Microphone Preamplifier Design Theory and Discussion

D

drtechno

Active member
Today, I am starting this thread because mic preamp design is a complicated subject. Mainly because the end user frame of reference is way different than the though behind designs.

First I will explain the ideal model, then walk through some of the basic designs and methods applying them.

The ideal model and conditions, and design goal for a mic preamp:

The main purpose of the mic preamp is to provide voltage gain from a low voltage source, which is a microphone. There are two basic types of microphones. These are passive and active microphones. Passive microphones work using passive elements. Such as a diaphram or ribbon element to generate a voltage from sound pressure waves. This type of microphone has a low DC (resistance) and AC (output impedance) source resistance. These types of microphones are in the category of passive voltage sources. Another type of passive voltage soure in electronics is the thermal couple. All of these types don't require a load for their operation. Examples of these passive mics are dynamic handheld and ribbon microphones. Active microphones are microphones that have built in amplification circuit(s) which some are externally powered and others are powered by a universal mic preamplifier circuit. They have a moderate source impedance and Dc resistance that in phantom power mic preamp designs is variable due to the requirements it must have a load and front end coupling circuit of phantom power preamps that load the microphone are not always the same. Self powered types like a Telefunken ELM251 utilizes an output transformer to provide loading for its output circuit and establishes a low DC and AC source resistance. In the Ideal circuit model, the source resistance and impedance is considered 0 ohms (others consider this as infintecimal in resistance & impedance in their ideal models) but there are physical limitations to devices. The mic preamp circuit ideally would have infinate impedance as it would accept the voltage from the source without loading it down and reducing the source output volatge. It would also amplify the signal without adding significant distortion to the desired output voltage amplitude. Different circuit designs as well as the type of amplifying device as well as signal coupling design differentiates the physical design from the ideal model. So different goals are established to satify univeral compatability with all microphones in the case of the universal or the mic preamp with phantom power and some circuit designs are better than others when it comes to achieving this goal.
 
Ujn Hunter said:
Check out the Cranbourne Audio Camden 500 Mic Preamp. They seem to have nailed what you should be looking for in a Mic Preamp.
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I wasn't looking for a mic preamp. But if I wanted one I would just build something like a Millennia HV-35P where the input coupling switches from AC coupling with phantom to DC coupling.
 
Ujn Hunter said:
Check out the Cranbourne Audio Camden 500 Mic Preamp. They seem to have nailed what you should be looking for in a Mic Preamp.
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Yup, just about state of the art and not a silly 'boooteek' price!

Cranborne Audio Camden EC2

With added functionality and improved specs, this great preamp just got better.
www.soundonsound.com www.soundonsound.com

^ Even better if you want stand alone and two channels. They have even improved the specification a tad as well. Why also do so few mic pres have a headphone facility? Makes tremendous sense in a home recording setup

At just over a bag for two channels, a bit of a steal!
Dave.
 
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While we wait for drtechno to get his minces fixed (and I really do wish him all the luck in the world) if anyone wants a good book that describes a very good pre amp design as well as a wealth of information on many aspects of audio electronics look for .
Small Signal Audio Amplifier Design by Douglas Self. He has also produced a very good book about power amplifiers. Very interesting but I confess I found some of it a bit beyond me.

Dave.
 
Sorry for the pause, I have good news. They will be able to restore most of my sight back after a couple of operations. Which I'm excited to see again and be able to type instead of having someone else type for me.

ecc83 said:
Small Signal Audio Amplifier Design by Douglas Self.
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I'll have to check it out. Writing books about this subject is difficult without making an opinion at times. So you have to put yourself in the mindset of presenting all options and all methodologies without over whelming them with opinion to neutrally convey this subject.

Since my background is of technical in nature, my challenge to myself is try to convey this in a way that everyone can understand without having taken a couple of classes of college so I might have to reference or explain general info at times for those who have not taken a course of two in small signal amplification and basic electronics and circuits.
 
