So I decided to create a mic preamp.
In fact, it's not entirely true, I'd like to design a PCB for it. I bet it's not that easy and I have to say that I have no clue about how well it will go, but still, I find it worth a try.
The beginning of a mic pre
Everything started when ForthMonkey on GroupDIY posted some schematics and sound files of a mic preamp. It's a JFET design with only 3 active components in the entire circuit: the 3 JFET.
The part count seemed low so I thought about doing a little PCB because I'm curious. I never ever designed any PCB, I have no clue about doing such a layout, but GroupDIY is a community insisting on experimenting things so let's go.
The schematics of the FETREDD
They seem pretty easy: a first JFET, a bunch of resistors and capacitors, two other JFET and an output. No funny part, everything seem quite clear.
If I were into marketing, I'd call it a "pure class A preamp delivering an outstanding sound with personnality" and I would sell it a fair amount of money!
As a bonus, ForthMonkey included an extra little circuit called TNT. This switchable circuit kind of boosts the lows and highs. A kind of loudness thing as far as I understand. If I find it boring, it's still a place where I can add any kind of processing.
Here the schematics from the main circuit
(thanks to ForthMonkey)
And the TNT circuit:
The power supply & format
You need a single rail +24V power supply and +48V for phantom power. Well I'm lazy, I do own a 51X rack which provide 24V and 48V so I'll do a 51X layout. This way, I don't need to create/borrow an extra power supply, a complete 19" rack and so on. It will also help me with layout decisions, because the format is already there and it's a good start.
What I need to add
It would be too easy to just copy the schematics and call it a day, the FETREDD mic preamp wouldn't be complete without a few things:
an input and output stage
that's where it gets a bit complicated. At the moments, the pre is (as far as I understand) high impedance unbalanced In and Out. So I can plug a high impedance instrument directly in like a guitar or a bass and it would work fine and I can get a nice sound on the output as long as I don't need to drive long cables. Right now it's a perfect preamp stage for Instruments, but I want it good for mics and I want it to be able to drive long cables. SO I need an input and output stage.
The easiest for me is to put 2 transformers in the circuit. They aren't cheap but they sound good, they make my life easy and might even add some character to the sound.
I could add a kind of little circuit to adapt impedance and balance the input and output. It would probably be a bit cheaper, but even though they are easy circuits, I still need to interface and adapt them to this circuit. And I can't do this at the moment, because I don't understand electronic deep enough to know exactly what to do.
So transformers.
I decided to use the Haufe ST 8456 as an input transformer because it's small and sounds at least as good as the lundahl equivalent and it's cheaper. Oh and I can source it here directly in Germany which is quite convenient in regard to porto and taxes.
I haven't decided yet what I will use as an output transformer. I will need to test the circuit with several. ForthMonkey said I would need a 10k:600 transformer, but if the circuit could drive 600Ω I could use a 600:600 one. Which would be quite convenient because there are more 600:600 transformers than 10k:600.
So I need to build this circuit, try it with different output transformers and select the best. Well, let's see how far I go.
Adding transformers to the circuit seems to be easy, just put one before and one after the circuit, connect the right pins to circuit and ground and there you are!
Oh and it's easy to test transformers as they basically don't really make the preamp different.
48V phantom supply
this power supply is necessary for condencer mics. It's not a difficult thing to do because it's basically a switch, 3 resistors and a capacitor, but it needs to be done.
While informing myself on preamp design/layout I stumbled across the FETboy. It's also a class A preamp (from
Scott Hamptone) and the schematics were kindly released in an audio magasine. And it shows a nice schematics on how to transform a basic gain stage (like the FETREDD) into a full blown mic preamp. Thank you Scott.
As I'll use an input transformer, I don't need to protect my circuit against phantom power with extra capacitors.
You can find more infos on the FETboy on
DIY recording equipment. A great DIY audio wiki by the way.
Input PAD
Well I shamelessly copied Scott Hamptone input pad. It's a -14dB pad placed on the secondary of the transformer. That means that the pad is between the transformer and the preamp. Which means that the transformer still get the full signal before it's attenuated and it will add
distorsion character to the pre. Scott writes that most modern good quality transformers can cope with this.
Instrument input
Do I need this one? It seems pretty easy to implement because it's just a jack between the transformer and the gain stage input, but I still need to figure out how to do this. I need the instrument input to bypass the mic input when a jack is in, that seems pretty easy. But I also need that the jack plug let mic signal through when no instrument is inserted. Do all the jack plug do this? If so, why do a lot of preamp have an extra switch for this? Because the instrument input goes through the transformer? It's such an easy question that I barely dare asking it...
What else?
At the moment there isn't any kind of LED to show anything to the user. Do I need shiny LEDs? I find it good for 48V as you don't want to fry your expensive ribbon mic. I also like the idea of a signal LED. Something that shows if a signal is coming in (or coming out?), I find it easier to troubleshoot if you at least know if there is something coming in, even though I don't believe in metering for analog hardware in our digital world.
There isn't any kind of phase inverting switch. They are quite handy for stereo/multitrack recording. How and where can I implement this?
Check list
So in the end I would "just" need this:
the PCB
the electronic components
the input and output transformers
the frontpanel (made by my brother in law)
a metal L bracket to hold the PCB and frontpanel together (also made by my brother in law)
Time and so on...
A word about the g7: it's a tube mic designed by Jakob Erland, the guy at Gyraf Audio in Denmark. His contribution to the audio DIY community is invaluable, and beside offering some high end audio tools, Jakob has a nice DIY page. The mic is an original design based on the famous Neumann U47, AKG C12, with the Neumann U67 tube (EF86). That's quite a pedigree...
