Easy IC Opamp based Mic pro

Hey everybody!

Since this is the first post I'm making, and I hope for a longer relationship in the forum, I'll start by introducing myself, as is common in many other forums.

I'm Philipp, and I started studying electrical engineering in Berlin, Germany. I used to do work as a studio engineer and session bass player. Now the only work I do besides studying is producing and playing for a small number of bands. Studying is hell of a lot of work, as some of you may know

Now when I started working in the studio, I got acquainted with a whole lot of outboard analog gear, consoles, GDR mics and more. Since I'm a curious person, I started building some simple electronics, like an L-Tap power attenuator and some effects pedals, as well as some basic mods and repairs to different amps.
Now, I would like to dive a little deeper into the material and build myself a mic pre or several mic pres.
I'm really looking for a build thats very easy to understand, so I learn how the different ingredients of a mic pre are implemented. I've seen the Green Preamp and while it seems that I can't get hold of a supplier for the PCB, it seems simple enough for me to try etching the PCB myself. We have a lab at the university where we can do the etching and I love to try it out.
Also, the Green Preamp seems like a good basis to try out soldering SMD discrete opamps later in time.
I'd also like to put 8 or so vertically in a rack, so I have a basis of preamps for some musical ventures.  (possibly with different opamps)

Now the thing is, do you think that the Green Preamp would be a good choice for that, or do you know an even simpler build than that? Also, do you have any good referral pages that would help me in general, for example for etching? I've already found the gathered page for the green pre, but they are rather specific, as far as I've seen.

Hi Philipp,

welcome to groupdiy.com!

Here are my suggestions:

- a simple and cheap transformerless ADA797 based design, for instance here as a kit or PCB only http://diy-tubes.com/index.php?route=product/product&path=59&product_id=134

- you will find some really good mic preamp ideas here http://www.nanovolt.ch/resources/microphone_preamplifiers/

- check for DIY Api 312 style mic pres

cheers

Hey rock, thanks for your reply!

I've checked what you suggested, and the ADA797 build looks very convincing! From looking at the schematics of the PSU that they also sell with that kit, you can power 2 pres with one PSU. Since I don't see them selling a transformer with that PSU kit, which is why I suspect that I need to get that myself and scale it accordingly to my plans with putting 8 pres into one rack.

According to the schematics, the PSU requires 15-34V *2 for running. Is my assumption correct that I would also need a transformer to bring the voltage to the right level? Also, if I  would want to run 8 pres, using 4 power supplies, would I even need to use a transformer, since 240V/8=30V (240V is the German outlet voltage). 30V falls into the range that the PSU needs per channel. I would run the PSUs in series.

Now I'm pretty sure that my simplified attempt to understand the matter is highly flawed. Could you point me into a direction to understand the matter better?

Best,
Philipp

Uhhh...You need DC voltage rails 
Really need to review some basic ideas I think...

Something based on the That or INA mic preamp chips might be even easier to build. Look for 5$ preamp as a a start. And you really need to develop some basic understanding of power supplies. The basic concept is to use a transformer to bring down the mains voltage, have a rectifier to turn ac into dc, put a capacitor after that to smooth out the gaps between the peaks and use a regulator IC to get rid of the remaining ripple (That is the very basic short version). I'm sure you'll find plenty of information about that on the net.
I remember that there was a version of the 5$ preamp running on two 9V batteries, this might be a good start without too much possible damage to some innocent silicon or even your health ;-)

Michael (also from Berlin)

> use a transformer to bring down the mains voltage

*AND* provide galvanic isolation between utility mains and the artist!

The idea of stacking audio gear *directly* on the line is so absurd that it was probably not mentioned in EE classes. In old home radios there was a class of transformerless radios, but there were no input output connections, no way to touch the electronic signal. Only radio waves in and sound waves out.

PRR said:

> use a transformer to bring down the mains voltage

*AND* provide galvanic isolation between utility mains and the artist!

The idea of stacking audio gear *directly* on the line is so absurd that it was probably not mentioned in EE classes. In old home radios there was a class of transformerless radios, but there were no input output connections, no way to touch the electronic signal. Only radio waves in and sound waves out.

Click to expand...

+1

Hey Philipp,

you need more information about that topic. I would start, as mentioned before, with a small project.
A good idea is to avoid working with 230Volts main voltages in the beginning and use batteries or wall-warts (Steckernetzteil).  You can buy a 9-24VAC power plug at Segor here in Berlin.  With this power plug you can build up your needed single- or dual voltage psu easily and safely.

Here is the link:

http://www.segor.de/#Q=NT9-24VAC%252F1A&M=1

Cheers

You could try thr $5 mic pre project as a starter:

http://www.audiomasterclass.com/newsletter/the-famous-5-preamp-everything-you-need-to-know

Cheers

Ian

I'm stunned by all the helpful input that you guys are willing to take the time to write!

Now I realize the absurdity of my suggestion to not use a transformer to run the pres. I have actually realized that before, but I forgot that I do not only need a voltage reduction from the transformer, but also the rectifier to turn the AC into DC. I hope you don't understand my brainfart as a reluctance to learn the stuff

Now, the 5$ pre also seems like a good idea to try out! It's exactly what I'm looking for. Also the suggestion to just use one of those Segor  PSUs is great. I'll try out building one of those for now and see where the journey takes me!

Hi Philipp
Welcome aboard. Feel free to check out our german forum with a lot of Berlin based members here: http://www.energyrecording.de