How to Add an Input Impedance Control to Any Mic Preamp
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- Опубликовано: 29 авг 2024
- Build an "Input Z" control that works with any mic preamp. Estimated build time: 30 minutes. Cost: $20.
01:00 What is impedance?
01:30 Circuit diagram
05:28 How it works with John Born of Shure Inc.
09:20 Build instructions
You can see the graph John mentions here: www.diyrecordin...
Today I finished this project by myself (as dual channel version). The result is worth the effort. Many many thanks!
I tried it with two cheap ribbon mics to record my guitar in Blumlein stereo configuration. Man, the muddieness is gone. Awesome!
I can't wait to build this and report back, because I bought a CL-1 a few years ago, love the hell out of it, but wish I had a CL-Zi, and this is about as close as I'm going to get, for the time being, since I refuse to spend $379 to get variable impedance. Thanks for making this video and lots of respect for not worrying so much about the background noise of the video and instead focusing on content.
Cloud Microphones makes the CL-Z variable impedence clean gain pre-preamp. Check them out. You are a very smart fellow. I liked how descriptive you were and illustrative.
Very cool. Just seeing your channel for the first time. This is one of those projects that I have thought about on and off for many years, and I've never actually built. But I guess I'm going to do it soon. You just made it all look so damn easy. Thanks for doing that.
If you put a 1Kohm pot in parallel you limit your range of impedance to 600 ohms or less. I would use a 1 Megaohm or even a 5 Megaohm potentiometer for more range.
I would also consider what the lowest useful impedance would be and calculate a parallel resistor value and include it in serial with the pot so as to not short out your input when the pot is turned all the way down.
Usually volume controls are 10k pots and tone controls are 100k pots. Why this is so Sir
I Agree, if you add a parallel resistor to the capsule, the overall resistance will only be less than its original, if the capsule is 600ohm and the resistor 1K it will be 375ohm max. So if you want to add resistance, it must be a variable resistor in SERIES with the mic capsule...
Both you and peterson are a bit off on this. First, many preamps are higher than 1.5k...often like 3k to 5k, some even higher. You dont want to limit your highest imp choice after adding a pot, so yes choose a larger pot than 1k or your new max will be much lower than your orig max. On the other end of things, 1Meg is far too high and 5Meg even moreso...they will have little to no effect til you get way down on the control, with a very narrow touchy range there, so thats not good either. A better overall choice would be a pot somewhere between 10K and 100k. There are only 4 common pot values in that range, so 4 choices...10k, 25k, 50k, 100k. Try a few of those to see which works best for your preamp of those 4 choices. Forget 1K and 1M they are too far on either extreme, either doing too much or too little
Good call! I'm putting together an in-depth blog post right now with the graphs.
Just ran across your pages today after an epic struggle trying to procure a stereo amp with balanced inputs for my main outs on the old audio interface. It shouldn't cost that much. Anyway, I'm stoked this information is available and I'm in the process of ordering the balanced line input module in order to connect the unbalanced outputs to the l/r inputs on a cheap goodwill receiver. I'm fairly confident that this is the proper solution in order to reverse polarity on one of the balanced signals being sent from the interface. (Right?) Anyway, thanks for the video, keep em' coming!
Hi Frank, that's a great idea. A 200-Ohm or so resistor in series with the pot would be a great addition to this box.
Noob question: When you say in series do you mean that the resistor needs to be soldered to pin 2 and pin three of the lead points on the Potentiometer or between pin 2 and pin 3 on the XLR input, or does it matter?
I thought there would be a demo of it...
Nice work, Peterson!
Good to see a clear explaination of how this works.
I am also seeing this referred to as the same selectable input impedance function as many mic preamps have, when it is true that most of those are SERIES resistors which INCREASE the resistance, NOT parallel resistance which is LOWERING the resistance, in other words loading down the mic, attenuating it, and DEPENDING ON THE MIC, changing the mics natural frequency response, either to a huge, or maybe just a slight degree.
Correct me if I'm wrong but when you demo the actual assembly I believe you have just put the pot inbetween pin 2 & 3, kind off shorting it via the resistor. This is not really a parallel connection to the pre.
