adm780 w/jlm input transformer

[kakumei47]

Ok so kinda newbie to figuring this stuff out but I'm changing out the UTC transformers in my ADM 780s with the JLM input transformers. I know I'm right on where I'm putting the leads, but what I'm not sure of is the resistor value across the secondary.
Check out the ADM schematic posted here:



Going by info on other preamps from Eisen audio (http://www.eisenaudio.com/diy500/tables/ipxfmrs/) and JLM (http://www.jlmaudio.com/Baby_Animal_Mic_Pre.htm), the JLM doesn't need a zobel network and should have a 10k resistor across the secondary.

Applying that to what I see on my schematics, it looks like r55 (75k on schematic) should be the 10k, r53 and c25 in series are the zobel network that can be removed. Am I right? C 26 should be left as-is, right?
Also, the mu-metal can on the UTC had a screw terminal to ground. Should I do this with the JLM too or not needed?

[kakumei47]

BTW for others doing this the JLM input tranny should be hooked up as follows:
6-blue 7-yellow 8-none (or shield, which can also be tied to the shield ground from the UTC) 5-red 3-green 1-black
Thanks stace for the info a while back! (edited to reflect problem + solution below)

[kakumei47]

Did as above and it sounds great. Would have posted AB clips but I made a stupid wiring mistake and had to open it up again, so instead of making two trips I finished both of them.
Next up: tring to put phase, 48v, and pad switches where the assign switches or led lights are and wiring up an output fader pot.

[kakumei47]

Spoke too soon in saying these were working right.

Works fine with dynamics. BUT, when I apply phantom power (via a 48v switch on the PSU), I lose signal. Doesn't power condensers, and cuts out signal even with dynamics. Not good.

Obviously I screwed up somewhere, but I'm not sure why this would be happening.

I wired it up as above, with the primary of the jlm going to pins 5 and 1, the secondary to 7 and 6, and the CT to 8. The only other modifications I made were changing r55 to 10k and pulling r53 and c25, which I assumed was a zobel network the JLM doesn't need.

Any idea where I went wrong? I have more info (schematics, pics, etc) if anything would help.

[Nate Dort]

If you're cutting out dynamics when you turn on phantom, you're probably connecting the + and - of your balanced input. How are you applying phantom? Through 6.81k resistors at pins 2 and 3 of the XLR? Through the transformer's primary center tap?

[Nate Dort]

also, looks like your center tap should go to 3 (i.e. floating). 8 is ground, and is for the transformer shield.

[kakumei47]

I'm using the original edge connectors and phantom power input points. I don't have switching on the module yet, but I installed a switch on my power supply box to send 48v to my two 780s. Worked fine with the stock pres. 48v still comes in at #27 on the pinout in the schematic above (upper far left on the schematic above). I didn't change anything about that section at all (at least I think I didn't). hmmm.

[kakumei47]

Ok I'll try switching the CT to 3. Could that be causing this?

[kakumei47]

Followed Nate's suggestion and switched CT to 3 (floating not ground) and made sure the shield ground was solid. Works great! Glad I didn't fry anything. Thanks!

[orphanaudio]

Looks like you got it solved, but here's what I posted on the Orphan Audio ADM forum in response to your question: http://cms.orphanaudio.com/index.php?id=86,0,0,1,0,0


According to the wiring you show above, it shorts out one half of the mic transformer input coil (and phantom power) to ground, and as a result one side of the incoming mic gets shorted thru the transformer primary as well and the input tranformer get biased with phantom from only one side.
It will pass audio, BUT... when phantom is turned on, it applies power only to one side of a condenser mic, so they will not power up.
With dynamics, it puts a phantom voltage charge across the mic and, as you experienced, it biases the microphones output transformer or directly charges the microphone coil with DC and tries to launch it out of the windscreen, either effectively shuts down the microphone.
A ribbon mic would be turned into a confetti-cannon with this wiring if phantom was turned on.

Try this wiring:

JLM-ADM
RED- 5
BLK- 3
GRN- 1
YEL- 7
BLU- 6


I also strongly suggest a termination resistor on the secondary of the JLM (65k to 360k?) Use whatever JLM suggests or you may experience ringing, distortion and overshoot at higher input levels, although the 75k in R55 may be enough.
Also, If you are powering more than one preamp from your phantom supply, put a 200 ohm resistor in series with the 48v input to each module. (ADM pin 27/*27). this will limit the max current drawn on each channel, and keep a accidental short on one channel from loading the phantom supply and affecting other channels.

Beware:
If the JLM mic transformer is significantly more wideband than the original UTC, you may wish to at least double the value of feedback compensation caps on all the IC opamps, or you may experience oscillation at higher preamp gain levels. The feedback cap mods are from Deane Jensens engineering notes to KNBC LA on exactly this problem with oscillation.

[kakumei47]

Thanks for the info Ken!
On the JLM, black goes to the primary and green is the CT, so I'm pretty sure 3 and 1 should be switched in your post (it's hard to see from the jlm wiring picture online). My new working setup is:

6-blue 7-yellow 8-none 5-red 3-green 1-black

I'll add the resistor and check out the feedback cap changes. Thanks a lot!

[orphanaudio]

On the JLM, black goes to the primary and green is the CT, (it's hard to see from the jlm wiring picture online).

JLM is pretty fuzzy in the input transformer color codes, shows several different pinouts on schematics, (some with missing wire colors), and the color codes do not conform to the standard sequencing used by Jensen, Cinemag, UTC or many other industry leaders. They also call the transformer out by at least two different part numbers on the website, so I went by the physical description and the schematics that agreed with each other.
In any case, transformer input-hi to ADM pin 5, transformer input-lo to ADM pin 1, and for safety, just insulate and do not connect the transformer CT wire. (Hopefully there is only one CT wire???)

[Joe Malone]

JLM14 input transformer

Red + Primary
Black center tap
Green - Primary
Yellow + Secondary
Blue - Secondary