Mic Loading Confusion

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chris319

Well-known member
Joined
Sep 4, 2006
Messages
110
I am confused about loading a dynamic or ribbon mic on the way into a preamp circuit. Everything I've read (and Ohm's law) says that the higher the load impedance the less loss there will be due to loading. So why aren't all preamps designed to terminate a mic with 1 meg and be done with it? What is the downside of using too high a value of load resistor? Also (n00b question) what is the difference between a resistor to ground on each side of a balanced mic, and a single resistor across a mic?

I have read Richard Werner's AES paper from 1955 dealing with ribbon mics.

I am building this circuit (figure 2) using the THAT 1512 IC:

http://www.thatcorp.com/datashts/1500data.pdf

They use a 1K resistor on each side of the mic input to keep noise down. I've tried resistors all the way up to 5K on each side (10K combined) and do not see a change in the noise floor.
 
Microphones are typically designed for optimal frequency response with a nominal 10x source impedance termination (1.5-2k). Indeed you could pick up a very slight improvement in S/N by lightly loading, but could see a commensurate deviation in frequency response from specified.

JR
 
What John said...

Also, if matching to the manufacturer's recommended termination were NOT the consideration, one reason people use lower values at the input is to reduce the noise when the preamp is not connected to a source. Of course the diligent user should simply disable or switch off that preamp's output, but sometimes that may be overlooked.

Also, if you have a bipolar input preamp with a lot of bias current out of the input(s), a high termination resistor will lead to a substantial input voltage shift in the absence of a lower-Z source. A balanced input preamp will reduce this to just the difference between the two input shifts, but it could still be significant. As well, the source may be a.c.-coupled, maybe with a highish value local termination, so unless the preamp has a d.c. servo, at higher gains it may have a substantial d.c. offset even when connected to a source. This could reduce dynamic range in extreme cases, and possibly increase distortion by biasing internal stages away from a balanced condition.

As remarked in here before in off-topic excursions, one can generate the equivalent of a given impedance over a useful frequency range using feedback techniques, but resulting in a termination that has substantially less than the thermal noise of a real resistor. The improvement is likely quite small though for a microphone that wants to see a couple kohms. In other situations it can significantly enhance signal-to-noise ratio.
 
[quote author="JohnRoberts"]Microphones are typically designed for optimal frequency response with a nominal 10x source impedance termination (1.5-2k). Indeed you could pick up a very slight improvement in S/N by lightly loading, but could see a commensurate deviation in frequency response from specified.[/quote]
Thanks, this is very helpful.

There are a number of dynamics with rated impedances of 150 ohms (Shure SM57, SM58, E-V RE20, RE27 and RE50). Then there is the Audio Technica AT804 with a rated impedance of 600 ohms. So if I build the THAT 1512 circuit with a 3.3K resistor on each side (6.6K total) it should handle both the 150 ohm and 600 ohm mics reasonably well, correct?

Here is where my preamp is used:

www.miclisteningroom.org

I'm after a preamp which is as transparent as possible so as to let the sound of the mic come through with as little of the sound of the preamp as possible.
 
Keep in mind if you use phantom power, the nominal termination impedance includes those 6.81Ks in parallel.

I don't have much love for 600 ohm mics, and wouldn't favor them over the typically higher quality Low Z mics. If the Hi Z mic uses a 1/4" plug, you could wire up an additional 1/4" input jack with switch contacts that disconnect a parallel set of input termination resistors that would only be in circuit when no Hi Z mic is plugged in for using the low Z XLR input. This way you could have 2K termination for low Z mics, and whatever you want for hi Z.

Another common approach is to use a small in-line step down transformer to match hi Z mics to low Z inputs.
------

If you're willing to experiment, build it for the higher impedance termination (or higher) but with a variable potentiometer in shunt across the input. This way you could listen to different terminations and use what sounds good to you. Just be alert for frequency response or perhaps more subtle interactions.

JR
 
In my arrangement, condenser mics are powered with an external phantom supply.

Trying to capture the "true" sound of the mics as I am, transformers and hand tuning are not options. What I could do though, is wire the preamp for one impedance and then wire some other resistors into an XLR barrel which simply plugs in line when I want to record a mic with an oddball impedance. That would work just fine.

