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FET mics with high Z preamps
Old 5th July 2015
  #1
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A.alden's Avatar
 

FET mics with high Z preamps

I came across this article about mic/preamp impedance matching:
- When Mikey Met Ohmy : Recording Magazine -

Quote:
Transformers, like moving-coil microphones, care about what load they face, and loading a transformer with a too-high impedance can make it ring as thoroughly as an insufficiently-damped moving-coil mic.
Would a FET mic like the Telefunken M60 (listed as <120Ω) or the km84i (150Ω) ring if connected to a high-impedance mic pre like the Forssell SMP2 (13kΩ) or Gordon model 5 (16.5kΩ)? Has anyone experienced this kind of problem?

FWIW, I recently tried an M60 with a Grace pre (8.1kΩ) and it sounded fine.
Old 5th July 2015
  #2
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Hi
By the time you have 30 ft or more of cable attached to a mic the 'top end' (20KHz +) will be either starting to 'roll off' or at least be tamed so the effects are not as dramatic as may be suggested.
Ringing is most noticeable when it is from an INDUCTANCE when combined with capacitance without a significant resistance to 'damp' the oscillation that you get when you have a coil and capacitor together.
In most cases it would be similar effect to having a couple of dB 'boost' at 20KHz (or thereabouts) which would just be considered a touch 'bright'.
The interactions between a microphone, the cable and the input of a preamp are pretty complicated so you could spend all morning trying to calculate what is going on, or you could simply turn a control on an EQ and have any 'oddity' corrected.
Matt S
Old 5th July 2015
  #3
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A.alden's Avatar
 

Hey Matt!

Thanks for your reply!

So if I understand correctly, any ringing that could result from insufficient electrical damping would be heard as just a slight gain boost, rather than prolonging or smearing of high frequency transients?

Could low resistance actually yield more detail from the mic?
Old 5th July 2015
  #4
Yank it out and test it. Use a square wave and an RC decade box. If the symetry of the square wave improves, consider yourself more concerned than the folks that built the mics that used them.

Also consider you may need to adjust those secondary damping networks for every mic preamp input impedance you see, from 2 meg Gordons down to 1200 ohm Neve.
Old 5th July 2015
  #5
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What I have found is that a lot of vintage mics were designed during an era where the preamp impedance was significantly lower, and their tonality can change for the worse when used with a modern high-impedance preamp. Take, for example, the the vintage AKG C451, a lot of people use words to describe these like "shrill" "ear-splitting" and "ice-Pick." However, when used with a 1970s preamp like the Neve 1073, or a Flickinger -- which has a 150-Ohm input transformer -- they sound rich, full, and well-balanced.

The most extreme case I have found is with the old 30-ohm dynamics like the Altec 633a or the STC 4021 Ball & Biscuit, where the impedance of a modern preamp makes them sound thin and useless. However, some preamps like the UTA MPEQ-1 actually have a 50-ohm transformer setting, and these mics sound amazing at that setting, with additional gain and fullness of tone. I think it is no coincidence that Eric Valentine has a few of the old 30-Ohm Altec dynamics in his mic locker.

It is fun to experiment with the mic pre's that have a variable impedance setting, and in that way you can see at which impedance any particular mic performs best. The Warm Audio WA12 has a great TONE button which drops the impedance to a 600-Ohm setting, that gives a pretty dramatic A/B difference for many mics. Other variable impedance implementations can be more subtle.

Last edited by Cathedral Guitar; 5th July 2015 at 10:02 PM..
Old 5th July 2015
  #6
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Impedance is more complicated than just a number. It is a dynamic variable and sometimes a range of values but it is almost never static when connected to a system. There is also more than one factor involved. Capacitance, inductance, inductive capacitance, etc etc. It's a tough concept to wrap your head around if you don't have direct experience in measuring impedance.
Old 5th July 2015
  #7
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transformerless mic pre's are higher impedance seeking sources. the lower the impeadance, the more gain you have to apply, the more noise floor will happen. the input coupling caps that are installed to do the phantom blocking are the culprit to most of the frequency response issues. There was another thread about adding transformers to a midas board recently and I suggested 200:47K and most modern inputs like 47K regardless they spec around 1K stock they don't have to work hard at their job therefore a great reduction in the noise floor.
Old 5th July 2015
  #8
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Anyways, yes it's important to match mics to preamps that will produce an optimal signal. This can't always be predicted by specifications though. The best way to tell if a mic matches a preamp is whether it sounds full and detailed; or not. If you have to make measurements the most revealing would be to examine the frequency response of the resulting signal.
Old 5th July 2015
  #9
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Quote:
Originally Posted by psykostx View Post
Impedance is more complicated than just a number. It is a dynamic variable and sometimes a range of values but it is almost never static when connected to a system. There is also more than one factor involved. Capacitance, inductance, inductive capacitance, etc etc. It's a tough concept to wrap your head around if you don't have direct experience in measuring impedance.
Inductive capacitance? :-)
Old 5th July 2015
  #10
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Quote:
Originally Posted by Audiop View Post
Inductive capacitance? :-)
haha I meant inductive reactance. But it does have to do with the capacity of a coil so it's not that big a mistake. ;-)

Last edited by psykostx; 5th July 2015 at 11:10 PM..
Old 6th July 2015
  #11
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A.alden's Avatar
 

I don't have access to much measurement equipment, nor a perfectly controlled space, so that will have to be a project for a future time and place.