I would have thought that a better mod would be on the preamp side not the mic side, otherwise mic manufacturers would add switches in their mics to change impedances as a gimmick.
Also, the 10:1 ratio is actually not that bad, it might be overkill for some mics but my experience is that transients "seem" more responsive with a higher ratio.
I'm actually wondering whether you couldn't increase the output impedance with the a dual pot in series with the pre.. rather than parallel.. hmm..
I think this idea seemed great but you haven't convinced me that you've fleshed it out and also actually tested it properly using spectrum analysis.
Cool idea tho!
You legend! This is exactly what i needed to know. Thankyou!
Now just to connect a VU meter.
the point of a selectable input impedance is to allow the user to optimize for the mic you are using- specifically to AVOID losing signal that exists in the natural output of the mic, and to AVOID altering the natural frequency response as well as creating harmonic distortion. dulling a mic down, or thinning it out by mis loading it is not really the same as proper impedance bridging, and then using an EQ. I just find it misleading to not include all this info, and I do not agree it is a "service" to encourage people to build things that they have no idea will tangibly degrade the quality of the mics they are using and basically educate them to think of that as tone shaping, which seems like a great way to confuse people more than help them.
It simply changes the sound. Whether or not this is a degradation or an improvement is up to the user.
What you get with lowering the impedance is more 3th and 5th harmonic distortion. That is not tone correction but added distortion. Even the 6K8 resistor in the phantom loads the emitter follower in the microphone. When you use the True Phantom powersupply, you no not load,, and the distortion of the microphone is 4-20 dB lower. See truephantom.nl/ and the chip: www.prema.com/index.php/en/assps-en/audio-ic
J. Kist
Thank you for this. Great presentation and good clear instruction .
thank you for this information my friend!
The first time I tried one was on an Art Pre Amp. It was nukn futs.
I gotta make this box now bro.
Two things: [4:47] You need to put a resistor between the wiper contact and the connection to the the XLR or you will dial in a short. The link to the graph referred to by John, doesn't work.
good video, but the background music and fan is terribly distracting
+Convivial Cannibal Clan "they have Ritalin for that." 😂😂😂😂😂
@၃ပ၄ That's a disgustingly condescending reply...
Those who can’t appreciate the sound of pre-tape recordings don’t really appreciate analog sound at all.
I was waiting for a demo!!!!!
Thank. Sounds like an interesting project, any chance you could make a video with some factual examples of the effect of impedance on the sound of a mic?
great DIY project. thanks a lot! ; i'm building one..... one afterthought; consider a PEC potentiometer or someting else of really high quality, the contacts on that alpha pot could be an achilles heal that degarde signal quality. regardless. thanks again. great video.
Thanks! Honest question for you: how do you think the contacts would degrade the quality? (Besides losing contact with the track altogether). Added resistance, capacitance?
maybe the pot you used is fine, but if the contacts aren't a high quality like silver, gold or a good nickel alloy, they will likely not conduct the signal and all its bandwidth which as much integrity; less inherant capacitance for the task, or you could say comparatively more resistance to that of a higher quality metal for audio. i'm doing your build with one of these.. PEC 6311-351 1K D5 TYPE K. It has gold contacts; worth spending $10 give or take on the pot. try one yourself and let your ears be the judge. thanks again.
Thanks Brian. My understanding is that the terminals of your PEC pot are gold plated not to improve conductivity or any other spec, but to prevent oxidation over time. That is, there's nothing special about gold as a conductor, it just doesn't tarnish. Every pot will conduct the entire bandwidth of an audio signal.
I'm not sure what you mean by "less inherent capacitance for the task," but there is no need for any capacitance at all in this circuit-the the less the better, in fact.
