Introduction: Build the $5 Microphone Preamp

Picture of Build the $5 Microphone Preamp

A little while back(2 years) the guys at record-producer decided to do another equipment test; this time with mic pre-amps. They chose three different ones ranging from $5 to over $1500. They recorded samples and allowed people to have a listen. Here is the link:
http://www.record-producer.com/learn.cfm?a=3062

Today I will be writing an Instructable on how to build the $5 pre-amp.

Mine cost a bit more then $5 but not that much since the majority of it was samples :D

Step 1: Prepare the Encloure

Picture of Prepare the Encloure

This time, I was able to borrow a drill for long enough so I could take picture of the project. Since the XLR jacks were not perfectly circular, i found i used my dremel a carve out most of it after a hole had been made.

Since the enclosure is made of plastic, I was able to use my wood drill bits. These bits are sharper and cut faster through the plastic.

Step 2: Planning

Picture of Planning

Planning is an important part of any project one builds. As for what I did was:
1. I found all the technical data for the semi-conductors and figured out what I could use for this design. I found three chips from Texas instruments that would fit the application nicely. These are the OPA137, INA217 and the TLE 2426.
2. Using the data sheet for each figure out how they will work together and make a schematic for the project. In my case the schematic was already made from TI, so all I needed to do was fit the third chip in.
3. Take the schematic and convert it to something that allows you to build.
4. Obtain the materials that you will need for this project.
5. Start building, double check everything as you build the project. This allows you to fix errors as they happen rather then unsolding connections after.

So there are still a number of thing that i’m not sure what I should so with.
The first is that I used two electrolytic caps for the input, what other types of polar caps can be used?
Secondly, for off board wiring, I connected pin 2 and 3 to positive and negative inputs, however for the output, I only have one out. Does this mean I no longer have a balanced signal and pin 3 and pin 1 are both connected to the ground?

Step 3: Off Board Wiring

Picture of Off Board Wiring

For this step what has happened is the three chips are mounted to the enclosure and wires are connected inside. I would draw you a plan but everyone is going to be different.

If you are not using shielded wire, it is good to twist wires together; this protects from electromagnetic interference by cancelling out.

I also stacked the two boards on top of each other for a neater appearance and so that one is not randomly floating. It is the second picture.

In the end I didn't use the 4 diodes for clipping protection as it was just to much of a hassle to connect it all up.

Step 4: Power Supply

The original Mic pre-amp used all batteries for both the chip power and for the phantom power. I figured that if I could get the power clean and stable enough from a wall jack that is what I will do.
The first thing is to get clean AC power from the wall. What I use is the Monster HT800, which is just a line filter and surge protection. From there I use an old laptop power supply to give me a DC current. The last step since there is still some rippling of the DC current I built a voltage regulator from the LM317. This is an adjustable voltage regulator which allows me to get the exact voltage I need.
There are no pictures for this step as I will be writing another instructable on how to build the voltage regulator.


UPDATE


I decided to change the power supply a little bit but for the most part, it's the same. I am just using a laptop charger which just connects to the TI's rail spliter(TLE2426). i'm thinking in the future of actually using the LM317 but for now i just have the rail spilter. the schematic for the TLE2426 is found on http://tangentsoft.net/elec/vgrounds.html
it's about half way down the page.

Comments

OttoR4 (author)2016-07-10

Hey there. Did you wire the XLR-output as you mentioned as such:

INA217 pin-6 to XLR-3, then XLR-1 and XLR-2 to GND? Does this lower sound quality or could the preamp be rebuilt with an XLR-output in mind?

allegfede (author)2015-09-28

could you please update the instructable with more clear (computer generated?) schematic?

it's very hard to follow the hand writed one :-(

thanks :D

carlos.quintas.1232 (author)2015-03-09

Problems i find so far, i cant find the 47uF 60V capasitors i dont understande how to conect the xlr output, and i like to implement some volume control on the preamp how do i do that? controling the voltage?

Kenpaktuō. (author)2015-01-18

It seems that you are already speaking from the perspective of having a whole garage worth thousands of USD of spare electronic parts available to reduce the price to mere additional 5 USD for the one or two parts you do not have of over a dozen required. I tried to source only those 3 ICs which are about 25 USD in my country (including shipping). Then I would have to buy single XLR plugs and what not and end up with maybe 50 USD for the unit alone. 10x more expensive than "advertised".

I am a bit of a scrap part collector myself, so I have a few old boards and PCs and PSUs in storage and because of that also a notebook power supply. But this is already unusual. The average person will not even have that, raising cost to over 75 USD. I wonder how well the circuit performs in light of that, not even including the disproportionate amount of free manhours you need to spent on building it as well, or even the much increased risk of human or mechanical failure.

