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Impedance question with mic pres.

Discussion in 'Pro Audio Equipment' started by Paul999, May 12, 2014.

  1. Paul999

    Paul999 Active Member

    Mar 23, 2011
    Edmonton, AB. Canada
    I am mildly versed in electronics. I understand that when you plug a mic into a mic pre the mic expects to "see" or "react" to a specified impedance. I think that you want the mic pre circuit to be x amount higher impedence then the mic (maybe I have that backwards).

    I also know that sometimes in the "gold ol days" certain high output mics were directly plugged into a compressor and the mic pre was skipped all together.

    My understanding is that an 1176 or la2a doesn't have the same impedance as a mic pre.

    I have, many times, used high end mic pres, plugged directly into compressors as well as plugging mics into line inputs on my console with no substantial difference in frequency response or sound in general.

    So my question is do we really need mic pres. On big tracking sessions most mics need much less then 15db of gain in my experience which any line input, compressor or eq with a gain circuit can handle.

    So in theory a person could take 8 channels of mic pres, 8 channels of eq (with a line gain circuit) and 8 compressors with a good make up gain circuit and use them for tracking 24 channels of sources. This stretches gear MUCH further saving thousands for people needing 24 channels for tracking.

    What are others thoughts on this? I've been doing this for years one way or another and have many recordings to back up my claims.
  2. Boswell

    Boswell Distinguished Moderator Resource Member

    Apr 19, 2006
    Home Page:
    You can indeed get that sort of configuration to work, especially if you are working with high SPLs and use high-output valve (tube) mics or high-output dynamics, as neither of these categories usually need phantom power from the pre-amps. However, condenser mics that use FET or transistor buffers do need phantom power, and this must be supplied either from the pre-amp input or by a separate PP unit.

    When it comes to using the lower output dynamic types of microphone (both moving coil and ribbon) in a lower-SPL environment such as a concert rather than in a studio, then you can need considerable gain in the input stages, even as high as 70dB for some of the ribbon models. This amount of gain is simply not available with equipment inputs designed specifically for line levels, even if a variable gain control is fitted.

    Another major factor is amplifier input impedance. Dynamic microphones change their response and damping with load, and generally have a sweet range in the 500 - 2000 Ohm bracket. Although it's possible to pad a 20 KOhm line input with a resistor to bring its load impedance down to this level, just using the standard line input is not going to give the same sonics as a pre-amplifier designed for medium-impedance microphones, even if there is enough gain for the sound source being recorded.

    Most modern condenser microphone designs have active drive circuits, so they are relatively insensitive to load impedance. This was not the case many years ago when I first got into this game, as everything was transformer-coupled and you were shouted at in a studio by the Chief Engineer if you did not check that each connection (mic or line) was terminated with 600 Ohms.
  3. paulears

    paulears Well-Known Member

    Feb 7, 2014
    Lowestoft - UK
    Home Page:
    Boswell has it right here - the problem is that we're starting to use 'mic pre' as a specific term for a bit of kit, when for most people the mic preamp is just one feature in a device that does a lot more. The esoteric mic preamps that have as their centre of design, very, very low noise, and not a lot else, are products designed to appeal to people who need high gain and low noise from mics that may be a bit deaf compared to others - so they need to squeeze every drop of gain they can. For others, preamps in normal mixers are fine.

    Looking at my own mixers and microphones, I suspect my preamp gains spend more time in the higher gain positions than in the lower ranges. I have a few devices that have a bit more 'oomph', but looking across the mixer from the last recording session - snare drum, saxophone and kick drum are the only channels that have low gain settings - the rest are between 50% and 75% of the pot travel. I have, by necessity tried to use a mic direct into a hardware compressor, and the results were very poor - just not enough output, and none of mine have phantom power - which is important for me.
  4. RemyRAD

    RemyRAD Guest

    Sep 26, 2005
    Here's a couple of other things you should know. Regarding microphone outputs that have enough output, to drive a line level input. A couple of microphones that come to mind are the Neuman U-47, 48, 49, 50, 67, 87 and then some. These microphones put out such gobs of ample output voltage that a built in output pad, were built right into the guts, of those microphones. In their literature, they even informed you that they could be bypassed. Even dynamic microphones today that are tight miked, that utilize neodymium magnets, on high SPL instruments are also no problem.

    However, in the question of impedance matching. While most microphones are indicated as being designed as 150 ohms or 300 ohms, 250 ohms, 50 & 30 ohms. We don't need to load them into those input impedances. Many microphone transformer inputs offer those selections all the way up to 1500 ohms or greater. And where many folks have discovered that if you want that older times sound? Ya might want those lower impedance input load preamps? Because they'll damp down the signal from the microphone. It will become darker and less descriptive sounding. Hence that classic sound from the days of impedance matching. Most of that one out starting in the 60s in the 20th century. Low impedance source outputs sound best when loaded into medium-high impedance input loads such as 1500 ohms for microphones and 10,000-50,000 ohm line level inputs. Which by the way, if you're feeding from a transformer isolated output? You should load the transformer down with a 600 ohm resistor (620 is readily available and works fine. 600 is a special order. Don't go lower.) The output transformer requires this as the transformer will not exhibit the proper output, ring and response. The resistor with the transformer are part of a tuned circuit. There is usually also a capacitor in there. This tunes the transformer to its proper resonance. But the inputs of what you're plugging it into don't have to be 600 ohms. Just ask Neil Monsey. Oh wait... he's dead. But go ahead and search his name on YouTube. You'll learn something.

