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Author Topic: Need help - racking up some Langevin AM-16s  (Read 2818 times)

New Orleans Steve

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Need help - racking up some Langevin AM-16s
« on: November 28, 2004, 04:34:23 pm »

John, maybe you (or others here) can help.

I am trying to rack up some Langevin AM-16s.  I want to use them as mic preamps.

While I have only made cursory inquires, I have found no consensus, rather radically different opinions.

I have a schematic and PSU. In a test I was able to verify that all of the modules will pass signal at the same level.

For most of what I do the 45db of gain is really too much. That's due to modern high output mics and louder sources and closed micing than they would have ever imagined when this was a new unit.

For our purposes don't worry about the PSU or phantom.

A subject of contention involves the pad and output adjustment.

I have been told it can cleanly take any output that is basically mic level (even a kick drum) and properly add 45db. Therefore the pad should come after the pre; just to set the level to tape.

Others have told me to put the pad before the pre to give it a more realistic input on which to add 45db.

Still another recommended a pad Between 2 modules to make a single channel.

What's the real scoop?  Do you know of a link?

Thanks so much,  Steve

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Brian Roth

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Re: Need help - racking up some Langevin AM-16s
« Reply #1 on: November 29, 2004, 03:43:37 am »

I had to look up the AM-10 spec sheet to refresh my memory...

The gain is stated as 45 dB *fixed* and the maximum output (when strapped for "high power") is + 24 dBm.  Ignoring the slight tweaks possible depending upon input impedance strapping, that means that a mic level signal input of approx. -21 dBu will cause the preamp to clip.

For a close mic'ed amp,  or whatever, you will need to install a pad AT THE INPUT to the mic transformer if you don't want square waves at the output of the preamp.

Be SURE the pad properly "reflects back" a load of 1000 to 2000 Ohms to the mic while presenting 200-Ohms-ish into the primary of the input transformer.

As for levels...

Think of it this way.  Assume a REALLY hot signal from a condensor mic at 0 dBu (loud kick drum).  The preamp adds 45 dB of gain.  Thus, the output would need to be +45 dBu.  But, max output level from the AM-10 is +24 dBu, so you're overdriving it by 21 dB and making some nifty clipped square waves.

But, in these times, horribly overdriven signals are "the really cool sounds" vs. Ye Olde Days when only Marshall guitar amps were required to create similar sounds.  It seems that the entire recording chain needs to be spitting out square waves these days....sigh. Looking at the waveform on a scope, or in a DAW, it's like a line of toothpaste dropped onto the bathroom counter.

SO, "it all depends".  If you prefer the sound of seriously clipped signal waveforms, just let 'er rip.  If you don't want serious distortion from a high-level mic source, pad between the mic and preamp input.  This is audio-level-matching-101!  <g>

BTW, adding a pad AFTER the preamp is only useful if the recording device requires a lower input signal than +24...unless you need to clip the recorder!

Bri

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New Orleans Steve

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Re: Need help - racking up some Langevin AM-16s
« Reply #2 on: November 29, 2004, 10:52:43 am »

O.K. Brian Thanks, That seems correct.

I am thinking I will need a Pad on the IN and a Att. on The OUT.

Typacally the pad is a fixed value, say 20db. Should I go with that or can I chouse a variable pad?

It provides transformer taps, but I think 150ohms (or is it 600 ohms?) is what I will use with dynamic mics (57s, 421s, EV ndyns).

What would be a good part? I Don't want to go overboard, but I will spend to get something a step or 2 above cheap stuff.

It seems I have been messing around with these things forever - I ready to do some tracking with them!

Steve
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Timeline

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Re: Need help - racking up some Langevin AM-16s
« Reply #3 on: November 29, 2004, 03:10:27 pm »

How many do you have?  

I own a Boutique Audio rack of 8 and wouldn't part with them for anything.
I have compared them to V76 and Telefunken solid state preamp racks.

