Basic questions about microphones and mic preamp design...

"... peak readings of 1 volt or more were common, especially from close up microphones on voice and drums." Nice ref.

Speaking of tube vs transistor sound under overload, what's a good comprehensive article that's newer than that 1973 tome?

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 Thanks,
    - Win

I read in sci.electronics.design that Allan Herriman wrote (in ) about 'Basic questions about microphones and mic preamp design...', on Mon, 13 Dec 2004:

IEC 268-15 (aka 60268-15) was replaced by IEC 61938, and EN 61938 is a clone of that.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I suppose that few know that "SSM" stands for "Solid State Music", the same name given to a small surplus (and new goods) electronic store in Santa Clara, run by John Bergoun(sp?). John's interest was in organs, more explicitly solid state implimentations. As a result, he designed numerous solid state audio ICs, which he got Exar to make and sell. Other companies eventually copied those designs, using the same SSM designation that Exar used.

** I agree; a gain of about 10 is sufficent for the first stage; tends to prevent clipping and decreases the NF added by the next stage by a factor of 10. ** Any "micropnone" that has an audio output signal of 500mV already has, by definition, an amplifier stage built in, and that level is essentially LINE LEVEL.

I read in sci.electronics.design that Robert Baer wrote (in ) about 'Basic questions about microphones and mic preamp design...', on Mon, 13 Dec 2004:

The source follower or common-source stage in an electret microphone can only just be described as an 'amplifier'; it converts the capacitive source of the capsule, 2.5 to 10 pF, to a manageable lowish impedance (1 kohm to 5 kohm). But with a suitable supply voltage (e.g. 12 V), such a microphone can produce several volts if placed close to a brass instrument.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

snip

A dynamic mike works like a loudspeaker, a diaphragm moves a voicecoil in a gap with a magnetic field created by a permanent magnet. There are several different condensor mikes

1.) Electret, which requires only a lowish(1.5-5V) voltage to operate a built-in FET stage which is supplied by batteries or +5V on the center pole of the connector(Sony). The output voltage of 1/4" diameter size is around 3 to 10mV/Pa. Output is unbalanced jack or RCA. Price is very low. There are also musicians grade electrets with a built in transformer for balancing and phantom power or 2 AA cells. 2.) real Condensor, needs a higher operating voltage, normally +48V phantom power. Has balanced output with XLR-connector. Price from 150$+. 3.) RF-Condensor, made by Sennheiser, modulates a high frequen.cy. Balanced XLR, phantom powered. Price is high 300$+

Can do both if done properly.

It is only used with balanced lines. +48V, negative to Pin1(gnd), positive through 2 resistors 6k8 into both of the balanced conductors (Pin2 and 3).

Depends on the dynamic range you need. You can make a preamp with a +2.5V only supply, but then you are more limited with the max. Sound Pressure Level vs. gain setting. If the mike puts out 20mV/Pa (1Pascal equals 94dB SPL) and your pre can output 0.31Veff without limiting, you can have +26dB gain, if your input stays below 94dB. A singer can easily output 120dB SPL, which limits your gain to 0dB, generally you want some additional "headroom", so the gain must be even lower. This raises OTOH the noise floor. Depending on the sensitivity of the mike and the required max.SPL, you have to find the best compromise.

You can use any low noise opamp depending on the supply voltage range. You will need to decide for an electret with headphone jack only, because the XLR connectors are big and heavy.

If you are Jaye Foucher, you are very welcome. :-])

--
ciao Ban
Bordighera, Italy

This is correct, but electret mics are often 'line-powered'. One arrangement is: ASCII art; use Courier font

+----o-------o-+-4.7kohm------o +V mic | | |] +----100 nF----o audio +--------+------| FET cable | | |] pre-amp/mixer mic |O 470 Mohm | capsule | | | +--------+-------+---o-------o----------------o 0 V

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

snip

snip

well, you can get up to 0dBu out of a microphone,

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

I think this is really a very old article with a lot of unsubstantial myths. It is not true that you find tubes in mixing consoles today. Where you find them is in guitar amps and certain microphone preamps. The mics mentioned in the article are super expensive big sized condensor mikes, which have a lot more sensitivity than small 1/2 or 1/4" electrets. Generally spoken the discussion has moved to analog vs. digital recording gear not tubes vs. transistors any more.

ciao Ban

Have a look at Rod Elliott's Webpage. I've build his Low Balanced Microphone Preamp

myself. I'm running it from 2 9V Blocks (one for +9V and one for -9V) and it works just perfect. Added two 2k resistors from +9V to balanced inputs followed by two capacitors as phantom power supply.

Cheers, Nico

All the mics apart from the RX77 are condenser mics, the Rx77 is a ribbon. One problem with the 2019 is the high level of noise at unity gain

from the data sheet f = 1 kHz; G = 1 50 nV/ vHz f = 1 kHz; G = 100 1.7 nV/ vHz

So the less gain you use the more noisy the micamp is. Somewhat back to front!

