsule, the sm7 has virtually the same capsule as the sm57 but with some very subtle improvements and a far superior grill/case. I'm well aware of these "shure" effects as you call them . I'm not looking to create the sm7 again, I do'nt love the mic actually.

I want to fix issues that I'm aware of in cheap condensor mics and improve some otherwise boring ones! Should be fun, thanks again for your terrific chart, it will be very handy this weekend as I plot some values and try this out in reality.

can a dynamic mic's mid-boost circuit be added to a condensor mic or ribbon mic?

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