For Bass, kick, snare, guitars they have a tightness that's next to none.  A front end pad as mentioned previously is essential. I use a step rotary switch with three pad settings, 0, -18, -22, -27. these allow you to make the proper resistances to the  amp. I use 250 ohms shunt against the amp generally although I like them with no pads better if I can get away with it..

I also have 1108's & a pair of V72's to rack with Ian. Ian's been invaluable to me. http://www.boutiqueaudio.com.

Hope this helps.

Cheers

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New Orleans Steve

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Re: Need help - racking up some Langevin AM-16s
« Reply #4 on: November 29, 2004, 03:40:57 pm »

  Yes, I have been to that and some other sights that look they do some classy work. On this one I'm defiantly looking to go with the D.I.Y. approach. It's in the sprit of the deal surrounding the circumstances with which I got these modules. I got them totally on the brother-in-law plan from a friend / engineer (even took some time to pay him).

So, do you have a recommendation on the pad - that is a specific part or source?

What do you think about this?   http://www.rollmusic.com/projects/3125.shtml  
    down the page I'm thinking Input + Gain / control Boards  
  It's not cheap, but it will save install time, so...

THANKS, Steve
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Timeline

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Re: Need help - racking up some Langevin AM-16s
« Reply #5 on: November 29, 2004, 07:50:35 pm »

Steve,


Looks like you could save a few bucks there. But, beware of the 'ship monkeys'!

The only thing I would say is that your link provider looks a bit shy in the mounting department.  

I have REALLY had problems shipping lately, even in 2" foam shock cases,  when traveling.  

My first rack of AM-16's was bounced so badely they were all over the place
when I opened the cases.

Ian had to put in special bracing rail in all my racks because of the 'ship monkeys'.

Just a thought.

Good luck.

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Brian Roth

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Re: Need help - racking up some Langevin AM-16s
« Reply #6 on: November 30, 2004, 01:47:21 am »

You *might* need a pad (AKA attenuator) at the output of the preamp if the device being driven can't handle the max +24 dBu output of the Langevin.  There are no hard and fast answers since I have no idea of your setup.

As for the mic pad, having at least a 20 dB attenuator available for hot mic signals is perhaps the one-stop-shop choice.

My long-time friend Rick Chinn has written a paper on this subject:

http://www.uneeda-audio.com/pads/

Bri

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driskel

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Re: Need help - racking up some Langevin AM-16s
« Reply #7 on: November 30, 2004, 01:59:45 pm »

Talk to Justin at Roll Music

http://www.rollmusic.com

He racked up 8 of my Langevin AM16's, he might also have pictures available.

~driskel
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moogus

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Re: Need help - racking up some Langevin AM-16s
« Reply #8 on: December 01, 2004, 09:12:58 am »

http://www.jlmaudio.com/AM16e.jpg

When we racked these AM-16s we put a 45db variable pad up front and made the hi gain feature available on a front panel switch.  This adds 5db of gain.  Appart from no having enough gain for some applications (par for this particular course), I think the owner has been happy with them in this configuration.

Hope that aids your decision.



M
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New Orleans Steve

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Re: Need help - racking up some Langevin AM-16s
« Reply #9 on: December 04, 2004, 11:48:35 am »

O.K.,
  Several 'tips' as to who can Professionally do this job. Some really good options, I am sure. I'm flattered that you think of me as some jet set engineer. I might carry some of them in my mini van across town but that's it. I'm really low mileage.
 Again, I have used them with pigtails and good results. Now, I will put them in a rack with a pad for drum applications.

 Having said that, can anyone lend some advice regarding the input Pad??
The application is mic level, Balanced.

  I'm looking for a spec. Like, "10K audio taper"..... Is that it??

If I could get this info, and a link or source for a hand full of them, That would be great.

Steve

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Brian Roth

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Re: Need help - racking up some Langevin AM-16s
« Reply #10 on: December 05, 2004, 01:28:36 am »

No, a simple pot is NOT what you want.  I point you once again to Rick Chinn's excellent tutorial on pads:

http://www.uneeda-audio.com/pads/

Bri



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