BTW I found a good write up on protection of the front end of mic amps from 48V glitches in TI's PGA2500 data sheet.

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

I agree, I was only pointing out the that mics can pump out a reaonable level, and this has nothing to do with transistors or valve technology. if you ebay for akg microphones, you can get big sized condensrs for less than 200$, so not so "super expensive"

If I had a studio ( those were the days!), I would like to run the same tests with slightly more elaborate equipment.

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Hi there,

I'm wanting to be able to connect a microphone to an recording device I'm thinking of designing based around the AD1871 24/96kHz ADC. The AD1871 is great in that it's very easy to configure it to accept either single ended or differential line level input, however, I'd also like to be able to attach a microphone to this thing, and that seems to be where things get complicated.

Essentially, I need a microphone pre-amp of some kind, it's just a matter of what kind I want, and how on earth to scrounge a design for it. My questions are:

• There seem to be two kinds of microphones in use -- condenser and dynamic. Can a single pre-amp design support both of these? What is the difference in a electrical sense?

• There also seem to be two different ways of connecting a microphone to stuff -- single ended or differential. Again, can a pre-amp support oth of these, or only one?

• What is phantom power, do all microphones require it, and it is only used in differential microphone connections?

• All the pre-amp designs I've found seem to require at least +9V and -9V supplies to operate. I'm hoping to produce a portable device, and so operation from, say, 2 AA batteries is desirable. Is this impossible? What's going in things like my little Minidisc player that has a microphone input, and thus a preamp, but runs from a single AA?

• Are there any IC based solutions to the above, since, as I'm hoping for a portable device, board space is at a premium.

Any help, opinions or rampant speculation greatly appreciated. Miraculous delivery of useful schematics may be rewarded with sexual favours ;).

Thanks, Jaye.

I always associated the "excess" noise at unity gain with the highish

5K feedback resistor causing the noise. Change this to zero ohms, as in a voltage follower, and the noise would (might?) decrease significantly ,(I havent done the sums,)

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Try this link-

Elliot has many other links to designs on the site including limiters etc, all of which are low cost He also has good answers to the questions you asked. Regards Anthony

You may need to supply phantom power to a condenser mic. (I say 'may' because you have the option of using an external phantom supply and some condenser mics contain batteries.)

A single preamp design can support both of these.

Any serious mic will be differential. A differential mic preamp can support a signal ended mic by grounding the other input. This is sometimes achieved with a special cable (i.e. the preamp is always run as differential).

Phantom power is a way of supplying power from the preamp to the mic using the same wires that are used for the signal (hence 'phantom').

It is needed if (1) there is a condenser mic that needs a high voltage bias (typically 48V) or (2) the mic has a preamp in it (typically 12V) e.g. Sennheiser MKH series.

There are standards that defined the electrical characteristics of the "P48" interface: IEC 268-15, EN 61938, etc. It is a three wire system, supplying +48V as a common mode signal on the signal wires, with the common as return.

The 12V phantom is a true two wire system. The third connection is used solely for shielding.

I expect it doesn't have very good performance.

AD, nee PMI SSM2017 (now obsolete!) or SSM2019

Thatcorp 1510:

These are very high end devices. Since you say minidisc, you might be after something with lower performance.

The app notes for those parts should give you some ideas though. I suggest you read them very closely (particularly the thatcorp ones).

The biggest mistake I've seen in supposedly 'professional' preamp designs is too much gain before the first variable gain stage. There's nothing you can do if the first stage in your preamp clips.

WHAT DO YOU MEAN, THE MICROPHONE IS PUTTING OUT HALF A VOLT?

Regards, Allan

Sorry Win I gave up my AES membership years ago, Icouldnt keep up with the maths Maybe Woodgate would have something more up to date.

Urban myth...About triggering an old type photgraphic flash bulb from a mic in a bass drum, I wonder if they are true?

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

There is a huge amount of stuff on this, about 80% being in the audiophool class. I don't recall any recent AES treatment of the subject.

My own view is that since transistors have pentode-like output characteristics, there is little point in comparing them with triodes. And if you compared them with pentodes, VERY carefully adjusting the measurable parameters to be equal, there wouldn't be an appreciable difference in the subjective effects of overload.

No problem at all with a carbon microphone. You don't even need the bass drum - a loud shout would do it.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I was actually thinking about a dynamic mic when I wrote that.

Yes, I have done the measurement (Sennheiser MD421 + very loud noise) and seen the results on an oscilloscope.

Regards, Allan

The mics used in that test are (I believe) all condenser types, and would be expected to output robust levels.

An advantage of preamp designs such as the SSM2019 is that their gain can be reduced to 0dB, which avoids clipping on such high level inputs. By switching only one resistor, the same preamp can be used for both mic and line level signals.

Regards, Allan