That is very good news and once again my very best wishes for a successful outcome go out to you.

As far as explaining technical matters to others, I have had at least 40 years personal experience of this!
My son, now 52 is, even if I say so myself, an excellent musician. Not only very competent on 'rock' guitar but jazz, and classical, Bach etc, as well. He reads well and has a very good grounding in musical theory. As well as guitar he is a good piano player and could 'dep' on keys in a pop band if necessary. Also very good on Clarinet and trombone! He is also bilingual in French.

But! Despite his dad's best efforts he finds electronic concepts hard to grasp or at least retain the knowledge. A good session on Ohm's Law and decibels and for a day or so he is fine but two weeks later? Gone.

I am confident I can explain enough about 'gain, noise, distortion impedance, power, and other audio electronics matters to any reasonably intelligent person. I always say, if you care for yourself, have.had a job, can read, drive a car* (legally!) work out MPG and cook a bit, you can understand basic audio concepts enough to be able to dismiss the BS merchants and the Russ Andrews types trying to rip $100 per m of speaker cable off you!

I am also retired, have time on my hands and am relentless! I will just bloody go on breaking a technical subject down until they WILL fekkin' understand it!

*I do understand most of those things are beyond you at the moment.

Dave.
 
Microphone Preamp Design Objectives and Challenges :

When someone designs an electronic circuit, they are creating something for a particular purpose. In this case, we are focusing on a circuit that amplifies a signal that comes from a microphone to a suitable level for the next device.

The signal that originates from the microphone, the voltage source, can vary widely due to the engineering of the microphone and the acoustic energy it has transferred from three dimensional space to two dimensional electrical current. Sometimes this electrical voltage is too high for some amplifying circuits and a switchable fixed attenuator is added to lower the relative input voltage. On the control panel it is usually called a "PAD". Other circuits are deployed before the amplification device as well to protect it as well as prevent interference and most add a circuit to supplement powering a certain type of microphone that has an amplifying stage within it. This situation moves the preamplifier device in the signal chain from the 1st amplifying stage to the 2nd or 3rd stage of amplification. So the scaling of the gain in preamplifier changes due to what microphone is connected. But generally 40-70db of amplification is the range microphone preamps are designed. For most close proximaty microphone amplification, preamps with 40db maxium gain is used like what you would find in a PA mixer.
 
ecc83 said:
My son, now 52 is, even if I say so myself, an excellent musician. Not only very competent on 'rock' guitar but jazz, and classical, Bach etc, as well. He reads well and has a very good grounding in musical theory. As well as guitar he is a good piano player and could 'dep' on keys in a pop band if necessary. Also very good on Clarinet and trombone! He is also bilingual in French.

But! Despite his dad's best efforts he finds electronic concepts hard to grasp or at least retain the knowledge. A good session on Ohm's Law and decibels and for a day or so he is fine but two weeks later? Gone.
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Most techs are not musicians and reality there are far and few that can do both much less do both well. I know how to play a few instruments, but I'm not that good at it. Electronics on the other hand, I've done very well over the decades. Had worked in many repair shops and even had my own shop for a decade. So design and writing about it is the last evolution of those skills.
 
Brian May? The head of electronic design at the amplifier firm I worked for was a first class guitarists and a very clever bloke. Worked for Soundcraft before. Went on to found his own very successful guitar amp company.

I have known a few more so not that rare.

Dave.
 
ecc83 said:
I have known a few more so not that rare.
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Its very few and far between.
Because about 5% of people that work in the electronics field do anything with audio electronics exclusively like that. For the first 12 years as a tech, the only time I dealt with audio publicly is when someone brought in something at the TV repair shop I worked in. Then later, when some of the old tube radio/stereo equipment turned antique I started to get some business with a few antique shops for restoration projects they do.

