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Post by Johnkenn on Feb 22, 2014 10:44:26 GMT -6
Is there a fast, hard rule on this? e,.g. - low impedance, more rolled top, etc. Or is it really just dependent on the microphone? Jeff's Heider is somewhere around 160 without Hiz engaged, 3xx with it engaged and 7xx with the pad in...I believe Neves are 2k ohms, and loz is 500? (I could be waaay off) Can someone give a dumbed-down, layman's explanation? Please remember I'm math illiterate  |
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Post by Johnkenn on Feb 22, 2014 10:50:35 GMT -6
Heider - MIC Input Impedance: 167 ohms, 300 ohms W/Hi Z engaged, 765 ohms W/PAD engaged Heritage - Microphone input impedance: HI, 1200 Ohm minimum, LO, 300 Ohm minimum. Higher gain positions gradually have greater impedances, optimum for lo gain ribbon mics. Input is Carnhill transformer balanced and floating. Helios - Input impedance is 1,200 ohms I'm not losing any sleep over this - just figured I'd ask the board tech-nerds |
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Post by jcoutu1 on Feb 22, 2014 10:53:23 GMT -6
Cool question. The Focusrite ISA One has a few different impedance selections too.
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Post by svart on Feb 22, 2014 11:05:55 GMT -6
Actually, it doesn't mean that you'll have any rolloff in all situations, it could mean that you have reflections, noise, distortion or boosts/dips in the response, or any number of other issues. It's all about power transfer between the mic and the preamp. Maximum power transfer is when both the source and load have matching impedances, but the optimal power transfer is 50% of the source at the load.
Anyway, what happens is that the load can be too high for the source and the source runs out of current to drive all frequencies. Low frequencies take a lot more power to generate so usually the driver runs out of current headroom and starves, effectively lowering the slew rate to the point where it's only driving lower freqencies it can handle. You'll get compression and distortion and overall you'll likely get reduced frequency range, and therefor probable roll-off.
Low Z is around 500R, HI-z is 10000R or higher. Typical preamps are around 1.2k-2.4Kohms on inputs.
160R on input seems excessively low. That would be lower than most microphones are able to drive..
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Post by popmann on Feb 22, 2014 11:06:56 GMT -6
I can tell you it's why I don't mess with vintage design preamps. I don't have any interest in playing mix and match with this and that combo until I find one that works. That said, I will not pretend to understand the mechanism....the effect isn't simple, though...we could say the low the impendnce, the less high frequencies....theoretically the less ultra lows....but, reality is it's much more complicated than that. And a single impedence is not good vs/bad...case in point: My LA610...has two settings. 500ohm....and 2k. They "added" the 2k to give a more "modern sounding option"....WTF? 500 sounds great and natural fairly across the board...and 2k the opposite. And, yet it's not simply "higher is worse"....because my Millenias run 6500 or something that for all intents and purposes is "zero loading effect on the microphone". But, 2k with the design of the 610=weird and harsh...so, point being, it's a more complicated equation than "higher (or lower) is better"....until you get so high there's no mic in the world it's gonna load. EVERYONE moved to that in the 90s...but, if you keep using the same mics the same ways...you might prefer to put some loading back onto them. |
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Post by popmann on Feb 22, 2014 11:20:01 GMT -6
Cool question. The Focusrite ISA One has a few different impedance selections too. Leave it on the one labeled "classic 220" or something. Promise. When I had one here, went through the higher ones and thought "this preamp really can't be THIS bad"...and I could not find a mic that didn't prefer whatever the original design input impedance was. But, then--except the Royer...all my mics were likely in production when the original ISAs were top shelf stuffs. So, YMMV...but, at least use the classic as the starting point. |
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Post by jazznoise on Feb 22, 2014 19:35:10 GMT -6
It depends on the type of microphone, firstly. Condensors are active and so you'll see as Ward described - changes in slew, distortion etc.
But if we view a Dynamic Microphone or a Ribbon as a series inductor (with a resistor in parallel to it) and the input Z of the pre-amp as a resistor going to ground we can see a passive microphone connected to a pre amp is a self contained hi-pass filter. By varying the Input Z we can change gain, but we can also change cutoff of the microphone. Higher Z's will generally make them brighter, but higher Z's also increase the Q of the filter. This usually means the presence peak of the circuit will become pointier - which is why so many people opt to do transformerless mods to SM57's, I think, to reduce the peakiness with modern pre's.
Obviously that's not the whole story and there's smaller amounts of shunt and series capacitance, and also inductance to ground meaning it's all a big messy bandpass filter really. But the main thing you'll probably hear is the first bit.
Or, at least, that's my understanding! I've been planning to make some Vari Z buffers. Will do..eventually!
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Post by keymod on Feb 23, 2014 7:22:08 GMT -6
Our friend Peterson, at DIYRE ( who is introducing the Colour Modules ) has a simple little kit available for altering input inpedance.