There's absolutely nothing wrong with using a $10 pot for this project-it may last longer-and I appreciate your feedback. I just wanted to make clear to anyone stumbling on this thread that any pot of reasonable quality will work with near-perfect fidelity in this circuit.
hi peterson, by 'less inherent capacitance' i'm not talking about 'capacitors', just capacity to conduct the sound (my bad terminology)... nonetheless, i don't agree that one metal is 'like another is like the next' in terms of how they conduct audio... in my experiecne different metals and alloys have very different sound characteristics and some are better than others... regardless, as i said at first, it wouldn't surprise me if the pot you used sounded great. thanks again regradless for posting this great DIY build.
Brian Gager do _your_ ears hear the difference?
Have you done a blind test ? Build one with each pot and then have someone swap without you knowing ??
Just curious
In my experience, at low impedance (1.5K) and low bandwidth (
Can you use the analogy of plumbing and water to explain output impedance please? I'm not understanding the term based on it's part of a driving force coming from the mic? Not a resistive force as assumed by the word impedance. Nice video despite the Trumpet. Kidding. I love trumpets.
Hi Andrew, I believe it would be safe for a 1/4 Watt potentiometer like I used in the video, but you will not see any effect on the behavior of a condenser mic in theory.
Hi ALL.. A good Discussion regarding "Matching Impedance" would be Considering using "T" Pads and ""H" Pads.. Using them would keep the MIC Side and the Pre Amp Side constant Resistance.. The "LOAD IN and the LOAD OUT" would never change BUT to recover LOSSES the PRE AMP would DO THAT.. SO Research "T" Pads and "H" Pads in the Radio Armature Hand Books.. Something to BUILD on the Knowledge Base..
You should show how it sound before and after modification.
What would be the result, and the method, of increasing the impedance?
Super cool man totally going to try one out!
Yeah that's the obvious thing, that unless you want a mute control, shorting out the signal is not too helpful. How did the Shure guy not catch that?
If it's not too precious you could install this on your preamp, they sort of touched on it but this will make more of a difference on the average dynamic compared to a condenser mic [ buffered electronics ] Paul Stamler had an article about simply adding a resister to your sm57 , same idea.
This is misleading. You're adding an additional resistive component to an impedance circuit that will change the corner frequency but disproportionally reduce gain compared to a proper transformer. You are changing the impedance of the CIRCUIT but not of the component in which the signal is actually being read across by the preamp. This is just adding a passive filter inline, no more - no less. -MSEE
This is incorrect. If you look at the circuit or the soldering and assembly part of the video, the pot is neither grounded (volume control) or capacitor and ground (filter control) He is simply, as described, adding a parallel load.
You are right, this will impair the frequency response, maybe not noticeably but it will reduce response. Either use a transformer or leave the impedance high.
I am noticing many comments where people incorrectly compare this to the cloudlifter Z. Your circuit is not a similar device at all and I would think that would be important to tell people the difference, and to go deeper in the theory you are introducing.
I tried this today and it doesn't work , it was basically a volume pot, what went wrong ?
Late to the party but same here
this way can only reduce impedance, loosing the fuller sound of an higher input impedance
What is the deal with the horns. CAN'T hear you.
With the potentiometer all left, won't the impedance be zero and the mic signal be "shorted"?
Shouldn't there be a resistor in series with the pot?
In the current situation it is possible to short the signal.
I don't think anybody would be interested in an input impedance less than 10 ohms. (or so.)
Your electronic theory is a bit wonky. The circuit you have made will only give you impedance adjustment equal to or lower than the old standard. So the max impedance will be 600 ohm, but when you turn the dial fully the other way it will create a hard short, the impedance is zero, and you will get no signal from the mic. This might be an interesting area to explore, but it limits the impedances you can try by excluding everything between 600 and 1.5k. If you change your pot value you can try impedances above 600 ohms as well.
Background music is killer.
Gone searching a resistor. UPD It worked! Thank you! A Question: If i shorten the wires like that, when potentiometer is opened, is it painful for electronics (condenser mic or preamp)?
AWESOME video, thanks for making it!!
Is the graph mentioned in the video still available somewhere? The link is broken. Very nice video, thanks.
Great video. Thank you for sharing.
would this be safe to use with a condenser using phantom power?
You're cool man. I like your video. Well done sir.
