JLM Phantom Powered Active DI

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bluezplaya
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Post by bluezplaya »

How could I wire an extra output on this (thru) like on most DI's? Would I just jump the wires from pins 2, 3, and 1 to another XLR or 1/4" out?

chrisp
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Post by chrisp »

bluezplaya wrote:How could I wire an extra output on this (thru) like on most DI's? Would I just jump the wires from pins 2, 3, and 1 to another XLR or 1/4" out?
In general terms, Yes, it is that simple. In the vast majority of cases, that's all that's needed, and its certainly where I would start.

There are possible line isolation issues that can arise (as with any gear) - but short of wiring 1:1 isolation transformers at every output, you're not going to avoid that. Better to just fix it if, when and where it happens.
Chris P
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Joe Malone
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Post by Joe Malone »

DI's usually just have the 2 jacks wired to each other for plugging into and out of the DI as a loop. You can do this by just wiring the second jack across the DI jack. Or if you want a driven output to isolate the load connect the output jack to the +out on the DI PCB with a 1k resistor.
Joe :-)
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Blake
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Post by Blake »

i need a better DI for live use and this looks like a simple project to cut my DIY teeth on before i jump into some pres

anyway chrisp i was wondering where you got case from ive seen a few people use similar cases for single channel pres but i cant seem to find anything similar locally

chrisp
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Post by chrisp »

Hi Blake

Its a Jaycar instrument box: HB5441 is the catologue number.

It does the job for studio work, but for stompboxes / live things I personally go for the diecast aluminium cases: maybe HB5042. They are much more sturdy. Its a pity that they are so hard to drill for nice socket mounting, the inner ribs make placement a real hassle sometimes - but spending hours with a rasp and making metal shavings is all part of the DIY game, I suppose.

Good luck, and post your build here for all to see.
Chris P
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Blake
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Post by Blake »

i know its been awhile but i wanted to update this thread. i built the DI and it sounds great on guitars (acoustic and electric). i havnt tried it on anything else yet but when the time arises i have high hopes for bass as well.

i did use the stronger case, and it was a PITA to work with, still having a bit of trouble with the nut coming off the lack socket but aside from that it work fine.

conleycd
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Post by conleycd »

Is there any way to have a phantom powered DI using this circuit and an audio ground lift? I can't think of a way because the ground is required for phantom power right?

Any suggestions besides an external power supply?

Christopher Conley

chrisp
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Post by chrisp »

Hi ConleyCD

I'm not 100% sure about this, but here goes.

Joe's schematic does show a ground lift switch on pin 1 of the XLR, and yes, when that is lifted, the +48 V on each of pins 2 and 3 isn't referenced to anything else on that connector.

Does that mean it doesn't work? Not necessarily - it depends what else is connected, especially at the input.

If you just have a guitar plugged in, then it wouldn't work - there's not independent ground reference through the guitar you'd care to use. But then again, in this case you wouldn't use the ground lift, would you? There'd be no separate path to ground that could cause any problems.

But suppose you wired the input with a thru jack and plugged it into an amp. Then, there is a separate path to ground through the amp lead and the amps own ground circuitry, and that's why you might get some hum if there's a ground loop problem. But the point is that this separate path to ground through the amp provides the missing 0V reference for the phantom power, and bingo, you're back in operation even if the ground lift is activated.

The same is true if you are using the DI as an isolation / impedance box with a line source such an a synth. The lead to the synth provides a separate source to ground that can provide a 0V reference for the phantom power.

So that's the beauty of it - the only time you need to use a ground lift is when there is some other path to ground in play, and in that case, it is that very same path to ground that makes the phantom power work.

The small downside is that you need to know what you're doing - if you have ground lift activated when its not needed, you'll get nothing out of the box. So wire away, my friend, get that little SPST switch a flickin'.

