Can someone explain a mic preamp in non technical terms?

Hard2Hear

Hard2Hear

New member
I have been wondering about this for some time. I am a musician and artist, not an engineer by any means. I'm wondering if someone out there knows enough about how mic pre amps actually are built and work to explain all the components in laymens terms...no really technical jargon.

Lets take for example a simple burr brown based mic pre amp. What is in that thing to make everything work? I know there's in and out connectors, and a chip to amplify the signal (this is the burr brown?) but honestly, outside of that I get pretty lost.

I need someone who has enough knowledge to explain it simply, input to output. Any builder or manufacturer welcome in this thread. If you can explain what happens in your product that would be helpful. Let's not get into eq and compressors, just a mic preamp. Let's try this question too. What is the bare minimum of components you have to have to properly amp a mic?

H2H
 
Most of us recording folks don’t build them; we just turn them on and use them.

Is this your homework assignment? You might want to try an electronics engineering forum of some sort.

Sorry man.
 
I think I've been here long enough to know what this forum is about, thanks.

I like to know how stuff I pay lots of money for works.

H2H
 
Hard2Hear said:
Lets take for example a simple burr brown based mic pre amp. What is in that thing to make everything work? I know there's in and out connectors, and a chip to amplify the signal (this is the burr brown?) but honestly, outside of that I get pretty lost.
Click to expand...

Yes, the chip amplifies the signal. The chip is an operational amplifier circuit ("opamp"), so called because apparently they are useful for math :confused: but who cares? This is audio!

Anyway, an opamp is an integrated circuit that contains, among other things, transistors. It's the transistors that amplify the signal.

Lots more on opamps:

http://en.wikipedia.org/wiki/Opamp

If you scroll down to the section "Internal Circuitry", it lists three stages of an opamp. That's also a pretty good description of the stages of a mic preamp.

Note that technically an opamp doesn't have to be an integrated circuit, it can be a discrete transistor or tube circuit (just as mic preamps can use opamps, discrete transistors, tubes, or any combination of the three). But in common parlance, opamp is used to refer to an IC.
 
This is gonna be the short but easy version:

Most equipment connects (talks to each other) with a "standard" signal, usually around 1 Volt. To get a mic level up high enough to talk to other equipment, you need to boost the signal around 100,000 times louder (or more), typically. That's the basic role of a "pre" amplifier. There's usually some other stuff going on inside, but that's the main function.

What other stuff? Well, phantom power going out to the mic on the same wires that the mic signal is coming in on, for one. That means there has to be some thing inside that will put out 48 Volts DC, while you put in a couple of blocking capacitors to prevent the DC from getting into the amplifier circuit, or you use a nice expensive transformer to block the DC.

If it's a balanced signal, usually, there's a small inverter circuit that will take the mirror image negative signal on Pin 3, flip polarity, and add that signal to the already positive signal on Pin 2. That makes the signal unbalanced but increases the level 6 dB and cancels out any hum and noise that was induced in the balanced line coming into the preamp.

It then goes to one or more stages of amplification, either by tubes or SS circuitry. As the signal leaves each stage, it's usually riding on the back of a DC signal, which needs to be filtered out with more capacitors. Some of these op amp circuits require negative feedback (where you loop a little bit of the output back into the input) to widen the bandwidth of the amplifiers.

If you want rolloffs in the preamp that may take a few more capacitors and resistors to set a on/off "eq" for a specific slope, a variable resistor for gain control, another small inverter circuit to take the unbalanced signal, put out a twin, but reversed signal, which will go to another balanced output jack, either direct, or thru another transformer.

Add a Power switch, some voltage regulators, a polarity switch, and you've pretty much got a typical preamp.

Simple enough?
 
Harvey, you're an invaluable person. I have to get to Texas some day just to shake your hand and buy you whatever you like to drink.:) I read that a couple times and it mostly made sense to me.

So a transformer based pre amp (which are the nicest ones, right?) are using that transformer just to block out current from getting back into the unit and messing stuff up?

This leads me to a part two of sorts. Going back to the burr brown thing. There are tons of amps out there that use the burr brown amp chip. From the syteks to the joemeeks, they are used all over the place. This question may be way over my head but I'll ask anyways. What makes different units, with basicly the same amplifying device in them, sound different? Or what makes one worth more than others? I think it's a loaded and big question. But thats one thing I don't understand.

Thanks!
H2H
 
Hard2Hear said:
Harvey, you're an invaluable person. I have to get to Texas some day just to shake your hand and buy you whatever you like to drink.:) I read that a couple times and it mostly made sense to me.

So a transformer based pre amp (which are the nicest ones, right?) are using that transformer just to block out current from getting back into the unit and messing stuff up?
Click to expand...

No transformers have other properties too. They 'unbalance' the signal, for example.

This leads me to a part two of sorts. Going back to the burr brown thing. There are tons of amps out there that use the burr brown amp chip.
Click to expand...

There's more than one BB opamp that is commonly used, anything from the 2228, 134 series, up to the pricy & high performance 627.

What makes different units, with basicly the same amplifying device in them, sound different? Or what makes one worth more than others? I think it's a loaded and big question. But thats one thing I don't understand.
Click to expand...

All the other components :) Good audio transformers, in particular, are very expensive. On a smaller scale, better quality capacitors and closely matched resistors are also more pricy.