    LOL one of the other cheap ways to go... is to simply, get yourself a quality microphone input transformer. This ultimately can be the game changer if you want to NOT use microphone preamps. You see, microphone transformers are what we call a step up transformer. The output windings outnumber the input windings by a factor of 10 or 15. What this means is, a microphone transformer is a passive amplifier. Capable of amplifying the microphone from between 10 and 15 db. Going into a line input, with again trim, capable of accepting a -10 db consumer output source and you'll have more than enough gain and the proper impedance matching for your microphones. And it will only cost you from between 50-$100 per transformer to do that. And you can get different transformers! Such as the most widely used Dean Jensen's and later similar ones from CinemaMag, St. Ives/Marinair, UTC, Triad, Hammond and others. Each one has their own particular classic sound. So while you might not have the same circuitry as a Neve or API or anything else? You'll always have those transformers that they use which is a big part of their sound. And that's a hell of a lot less money to spend than it is on the actual original modules. Include an API 2520, Melcor 1731 A, Dean Jensen 990, Neve 438/444 between 50-150 bucks and you get even closer.

    Now you're right in that you don't need that much gain. But here's why you do. With those classic items that I just mentioned, they all did something acoustically magical. The acoustic Magic came when you would slightly begin to over gain them. As their little discrete transistor operational amplifiers start to go nonlinear, they make for the most incredible sound. Frequently making equalization and dynamic range limiting or compression moot, unneeded, superfluous, doesn't add anything, takes away from... the sound. Which is what we all do with those classic older pieces like that. That's part of the ART, along with the science, of recording arts and sciences. It's actually knowing how to do everything wrong the right way. It's understanding how to push the envelope and which envelope to push.

    Now of course that actually adds, measurable distortion. So what? You can't mix with your eyes. Just ask Stevie wonder. Sound is subjective, 100%. What we want for modern day pop music is to utilize the musical distortion to offset the nasty side of PCM digital. When it comes to audio hygiene, my audio is generally bad ass dirty. My S H I T, sounds like rock 'n roll is supposed to. With careful use of musical distortion components.

    I'm a risk taker. I'll actually do that during tracking. These discrete transistor microphone preamps can do things that integrated circuit chips, cannot.

    Sure, you can plug a microphone right into an 1176. The old ones were all 600 ohm inputs. Later versions were 20,000 ohms or was that 10,000? I forget? It don't matter. An 1176 has 40 db of gain. Loading a dynamic microphone down to 600 ohms certainly won't sound bad. It'll sound more open at 10,000 ohms, however. Big deal. It'll still give you that discrete transistor sound and with an output capable of better than +24 dbM. But it may not be the quietest, way to go? But then there are those of us also use, judiciously, downward expansion. It can be used for noise reduction quite effectively. And without the need of completely turning microphones off, slamming the door closed, like a gate. That sounds damned unnatural. So the downward expander just ducks things down by a preset amount such as 6-15 db. You set the threshold so it ducks, when direct sound of that microphone is lowered or momentarily interrupted. Which is also one of the reasons why folks love mixing on SSL's. You get all that stuff on each channel input. Because there is nothing natural sounding about making electrical recordings. What you do naturally doesn't necessarily sound natural, in a good and intimate way.

    In fact, not only might I be over gaining my microphone preamp? I might still be running that into a compressor or limiter, also during tracking. I won't hit it hard. And I might even also adds some slight EQ and high pass filtering? When I'm tracking. I've been doing this long enough to have that kind of confidence in my capabilities so as not to screw up. Just because you want to see the meter move on your limiter doesn't mean it should. So it all just comes down to how we like painting our pictures. Some people like a full roster of color. Others like pastels. Some like oil. Others like watercolors. Some use thick paint brushes were others use thin. And Robert Crumb, liked the purity of pen and ink. Clean, clear, transparent, black & white. Lacking any color. Both colorful and colorless have their place in audio. Individually, separately or together.

    We make things sound the way we want them to sound.
    Mx. Remy Ann David
  5. audiokid

    audiokid Chris Staff Resource Member

    Mar 20, 2000
    Prince George, BC
    Home Page:
    Remy, I wish you would respond like this all the time.(y)

    Just saying...
    DonnyAir likes this.
  6. DonnyThompson

    DonnyThompson Moderator (Distinguished Member) Resource Member

    Nov 25, 2012
    Akron/Cleveland, OH
    Home Page:
    Yup. A solid post... accurate, fact filled, clear and concise.
  7. Davedog

    Davedog Distinguished Member

    Dec 10, 2001
    Pacific NW
    Certainly IS the major reason I bought a ViPre. Nothing quite like different lengths of copper surrounding a core of metal at the input. Not a resistor in sight. You get your 300, 600, 1200, and 2400 ohms balanced. Want more modern? Turn off the transformer and run your mic through the balanced bridge input. Just like a lot of consoles. But thats not the end. Just the beginning. How about being able to adjust the slew rate? Fast or slow. So, you could get that wonderful sag just like the old days. Terminate your ribbon mic at 300 ohms, set the rise time to slowest (five settings), adjust the mic preamp to hit the recorder just so and sing "Moon River"...

    I like the ViPre hitting a channel of the DBX 160SL for vocals. With the impedance selections, every mic is four REALLY DIFFERENT mics. I've said this before, you should hear an SM57 on this thing.

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