If I've got any of this wrong, I'm sure Matt or Joe or one of the other experts here will chime in.
Chris P
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Joe Malone
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Post by Joe Malone »

Chris is correct. If there is a ground loop causing hum and a ground lift switch on pin 1 of the XLR is opened then the DI will keep working on phantom power due to the ground coming into the jack. If the DI turns off then there is no ground loop and the ground lift switch should be closed anyway.

Hope that makes sense :D
Joe :-)
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greenmanhumming
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Post by greenmanhumming »

I have DI questions,

1. can the input impedance be raised, and if so by how much? is it possible to raise the input impedance to 5-10Meg to use as an input for a piezo pickup?

could it be switched between the higher setting and the standard?

2. I have some OEP transformers i took out of my BA's because i preferred the JLM14, could I use these as output transformers for standalone DI's? will they colour the sound in the same way in this application as in a BA?

3. I have no room on the front panel of my BA for a DI jack, if I make a standalone DI without transformer will it sound the same plugged into the BA as installing the DI in the box (seems to me it should)

4. what are the pros and cons of using a DI without transformer for live use?

greenmanhumming
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Post by greenmanhumming »

oh, one more question, what is the gain of the DI circuit without transformer?

what would be typical input and output voltages? this seems an obvious question, so sorry if I am missing an obvious answer somewhere...

Can the DI be used to drive an AMP kit directly?

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Joe Malone
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Post by Joe Malone »

1. can the input impedance be raised, and if so by how much? is it possible to raise the input impedance to 5-10Meg to use as an input for a piezo pickup?
Yes two off the 2.2Meg resistors which set the input impedance to 1.1meg can be changed to about 100Meg each to give 50Meg input impedance.
could it be switched between the higher setting and the standard?
Yes by just switching the lower resistor values across the input.
2. I have some OEP transformers i took out of my BA's because i preferred the JLM14, could I use these as output transformers for standalone DI's? will they colour the sound in the same way in this application as in a BA?
They should work wired backwards but I have just never tried them.
3. I have no room on the front panel of my BA for a DI jack, if I make a standalone DI without transformer will it sound the same plugged into the BA as installing the DI in the box (seems to me it should)
If you mean battery powered then yes it will sound the same up until the battery voltage limit of say 12v to 18v where as when powered from the BA PCB it recieves 48v so has more headroom.
4. what are the pros and cons of using a DI without transformer for live use?
Battery power live means you need to make sure the battery is going to last the proformance. Where as phantom power you do not have to worry. But the quasi balanced out of the DI kit will cancel noise into a balanced intput desk almost as well as the transformer so noise isn't a real issue. And if you have a earth lift switch then hum loops can be fixed at a click of a switch which probably the most important issue.
oh, one more question, what is the gain of the DI circuit without transformer?
Unity gain. 0dB. It is only a high to low impedance converter.
The JLM111DC can have a gain of +6dB
The JLM14 will give a loss of -12dB.
The OEP will give a loss of -15dB
The loss is actually good when phantom powered as you are usually going into a microphone input to get the phantom power.
what would be typical input and output voltages? this seems an obvious question, so sorry if I am missing an obvious answer somewhere...
What ever voltage you put in you wll get out without a output transformer except if it is higher than the power rail running the DI.
Can the DI be used to drive an AMP kit directly?
It could drive our AMP kit to give you a high input impedance AMP but it will give no extra gain than what the amp is already set to. It could also be powered from the AMP kits positive rail of +25v to +35v.
Joe :-)
JLM Audio
Capturing Audio without Injury

chrisp
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Post by chrisp »

With a nod to the PM from ConleyCD, here's an updated diagram with thru jack, ground lift and a phase switch.

http://www.legalfinesse.com/webpics/DI.pdf
Chris P
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Kev
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Post by Kev »

just thought this might help someone

http://www.diyfactory.com/projects/jlms ... mpledi.htm

astrovic
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Post by astrovic »

Kev wrote:just thought this might help someone

http://www.diyfactory.com/projects/jlms ... mpledi.htm
It helped me Kev - was the first DIY project I built :D

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