A quality power supply makes a big difference in preamp quality, which requires a good transformer, voltage regulators, and large capacitors.
 
Ok, that all makes pretty much sense to me.

I didn't realize there were different Burr Brown chips. The term is used pretty genericly, isn't it? Could someone help me compile a list of the Burr Brown units? I know some of them, but I do not know which version of the Burr Brown is in them. Maybe someone is smart enough to figure that out. Or maybe it doesn't matter?

What (I think) uses them:
Joemeek pres (the new ones)
Toft pres
Grace 101
Drawmer 1969 Mercenary
Studio Projects 828
Sytek mpx4aii(with bb option)
Some of the Rane stuff


H2H
 
Op amps are pretty interesting components, to me.

Other than straight out amplification, they have uses in summing, balancing and unbalancing actions, filters, phase change circuitry, buffers and line drivers.... all sorts of stuff you can do with them.

As an electrical engineering student with an interest in the audio area of electronics (for obvious reasons, being a musician and recordist), op amps are some of the most interesting things I come across.
 
Hard2Hear said:
I didn't realize there were different Burr Brown chips. The term is used pretty genericly, isn't it? Could someone help me compile a list of the Burr Brown units? I know some of them, but I do not know which version of the Burr Brown is in them. Maybe someone is smart enough to figure that out. Or maybe it doesn't matter?
Click to expand...

Oooh, I only listed a few of them, there are lots. It's easy to tell though: just open up the gear, the model # is printed right on the chip.

How much difference does it make? Going from a real cheap opamp like a generic TL072 makes a pretty big difference. Between the BB opamps, the differences aren't as large. It really depends on which opamp is appropriate for the circuit, which is beyond my knowledge.

I will say that Dan Lavry published a schematic for one of his analog front-end circuits for his converters, and it used 3 OPA627s, which is a $12 opamp, whereas mid-range gear tends to use opamps more in the $1-$2 range, or even less expensive opamps like the NE5532, which is $0.40.
 
Have a look at these schematics, they are pretty simple:

http://www.jensentransformers.com/as/as016.pdf

http://www.jensentransformers.com/as/as018.pdf

The first link is an input stage, although the transformer in the second schematic is usually considered part of the input stage. Anyway, you can see the phantom power supply--a 48V supply run through current limiting resistors (R2, R3), which keep you from getting a nasty phantom shock, and also protect your gear as well as the power supply. As Harvey mentioned, the 48V DC is blocked by the transformer (transformers don't pass DC). There are a few other niceties in the input stage; an impedance switch and pad, which are just a few resistors and switches.

From there into the second schematic, which has the transformer, an RF filter (RN, CN), the opamp (in this case, an Analog Devices chip), a pot or stepped gain control, which bleeds off the feedback loop to ground (less feedback = more gain). Then you have the output stage, which in this case is a simple pair of resistors (an "impedance balanced" signal).

Not shown is the opamp power supply.
 
Burr-brown is just the name of the company who manufactures some of these chips. Saying that your mic pre has a Burr-brown in it is almost like saying you have a General Electric in your house. A General Electric what? Toaster? Microwave? Light bulb?

The Sytek, for example, uses a type of JFET opamp that has a completely different function and usage than one of the INA all-in-one instrument amps that most of the budget mic pres use. They're both silicone of some sort, so they share that in common, but they perform different functions within the design. The Sytek is kind of like a hybrid, and it's a more complex circuit with the opamp performing only the menial task of gain, from what I understand.

Most of the mic pres that you listed utilize the all-in-one type instrument amps that kind of do everything within one chip, moreless (putting it in a very simplified manner that is). They're much simpler and therefore less expensive designs based around an instrument amp that performs most of the functions. This is why such designs are so popular among do-it-yourself ers making their first woodshop class mic pre.

There are two basic types of instrment amps made by Burr-brown: The INA-217 and the INA-163; neither of which are compatible with one another by the way. The 163 has been around longer, and has been used in the Grace Design, the Rane stuff, and the DMP-3. The 217 is somewhat newer, and was designed as a replacement to the old Linear Technologies SSM-2017.

Now interestingly enough, the Linear Technologies 2017 used to be in every budget mic pre, basically. And it pretty much sucked ass, and is mostly responsible for cheap mic pres getting a bad rap. Eventually, Burr Brown came along with the INA-113, and this changed things. It had much less HF distortion specs and sounded superior by most standards. Basically made it possible to make a truly good and functional mic pre at a modest cost. The 113, by the way, is now the 163.

The 2017 was since discontinued, so Burr-brown developed a drop-in replacement for it in order to fill the void, and called it the INA-217. Luckily, the 217 is a big improvement over the 2017, which is good news. Guys like Joemeek, Toft, etc. all use the 217.

By the way, Linear Technologies also came out with a pin-compatible replacement for their own discontinued 2017 ... the 2019. I don't know of anyone who's ever used it.
 
ms, thanks for the links and chess thanks for the bb explination. I think there are way more people out there than just me who weren't aware of the large possible differences between all of those. It's a term thats getting tossed around alot and I think it's important for people buying stuff out there to know what the yare buying. Same with the basic designs. Its always better to be as informed as you can be about things you use or buy every day.

Thanks!
H2H
 
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