Newbie question - DI box is going to be my first project

[moosapotamus]

I etched myself, to retrofit a Whirlwind EDB1.

BEFORE

AFTER

Still doing a little bit of head scratching, trying to figure out how to make use of those two switches. But, sounds fantastic in the stock config.

~ Charlie

 

[CJ]

Errrr, I vote for the AFTER di!!!
:grin: :guinness:

 

[moosapotamus]

:guinness: :sam:

OK, here's where I ask a question that gonna give me away as a complete noob...
RE: Reversing output polarity, in other words, switching XLR pins 2 and 3...

With the LL1538 set up in 1:5 config, I'm getting continuity between the junction of pins 1 & 4 and the junction of pins 2 & 3. So, that means I'm also getting continuity between XLR pins 2 and 3 because, for normal output, 1 & 4 of the LL1538 go to XLR pin 2 and 2 & 3 go to XLR pin 3.

So, what's going on? Am I doing something wrong? I mean, if everything is cool, and there's continuity between XLR pins 2 and 3, is there REALLY a balanced output? And, what difference would switching the feed to XLR pins 2 and 3 make? Know what I mean... ?

~ Charlie

 

[Bo Hansen]

Charlie,

Very nice work.

Regarding the extra switch, you can use it for a input pad (good for high output synth, sampler or line levels as amp slave output), take a look on my tech page http://w1.316.telia.com/~u31617586/ and on the API 312 or 325 DI input modification, you can use same pad circuit for the "1975 DI-box"

To use the second step on the extra switch you can have a -20 dB pad, and this is easy to do because it is same pad circuit as the - 10 dB pad, but you also add a extra 330 kohm resistor after the 2,2 mohm resistor to ground (the junction 2,2 mohm, 18 pF and 10 kohm), and you have a 3 step pad switch with 0, -10 dB and -20 dB.

Your question about the links on the transformers output and connection to the XLR, I must say I don`t understand, because you have don right on the PC-board so I can not see some strange in your circuit design.
Can you explain one more time what you are thinking about ???

--Bo

 

[moosapotamus]

Thanks, Bo!
Adding the pads like in your API mods sounds like a great idea.

Let me try to restate what I was asking about the transformer output...
I wanted to use one of the switches (dpdt originally for ground lift) to switch the output polarity by swapping the connections from the transformer outputs to XLR pins 2 and 3 (+out to pin 3, and -out to pin 2). Makes sense? I'm pretty sure that's done properly in my PCB layout, yes/no?

But, after putting everything together, I checked the actual connections that the switch was making by using the continuity tester in my DMM and found that, no matter which position the switch was in, there was always continuity between XLR pins 2 and 3. That doesn't seem right to me.

So, I pulled out the switch to check it for a short but there's nothing wrong with it. Back to the populated circuit board... With the switch now removed, the transformer outputs do not connect to anything on the board. But, I find that there is continuity between the transformer's outputs. Even though there is no such connection made on the PCB, my continuity tester tells me that the pin 1 & 4 junction on the LL1538 is connected to the junction of pins 2 & 3. That's not right, is it?

Thanks
~ Charlie

 

[mik]

Hi BO, as I told you your DI box works well, but isn't so large, is possible to use more power to avoid this peculiarity, I use it with +24v diretly connect with 100 ohm resisor, may I use 48 with 1K or more ?

 

[BYacey]

Charlie, If I understand what you are asking - this is perfectly normal. You are reading the dc resistance of the transformer secondary between xlr pin 2 and 3. If you change the phase reverse switch you will still read the resistance, albeit the transformer winding is just flipped end for end.

 

[moosapotamus]

cool. Thanks!
For my DMM, I'm not sure how low the resistance needs to be before the continuity tester beeps (less than 10 ohms, maybe?). So, DC resistance is not the same as static resistance (975 ohms for the 1538XL secondary, according to the datasheet)? I guess I need to pick-up on some trafo terminology, too.

Thanks, again!
~ Charlie

 

[Bo Hansen]

Charlie,

If you ohm measure a transformer winding with a DMM you measure the the "copper wire DC resistance" and not the AC impedance that the transformer are design for (200 ohm for the LL-1538/5:1)
The DC resistanse can be very low for a microphone transformer primary (around 10 to 100 ohm becomes of the design)
So it can be easy to think there are a short circuit when you have a beeper or a simple DMM.


Mik,

To not mislead other reader, this is a custum modification for Mik, because this DI circuit are use inside a Telefunken microphone pre amp and take the voltage feed from the TFK preamp supply, not from the 48 volts phantom power.

