JLM Phantom Powered Active DI

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greenmanhumming
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Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

I am building my DIs - the ones in my BAs sound great, and i've just done a battery powered one that also sounds great.

questions:

* i was looking at using the OEP263A2 as an output transformer, but the data sheet says don't put DC through the windings. What is that about? is it unsuitable for use with phantom power from the centre tap? I'm also bewildered by the dual primaries and secondaries, can anyone tell me the best way to hook it up?

* the DI kit doesn't seem to switch the power 0v on the input jack, to do this i have to solder the battery negative directly to the pcb pad/solder lug that goes to the ring of the jack... I've done this and it seems to work , but i wondered if i'm missing something... is this how its intended to be done?

* i neglected to ask Joe to send me any TL072 opamps, however i have a variety of others - OPA2134, OPA2132, LM833, NE5532, would any of those be suitable for phantom powering without the transformer?

chrisp
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Location: Sydney, Australia

Post by chrisp »

Do you mean an A262A3?
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

greenmanhumming
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Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

chrisp wrote:Do you mean an A262A3?
I guess I do. sorry. the one that is an option with the BA's, i wasnt that keen on the sound so I replaced them with JLM1:4 and now have a couple looking for a use...

chrisp
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Location: Sydney, Australia

Post by chrisp »

Here's my guesswork - I'd wait for Matt or Joe to confirm in case I'm about to burn your tranny, but FWIW

i was looking at using the OEP263A2 as an output transformer, but the data sheet says don't put DC through the windings. What is that about? is it unsuitable for use with phantom power from the centre tap? I'm also bewildered by the dual primaries and secondaries, can anyone tell me the best way to hook it up?
In general terms, DC through a tranny simply makes the thing act like a bar radiator - it turns electricity into heat. I couldn't find the reference in the online datasheets, so I'm not sure about this, but I don't believe the amount of DC in the DI project will melt your OEP. And ALL trannies deal with at least some DC - that's part of what they do.

In terms of hookup, the OEP has two identical primaries P1 and P2 and two identical secondaries S1 and S2, each with its own + and - pins. There is also a metal case grounding pin, which you can connect to ground. Connect P1+ to output XLR pin 2, P2- to XLR pin 3, and P1- to P2+, this becomes your centre tap for the phantom power. Connect S1+ to the PCB out+, S2- to PCB out- and S1- to S2+.
the DI kit doesn't seem to switch the power 0v on the input jack, to do this i have to solder the battery negative directly to the pcb pad/solder lug that goes to the ring of the jack... I've done this and it seems to work , but i wondered if i'm missing something... is this how its intended to be done?
The kit comes with a TRS DPDT switching connector, and is supposed to switch the power on only when a TS jack is inserted. Please don't be offended if I'm checking the obvious, but are you using a TS (mono) jack? If you have a TRS (stereo) jack the power switching won't work.
i neglected to ask Joe to send me any TL072 opamps, however i have a variety of others - OPA2134, OPA2132, LM833, NE5532, would any of those be suitable for phantom powering without the transformer?
There are complicated answers based on the power draw and available voltage for the ICs. On the other hand, a TL072 costs about $2.35 from Jaycar or Dick Smith. They are very common, you'll be able to find one dead easy.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

greenmanhumming
Posts: 117
Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

thanks! I will try it when i get a chance

I'm using the jack that mounts on the board which came with the kit.

i was surprised that it didn't seem to switch the power, but it didn't, although i have worked around it by soldering the negative directly to the ring tab of the jack

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

Post by chrisp »

I'm sorry, Green Man, I think I misread your original post.
i have worked around it by soldering the negative directly to the ring tab of the jack
That's exactly what you need to do.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

greenmanhumming
Posts: 117
Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

another question, mostly for my learning process, rather than an actual proposed mod....

looking at the schematic, what would be the consequence of inserting a resistor in the feedback loop(s) for each opamp?

i presume this would raise the output impedance and increase the gain, but given that the output impedance is so low anyway, if some gain was needed is there anything wrong with doing that?

