Microphone Preamp Circuit

Mr google tells me that there are thousands of ways to do this.

I would suggest you only do a project like this for fun...If you want it for serious professional work I'd say buy one.

The high input impedance and correct equalisation make it a tricky diy project unless you can get a pcb made up..doing it on stripboard or as a 'birds nest' will give dubious results. I started out with diy hi-fi, my home build amp worked a treat but it also picked up the local taxi radios and would 'motorboat' until I tinkered with circuit. (Mind this was all discrete components about 35 years ago!)

Great for learning, but no so good for professional work.

If it's just for fun there should be loads of circuits on the web..try op-amp manufacturers websites NS are always good...

(Hope that lot doesn't sound patronising... I don't know what facilities you have and how good you are with this stuff)

...anyhow...have fun

Del

I tried the simple circuits and they worked pretty well...

Now I want to try something bigger. I have access to some really good audio op-amps and am willing to put in the work required.

The cost of really good preamps available in the market is way over my budget. I actually think it is easier to just make one. The PCB can be made as I work in R&D and thus have enough resources.

That s good spirit, keep it up.

What you can do is log on to www.datasheetcatalog.com and search for microphone preamps, you will get some good ICs and you can download the datasheets for free. Go ahead and do your project with that high spirit of your.

What type of microphone (levels, impedance, frequency response), and what level of common mode rejection do you need? Is this a high-interference (RF) environment?

BTW, it used to be that the best performance was achieved with the the microphone amplifier in the same housing as the transducer. As the transducers themselves were not intrinsically balanced, neither was the input to the amplifier - but its output was balanced to make the most of the balanced connection. In many caces, the head amplifier and the line amplifier were designed as a pair. The "ground" for the head amplifier would provide the reference for the balanced pair.

how about a nice tube circuit

Contradiction in terms, I think

There is an enormous amount available on the web, but one of the best free resources is Douglas Self's website which provides a good primer on audio circuitry and objective comparisons of various op-amps. I would recommend his book "Self on Audio" if you need more detail.

He avoids the Hi-Fi subjectivism that is common in many sites with a thoroughly researched approach based on many years of mixer and amplifier design.

Last year I built a similar type preamp. Mine was also for an electret condenser mic, and after a couple of "practice runs" I came to the same conclusions as Physicist stated above. Go ahead and use some sort of non balanced head amp as close to the mic as you can make it. phantom power it from a secondary unity-gain preamp with balanced output. The circuit I finally used would not work for you because mine was frequency tailored for a high noise environment. However, the web has many two transistor (either low noise bipolar or FET) circuits that would work. What I would suggest is that once you've got your circuit, use EAGLE CAD software to work it onto a PC board. The program has a good auto-router and will gin up instructions your board shop can use, or artwork for you to make your own boards. EAGLE is a free download at www.cadsoftusa.com/freeware.htm. I used the board artwork from EAGLE, laser printed it on ink jet type photo paper, transfered it to the board with a clothes iron, etched the board and the whole thing worked just fine.

Hi Zam',

Nice reply, I hope you don't mind if I chip in and say an auto router is overkill on a small pcb.

Del

In my experience using auto routers for audio design is asking for trouble especially with high gain circuitry such as mic pre-amps (even more so with digitally controlled pre-amps!). Poor layout leads to noise, cross-talk, distortion and sometimes instability. PCB and wiring layout is critical to the quality of an audio design.

Most preamp designs use unbalanced circuitry internally even if the external I/O is balanced. This means there are return currents flowing in the 0V or common rail. The common has a finite impedance so the return current will create a potential difference along its length. There are other return currents circulating including power supply return currents which are usually harmonics of the original signal, these produce larger potential differences which can be superimposed on the original signal. These are all forms of common mode cross-talk.

Capacitive coupling is also an issue but is generally better understood. Inductive crosstalk between traces or external magnetic fields can also be an issue.

Careful hand layout is the only way to a high quality mic preamp.

Well said... over the years I've reverted to manual routing...even on my last two-processor + half a dozen other chips and a 16 char display, double sided board. (I still had to tinker with the gnd plane/tracks on the prototypes to keep the noise out [mice I think getting in somewhere] )

Auto routers (unless you have a real expensive one) add in loads of unecessary vias and can spiral themselves into a mess so you end up tidying up after them anyway.

(Appologies if I'm out of date on this...it was a while ago I bothered with an auto router)

Del

balance and unbalance can be converted freely witha transformer.

tube amplifier is rare using now, cas its very waste power, although its good.

what condenser mic do youi use? does it has resistance changer inner?

in fact you can use an oscillator to modulate it to get a MF signal and then add rf amplifier. its good. and simple.

The project is actually a temporary solution for a studio in the making.

I am using a Shure WL 93 Lavalier microphone

(so you can get the specs from... http://www.shure.com/ProAudio/Products/WirelessMicrophones/us_pro_WL93_content

I am starting work on a few existing designs that I came up with after all your help.

Meanwhile if anyone cud send me any designs that you would have tried and the problems that were faced or how to resolve them even.....

Check out forssell technology website http://www.forsselltech.com/main.shtml Some great circuits using his own discrete jfet modules. I have built several and these are top notch preamps that will stand up to any. I used vero strip prototyping boards that make circuit design simple and low cost. OK if you arent doing many of them.

Check out Forssell Technologies. Schematics there. I have built several based on his design that work very well. It is based on discrete Jfet Modules that are made by several sources including John Hardy, API and the above reference. It has a balanced transformer input and DC servo output stage. If you are interested I can provide all you need for putting one together. Go here for pic of what I built at www.kbaudio.imajon.com Have fun and good luck