Here is the 600-ohm mic in question. It is basically Audio Technica's idea of an E-V 635A. It has a balanced output and a pretty hot one at that for a dynamic.

http://www.audio-technica.com/cms/wired_mics/e48a2ceb72728b97/index.html
 
> there is the Audio Technica AT804 with a rated impedance of 600 ohms.

Audio Technica AT804 is good for pounding nails. Seriously: it is (as you say) a so-obvious copy of the E-V 635A, which was made for TOUGH duty. One of the E-V ads showed a 635A pounding a nail into a board and still getting the interview afterward. I'm sure the A-T version is equally robust. And very high output. Which means no extended bass or treble. EXCELLENT interview mike. Great stage omni mike because it is reasonably flat where it matters for most instruments, aim hardly matters, it does not muddy-up with stray bass, and it can't be broken.

I done a lot of good work with 635As, and think it should be used more. But it is what it is, and if it isn't right for a situation I'd use somethng else.

I can't see "optimizing" a preamp input for 635A or AT804. The mike impedance is flat enough that 2K or 200K loading won't change response more than such mikes vary one to the next. The output is HIGH to optimize S/N with very cheap porta-recorders, so any good clean input will not add noise.

Mikes "should be" 100-200 ohms, inputs "should be" 2K ohms. That's "unloaded" enough, and does cap your buzz reception when a mike cable gets pulled-loose. (50 feet of open cable on a 100K input will make an atrocious racket.)

The AT804 violates "should be" for its own good well-considered reasons. Don't triple-think their engineers. Use it like the techno-idiots it was made for, people who have no idea what impedance is.

> Trying to capture the "true" sound of the mics

What Is Truth?

What use is Truth in Music?
 
Visit my site (the one I linked to). There are samples of 65 mics, from a 44BX to a Neumann U87a to cheap Chinese condensers, as well as an AT804 and a 635A. My preamp has to be able to handle anything I throw at it, not optimized for a single mic, and it must do so without altering the sonic characteristics of the mic in any significant way.

I done a lot of good work with 635As, and think it should be used more.

In my line of work (TV news) we use RE50s extensively. As you know they are the shock-mounted version of the 635A. My biggest disappointment with them is that they're too susceptible to plosives/p-popping, and of course they do not reject background noise.

In the next batch I want to add to my web site a modified Nady RSM-2, a Fathead ribbon, an RE27, a Studio Projects mic, and maybe another Oktava (the SDC that everyone likes).
 
If you don't terminate them all in a standard way your recordings are not comparable.

What is your reference source, a spark gap or some other sound standard?

JR
 
No reference source, just a guy speaking into some mics. It is a very limited test meant only to compare many different makes and models.

For my standard loading I may settle on ten times the published impedance. For many dynamics the published impedance is 150 ohms. This I can easily handle with the XLR barrel I built since last we spoke. I would like to do a test recording with a typical dynamic (say an SM57) at 1500 ohms and at 6600 ohms termination and see if I can hear a difference.
 
You will; the SM57 has proven to be the most load-sensitive dynamic microphone in any of the tests I've run.

Most modern microphones use mechanical damping to keep the capsule's motion under control. The "Unidyne" mics from Shure (SM57, SM58 and relatives) are from an older school of design; they were intended to be used into lower-impedance loads, which supplied electrical damping.

As a result, when used into standard (1.5-2k) loads I find them excessively harsh-sounding: not only brighter but also more distorted, as if the capsule's excursions weren't being well-controlled. I found that a 500 ohm load impedance turns the SM57 into a very different (and IMHO, much nicer) microphone.

As you said, you could do the impedance change with a resistor in a barrel connector. For my experiments, I did that, except that it was a 1-foot XLR-XLR cable because that was easier to get locally.

Most transformer-coupled condenser mics want to see 1.2-2k; higher, and their transformers will ring; lower, and their amplifiers will distort more. Most transformerless condenser mics will work well with anything from 1.2k up, and will distort slightly less at higher impedances.

