Langevin AM-16 Racking

[hoodun]

I know there are some other posts regarding this topic but I am not getting the info I need and have decided to try to consolidate some of the info I do know, or at least think I know.

Wiring an AM16 Module

In the past I strapped the in and out to 600hms, added a 20db pad switch at the input, and put a stereo 500ohm pot on the output. Is this the best way to wire these things?

Do I need the 500ohm pot on the output? Do I need to add a 680ohm resistor on pins 2 and 3 of the out?

Any info some someone with some knowledge they are willing to share is greatly appreciated. I am good with a soldering iron but a lot if this is, honestly, over my head. Generally I get power supplies kits from JLM and wire up inputs and outputs with a pad and 48v switch. I do not entirely understand the optimal way of wiring these AM16s.



Also, I found this old info about swapping transistors for extra gain and have actually done this myself but I never tested it:

Replace Q1-Q6 with 2SC2545
Change R1 from 680 to 560
Change R4/R5 from 150k to 390k (matched to .1%)

 

[gridcurrent]

In the past I strapped the in and out to 600hms, added a 20db pad switch at the input, and put a stereo 500ohm pot on the output. Is this the best way to wire these things?

Do I need the 500ohm pot on the output? Do I need to add a 680ohm resistor on pins 2 and 3 of the out?

Strapping the input for 600 ohms will reduce the microphone loading, as the input is terminating;
a topic that has been discussed here ad nauseam. 
A 500 ohm pot on the output will allow you to adjust output level; only you can answer whether or not it is needed.
It can quickly reveal headroom limitations.
If an output control is desired, a 1000 ohm audio taper would be preferable to a 500 ohm unit.
An additional 680 ohm termination is not needed.
A straight or bridged T attenuator would be better yet.

 

[emrr]

needs an output load.  1K is fine.  otherwise there's a huge treble boost. 

What terminated load would you assign to the input?  I'm not seeing an obvious termination, but then I also usually don't think transistors.  I've  wired 150 and been fine. 

 

[hoodun]

Im not finding too much info by searching. There are a bunch of discussions with no real answers.

I guess the best (only) way to wire these is with a pad (switch or variable) on input and an attenuator (T -pad)on the output for more control into the converters. Then ad a switch that goes from 600 ohm to 150 ohm, possibly a three way switch to 50ohm. Thats it.

This plus a transistor mod could possibly get up to 70db.

 

[gridcurrent]

.

 

[emrr]

From previous attachment:
Input Loading:  2 dB (150 ohm source into 600 ohm input)

Yes, that does still look significantly loaded.  Is that broadband, or single/multiple frequency? 
How does response look with a typical 150 ohm input 20 dB U-pad terminated into the 600 strapping? 

 

[hoodun]

so would it be a bad idea to have a -20db 600ohm pad along w a switch that selects 150 or 600 input strapping. since the -20db pad could ne activated with 150'ohms selected. also, does anyone strap to 50ohm. any modern ribnons at 50?

only way I can see doing alk of this is with a 4 pole 6 position switch with 50,150,600,-10,-20,-30. im still coknfused as to how to wire it but ive seen it done so I know its possible. this solves someone selecting the pad outside of 600ohms.

 

[hoodun]

After reading countless posts, its safe to say Im losing my mind at this point.

What I learned. "all thats needed is an input pad". - which is contrary to everyone saying to just put an out put attenuator on it.

I did purchase a T-Pad attenuator 600ohm, though its unbalanced. So I may go the variable pad route. If I do , I believe putting a resistor across 2 and 3 on the xlr will load it at 600ohms?


Also, I measure R1 and I get 580ohms on all my modules. It says it should be 680ohms. I'm tempted to put a switch in for a 680ohm resistor and then having 580ohms the high gain.

 

[emrr]

Here's another point that's just come to me:

What are the DC resistances of the input transformer?

There are Langevin and RCA transformers that follow Western Electric design principals, with very high DC resistance, some almost equal to rated AC impedance.  If the AM-16 input is higher than bridging DC versus AC, then the loading may be simply insertion loss from wire resistance, and not transferred loading.  If you look at the venerated WE 618-B, you'll see that is has DCR of 214 ohm for the 600 ohm winding.  The RCA BA-11 600 output is 440 ohms DCR.  That breaks all the rules as we view it today.  I suspect it may give a more linear AC impedance plot as compared to the way we do it now. 

 

[Biasrocks]

Unfortunately Hoodun keeps posting the same question over and over again. No wonder he's confused.

The answer was in the original thread from someone who actually owns AM-16's and are using them daily.

http://groupdiy.com/index.php?topic=50215.msg744247#msg744247

20db input pad, followed by a variable pad on the output consisting of two 301R resistors and a 1k pot.

You're not talking a huge investment in parts, so try it and see if it works for you.

Mark

 

[hoodun]

Unfortunately Hoodun keeps posting the same question over and over again. No wonder he's confused.

The answer was in the original thread from someone who actually owns AM-16's and are using them daily.

http://groupdiy.com/index.php?topic=50215.msg744247#msg744247

20db input pad, followed by a variable pad on the output consisting of two 301R resistors and a 1k pot.

You're not talking a huge investment in parts, so try it and see if it works for you.

Mark

This is how I have always racked them in the past... I am just trying to find a better way. Me saying "Im losing my mind" means Im having fun.