The reason I asked is that I just ordered a pair of Telefunken M60s. They will be the first transforer-output mics in my kit, so I was doing a little reading on the idiosyncrasies of transformer-coupled mics (as compared to the transformerless mics I've been using).

Since the M60s are brand new, I would assume that the engineers at Telefunken have tested them with a variety of modern, high-impedance preamps. And unlike AKG, they don't list a recommended load impedance.

I auditioned them with my mobile interface (a Focusrite Forte at 2.4kΩ) and liked what I heard, so I'm mostly asking out of curiosity since I like to know how my gear works. When it comes time to get higher-end preamps I'll just try a few and use my ears.

Last edited by A.alden; 7th July 2015 at 01:26 AM..
Old 6th July 2015
  #12
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Quote:
Originally Posted by A.alden View Post
I don't have access to much measurement equipment, nor a perfectly controlled space, so that will have to be a project for a future time and place.

The reason I asked is that I just ordered a pair of Telefunken M60s. They will be the first FET mics in my kit, so I was doing a little reading on the idiosyncrasies of transformer-coupled mics (as compared to the transformerless mics I've been using).

Since the M60s are brand new, I would assume that the engineers at Telefunken have tested them with a variety of modern, high-impedance preamps. And unlike AKG, they don't list a recommended load impedance.

I auditioned them with my mobile interface (a Focusrite Forte at 2.4kΩ) and liked what I heard, so I'm mostly asking out of curiosity since I like to know how my gear works. When it comes time to get higher-end preamps I'll just try a few and use my ears.
I would say get a preamp that has an input transformer. Vintech Dual72 would probably work well in that application. Not very expensive either. I've always like the sound of 1272s on sdc mics.
Old 6th July 2015
  #13
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Hi
FET microphones and FET mic preamps are different animals.
A mic with a FET in it uses a FET as an impedance convertor from the 500Meg Ohms (upwards) of a condenser element, to the very low impedance of cabling and subsequent mic preamp.
A FET PREAMP would typically be used for guitar or other high (medium) impedance sources which prefer a 'light' load.
Since microphones can be plugged into a whole variety of preamps, they are usually tested into a specified RESISTIVE load so that comparisons can be made between them. Preamps with transformers on the input are NOT resistive and will interact between the mic output impedance, the cable (principally capacitance) and the inductance and resistance of the mic preamp transformer.
Traditionally the various mic manufacturers worked with differing 'load' and output impedances, also influenced by the mic 'element' Thus a ribbon is 'naturally' a small fraction of an ohm, but usually had it's own internal transformer to bring it to 30/50 Ohms. A dynamic mic would be 150 Ohms or thereabouts whereas any 'phantom' powered mic with an amplifier of some sort could be anything the designer wished although for 'convention' sake it tends to be around 150 Ohms or less.
Since good amplification was 'expensive' and originally favoured transformers to get the best signal to noise ratio the design goal for the first preamps was maximum 'power' (matching impedances) but then as transistors became available the input transformers and then later transformerless inputs favoured 'bridging' inputs where the preamp input impedance would be 10 or more times the microphone output impedance.
The combination of mic, cable and preamp, with it's various complex impedances ought to be 'critically damped' to give the most 'true' response. Using preamps with input impedances significantly more than say 3K Ohms is likely to 'load' the mic incorrectly with the effect of adding 'bumps' to the intended (designed) response and depending on how short cables the cables are, HF 'rise' and peaking.
You should experiment by using a mic with a very short cable (less than 3 feet) and again with cables of 40 or 50 feet between mic and the preamp and listen for the difference. You could even try with 100ft or more, a distance you can easily get between a mic and a control room desk preamp.
Matt S
Old 6th July 2015
  #14
Quote:
Originally Posted by Cathedral Guitar View Post
What I have found is that a lot of vintage mics were designed during an era where the preamp impedance was significantly lower, and their tonality can change for the worse when used with a modern high-impedance preamp. Take, for example, the the vintage AKG C451, a lot of people use words to describe these like "shrill" "ear-splitting" and "ice-Pick." However, when used with a 1970s preamp like the Neve 1073, or a Flickinger -- which has a 150-Ohm input transformer -- they sound rich, full, and well-balanced.
That's because those preamps roll off some of the tops that are offending in that mic if allowed to be heard full bandwidth. You can do the same thing with any modern preamp by loading down the inputs.

451's shrillness comes from the cheap ceramic 470 pf coupling cap used off the capsule to the jfet gate. Replace that with film and foil polyprop or polystyrene film and that edgyness goes away. Then it's a wonderful sounding mic in even feeding fast wide bandwidth preamps.
Old 19th July 2015
  #15
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Quote:
Originally Posted by Jim Williams View Post
That's because those preamps roll off some of the tops that are offending in that mic
Here is a shot of the frequency response of the Flicking channels we racked up. We bypassed the remote switching feature, and of course, the rest of the console signal path is absent. While I don't dispute what you are saying about the offending cap in the 451, but I want to show this because the Flickinger frequency response is rising slightly at 20K, and is not rolling off the highs in this racked channel configuration.



If the frequency response is rolled off on a 451, I pretty sure it is primarily due to the transformer's 150-Ohm input impedance.
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