Ribbons are a whole 'nother story. Classic ribbons were designed to work into a high-impedance input: a transformer with the secondary connected to the tube's grid, and no terminating resistor. Use them into a 1.5k load, typical of modern mixers, and you overdamp them, leaving you with muffled highs.

AEA makes a preamp (designed, I believe, by Fred Forssell) with a high input impedance. Scott Dorsey tried it on his classic RCA ribbons, and reported much, much better high-frequency response, as theory would predict. (Oh, he also tried an SM57 into it. As theory would again predict, it sounded awful.) For RCA, AEA and similar classic mics, I'd expect best performance into a high-Z load. Modern ribbons, such as Beyers, work fine into modern loads (1.5k or so).

Since you're building a transformerless-input preamp, you have a good deal of flexibility. If it were me, I'd do something like a 10k input Z, with barrels to drop it to 1.5k (for transformer-coupled condensers and most dynamic mics) and 500 ohms (for SM57-family mics).

BTW, you asked about placement of loading resistors: connecting between pins 2 and 3 of the XLR is fine. No need to use two and connect to ground.

Peace,
Paul
 
This is great stuff.

For the purposes of my shootouts I don't think I want to go down the road of hand tuning load impedances by ear for each mic. OTOH I don't think I want to have a constant, one-size-fits-all load impedance. This is why I'm thinking at the moment of a fixed ratio (1:10) of published impedance to load impedance. I am, however, open to suggestion on this. Naturally if I were doing anything other than a mic shootout I would carefully tune each mic and load combination.

I own an AEA ribbon mic preamp and it has many wonderful virtues: plenty of gain and a great S/N ratio, but I noticed it was making some of my dynamics sound "warm", a little bass heavy. It has an input impedance of 18k -- yes, 18k.

I wonder if the popular SM7b has the same matching issues as the Unidyne family. Manufacturers should publish a "recommended load" specification for their mics, IMO.
 
While I've already suggested this, most mic manufacturers design their mics to interface with the defacto standard terminations (1.5-2K) albeit with varying success. They don't publish a optimal termination, since if it was nonstandard it would discourage people from buying their mics.

Most preamps do not offer adjustable or switched input terminations.

The sound signature of a microphone is arguably how it sounds with the standard termination, since this is how it will be used by most owners.

Of course obscure or unconventional mic technologies may have different termination preferences, but that is not accomdated in mainstream preamps.

If your samples are made with widely varying terminations they will not be very meaningful to users of conventional preamps.

A perhaps interesting parallel, back in the days of black vinyl discs, MM phono cartridges were quite sensitive to capacitive loading/termination. The length of phono cables used between turntable and preamp could make measurable differences to top octave frequency response, while few understood the simple interaction and instead assumed magical properties for the wire.

JR
 
[quote author="chris319"] Everything I've read (and Ohm's law) says that the higher the load impedance the less loss there will be due to loading. [/quote] No. Overloading to get less current amplification needed you loose voltage. Underloaded to get less voltage amplification you loose current available. Less loss of power means equal impedances. However, sometimes manufacturers specify different load impedances that are needed for less losses in order to achieve an optimal damping factor for desired frequency response on standard load impedances since impedances of real microphones vary with frequency and sound pressure.
 
Mike inputs DO come in various impedances and it does make a difference.

While it enormously complicates your shoot-out, it would be useful to know "what changes" at various impedances. With some condensers, it does not matter (aside from lower output voltage). Paul has cited a couple classic mike types where the response changes significantly.

If load makes "no" difference, the user can forget that aspect. If loading changes the sound, the user should consider what odd inputs she has and if the sound-shift is acceptable.

> under what circumstances do you think there would be a need for a 10k load?

There are inputs that high. Particularly for single-ended inputs such as found in news gathering where the mike is on a short cable and gear is super compact (or extra low price).

OTOH, I have a board which is 1:1 transformer into 470 ohms to an inverting op-amp, just about 600 ohm input impedance. It is not real low noise... it was sold for affordable rock and roll, not fine harpsichord taping.

One vendor designs for a very low input impedance. There's some interesting theory to go with it; it also happens to be an elegant way to get a differential input with minimal circuitry.