Btw, the first surgery was a success, I can see but its dim and blurry around the perimeter of my sight, but they told me once they do the cataract surgery I should get most of my sight back.
 
drtechno said:
Btw, the first surgery was a success, I can see but its dim and blurry around the perimeter of my sight, but they told me once they do the cataract surgery I should get most of my sight back.
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Glad to hear that. I had cataract surgery about 8 years ago and went from 2/400 and 20/600 to 20/20 in both eyes. They can really can do amazing things these days. Good luck!
 
drtechno said:
Its very few and far between.
Because about 5% of people that work in the electronics field do anything with audio electronics exclusively like that. For the first 12 years as a tech, the only time I dealt with audio publicly is when someone brought in something at the TV repair shop I worked in. Then later, when some of the old tube radio/stereo equipment turned antique I started to get some business with a few antique shops for restoration projects they do.

Btw, the first surgery was a success, I can see but its dim and blurry around the perimeter of my sight, but they told me once they do the cataract surgery I should get most of my sight back.
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It depends on how you pick your sample! You could aver not many car mechanics become F1 drivers! But I bet all F1 drivers know F sight more about cars than you and I?

I can only speak about UK bands but because everyone in the 60s and 70s were skint as teenagers the people that got into music tended to coalesce around an electronics guy. Me in my case as bass player amp and speaker maker. The drummer was also a TV tech and he could get us free (used) valves!

I'll bet if you look at the personnel in many bands from that era you will find plenty of blokes who know one end of a solder iron from the other?

Great news about the eyes! I wish you all the luck in the world.

Dave.
 
Part 1-1: Input Circuits Introduction.

The input circuits of the microphone preamp are split into four main circuits to preform the functionality of preparing the signal from the microphone through the various stages of amplification thereafter. These circuits are assembled inline as fallows: the input protection circuit, the phantom power injection circuit (optional), the input coupling circuit, and the first stage bias. Below is the block diagram describing this:
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The fist stage, input protection, is a circuit made to take power spikes on the signal wires and shunt them. This is usually done either with reversed diodes tied to power supplies. Others use specialized diodes like TVS diodes and devices like varistors were used in the past. Voltage spikes from static discharges, and unloading of the phantom power that has charged up the coupling capacitor are examples of what the protection circuit was designed to handle so it doesn't inadvertently damage the amplification device.

Next is the phantom power injection circuit. It is designed to power externally microphones that have internal amplifiers in them. When these microphones are used the preamp is not the 1st amplifying device so contour of gain has changed, or how much you twist the knob to get the desired signal level as well as its fine adjustment. I write this down as an optional circuit because not all microphones need it. However, it is adopted as a standard incorporated into universal microphone preamplifier designs.

Input coupling is the next stage where the signal is passed into the amplification circuit. Here the design choices of AC or Dc coupling is utilized and signal termination either before or after is decided as well as incorporating the gain and signal loss of this circuit. AC coupling is the standard method chosen because it isolates the phantom power. However the choices regardless if its a step up transformer or a capacitor, there will be losses and phase shifts. The capacitor coupled method is widely used because of the low cost, however, different microphones will occur losses differently due to the interaction between the coupling capacitor adding series resistance at different frequencies.

The last stage of the input stage we will discuss is the first bias stage. This is where the incoming signal is applied to the bias potential of the amplification circuit. With some microphones, like condenser microphones, this is the circuit point where the microphone's output signal receives a load.

Next in part 1-2, we will go further into discussing input protection circuits.
 
Microphone Input Protection Circuits

Over the years, there has been designs for protecting solid state devices from the surges from plugging in a microphone into an energized phantom power circuit or sudden electro-static discharges. The first transistor mic preamp designs incorporated zener diodes that limited the waveform amplitude, but would pass the limited discharge through the preamp. Then later, as other amplifying devices were created such as op amps and instrumentation amps, they used diodes reversed biased and tied to the voltage rails to shunt away the surge from the circuit. This is in addition to the ESD protection diodes that are in some of the IC amp chips.
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