JLM Phantom Powered Active DI

Support for JLM Audio Kits

Moderator: Joe Malone

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

JLM Phantom Powered Active DI

Post by chrisp »

Hi y'all

A copy from my post over at Turtlerock, and one of the lesser known JLM applications - it uses the Active DI input to make a standalone DI unit.

Parts: DI kit and JLM 1:4 tranny from JLM Audio, an XLR(M) socket, an LED and a box.

Here's the hookup, based on Joe's webpage schematic -
[img width="10%"]DIJ1.jpg[/img]
The genius is in wiring the tranny backwards, so you get a 12db drop rather than a 12db boost. This returns the opamp output to a level that is just right for plugging in to a preamp, while balancing the output so you can use the OPA2604 opamp on phantom power (without the tranny, Joe recommends swapping to a TL072 FET opamp.

So simple, yes? Here's my build ...
DIJ2.jpg
You can add in 9V battery power if you want, which also allows you to put in a ground lift. You might want to do this is you are DI'ing something with a power source, but I wanted this just for guitars, so I kept it as simple as possible.

And what would you call such a military grade DI from Mr Malone?
DIJ3.jpg
You do not have the required permissions to view the files attached to this post.
Last edited by chrisp on Mon Nov 15, 2010 3:48 pm, edited 3 times in total.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

User avatar
Joe Malone
Site Admin
Posts: 1690
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Post by Joe Malone »

Great example of using the JLM DI as a external phantom powered DI.
Thanks Chris for putting a copy up here :D
Joe :-)
JLM Audio
Capturing Audio without Injury

astrovic
Posts: 17
Joined: Sat Mar 03, 2007 12:14 pm
Location: Melbourne, Australia

Post by astrovic »

Ok, I've only had 2 coffees so far today so my brain isn't on yet..but I have 2 questions and I think I know the answers.

1. How do the led and opamp get juice without a DC supply?

answer:
chrisp wrote:while balancing the output so you can use the OPA2604 opamp on phantom power
2. If I've got that right - why a TL072 if not using a tranny?

Answer (this I'm guessing) - a tranny wired backwards drops the voltage across to a level acceptable for a OPA2604. Without the tranny, you'll get a higher voltage at the OUT- pad on the board, which will fry the OPA 2604. The TL072 can handle higher input voltages so is better for this purpose.

How'd I go?

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

Hey Astro - we've got to stop forum-ing like this

(1) The OPA opamp and LED run off the phantom power that comes in through the XLR from the preamp, so pins 2 and 3 of the XLR are both at +48V compared to Pin 1. The 1:4 centre tap black wire is connected to both these pins (remember that the tranny is wired back to front), and can thus provide the +48 supply once PCB 0V is linked to XLR pin 1.

Part of my design goal was not to have to mess around with a separate power supply but still get the benefits of an active, rather than passive, DI, and the phantom power option works fantastically.

(2) I think its more an issue with load and impedance. Joe will have all this down straight, more than I do. However, leaving off the tranny involves more than just swapping the opamp. You need to take the phantom power off using some resistors as well. The JLM product page for the DI kit includes 2 schematics, one for tranny and one for without.
http://jlmaudio.com/JLMDI.htm
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

astrovic
Posts: 17
Joined: Sat Mar 03, 2007 12:14 pm
Location: Melbourne, Australia

Post by astrovic »

The more forums the better!

Thanks for reminding me to do something obvious like look at a schematic. I would have thought of that after my 3rd coffee :oops:

The schematic clears everything up for me. Thanks (as usual) for explaining stuff in a clear and simple way.

astrovic
Posts: 17
Joined: Sat Mar 03, 2007 12:14 pm
Location: Melbourne, Australia

Post by astrovic »

I just popped a TL072 in my (non-transformer) JLM DI...works perfectly. I ran it into the mic input of my baby animal, and then compared it to the DI in my baby animal (same unit), which has a OPA in it, and they sounded pretty similar - if anything, the TL072 sounded a tad brighter FWIW. But with a OPA in the DI Box, I found the unit to push too hard and break up fairly quickly, so I can see why Joe recommends the TL072.

All good stuff :D

User avatar
Joe Malone
Site Admin
Posts: 1690
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Post by Joe Malone »

2. If I've got that right - why a TL072 if not using a tranny?
It is all a question of how much current the opamp will draw.