The dual emitter followers circuit in the "1975 DI-box" can handle higher supply voltage and work fine, (we talk about the voltage on the inside voltage rail marked 24v 3,5mA) you get more headroom before output clipp, but you get no more gain if you have 48 volt supply compare with 24 volt supply because it is a 1:1 emitter follower circuit with unity gain.
The 24 volt supply are enough headroom (hundred times more that you need for a microphone input) but if you think the signal is to low from the DI output for your especial application, you can use the 2,5:1 configuaration instead for the 5:1 configuraton on the Lundahl LL-1538 transformer and you get only 10 dB loss instead for 15 dB, so you get 5 dB more gain out from the DI-box.
(I use the LL-1538 in 5:1 configuration with parallel connected low impedance windings, because it is a better match to transformer coupled microphone inputs)
10 to 20 dB loss are usal for DI-boxes, and this is good because the output from a HI-Z instrument have a lot more level that a microphone input are design to handle.
(there are a natural loss in a passive DI-box transformer approx. 20 dB or more, and a backwards microphone transformer aprox. 10 to 15 dB for active Di boxes)

--Bo

 

[Guest]

This is going to be a really silyl question but I can't get my head around this at the moment
The three pin XLR connection to the mixing desk - the one that supplies phantom power but also transmits audio back to the mixing desk...
I want to go direct to tape with this...with phantom power being supplied from a PSU that delivers phantom power.
How do I wire the three pins then?
One more question and a very simple one - if my PSU delivers phantom power and I want to send it to two or three different devices - do I just wire in three cables to the phantom power out of the PSU or is it more difficult than that?

 

[Bo Hansen]

uk03878,

The Di-boxes are designed to overcome the problem to connect a unbalanced high impedance instrument to a balanced low impedance microphone input, and a transformer with a high impedance primary and a low impedance secondary can do that work, but not with enough high input impedance for some instruments.
So therefore this can be overcome with a high impedance input amplifier before the transformer, and because this amp must have a voltage supply to work it is comfortable to use the existing 48 volts phantom power on the microphone input that are very usual on normal recording consoles.

If you shall use this DI-box to feed a not phantom powered microphone input, or a line input or simular, it is not practical to use a phantom powered DI-box, because you must build a adapter with a 48 volt power supply and some type of dc blocking circuit so you only get phantom power to the DI-box and not in your "tape recorder input".

Tell me what type of tape recorder you have and what type of input you whant to use.

Is it a Guitar or synth/sampler you want to use trough the DI-box ??

Maybe I can do a custom variant of this DI-box that can fit your application better. (with battery or wall adapter supply, and maybe a other output that match your input better)

--Bo

 

[mik]

thank you BO.

 

[CJ]

Err, the country hick has.......
never mind!
(Thank God Kev is up to his neck in concrete!)
Vitamin D Kev, it comes from the sun you know!
Take you vits!
:guinness:

 

[Kev]

:green:
I don't see much of the sun at the moment
can't wait for the Olympics to be over
concrete is firm but it will need a few more weeks before the factory fit out can start and control of the storm water must come first


you can never have too many DI units
there is enough info around these pages to get you started
312/325 , H-FET , C-Hick ... a number of guitar stomp sites ... a Jensen Application Note
the single tube powered by phantom from a couple of days ago

many choices

 

[Guest]

Bo
many thanks - it was just an idea to expand the di box
Note to self - stick to one project at a time........
I will finish this in the next few days and not get distracted - honest!!!

 

[tony dB]

Thanks Bo for sharing this! :thumb:

anyone made any progression?

Kubi,

do the OEP's work out well? Me too is interested in something to colour sometimes instead of just having the cleanest option ...

on the other hand i'm sure i want to build a few using the Lundhall's too.
Is the Sowter a good substitute for the Lundhall's here mentioned?

Can't wait to get this started :twisted:

 

[12afael]

www.muziek-gg.com/koerier/koerier%203%2001.pdf

here I find a schematic using the ssm2142 balanced line driver it could be an alternative for the transformer circuit .

I think it is better than traditional op amp designs because it don´t need external resistors for balance so the both sides are almost perfectly balanced.

 

[tony dB]

yeah, include me on those Edcors... :grin:
not tried in DI setups but for my (well actually NYDave's) reampdevices they ad a very interesting upperrange shine as well as some extra headroom to the signals i send tru.
Not the very cleanest either but i'm more one of those looking for some nice distortion then a one on one solution. Digital is clean enough for me :grin:

Could we order a few Edcors together?
Noticed another Belgian, André Tchmil, might be interested too on some ... (if you read this and feel to participate Kris, jump in! :grin: )
I want some other stuff from Edcor too like a gtr splitter/buffer and recall from my previous order it being a bit expensive on tranport. Since you're only something like 300 km away from me, sending from yours to my place or the opposite couldn't cost that much. I even visit Luxumburg from time to time, dunno how far you're away from there?

Cheers,

Tony

 

[tony dB]

i want to organise this if it looks ok for you guys...

 

[tony dB]

i'm sure we could seriously reduce costs if we get one package to come over the ocean.

In my previous order, transportcosts almost doubled my ordercost.

ordered 8 btw. Maybe it would be more interesting to pass tru Germany?
I know EU like to cream of our benefits from import from the US overhere...