Gh0sTly (author)2014-04-16

I have to ask, is it possible to replace the jacks with say, 1/8th or 1/2 jacks for those kinds of mics? and would I just double this circuit for a left/right channel setup? I am using 4 mics into a stereo level mixer, then I need a preamp to feed that signal into my recording computer. This would be fantastic if I could.
Thanks for the great writeup!

derkoettner (author)2009-01-18

Hello, i saw your project. I would like to build my own one. But the probleme is i don`t know how to build a balanced output stage for it. Do you know how to do that? Thanks

jonboytang (author)derkoettner2009-06-28

pin 5 is negative (ground) while pin 6 is positive. ground is just connected to the plug casing.

sir_phobos (author)jonboytang2014-03-06

Hey, I'm considering building this preamp and want to make sure I understand your answer. What did you connect pin 5 of the INA217 to? What is connected to pin 3 on the XLR output connector?

tqwerty (author)2013-11-21

Would it be possible for you to write up a [very clear] tutorial on how to do this?
I really want to build it but I don't have the circuitry and electronics knowledge to figure it out myself.

Thanks :)

Lacrossanator1 (author)2013-01-07

could you please tell me what the gain is on this amplifier?

zerocool924 (author)2011-11-15

Do you have access to the original schematic? The website took it down and it cant be found.

jonboytang (author)zerocool9242011-11-16

Should still be on the website: http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sbos247b&fileType=pdf
It's on page 7

zerocool924 (author)jonboytang2011-11-22

ahhh i got it. thanks!

pigeonpants (author)2011-02-13

I was wondering, any chance you could just add a quick parts list?

tigerdog330 (author)2010-04-07

 what chip is that that you use for the power supply?

jonboytang (author)tigerdog3302010-04-08

The documentation for the chip is here: focus.ti.com/lit/ds/symlink/ina217.pdf
This shows the two chips ina 217 and the OPA137.
The last chip TLE2426 is the ground splitter

PuddleOfMudd25 (author)2010-02-21

I am trying to make a microphone amplifier, and I'm short on cash and parts. I don't think I'll find all of these parts in my town anyways. I do however have two broken (not really broken, just old) computer amps lying around. Could it be possible to use one as a preamp (scavenging parts from it) and the second as the main amp? What will need to be done?

Kyosho (author)2009-06-27

What voltage was your power supply? Also, what exact resistors did you use to between pins 1 and 8 of the the INA217? I'm building a really simplified version of this amp and the datasheet has so many options that I'm totally confused. So, your answers to those questions would really be a help.

jonboytang (author)Kyosho2009-06-28

i just used a laptop power supply which was 18 volts it then gave me +9 and -9 volts by using the TLE2426. What you can actually do is cut out the four diodes, cut out the phantom power resistor (47k) and the cap(47uF), connect pin 5 to ground and just have pin 6 as out and cut out the 0.1 diodes.

Kyosho (author)Kyosho2009-06-27

I should mention that the simplified version I'm making leaves out pretty much everything else.

Allemd15 (author)2009-05-08

does this have phantom power

djfrosty214 (author)2009-03-18

hello I am a noob to all this stuff but can you but a list to all this stuff since i want to try to build it since i messed up my pre amp

jeandeau (author)2008-12-22

Have you posted the power supply for the preamp? I searched the instructables site and didn't find it. thanks

OneHarp (author)2008-11-14

hey bro, I took a look at your schem, and if you are not using phantom power, then you do not need to use those large (polarized) electrolytic caps on your differential inputs. Large value caps like that are needed only to filter the power supply (and to act as power "resevoirs" to prevent sag), which, in a phantom powered arrangement is coming in on the same wire(s) as your signal. If you are not phantom powering it, you can (and probably should) use standard, non-polarised film or even ceramic capacitors on the inputs instead. These will probably sound a little better, but more importantly, you won;t have to worry about them ever leaking or blowing up if you accidentally cross wires or have a power spike. Any moderate value will do: 0.047 is fairly standard, 0.1 uf will let more bass through. I wouldn;t really go much higher than 1uf or too much lower than 0.022.

jonboytang (author)OneHarp2008-11-16

alright sweet thanks buddy, i wasn't really sure about the electrolytic caps in the first place because they can colour sounds. i was thinking for the next project i would build one for dynamic mics and this seems very useful. thanks again

gnargnar (author)2008-11-03

i really need a friend who can teach me to make this stuff, because i would love to do so. great project btw, i like the look of it even

jonboytang (author)gnargnar2008-11-04

thanks, funny thing, I was originally planning to use another case until I realized the XLR jacks wouldn't it, so I randomly found another. I learned almost everything from google as well as other people's projects. Just make sure that you double check more then one website, But if you have questions about stuff, I can try my best to give you answers.

About This Instructable

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