Hi Peterson, please could you tell the formula you refer to at 3:35 for calculating the impedance needed to add in order to achieve the wished final impedance? Thank you
Wow, great video man! Thanks for the post..lots of good stuff in here. Well I had a question that I was hoping you may be able to point me in the right direction with your knowledge base. I want to create an audiophile grade/vacuum tube buffer to be used with my acoustic guitar and lap steel instruments but would like a wider range of instruments to be able to utilize its capabilities as well. I am using a 1992 Trace Elliot TA100R 4x10" acoustic amplifier and a dimarzio dp234 black angel magnetic passive sound hole pickup and just curious what your suggestion would be about going about this if you have already built one before. If not, any suggestions would be much appreciated. Definitely got yourself a new subscriber. Thanks!
I recently bought a mic pre (input and output transformers as well as tubes) from a tiny boutique company. It has a 500 Ohm and a 2K switch for the mic input and the builder told me I should be using the 500 Ohm input for my capacitor mics, which output the usual 150 Ohm to 200 ohm. I told him I would expect the 2K to be the "correct" selection, but he said no. I don't like to question him, as I know nothing really and he did build the thing, but is there any circuits that would make this 10x rule not correct?
In theory you are right that 2kOhm input impedance is more appropriate in order to minimize loading (the impedance rule of the input impedance to be at least 10 times the microphone output impedance) but I wouldn't say that setting the preamp input impedance to 500Ohms is a wrong thing to do. Like Peter mentioned in this video, it's just going to sound different with lower overall output.
Shouldn't this be called, input resistance? In order to change the impedance, it would have to be a coil, an inductor.
Impedance is reactance + resistance, and the reactance comes from the coil. So if you don't have a coil, then Z=R
I tried this, but the potentiometer does not adjust the volume. It only cuts it at the leftmost point. What could have happened?
Hi!
Just a quick question. I recently bought an Shure 520DX and I can't get it to work with my audio interface/amp. And I gathered that is because it is high impedance.
I wasn't informed by anyone when buying this microphone. What do I need to intergrate it to work with my Focusrite 18i6 interface? What do I need to buy? Any ideas?
Have you tried a 5k Linear pot? I have all the parts on my workbench but not a 1k pot. Im going to try it out anyhow and see what happens
Would this idea work if I use it on instrument input with JFET design? Interesting enough IK multimedia Axe audio interface features variable impedance guitar input, as well as Fractal Axe FX "secret sauce" input circuitry.
Great video! I loved the early Hot Jazz too! I've got an impedance mismatch of a standard Shure sm58 mic 150 ohms (300 ohms actual) to 1M ohms guitar effects pedal input. As a result of the impedance mismatch the signal is not tickling the distortion effects as much as I'd like it too. I bought a Livewire Lo-Z to Hi-Z transformer adapter and I actually think that's tickling the pedal too much...rather I'm getting wicked feedback more so than without the transformer. I'd like to dial back the resistance just a hair rather than dialing back my pedal's drive. Someone told me I shouldn't as it would ruin the mic. The Livewire transformer probably still isn't right for the input as the thing says 250-50k ohms on it. Any advice?
resists AC current?
That would be great for the guitars as well. Any projects like this for the guitar DIs?
I agree. IKmultimedia has something like this on the front of their axe I/o unit. It’s a game changer for passive pickups.
Thanks! Yep, this box should be safe with any kind of mic preamp.
Hi Peterson, your simple and good "add-ons" make verstile the stock audio!!! thanks for that. So, would this be safe to use with a preamp with input tranformers like vintage API 312 o tube preamps like hamptone or Altec 2566A?
thanks for the video！
but i do wanna ask can it help to change the tone of dynamic microphone in…“insert” in a mixer？thanks
No. Impedance is not simple resistance. It's resistance to an AC signal, typically frequency-dependent.
Will lowering the value of the potentiometer increase how high the impedance can go?
Good video keep it up
I've got an Allen&Heath MixWizard WZ3 16:2 which has a 2Kohm XLR (pad out) connection and I've got several sm57 and sm58 mics, which I would like to do a fixed resistor mod on because I only use those mics with this one mixer. I'm good with a soldering iron but I'd like confirmation from a couple different sources what value of resistor that I should install in these mics to get the most optimal performance/sound out of them. Thanks!!