Kev
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Joined: Sat Mar 17, 2007 6:42 pm
Location: Melbourne

Post by Kev »

before answering that question

a question back at you
... why do you want more gain ?

the following Mic-pre is where the gain can be added.
DI's are more about the impedance change than they are about providing gain ... and a way of providing an instrument split or earth loop break

so back to the " if some gain was needed is there anything wrong with doing that? "
why ?
what is the specific sutuation ?

chrisp
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Location: Sydney, Australia

Post by chrisp »

I'm thinking the same, Kev.

But, to try to answer Green Man's question, the IC is currently a non-inverting, impedance changing buffer, right? In principle, you can configure this to add a gain element, but it involves two resistors for a non-inverting amplifier: one in the feedback loop (Rf) and one that goes from the opamp- (and the end of the feedback loop) to ground (Rg). The opamp gain is then 1+ Rf/Rg.

You'd need to do this on BOTH of the opamps in the circuit with pretty well matched resistors. The output could be raised up to line level, for example. But I would not put line level into a microphone input (and my DI is phantom powered), so I'm happy just to have a nicely toned DI and let my BA's do the talking.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

greenmanhumming
Posts: 117
Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

why? to use it as a hi Z input for a line input on the mixer and avoid the mic pre, then a little gain is good too since you are not going through a mic pre. and the super low Z out is not needed since the input impedance of the line iput is say 10k rather than 600R

greenmanhumming
Posts: 117
Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

to be honest i'd probably build something new rather than try to adapt a good DI PCB, but i was just wondering about the principle of it..

Kev
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Joined: Sat Mar 17, 2007 6:42 pm
Location: Melbourne

Post by Kev »

potentially it is all do-able

if you have 10k and not 600 ... or even lower mic-pre inputs then it can make sense for the specific situation

it would still be interesting to know what instrument and situation you are looking at

for simplicity it can work

but
generally a mic-pre will be chosen to bring something more to the signal chain

a very simple double opamp like these BBs could be a little bland straight into the desk
BUT
it could be just perfect if that's what you are after

for the a simple change and match a couple of resistors try it
why re-invent the wheel
try it and if you like it - stick with it
if not - just convert back to the standard DI
...
can never have too many DI's

final thought
think about your rail voltage and how it relates to your line level and required headroom
I say your line level as people do seem to have different views about what line level really is
don't just look at averages but the PEAK levels you want

greenmanhumming
Posts: 117
Joined: Tue Apr 10, 2007 1:19 am

Post by greenmanhumming »

i'm thinking of making a pickup blender box, for a guitar or bouzouki with 2 pickups - one pickup is a magnetic, the other currently a B-band, but might be piezo or anything, so 2 hi Z inputs, passive mixer afterwards, followed by a true DI in mono if i actually need 600R out

maybe even add some really basic passive eq on each channel too..

just idle thoughts at present...

conleycd
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Joined: Mon Aug 06, 2007 9:26 am

Re: JLM Phantom Powered Active DI

Post by conleycd »

Hey guys,

Just finished up my discrete FET with transformer 'stage' DI. Fit into a Hammond Pedal box. Tight but it worked. A question, I have pin 1 connected to the box. I do have a ground lift which lifts pin 1 from ground. These DIs are plastic inputs so the grounds don't meet via the box. Is there any problem with this? I know most stuff does use the chassis as shield/ground. Should I have left the box floating?

CC

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Joe Malone
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Re: JLM Phantom Powered Active DI

Post by Joe Malone »

conleycd wrote:Hey guys,

Just finished up my discrete FET with transformer 'stage' DI. Fit into a Hammond Pedal box. Tight but it worked. A question, I have pin 1 connected to the box. I do have a ground lift which lifts pin 1 from ground. These DIs are plastic inputs so the grounds don't meet via the box. Is there any problem with this? I know most stuff does use the chassis as shield/ground. Should I have left the box floating?

CC
Just make sure the 0v from the DI PCB goes to the case and only switch the wire to pin 1 on the XLR and all will be fine.
Joe :-)
JLM Audio
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