Help with passive Direct Box

[jimosity]

OK - So let me preface this by saying that I'm a complete dummy when it comes to transformers.  I don't quite understand the pinout wiring for a transformer so I'm sure that doesn't help me.  
Here's the end goal.
I have a bunch of 42TM018 transformers that I would like to be able to use.
What I need is a simple passive direct box.
I want to take a cabinet simulator circuit and run out of that circuit into the passive DI and then have an XLR output.
I tried to modify a Jensen diagram with the 42TM018, but I'm not quite sure of my wiring.  Can someone verify this or give me some other ideas?


Also, if I want to have a volume control for the XLR connection, would it go before pin 2 on the XLR?  Would A100K be best?

[brett]

Hi
your transformer is completely unsuited to this task.
You want a transformer with high primary impedance (many kohms), and LOW output impedance (ideally a hundred ohms, but maybe ok up to 1kohm).
Alternatively, you could put buffers between the transformer secondary and the XLR plug. A dual op-amp, or 2 x JFETs would do the trick.
Personally, for a few bucks extra, I'd go with a higher-quality transformer with Zout of 600 ohms.
cheers

[jimosity]

I notice that Small Bear has the 42TM018 that I mention and lists it as a 'Transformer 1:1 For Direct Boxes'.
 Is that because I'm using it in a passive capacity?
I also have an 42TM022.  Would that be better suited?
Do you have any suggestions for something else that would work better?  Preferably from Jameco or Mouser.

[PRR]

What is "Cab Sim"? Isn't it active?

[jimosity]

Yes, it's an active, powered Cab Sim.

[ubersam]

I looked at the datasheet for the 42TMxx transformers sometime ago and saw that the frequency response was 300Hz~3.4KHz, so kept looking and found Jensen and Whirlwind. I ended up getting a Whirlwind TRHLM (20Hz~20KHz, 20K input Z, 150 output Z). Its basically the same transformer inside a Whirlwind DI box.

[jimosity]

Hmm.  Both the Whirlwind and Jensen are pretty pricey, any ideas on something more cost effective?

[ubersam]

This might be of interest: http://www.jensen-transformers.com/an/an003.pdf - go to page 3, section 2.4, no transformers involved. Other examples I've seen of that circuit use a dual supply, not sure how to or if it can be adopted to a single supply.

[fuzzy645]

I don't know any answers here, but I posed a similar question a while back. I noticed that Small Bear sells a 42TM018 that is advertised (by them) with the purpose of "Transformer 1:1 For Direct Boxes"

I believe the Jensens are upwards of $85 and this is only $2.75.   It would be great a passive DI can be made with an inexpensive transformer like this one.

[PRR]

Your diagram is fine.

If you must have a voltume control, use a 5K pot between the cab-sim and the transformer. However with the low-price '018 iron this will reduce the bass response. The '018 will reach 80Hz if driven hard (direct from a chip) but not with pickup or large resistance feeding it.

[jimosity]

Thanks PRR.

A couple more questions;

1) I found this on a website: "The Whirlwind IMP2 DI box is about as simple as it gets. The two 1⁄4-inch input jacks are connected to the ­primary of a 10:1 step-down transformer, and the secondary is connected to an XLR output jack. Notice the ground-lift switch. The unused input can be used as a thru jack."

If the 42MT018 is listed as a 1:1, so if the Whirlwind is listed as 10:1, how could the 1:1 be listed as "for direct boxes"?

Source: http://emusician.com/mag/emusic_going_direct

2) Instead of using a transformer, would something as simple as an AMZ buffer be just as good to add after the Cab Sim (or even before it?)?  The Cab Sim has enough power to drive the XLR pretty well and I've tried the AMZ buffer before the Cab Sim and it works great.

[ubersam]

If the 42MT018 is listed as a 1:1, so if the Whirlwind is listed as 10:1, how could the 1:1 be listed as "for direct boxes"?

I've wondered the same. The impedance ratio of the IMP2 is actually 133.33:1 (20K:150). For a 1:1 impedance ratio the primary and secondary windings have equal impedances, for the 018's both are 10K (10K:10K). Your cab sim might be happy with the impedance of the 018's primary but the device you connect to the secondary might not be (for example: the low impedance input of a mixer). I've seen 1:1 transformers used more as an isolation transformer to fix or avoid ground loop issues between devices. If I remember my conversation with a mouser rep about the 42TMxx's, he said they were primarily for telephonic/voice communication systems for which the narrow frequency response is sufficient. I have some 42TM017's (same frequency response as the 018's but with a 25K:600 or 41.67:1 impedance ratio) that I tried to use in a DI but found them to be severely lacking in the bottom end.

[Mark Hammer]

I scored a nice Pico audio transformer dirt cheap from a now-defunct surplus place near me.  Looking on their web-site it would seem to be a fairly pricey item these days.  Haven't done anything with it yet, but I'm hoping to.

[jimosity]

So, looking at this image, there are two questions that I have.
first, what Opamp would be a good suggstion if TL072s, which I normally use are not recommended.
Also, the part that says that the unbalanced output should not be used if the balanced output is used...
Not sure I like that.   Also it looks like my unbalanced out is running thru some of this circuitry.
Is that going to change the tone or is it going to just be used as an input to this circuit?
Can anyone help me with a layout for this?

Sorry if this message is weird or the pic is weird, Im on an ipad only and can only share pics on dropbox right now.

[PRR]

> how could the 1:1 be listed as "for direct boxes"?

Active direct boxes. Put nearly any chip between guitar and transformer, you can get away with a low-cost 1:1 transformer.

TL072 will work fine: as it says, use the 470 resistor not a 100 resistor.

Jensen's stuff is VERY VERY good, super high-end. You can get very fine results with $3 transformers; but if you Need The Very Best, then just pay Jensen what they ask.

[tempus]

If you want to use a transformer, I'd go poking around at www.edcorusa.com, and check out this page for sure:http://www.edcorusa.com/category/46-tpcseriesmatchers.aspx

[tempus]

Also, what is your reason for wanting the balanced out? Does it need to be transformer based? If we could see a schem of the cab sim, it might give us a better idea of the output impedance (and therefore drive capabilities) of the circuit driving the transformer, which might make your 018 work just fine. If the cab sim already has a low impedance output, the balancing act is a lot easier. R.G. was kind enough to school me on the subject a while back in this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=77449.0