JLM BA dedicated mic/line input difference with Pad

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kambo

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Apr 24, 2009
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Hi all,

i am pretty new here, and i did take your advice and read/searched a lot before i post this thread.
i wasnt able to find specific answer... may be due, there is no such issue :)

i am planning to build some mic pres such as JLM BA,  for line level usage. not for mic recording but typical fattening up samples etc purposes. using different trans on modules for sound options...

i understand that using Pad switch on JLM BA, makes it suitable  for this task. which is a great news for me.

my, question is, some preamps have dedicated line/mic inputs.
ie: on neve 8801 channel strip, mic pres uses the transformer input, and line input bypasses that transformer, 8801 also have -20 pad switch similar to JLM BA.

* is there any real difference, or benefit using such dedicated line input.... or is it just to give different color for the line input, on such devices...

* as far as i understand, by using, or adding a pad switch (even stepped for further adjustment)
i can use any mic pre as line level usage....  or is there something deeper going on, on this
dedicated mic/line inputs...

i would be really glad if you could enlighten me in this issue. .

thank you

kam

edit: soundcard lynx aurora 24k input, 600ohm output impedance
 
If you were using these strictly for line inputs, I'd maybe find some step-down line transformers for the job.
 
The BA does not give you the option to bypass the input transformer, unless you add blocking capacitors to keep the bias voltage applied to the opamp inputs (usually 24VDC depending on the opamp you used) from hitting the output line driving opamps of your soundcard. Also the BA could become unstable with the extra capacitance hanging off the opamp inputs. Suffice it to say you are better off padding down your line to mic level, though you may need something more like -40dB. Or, if you can, simply turn down your output level coming from your soundcard until it doesn't clip the BA's input.

Some of the older console designs bypassed the first gain stage for line inputs, many newer or lower budget consoles simply pad the line input down and feed it to the mic input. The BA requires you take the latter approach. Not a bad thing if you want the coloration of the input transformer. There is simply a noise compromising in dumping level in the pad and making it up with more gain from the opamp.
 
rodabod said:
If you were using these strictly for line inputs, I'd maybe find some step-down line transformers for the job.

that would be excellent

skipwave  -----
and thank you for the ınformation


i belıeve there is impedance problem too. G9 looks like has big bass roll off dur this problam...
 
I'd maybe ask Joe if it is possible to flip around his JLM 1:4 mic transformer for this purpose. That, along with the 99V opamp could be quite nice.
 
Joe is super good about answering questions and mods to his products.  I might suggest you either try jlmaudio.com 's forum (which is accessed in the middle of his front page) and post there.  You might get a faster answer there than by emailing him.

I think it might be possible to wire his input transformer backwards for a step down of 4.  Of course, if you just build the preamps the same and use the pad you always have the option of using if for a mic preamp if you need it or if you want to sell it.

The input transformer (I think) steps up 4x and then your pad is going to reduce that using resistors and then your signal is going to hit the OPAMP that you've selected.  The transformer might add some colour - but I find Joe's input transformer to be more neutral than something like a Neve (St. Ives) etc.

Drop Joe a line.

CC

 

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