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Post by cowboycoalminer on Feb 23, 2014 9:04:14 GMT -6
popmannWhat is the ideal load for my vintage 67? My guess is a something in the higher range because it is darker than most condensers. Thanks
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Post by popmann on Feb 23, 2014 9:55:02 GMT -6
I prefer the Millenia to anything I've used with old tube era Neumanns. That said--it's also something never intentionally designed that way...so, ymmv.
I would agree with the above that condensers vary more in the extremities of frequency and distortion. The 610 develops odd distortions...frequency spikes...sizzles...at 2k.
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Post by tonycamphd on Feb 23, 2014 9:57:20 GMT -6
popmannWhat is the ideal load for my vintage 67? My guess is a something in the higher range because it is darker than most condensers. Thanks looking forward to this answer, i just wanted to bump, and to say how cool it is to have questions like this all over this forum, we are reaching the summit of pro audio nerd mountain, i've learned more from you guys since last summer, than wading through the sludge on the other site for 4 years!..and its the coolest thing ever! Thanx fella's |
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Post by jimwilliams on Feb 24, 2014 10:27:08 GMT -6
Most of my mics have no transformer so I load them lightly at about 4.2k ohms through my preamps. Lower that impedance and THD creeps up as those curcuits have a harder time driving low impedance loads.
I prefer that higher impedance for my transformer coupled mics as well, they souud more open and airy. Loading down a transformer coupled mic is like loading down an electric guitar, you just roll off the top end.
If you want to optimize those loads, use a square wave oscillator at 2k hz and remove the primary lead in the mic and feed that square wave at around 30 mv into that transformer. Plug into the preamp/load under test and observe on a scope. Adjust the load until the top of the waveform is flat. That way you have found the sweet spot avoiding ringing and overshoot problems.
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Post by svart on Feb 24, 2014 12:56:44 GMT -6
It depends on the type of microphone, firstly. Condensors are active and so you'll see as Ward described - changes in slew, distortion etc. But if we view a Dynamic Microphone or a Ribbon as a series inductor (with a resistor in parallel to it) and the input Z of the pre-amp as a resistor going to ground we can see a passive microphone connected to a pre amp is a self contained hi-pass filter. By varying the Input Z we can change gain, but we can also change cutoff of the microphone. Higher Z's will generally make them brighter, but higher Z's also increase the Q of the filter. This usually means the presence peak of the circuit will become pointier - which is why so many people opt to do transformerless mods to SM57's, I think, to reduce the peakiness with modern pre's. Obviously that's not the whole story and there's smaller amounts of shunt and series capacitance, and also inductance to ground meaning it's all a big messy bandpass filter really. But the main thing you'll probably hear is the first bit. Or, at least, that's my understanding! I've been planning to make some Vari Z buffers. Will do..eventually! Hmm, I think you mean me.. Anyway, you are discussing "reactance", which is a complex relationship between an inductor, it's inductance, resistance and it's parasitic capacitance. I agree with you about the modelling, but it's a lot more complex than you describe. It can be mathematically modeled as a lumped element, with only the first orders of each parasitic approximating a whole system, but is generally broken down into distributed elements for simulation modelling. However, input transformers and their impedances are usually designed to be very flat across frequency. That's just part of the reason that the mic be matched with an input impedance that it can drive easily. Generally speaking, the strange frequency anomalies that happen from mismatches will happen due to the mic(source) inability to drive that impedance across all frequencies. It's the same for both solid state and transformer outputs. In any case, it's also the same for driving TOO high an impedance as well. The source was designed to be stable at a particular range of load, so operating outside of that range can cause similar anomalies as driving too low a load, and can in fact lead to other issues, such as reflections, which happen in cases of too light a load. Anyway, it's all rather academic. Plug in your mic and use it. If it sounds bad, find another mic or do something different. |
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awtac
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Post by awtac on Feb 24, 2014 21:12:26 GMT -6
when you load an amplifier with a lower impedance it is made to work harder. When an amplifier is made to work harder, certain byproducts result, distortion is usually to first noticable and its not always bad distortion either. Each amp will exhibit its own quirks when you push it to its operating edge. If you want a general outlook on impedance, imo, thats it. Everything has a window and a calculator is only going to suggest its parameters, you have to listen.
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Post by cowboycoalminer on Feb 24, 2014 23:14:40 GMT -6
This might be the single most important thread on RGO yet. I'm convinced that if I learn more about this issue it will drastically help me save time and a lot of headaches trying to find good combos. I'm going to really dig in to this topic and learn all I can. Keep the dialog coming. Great stuff!
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Post by Martin John Butler on Feb 24, 2014 23:22:38 GMT -6
If you perfectly match a mic with input impedance, does it mean that you will in fact, hear more cowbell, or just a different sounding cowbell?
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Post by matt on Feb 24, 2014 23:47:31 GMT -6
If you perfectly match a mic with input impedance, does it mean that you will in fact, hear more cowbell, or just a different sounding cowbell? Perhaps it is a manifestation of the search to record a More Perfect Cowbell. |
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Post by popmann on Feb 25, 2014 0:42:06 GMT -6
It means you'll FEEL the cowbell. Capture it's soul. It's essence....