Resistor Phantom powered DI
http://www.jlmaudio.com/JLM%20DI%20kit% ... Simple.pdf

48v Phantom power is sent via 2 x 6.8k to the DI which also has 2 x 6.8k resistors. This gives a 6.8k/2 + 6.8k/2 = 6.8k overall resistive feed from the 48v. So if you use a OPA2604A which draws 10mA it will lose almost all the 48v across the 6.8k resistance and only end up running on 1 or 2volts which is not enough for it to run on properly. The TL072 on the other hand only draws 2.8mA so will only loose 2.8mA x 6.8k = 19v across the resistor and still have 29v to run on.

Transformer Phantom powered DI
http://www.jlmaudio.com/JLM%20DI%20kit% ... %20out.pdf

With the transformer DI circuit shown in the link above the transformer centre tap is used to feed the power with about 10 to 15ohm resistance so only the 2 x 6.8k resistors at the mic pre will be the resistive path. This gives a 6.8k/2 = 3.4k overall resistive feed for the 48v. So if you use a OPA2604A which draws 10mA so will lose 10mA x 3.4k = 34v across the resistor and still have 14v to run on. The TL072 on the other hand only draws 2.8mA so will only loose 2.8mA x 3.4k = 9.5v across the resistor and will have 38.5v which is higher than the max 36v it can handle.
Joe :-)
JLM Audio
Capturing Audio without Injury

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

Thanks for the explanation, Joe - I knew you could do it better than I!

That approach of "work the current draw backwards through the circuit to work out the voltage drops" has me thinking about a solution to another circuit debugging issue I'm working on for a guitar pedal, so thanks for the heads up on that as well.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

astrovic
Posts: 17
Joined: Sat Mar 03, 2007 12:14 pm
Location: Melbourne, Australia

Post by astrovic »

Thanks Joe :D

BR
Posts: 2
Joined: Tue Apr 17, 2007 11:35 am
Location: Orlando, FL
Contact:

Post by BR »

Hi guys,
Great idea here.

for a ground lift, is it still necessary to have battery if I install a THRU jack?
Wouldn't ground reference come from the "amp" at that point??

also, what is the color code on the JLM 1:4? I'm trying to figure out how to this with another traffo.

thanks

Gil

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

Hi BR
BR wrote:also, what is the color code on the JLM 1:4?
Red = Primary +
Black = Primary CT
Green = Primary -
Yellow = Secondary +
Blue = Secondary -

As for your other questions, I'll leave that for the experts. But my gut feeling is that while you get "a" ground reference from the shield cable connection of your THRU socket, it might not be the one you are expecting - it will depend on how your amp deals with that connection at its end as well as how your other gear is connected / powered.

In my simplest of the simple implementation, there is no risk of hum because there is ONLY the ground reference that comes with the XLR pin 1. I used the phantom power option because I didn't want to fiddle around with batteries (also, the schemo calls for at least 12V, so you'd need to string a few together).

If I were going to add a THRU socket, that's where I would add the ground lift, rather than the XLR. It will be much simpler, as the schematic stays the same for the phantom powering side of things -

IN + to PCB and THRU +
IN- to PCB and GndLft switch pole
GndLft switch "ON" to THRU -
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

BR
Posts: 2
Joined: Tue Apr 17, 2007 11:35 am
Location: Orlando, FL
Contact:

Post by BR »

Cool,
thanks for the response.

I have an API output tranny that is a 1:1:1:1 . I'm trying to figure out how to wire it.
So in order to wire like you did, my wiring would end up being 1:1. Right?

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

You may be able to wire it to get 2:1, but the JLM is an input tranny wired to give 4:1.

A lesser ratio than 4:1 will (counter-intuitively) give you a hotter signal out of the DI box (like up 12db at 1:1), so be careful what you plug it into. Apart from that, it should work.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

User avatar
bluezplaya
Posts: 7
Joined: Wed Jul 25, 2007 4:38 am
Location: Planet Earth
Contact:

Post by bluezplaya »

I am interested in building two of these with trannies. However, I will not be powering from a 48v source, rather 9v batteries. Obviously I will have to pair up to get +18v, but wonder how I can get power for the LED with only 18v? Chris, what are you using to fasten the input transformer to the box?

Thanks,
Adam

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

When you order a JLM1:4, just ask Joe for a plastic mounting clip instead of the mounting screws it normally comes with. Its got a little adhesive pad on one side and a plastic loop to hold the tranny on the other. Thats all it is.

BTW, you should be able to run the DI on a single 9V according to the schematics
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

Post Reply