+WallOfSoundPedals Check out Uncle Doug YT chn he does some great videos that would answer your questions.
Hi what would be the difference with this and a cloudlifter Z as that takes phantom power and can Add 15k of impedance ?
Is that a Logarithmic pot?
thank you
Just tried this but it acts as a volume, not impedance control
Sorry if this is a dumb question. I just created the mod, and as the potentiometer is turned down, the volume drops to zero. Is this just because the resistance is so high, or did I do something wrong? Thanks!
where‘s the shure graph? :(
Why the music ? can't hear you talk
Great video and cool channel man. Duck that music a little bit with your voice.
Thank you so much you are the best
Aw shucks, thanks!
The link to the graph is now in the description, sorry for the delay.
Cool! Where can I find the graph you guys are referring to?
Hi, I was trying to find the blog post and it seems to be gone. The link to the Graph is also dead. If you could please fix it.
brilliant watch! boss liyefee!
Hey,
Could you suggest me some books or articles where i can find more information about how impedance affects the frequency response of dynamic mics???
I´m writting an essay about it and wanna go a bit deeper on the subject.
Cheers!
Victor Raso did you write your essay? Id like to hear it!
Did you finish your essay ? Can you post it ?
you rock! and what is that jazz like song in the background?
Why did they not show the graph??
Would have been good to show us the graph mentioned? As it stands, this is just "intresting" as you said. I unfortunately am none the wiser. I have a microphone that has a built in impedance modification, removing it makes it basser and it is already too bass. See, so do I need a bigger or weaker resistor? I get it, you are trying to sell your kit, but your kit is a bit big to go on my camera
Very informative video.Thanks mate!
Are you by any chance still selling these kits? I would be very interested in two boxes or a dual channel chassis. XLR I/O? If not maybe you could point me in the right direction online, sourcing the components etc... Thanks for the video, thanks for the gear!
Nice. But this can only lower impedance. And actually attenuate as you go lower because the in phase and out phase will be cancelled. What would work better would be a dual pot wired where each each phase is connected separately and resistance from each to ground is adjusted.
Impedance and loading is a powerful principle, but this is not exactly globally helping the masses understand how to make better recordings. On a technical level this is basically wrong, and though I am all for breaking the rules - I just tend to believe it is more of a valid "choice" to break the rules WHEN YOU KNOW THE RULES YOU ARE BREAKING ... (although JUST WRONG is, OF COURSE, very fun sometimes!)
How did you make the metal for the face of the unit?...
Mhh... Bad idea. Loading a mike with a low impedance device will bring distortion, low signal with degeneration of tone and weird frequency cancellations. You ideally want your front-end pre to have a very high impedance to load the mike the less you can... as every good old vintage tube preamp does! Variable impedance machines have the possibility to INCREASE the standard 10:1 ratio, not to lower it.
@Peterson Goodwyn So as luck would have it, your kit is sold out & I don't have a drill press or any other fancy tools other than soldering iron lol... Nonetheless, while I wait, i'd like some clarification: Lets say I have a mic preamp that has a 600OHM input impedance stock & I want to be able to create a 10:1 impedance match between my 200OHM output impedance mic & that preamp... Would your pot value be correct, does this device do this, & if so, by what means??? Does it DECREASE the output impedance of the connected mic or does it INCREASE the input impedance of the connected preamp? Also, whatever the above case may be, how do I know what impedance amount i'm at w/o keeping a multi-meter attached? Do you just have to guess???
Hi Brandon, sorry for my delayed response. As a passive device, this box is only capable of reducing the input impedance of the preamp. So, as in your example, if your preamp's stock input impedance is 600 Ohms, there is no way to increase it to 2k Ohms with this box. Similarly, there is no way to decrease the output impedance of the mic. Either of those things would have to be done either actively or with a transformer, which is beyond the scope of this project.
Regarding how you know the impedance of the preamp while using this device, yes, you have to guess. But you have your ears to guide you!
