La2a as Mic Pre

[mac]

Hello all,

I have read that you can use the La2a as a mic pre for dynamics mics due to the gain these units have. What I was wondering - is there anyone out there who have taken this idea to the next level?? I am thinking to boost the front end and include a DI, phase, phantom, pad etc..

This way I could use these as stand alone mic pres - comp - tube stages for live recording, as well as use them in my chain in the normal La2a way back in the studio.

As I continue in my information gathering phase for next years builds (pair of hopefully point to point La2a's), I have seen a few threads head in this direction - but havent found any examples directly (hence the new thread)...

Anyone out there using their La's like this? Installed additional front end tricks??

Cheers,
Mac.

 

[mrclunk]

We use ours as pre's occasionally.
I think they have about 40dB of gain. Haven't tried them with a dynamic mic but they work well with valve mics with a healthy output level.
Last time i used them as a pre was on sax and trombone and it sounded great.
Since that i've actually reduced the gain a touch to make them manageable on hot line source.
If u do intend use them as pre's you'll need to be really anal with you grounding to avoid hum.

I am thinking to boost the front end and include a DI, phase, phantom, pad etc..

I would just build a separate pre...
Or maybe one of NYD's one bottle's infront would do the job nicely.

 

[emrr]

JLM kits, of course. 

 

[mac]

If u do intend use them as pre's you'll need to be really anal with you grounding to avoid hum.

I assume this is because normally you have them running down low on line level sources - but crank them up the gain to use them as a mic pre - so the noise floor is a lot more obvious. Is this what you mean?

I would just build a separate pre

I already have a bunch of other pre's to choose from - I just like the idea of building flexibility into the design, to maximise usability..

JLM kits, of course.

yep...Joe is just down the road for me... ;D

Mac.

 

[drpat]

It works great as a preamp. Why complicate things? Just build the stock LA2A circuit.

 

[Kingston]

If u do intend use them as pre's you'll need to be really anal with you grounding to avoid hum.

I assume this is because normally you have them running down low on line level sources - but crank them up the gain to use them as a mic pre - so the noise floor is a lot more obvious. Is this what you mean?

That's it really. The LA-2A noise floor isn't great even if you tweak it to perfection. We're still looking at about -80dBu. Compare that to anything modern with noise floor below -100.

But it's rarely a problem. There are not so many quiet sources to record in the studio.

 

[mac]

Thanks for the insights guys - much appreciated...

mac

 

[CJ]

yo,  word down, whats shakin?

you could play with the feedback, maybe bring the feedback pot out to the front panel as a gain control, you won't have any tone control on this thing, unless you want to add a simple set of rc filters like the v76, could be easily done,

you could drive the utc input, a 10 , with a utc input, a 10.

have them switchable, core loss could be a problem, maybe an ouncer> a-10 would work.

since they would be right next to each other, hum would not be an issue,

i' jus sayin... 8)



what is he sayin?  i pity the fool who tries that cockaphony notion,

 

[Silvas]

I think i´m adding a +48 supply to my D-LA2A !


At least, i´m going to test adding phantom power to the input trafos and see how it works with condensers...one more "pre" doesn´t hurt.

 

[CJ]

there might be a way to make a exciter out of that, filter off the high end,

then, instead of compress on the peaks, amplify the peaks some how,

maybe just a matter of switching a wire or two?

how would i know , i ain't no machinist,

 

[mac]

cool CJ.... now you're giving me more to think about....and I was gettin accused of complicating things!!!  ;D

I probably need to organise what I want to use for input trafo first then design back from there...... I will keep hunting for input trafo's as I think I would have to sell my children to purchase UTC's here in OZ....

I have dug up some interesting Trimax stuff - but cant find any info on the net yet.

Silvas - it sounds quite feasible to bung a "go between" or similar up front http://www.jlmaudio.com/JLMDI.htm, and at least gives you the option of using it as a pre as well.

all good,
Mac.

 

[Jonte Knif]

Hi guys,

you know, Ma$$ey mic pre is essentially fairly straight copy from the circuit under discussion. V@x B@x still more because it has compression element just after input trannie, like in LA-2A.. The biggest difference is of course the adjustable FB which will not be too difficult to make.

The first think to do is make a bypass switch for the 68k series resistor to lower input related noise. After that the circuit is capable of extremely low noise if well built. I see no problems what so ever. If you keep the gain pot at maximum the noise is dominated by input trafo resistance and 12AX7 noise, which is low enough. If you turn Gain to about -6dB the noise is pretty high though, the pot will dominate the noise figure.

That's it really. The LA-2A noise floor isn't great even if you tweak it to perfection. We're still looking at about -80dBu. Compare that to anything modern with noise floor below -100.

But it's rarely a problem. There are not so many quiet sources to record in the studio.

What do you mean? Many modern tube pre amps have higher noise than can be easily achieved and I don't understand why. And certainly more than -100dBu. -90dBu A-weighted is completely adequate for _anything_ if the noise is resistor or tube related and not some nasty buzz. What we are interested here is EIN. Equivalent input noise.
The original input trannie is 1:8 if I remember. That 68k resistor bypassed and signal taken straight to 12AX7 grid the EIN will be about -130dBu with a good tube. Even -134dBu should be possible if everything is nicely implemented. So, with just one easy mod the circuit will be so low in noise, that the only problem is low gain.

In addition to these mods the pre I mentioned has output trannie of 1:1 turns ratio, which is nice if you drive moderns loads but a terrible compromise when driving low loads. Of course it gives more gain.

So this circuit routinely used by M (also in Mass Pass etc etc) and advertised as something of their own design is just LA-2A with FB adjustment. Which is exactly what you could do to make your LA-2A pretty versatile mic pre. I find the idea fairly sound. But DO make the bypass switch for that 68k resistor to capture even low sources with low noise.

Best,

Jonte

 

[mac]

Hey Jonte,
thoughts on de-essing circuitry?? or am i just getting carried away now?

Also, is it poor design to explore switchable output trafo's as part of the gain / noise issue, or am I just going to make something more complex than a nuclear power station...

I have a habit of trying to cram as many lollies into one jar as possible 

Mac..

 

[bernbrue]

Hi,
what an interesting discussion! I thought of using the LA2a as mic pre before. It has tons of gain and sounds fantastic. My initial idea was a combination/channel strip with a tube mic pre (orange 86)(obviously not needed), pultec style passive EQ and the LA2a. Volker (silentart) has developed a nice front end pcb with Phantom Power, Pad, Phase, additional PSUs and selectable passive/transformer based DI. The "68k" bypass/adjustable feedback" mod sounds great as well. I would like to hear more details about this mod. Here are some useful information on how they altered/implemented the LA2a design in the LA610.

http://www.uaudio.com/webzine/2004/november/text/content2.html

regards
Bernd

 

[mac]

Good article Bernd! I love block diagrams, I am more right brained after all!!

Mac.