Skull

LA-2A impedance
« on: June 08, 2004, 09:48:45 PM »
Hello,

I would like to use my LA-2A clone in my recording studio and my guitar rack set-up. Meaning, I'd like to plug my guitar straight into it OR plug my ProTools Digi 001 module directly into it. What impedance am I really dealing with? Is it the same? Do I need a different i/o transformer set-up than DJ specifies? Is the Digi 001 "professional line level" ? I am a total idiot about this stuff. I want to sort it out BEFORE I order the transformers.  Any place where this is already covered?

MUCHO thanks..

Lee


CJ

LA-2A impedance
« Reply #1 on: June 08, 2004, 10:07:39 PM »
Welcome, LEE!

A guitar won't drive the LA2 very hard by itself.
And the impedance is a lot higher than 600 ohms, which is the input impedance of most LA2's.

You could go line out from your guitar amp if you have one.
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

LA-2A impedance
« Reply #2 on: June 08, 2004, 10:13:21 PM »
Lee,
This may be a load of bollocks, so take with a pinch.

The 'standard' input impedance when using an A10 or HA100X is selectable 250Ω or 600Ω. I 'think' the Jensen transformer mod is a 10KΩ input, but either way these are not ideal for a guitar signal which typicaly likes to see 1MΩ or more.

While the LA2A has a goodly amount of gain (50dB ish), it really requires a fairly hefty input signal to do it's thing, so ideally you'd need a decent DI with a solid output to get some serious compression or limiting going on in the box.

peter
http://www.lazpro.com
I reserve the right to not sell to idiots

Moby

LA-2A impedance
« Reply #3 on: June 08, 2004, 10:13:45 PM »
Try DI box. If its HA100 input version, it has close gain to make it.
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com

Moby

LA-2A impedance
« Reply #4 on: June 08, 2004, 10:15:18 PM »
Peter, you are 5sec faster :green:  :guinness:
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com

Skull

LA-2A impedance
« Reply #5 on: June 09, 2004, 01:20:30 AM »
Thank you all so much. I figured DI was the answer.

Here's my signal chain: guitar > Matchless HotBox clone (all tube) > LA-2A clone > 1960 Gibson tube amp.  So all I need is a DI in? I don't need to change the impedance again when going into the amp? ALSOOO... I'd like to keep the signal chain all tube. Any leads on an all tube DI schematic (!)?

Sorry to be such a pest, but what do I need to interface with the Digi 001? Are home studio commercial tube mic pres enough? And, same question here, do I need a pre out as well as in?

I know that's like 40 posts in one. Thanks for your patience.

Lee

LA-2A impedance
« Reply #6 on: June 09, 2004, 07:52:51 AM »
Whoa back there Lee,
I'm no guitar player (saw one once, but didn't like him), but that signal seems a bit skew wiff to me. Surely that Gibson has a guitar input, no?

Guitar>Hotbox>Gibson>LA2A would seem to me a more likely candidate.

As for the Digi.....RTFM old son.
http://www.lazpro.com
I reserve the right to not sell to idiots

CJ

LA-2A impedance
« Reply #7 on: June 09, 2004, 01:58:18 PM »
I just saw an add for an Ampeg tube direct in box.
I don't know how old the add was, or if they even still make the thing, but you might ask around.
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

toby

LA-2A impedance
« Reply #8 on: June 09, 2004, 03:19:34 PM »
i think an LA2 might be a bit of an overkill in your guitar signal path, but never tried it.

groove tubes make an all tube DI box i belive, but what may be easier is adding a second input to the LA 2 in the way of a second transformer. the kind thats in a DI box should be fine. here in the UK maplins make one specially for that for £7. The HF rolloff of a cheap transformer should not be a problem since the signal is going through a guitar cab, which don't have much of a hf response anyhow.

If you were capable of building an LA2 clone building a tube mic pre should be pretty easy in compraision. Look at the design on the Gyraf site. Built one myself and it sound excellent.

Toby

Skull

LA-2A impedance
« Reply #9 on: June 09, 2004, 06:56:22 PM »
Thanks again!

Toby, that second trans idea is great. I don't need to use it as pro gear, so I might make the DI t-former my only one.

I didn't set out to make an LA-2A, I was just looking for a schematic for a tube opto-compressor with  no ICs or transistors. Well, that ended up being a very short list.

