LA-2A design with 48v phantom power.

[Zhyyra]

Hi,

I'm in the process of 3d and 2d drawing (engineering level geometry technical drawings) a chassis for an LA-2A that I intend to build in the future.
The main use that the unit will be used for is vocals.
So, today while designing, I was thinking about how I always use a condenser mic and how it needs the 48v phantom power.
My idea is to plug the mic directly into the LA-2A and then from its output, take the signal to my mixer where I have the ability to further process the signal using a Behringer 3d Virtualizer, Fx2000, which is also connected to the mixer.

My question is, how difficult would it be to redesign an XLR input port on a LA-2A to power the mic with 48v phantom power perhaps utilizing a push-button switch added to the front panel. Effectively I would like to end up with a unit that acts as a tube mic pre-amp, that is able to compress too.

I imagine that it might be quite simple but, I'm no electronics buff, (I'm a machinist) and would really appreciate some advise on this if possible.

Zhy

 

[Walter66]

Thats a compressor and limiter, not a mic pre. You may need a pre-pre.

 

[TwentyTrees]

There are a few posts around the forum on how to set up an LA-2A for use as a standalone mic pre. Stock, it's got more than enough gain for use with a hot condenser mic on vocals, and I'm pretty sure that at least one major manufacturer's preamp + compressor SKU is based on exactly this.

It's definitely possible to install a +48V supply here. I'd suggest looking into how Volker (Silent:Arts) does it with his MIO PCB. You'd need to actually generate the +48V somehow of course, for example an additional toroidal transformer and linear supply, or an SMPS. The LA2A is quite susceptible to induced noise, so you'll want to spend some time looking into a quiet layout.

One further thing to look into is the series resistor before the 68k series resistor after the input transformer - some suggest bypassing this in mic mode to reduce the contribution of the Johnson noise. You may also wish to consider an entirely separate XLR socket for the mic input, so there's no danger of accidentally feeding phantom power out of the line input into whatever's before it in your chain.

 

[Zhyyra]

Thanks a lot for your advice TT, that gives me good starting direction.

I was indeed thinking about where the +48V would come from and got the idea to design my own input transformer with an added coil for the +48V. I would make this transformer a toroidal one. I have the intention of winding my own transformers for input and output signals too anyway. I want to make my own clones of the old UTC ones.
I do like the idea of a totally separate XLR socket for the mic.

Much to think about and look into, but hey, it can be done, thanks for answering my question.
Will definitely study the MIO-PCB.
Zhy

 

[TwentyTrees]

You're very welcome, it's definitely a project worth looking into in my view. To clarify my earlier post, your +48V supply would be entirely separate from your input and output audio transformers - it could certainly be derived from a separate tap on your power transformer (often a toroid) if looking at a linear supply.

Here's a (very) relevant thread:

https://groupdiy.com/threads/la2a-as-mic-pre.35541/

 

[Zhyyra]

Thanks for the link TT, it made a very interesting read, and I'm sure to be referring back to it and the links within it. Surprisingly, the links still work after so many years.

Yes, I did get that you meant an added transformer (for the +48V supply) in your first post. It is however good to know that you agree that it can be tapped from a custom single power transformer.

Zhy

 

[HoneyBadger Audio]

An idea perhaps worth trying before you commit to modifying a 2A would be to use an external phantom power injector and see if you like the results of using the 2A as a pre. Objectively, there's nothing that stands out that would prevent the 2A from being a mic pre. The only issue you might run into is the gain reduction will probably never hit a level that the sidechain circuit will be able to come into effect. You may need a cloud booster or some active booster to get a strong enough signal to achieve any gain reduction.

 

[thomasdf]

Have a look at meanwell SMPS, they are very powerful and most important : compact and very quiet... That may be a good solution for your 48v supply. If you screw it outside the enclosure you can feed a clean 48VDC to your XLR input in the enclosure and it shouldn't mess (EMI) too much with the rest of the circuit.

 

[Zhyyra]

I like that idea HoneyBadger! No harm in trying that first! Thanks .
I will indeed look at some Mean Well power supplies thomasdf! If I like it, and as you say, it does not create a noticeable EMI I may just keep it like that .

Muchos gracias,
Zhy

 

[ZHOUJUN]

Add 48V to LA2A, you will consider more, such as inductive noise, the impedance matching of the microphone output signal and LA2A, and the signal output by the microphone is not enough to activate the threshold of LA2A, you can try to convert your preamplifier into a plug-in type, such as in the front The input and output are increased between the 1st and 2nd amplifying circuits of the preamplifier. The microphone signal will first pass through the first stage amplifying circuit of the preamplifier, then pass through LA2A, and then be connected from the second stage amplifying circuit. The input impedance can be very wide. Modifications can be a bit difficult, one of my tube preamps is designed this way to allow an EQ or compressor to be inserted between the 1st and 2nd stages of signal amplification.