Peterson Goodwyn Man I figured it was too good to be true! LOL You never know tho with technology these days, so I always ask! Thanks a lot man, I honestly think the mic/pre combination will work ok w/o changing the pres input impedance. I think the 10:1 ratio is more of a personal preference than anything, since i'm currently using a GAP73 pre at 1200OHMS input with a 200OHM output mic & it sounds great. Thx again man & congrats on the present & future inventions! That's an awesome gift to have! -B
Peterson Goodwyn You can add another potentiometer in series with your signal to increase the impedance.
If your mic preamp has no input transformer, (e.g. a modern transistorized/op amp type) you can add a 10:1 (really 1:10 or 1:4) step-up input transformer, which will both lower your impedance (to more like a ribbon mic's expectations) and boost the voltage (and signal) free. This is better than a resistor network which results (as video notes) in at least a 6 db loss, (and with the circuit he has made often a lot more loss!). The only drawback is that for good frequency response you need a good input transformer. The "10:1" reference is not to mics but to circuits in general, where when amplifying mainly voltages, you want the input impedance of the next stage to be 10x the output of the last stage. Mic preamp transformers are generally the opposite, in that instead of stepping down the voltage in exchange for better accuracy of voltage and making it up with a gain stage, you want to take the tiny signal of a mic and INCREASE the voltage before it hits the first amp stage, so it is higher above the noise floor. So mic input transformers usually have fewer primary windings and more secondary, boosting the voltage, but lowering the input impedance that would be seen by the mic. This is okay, because in a (tube preamp) the input impedance (without the transformer) is already so high that lowering it still keeps it well above the 10:1 ratio for voltage transfer. Example. Input stage of preamp is say 1 megohm (the grid bias resistor value in parallel with a near infinite impedance of the tube). A 1:10 step up transformer makes this 1 megohm look a lot lower, more like 100k to whatever is plugged into the transformer. But that is still way more than 10x the output impedance of the mic, which may be as low as 50 ohms, 100 -150 ohms or even 600 ohms.
+Peterson Goodwyn -- Where is the graph you are talking about? Your link is dead.
thaaaaaanks!
Remix this without the music
May i ask what is the formula?
+SiIentwater R1 = R2 × Rtotal / (R2 − Rtotal)
@@yngvai777 Hello Michael, could you apply the formula to the specific example of the video? What values R1,R2 and Rtotal would have? Which would be the "desired impedance of 600 Ohms"? which one " the resistance to put in parallel of 1kOhms (the pot)"? and which one the "pre input impedance of 1.5kOhms"?...Thank you
Nice video, but ditch the music it's distracting.
Where can we find the graph?
Sorry.... is Altec 1566A.
That Potentiometer, how many watts should it be?
WTF with all the background noise? Definitely a DIY recording ;oD
A 5k pot will work just fine. You'll simply be using less of the pot's rotation most of the time.
The thing about adding a pot with a higher resistance... Think for a moment about the mic before you do anything to it. That's electronically the same as the original mic preamp impedance with INFINITE resistance in series with it. That is, the higher the resistance of the pot you add, the LESS will be its effect at one end of its turn. Since the pot can actually go to zero ohms, at the other end you have no signal at all.
So there's nothing to be gained by making the added pot more than ten times (or so) the resistance of the mic preamp's input impedance. Anything more and you can't sense any difference.
For someone that knows so much about microphones, you could use one you built for your own videos! Also the guy you interviewed.. just can't take advice
Nasty background music disturbing commentary
"That was Sidney Bechet and a small pick-up band playing "China Boy," a favorite jam tune of the 1930s.
"Slightly in the background, one could hear people moving about, and the person on screen talking often just as loud as the music, sometimes not so loud."
Okay, I exaggerate, but the sounds other than your voice are completely distracting!
I've only seen this one video by you, but I hope you've learned by now that background room noises attract attention and make it difficult to understand the words you're saying.
I know I'm new to your channel, but the back ground music is not only annoying, but is really crappy music, jazz is garbage!!!
Haha I know! I have regretted that decision for over 5 years now!