Diy mic preamp

[JebemMajke]

Lately I'm into home recording, and I've acquired AKG d222 mic. And the sound is good. But the volume is too low when I record something ( sony sound forge audio studio 10 ). So i'm used AMZ mosfet booster as preamp. And the sound is nice, a bit compressed, but it's ok. I get much less noise with it. But i was wondering if there is a good diy mic preamp?

[brett]

Hi.
For a mic you'll want a balanced, low impedance pre-amp.
A JFET or MOSFET booster is high impedance and quite high noise relative to other options.
Maybe search (here or whole web) for 'balanced mic pre-amp'
cheers

[Seljer]

I was in a similiar situation and was pondering about building a mic preamp. Then I started contemplating adding phantom power and a second channel and panning and a bunch of other stuff.....in the end I decided the easiest solution was to buy a $60 Wharfedale mixer (or a Behringer or whatever your local store has in stock).

[wavley]

The API 312 is an easy and great sounding build, the transformers will set you back a some but it's totally worth it.

Just google DIY API 312, the groupdiy forum has more than you will ever need to build one.

[JebemMajke]

Hi.
For a mic you'll want a balanced, low impedance pre-amp.
A JFET or MOSFET booster is high impedance and quite high noise relative to other options.
Maybe search (here or whole web) for 'balanced mic pre-amp'
cheers

I have zero noise with AMZ mosfet booster. And the sound is pretty close to the original. Apart from that compression.

Thanx wavley, i will look into it.

[mhond]

http://sound.westhost.com/project66.htm

[therecordingart]

The API 312 is an easy and great sounding build, the transformers will set you back a some but it's totally worth it.

Just google DIY API 312, the groupdiy forum has more than you will ever need to build one.

I second this. Really low parts count, but three of the components can be pricey. To reduce costs you can get the transformers and DOA from classicapi.com . Jeff is a good guy.

[brett]

Hi

http://sound.westhost.com/project66.htm

+1
This design, based on a differential compound pair will do a great job. And only cost a few $$.
As others have mentioned, transformer-based designs can be good, but to get really good performance requires expensive transformers.

Concerning noise and impedance: mics are deliberately low impedance so that static and any induced noise (e.g. by flickering flouro lights) is 'conducted away'. The higher the impedance, the more the cable acts like an antennna, and picks up high-voltage, low current noise. At a Megohm or more, you can sometimes hear it when someone stands on the cable! Or hear someone welding in workshop 100 metres away.
Of course, if you live in an electrically 'quiet' neighbourhood none of this matters.
cheers

[John Lyons]

By chance does anyone have the values for the power supply?
http://sound.westhost.com/project05b.htm
I'd buy the board to get the schematic but it's fairly steep as far as boards go.

[brett]

Hi
You don't need the Aux section.
The diodes are all just 1N400x, such as 1N4004.
C1 and C2 are the big filter caps, but due to the low current draw can be about 220uF, but I'd use 1000uF because it's almost no price difference.
R1 and R2 are small, forming a 1-pole filter with C3 and C4. Looks like he used 10 ohms for R1 and R2. C3 and C4 are large - like C1 and C2, at about 1000uF. (probably not necessary to be that big)
C5,6,7,8 are all small metal film caps (for low ESR). 100nF is usual.
C9 and C10 are tiny according to his board (22uF?), but I would have made them bigger (minimum of 100uF). Of course, he knows a lot more about these things than me.

Not sure why he's used the LM317/337 regulators and those adjustment resistors. An LM7815 and an LM7915 would save all the fuss. That gets rid of R3, R4, R5 and R6.

I'm no expert, but I'd reckon that's about right.
cheers   

[ggedamed]

From the Project 66 page:

The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the regulator ICs. Some 7815 ICs could be sold as noise generators, the adjustable voltage ones (LM317, LM337) are very much quieter.

[boogietone]

Building a mic pre or two, either from a kit or otherwise, has been on my list for a while now. Cost and time have been the issue so far.

Currently, I use ART Tube MPs. They work nicely and are quite cheap. Plenty of gain. Plus, it's tube. GC has them for ~30 bucks. The only problem I have with them is some 60 cycle and harmonic hum. Though it is not horrible and can be easily eq'd out as needed. I use an in-the-box linear phase eq with 3 relatively high Q notches. I have not tried yet, but think that this should be fixed with improved power filtering. There is also the "Black Lion" mod floating around on the intertubes somewhere that provides some insight.

Plus, of course, it's tube. 

[John Lyons]

Thanks Brett
I figured the caps are pretty standard values but the adjustment resistances I'd have to figure out.
Perhaps textbook/datasheet would have it.

[davent]

Hi John,
Like Brett said R1 and 2 from the photo look to be 10Ω 1w resistors. The parallel resistors on the adjust pins would be there to fine tune each regulator, with some odd resistor value, to exactly ±15v out, each regulator will vary somewhat. I built a ±15v using a series resistor/trimmer on the adjust pin and it works fine. The values  around the regulators i used, this was from an article in AudioXpress, were 1k for R3 &5 with ~11k for the adjust pins.

Formula and calculator for resistor values here. http://www.whatcircuits.com/lm317-calculator-v2/

dave

[John Lyons]

Thanks Dave!