Basic questions about microphones and mic preamp design...

[Jaye Gallagher]

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.

 

[Allan Herriman]

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?

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.

* 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?

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).

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

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.

* 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?

I expect it doesn't have very good performance.

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

AD, nee PMI SSM2017 (now obsolete!) or SSM2019
http://www.analog.com/en/prod/0,2877,SSM2019.html

Thatcorp 1510:
http://www.thatcorp.com/1510desc.html

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).

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

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

 

[John Popelish]

Jaye Gallagher wrote:
(snip)

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.

There are quite a few variations of microphone standards. you should
be familiar with them before you decide exactly which you are going to
support with this design. I suggest you start with a google search
for [microphone tutorial].

 

[John Woodgate]

I read in sci.electronics.design that Allan Herriman <allan.herriman.hat
[email protected]> wrote (in <057qr09dbrcu8pv89utpqadfbigchnco
[email protected]>) about 'Basic questions about microphones and mic preamp
design...', on Mon, 13 Dec 2004:

There are standards that defined the electrical characteristics of the
"P48" interface: IEC 268-15, EN 61938, etc.

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

 

[Robert Baer]

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
http://www.analog.com/en/prod/0,2877,SSM2019.html

Thatcorp 1510:
http://www.thatcorp.com/1510desc.html

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

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.

 

[John Woodgate]

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

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.

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.

 

[Allan Herriman]

**
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 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

 

[Ban]

Hi there,
snip
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?

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$+.
3.) RF-Condensor, made by Sennheiser, modulates a high frequen.cy. Balanced
XLR, phantom powered. Price is high 300$+

* 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?

Can do both if done properly.

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

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).

* 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?

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.

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

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.

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

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

 

[John Woodgate]

<[email protected]>) about 'Basic questions about
microphones and mic preamp design...', on Mon, 13 Dec 2004:

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).

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

 

[martin griffith]

Allan Herriman wrote: snip
snip
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.

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

http://www.gprime.com/proaudio/tubes/tubes.htm




martin

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

 

[Allan Herriman]

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

http://www.gprime.com/proaudio/tubes/tubes.htm

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

 

[Winfield Hill]

martin griffith wrote...

well, you can get up to 0dBu out of a microphone,
http://www.gprime.com/proaudio/tubes/tubes.htm

"... 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?

 

[santec]

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.

Have a look at Rod Elliott's Webpage. I've build his Low Balanced
Microphone Preamp (http://sound.westhost.com/project66.htm) 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

 

[martin griffith]

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

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.

 

[Ban]

martin griffith wrote...

"... 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?

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

 

[Allan Herriman]

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!

This is normal for an amplifier with the noise referred back to the
input. It just means that (at low gains) the noise is dominated by
something further down the chain, e.g. the output amplifier. Output
referred noise doesn't change much with the gain.

I'm not sure if this "high" noise is a problem. G = 1 would be used
with +10dBu inputs, resulting in an SNR of about 110dB (assumes 20kHz
noise bw).

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.

Good find.

Regards,
Allan

 

[martin griffith]

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

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.

 

[martin griffith]

This is normal for an amplifier with the noise referred back to the
input. It just means that (at low gains) the noise is dominated by
something further down the chain, e.g. the output amplifier. Output
referred noise doesn't change much with the gain.

I'm not sure if this "high" noise is a problem. G = 1 would be used
with +10dBu inputs, resulting in an SNR of about 110dB (assumes 20kHz
noise bw).


Good find.

Regards,
Allan

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.

 

[martin griffith]

martin griffith wrote...

"... 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?

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.

 

[Anthony C Smith]

Try this link- http://sound.westhost.com/projects-8a.htm
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