Specifically, 10K un-loads your 600 ohm mike to get the hi-Z sound. You can change the usual ~~1K base resistors to 5K and get up near 10K input

Then tack 2.4K and 600 ohms on, to get the normal and lo-Z sounds. Since many mikes won't change much, I'd quick-check 10K 2K 500, and record a full sequence only on mikes which change timbre.

As a general test, I'd like to know the LOUD signal response in low Z. Dynamics won't care, but many of the $99 condensers can't drive a heavy load at high level, though they may do fine on speaking voice at voice-booth distance. (Or you could leave that to the music-studio guys.)
 
[quote author="pstamler"]
Ribbons are a whole 'nother story. Classic ribbons were designed to work into a high-impedance input: a transformer with the secondary connected to the tube's grid, and no terminating resistor. Use them into a 1.5k load, typical of modern mixers, and you overdamp them, leaving you with muffled highs.

AEA makes a preamp (designed, I believe, by Fred Forssell) with a high input impedance. Scott Dorsey tried it on his classic RCA ribbons, and reported much, much better high-frequency response, as theory would predict. (Oh, he also tried an SM57 into it. As theory would again predict, it sounded awful.) For RCA, AEA and similar classic mics, I'd expect best performance into a high-Z load. Modern ribbons, such as Beyers, work fine into modern loads (1.5k or so).

[/quote]

Paul,

I am wondering why would be that and what is the theory behind it? It seems that ribbon impedance is mostly resistive, raising only on a low end, around the tuning resonance frequencies.
There is an AES paper on electrical loading of mics circulating here (IIRC, PRR has posted it), where there are some graphs showing the top end the least affected. From memory, difference between unloaded and loaded with 250 Ohm (!) had difference only -5 db starting from 2K, and almost -20db at the tuning frequency.
I myself have played with loading ribbons quite a bit and from my experience the bass is the first thing to go.

The reason Beyers work fine with low loads is that they are tiny (about 0.06"x0.910") and work as very rigid pistons--only God knows what is the tuning frequency :roll: .

Best, M
 
A couple of years ago, I wrote an article on variable impedance preamps and did some tests, of course. It is in Sound & Recording issue 05/2006 for anyone who cares to read it - it's in German, though.

Overall, impedance does not affect frequency response as much as most people seem to think. Condensers are hardly affected at all, except in the very very low frequencies for the simple reason that the output caps in the circuit form a hi pass with the preamp's input impedance. You lower the impedance, the cutoff goes up. Condensers that have no output caps (i.e. the Schoeps circuit) are virtually unaffected by the preamp's input impedance (as far as frequency response goes, of course low impedances will load down the output stage and produce higher distortion/lower the maximum SPL figure).

Dynamic mics are a little more sensitive to impedance, but not enough to make you worry. I did not test any mic of the shure 57 family, though. The ones I tested were Beyers (an old M300 for instance). At low impedances dynamics tend to show some bass loss, but only about 2 dB at really low impedances. Changing the mic position will result in much higher variation.

The only mics that reacted to loading in any really significant way were ribbons. At low impedances, there was quite some bass loss, but also some treble loss. The models I tested were the only two Chinese ribbons available in Germany at that time, the large AEA-inspired type and the somewhat smaller Royer-inspired type in a lollipop housing. The Royer-ish ribbon was more sensitive to loading, mostly because the output transformer is a higher ratio and the resulting output impedance is about 400 ohms (vs. 250 on the AEA type). On these ribbons the treble loss was quite noticeable. And I might have gotten even greater differences, had my max impedance been higher than 2400 ohms.

Note: When comparing sound differences resulting from different loading, it is absolutely essential to meticulously compensate for the associated gain loss. I think a lot of people forget about that - those level differences are often small and thus likely to be perceived as sound differences.
 
[quote author="Rossi"]
Note: When comparing sound differences resulting from different loading, it is absolutely essential to meticulously compensate for the associated gain loss. I think a lot of people forget about that - those level differences are often small and thus likely to be perceived as sound differences.[/quote]

Some generalized version of this should be put on a plaque and hung on the wall in plain view wherever audio art transpires.

Having said that, the compensation can be a subtle matter. I guess the default source is pink noise, but even then what weighting should we use?
 

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