A guy at Jensen is actually helping me with this. If he comes up with anything y'all can use, I'll certainly share.

What's interesting to me is my hotbox clone has no i/o transformers. My vintage tube Echoplex has no i/o transformers. They are pretty much the same design. The input signal is applied straight to the tube grid.

I have used the Echoplex as a hardware send/return with my Digi 001 and it sounded great. I guess I ought to post some pictures and sound files.

I'm begining to wonder if I need any transformers at all.

As far as my signal chain... The accepted practice is to use a compressor right after the guitar so you don't compress all the noise coming out of the fuzz unit. But I like all that noise. And the noise coming out of my 4 tube fuzzbox is pretty spectacular. But you can really hear the narrowness of a transistorized compressor after all the air in the signal with 4 Ei tubes. So, a tube compressor was sought.

I don't understand the question about inputs to the Gibson amp. Yes, it has two guitar inputs.


rafafredd

LA-2A impedance
« Reply #10 on: June 09, 2004, 07:24:08 PM »
If I would do that I would put another stage in front of the LA2A circuit to make the gain that the transformer would do.

So, get the inpiut transformer out and use a tube stage. Like this:

Guitar -> grid / plate -> T4

You can omit the output transformer also, or maybe add an inter-stage transformer. Something like 2.5k/40k Z, So you would connect the output of the LA2A direct on the guitar amp HiZ input.

Just a crazy idea, but I have thought about it already...

Skull

LA-2A impedance
« Reply #11 on: June 09, 2004, 10:22:17 PM »
Thanks!

Do you mean to be that litteral? Just the signal into the grid and out of the plate? I would guess you mean a small gain stage complete with grounding, etc - but maybe you don't.

The guy at Jensen recommended pretty much the same thing: dispense with the i/o transformers. He suggested a cathode follower buffer at the input. I'm looking at White's cathode follower and I'm wondering if I'll need a pot or trimmer to dial in the level to the T4b. My T4b is a Vactrol clone for now. Got no idea if that will make some other sort of a diference. I can at least mess with the resistor values inside the module.

rafafredd

LA-2A impedance
« Reply #12 on: June 10, 2004, 12:52:14 AM »
no I would not use a cathode follower here. Neither a white cathode follower.

Try a simple double triode 12au7 line amp. It cannot be difficult to find someplace to borrow a circuit from. I will look at my archive and have a look. I think something like 18db would be enought, but variable gain  would also be nice, so you can drive the LA2A input harder or soft...

Skull

LA-2A impedance
« Reply #13 on: June 10, 2004, 02:22:12 AM »
Wow!

Cool, thanks a lot. So you don't think I need the CF as a buffer? Great with me, as I understand amps better.

I will look for a circuit. If you can find one, please send!

rafafredd

LA-2A impedance
« Reply #14 on: June 10, 2004, 10:58:26 AM »
Wait to see what others will tell you, as I´m no expert, but I think something like this would work fine:


Skull

LA-2A impedance
« Reply #15 on: June 11, 2004, 03:28:31 AM »
Hey!

Thanks!  Did you design this specifically for this problem? Certainly looks easy to do. I'll try it. Should I put it before or after the 68k input resisdors?

Just because the Jensen Tech mentioned it, can you tell me why a cathode follower would not  be good here? Is a CF an amp in it's own right? Why did he call it a buffer?

(I certainly understand if you don't have the time for all this..)

Thanks for the circuit!

Lee

sismofyt

LA-2A impedance
« Reply #16 on: June 11, 2004, 06:09:28 AM »
A cathode follower will provide less than unity gain and a low(ish) output impedance. That's a great idea if you want the gain through the LA2A to saty that same AND drive the input trannie as well.

If you do a straight plate coupled triode, you'll get some gain, 20dB or so, but you can't drive the input trannie. You'll need to bypass it or take it out.

And maybe you'll end up with too much gain considering that you're using an external preamp.

Both options will provide a high enough input impedance.

If you're ONLY going to use the LA2A for that guitar setup, I'd mod the whole sheebang to fit that task only..

Kickass setup you got there.. geetarz'n'optos... yummy  :razz:

Skull

LA-2A impedance
« Reply #17 on: June 11, 2004, 03:10:14 PM »
Yeah.

I'm dropping the i/o trannies. That way I can plug it straight in to my guitar or Digi 001. The 001 is  set up for  guitar effects.

Thanks!

Lee


 

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