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Post by jazznoise on Feb 25, 2014 5:35:21 GMT -6
Hmm, I think you mean me.. Anyway, you are discussing "reactance", which is a complex relationship between an inductor, it's inductance, resistance and it's parasitic capacitance. I agree with you about the modelling, but it's a lot more complex than you describe. It can be mathematically modeled as a lumped element, with only the first orders of each parasitic approximating a whole system, but is generally broken down into distributed elements for simulation modelling. However, input transformers and their impedances are usually designed to be very flat across frequency. That's just part of the reason that the mic be matched with an input impedance that it can drive easily. Generally speaking, the strange frequency anomalies that happen from mismatches will happen due to the mic(source) inability to drive that impedance across all frequencies. It's the same for both solid state and transformer outputs. In any case, it's also the same for driving TOO high an impedance as well. The source was designed to be stable at a particular range of load, so operating outside of that range can cause similar anomalies as driving too low a load, and can in fact lead to other issues, such as reflections, which happen in cases of too light a load. Anyway, it's all rather academic. Plug in your mic and use it. If it sounds bad, find another mic or do something different. Indeed, my bad! Assuming we're just trying to get an approximation of the effects of different loads, I think a Thevenin Equivalent is pretty sufficient. What I've always REALLY wondered is, since the Z changes how fast the flux from the magnet in a dynamic dissipates, does that mean it has a direct mechanical effect as to how quickly/slowly the diaphragm can move? Yes we are getting academic. But that's fun..right? cowboycoalminer - I don't have a pre with a Vari-Z, but if you would like to add important-ness maybe you could get a ribbon and a well known dynamic (SM57, M201), get them to do a recording of a static source at different Z's (A full band piece of audio playing through a full range speaker would be great), then volume match them and post. I imagine it'll help you see where your own opinion is on things, and everyone else might learn something too. Just think of all those little upcoming RGO's, still teething, wondering what pre goes best with their SM7.. |
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Post by svart on Feb 25, 2014 7:45:28 GMT -6
What I've always REALLY wondered is, since the Z changes how fast the flux from the magnet in a dynamic dissipates, does that mean it has a direct mechanical effect as to how quickly/slowly the diaphragm can move? Absolutely! Take a little DC motor and spin the shaft while the leads are unconnected. It spins easily. Now take that motor and hook the leads together and try to spin it.. |
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Post by jazznoise on Feb 25, 2014 8:01:48 GMT -6
HAH! I was thinking of Fleming's Left Hand rule. Glad to know I learn something good at school!
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Post by svart on Feb 25, 2014 8:03:18 GMT -6
I expect that the dynamic motor through the transformer would be a much more subtle version of the DC motor trick, but nonetheless, I bet that the dynamic mic motor is mechanically dampened through residual parasitics, like the flux, etc. Complex indeed.
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Post by Johnkenn on Feb 25, 2014 11:46:12 GMT -6
This might be the single most important thread on RGO yet. I'm convinced that if I learn more about this issue it will drastically help me save time and a lot of headaches trying to find good combos. I'm going to really dig in to this topic and learn all I can. Keep the dialog coming. Great stuff! I agree...it's a great Nerd topic! |
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Post by IngEng on Feb 25, 2014 22:00:32 GMT -6
There’s a really good article that quantifies differences in several measured parameters as you change the load presented to different mic types and brands: “Phantom Power the Modern Condenser Microphone Part II: The Effects of Load Impedance on Microphone Performance” by Mark Zaim of Audio Technica. It was presented at the 131st AES in Oct 2011. It lays out the effects in really good detail (or, in excruciating detail depending on your tolerance for tech papers). It confirms what you can hear when doing subjective tests, namely that it can affect the low and high frequency extremes. It all depends on the mic type and mic design, though. In general, the more tightly the mic mechanical components are coupled to the mic pre-amp (ie, not isolated by a buffer amp in the mic), load impedance changes will have a greater effect. The paper also shows how max SPL, sensitivity, noise, distortion, etc. can be affected.
Load impedance effect on mics can be taken advantage of to shape tones if the impedance is made selectable. Several manufacturers, including me, do this. When using transformer coupled input mic pre’s, probably the most familiar method is to change transformer turns ratio to do this. It can also be done by changing the transformer secondary load while keeping the turns ratio constant. People sometimes get cagey about this method, but it is valid and can work perfectly with a transformer that is designed to accommodate this scheme. Having a variable input impedance is a convenient way to get more sound shaping capability out of a piece of gear, if you’re looking for that sort of thing.
As noted in the posts, the mic pre input attenuator can have a huge impact on the impedance presented to the mic, and therefore the sound. You might dial in a really good sound with no pad and certain mic pre settings, then engage the pad and find that the sound changes a lot because the impedance changes. Just something to note when checking how the gear makers implement the pad.
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