Country hick*** Di Box

[aortizjr]

wow this thread is old. But appropriate for my question.

Has anyone used an Edcor transformer for this?

Cinemag and Jensen are too expensive. Xicon and other telephone ones have too limited frequency range. I figure the Edcor WSM series are cheap enough and hopefully more linear.

http://www.edcorusa.com/products/transformers/wsm/index.html

What ratio would be best? 600:150, 10K:600, etc?

 

[BYacey]

The 600:150 should be fine, it's about the right ratio for this application. I wouldn't be too quick to dismiss some surplus tranformers for this circuit. It doesn't really place any high demands on the transformer itself as far as power and loading. The transformer will typically be driving a mic preamp input impedance of 2K or more.

 

[aortizjr]

So I got it prototyped on a homemade PCB. I am getting sound and it sounds fine.

But check out the frequency response I am getting. A 3db boost in the lowend:

I am guessing I could probably just throw a cap on the output to even it out. But any other ideas as to what the problem could be?

I am using a J201 instead of a 2N5457 and an Edcor PCW600:150.

Thanks!

 

[CJ]

unfortunately, the transformer can not make more voltage than you put in, at the Low end at least.
if it could, a lot of designers would be in heaven, and the patent office would be buzzing.
kind of like cold fusion. remember those poor slobs?


This is because there is not enough leakage capacitance in your transormer to frome a resonant circuit, so the capacitance needs to cvome from somewhere else.

I know painfully and exactltily where it is coming from.

But this exercise is left to the student to figure out where.
Don't think too hard, it causes brain bubbles.

And please do not try to compensate the circuit with more capacitance!
Your power bill might drop from the 90 degree phase shift, (compare in and out signals in X-Y mode) but the amp will sound like s*** !

 

[aortizjr]

Well so I built a probe with a 10uF capacitor so I could tap anywhere in the circuit without DC distortion.

Turns out the circuit is perfectly flat until right before the 10uF cap before the transformer. But right before it, with the 10uF cap in the probe it is fine.

Did component swapping and it was the same. I also tried the probe with and without the 10uF cap.

I also cleaned up to make sure all the solder flux was gone. I also cut and rerouted a trace via a wire just in case it was that.

Typically I have found extra capacitance in a breadboard. But not on a PCB.

But as soon as it connects to that input of the transformer, I get that 3dB bump. And this is at the input of the transformer.

So yeah I don't know.

I understand you not wanting to "give" away the answer. But is there an article or a thread I can read or something like that?

 

[BYacey]

Have you looked at the waveform of the signal before and after the transformer with a scope? Sounds like possible core saturation to me.

 

[CJ]

sorry!
there is a cap inside the scope also, unless it is a new one with an ac/dc switch.

 

[aortizjr]

Finally digging this project back up. Been so busy with work and other projects I had to put this on hold for a while.

Anyway I messed with it some more and can't figure it out.

Taking the core saturation idea I tried changing the load on both ends with resistors, and that did all sorts of bad things to the response. I was thinking about maybe trying a zobel network. But how do you calculate the values?

Any other ideas? Or places to look?

 

[Mylithra]

I built a copy of this following the schematic as written. (the first time) I had no trouble getting it to work. I made some minor circuit modifications to get it to run on that "Phantom Backwoods Sewing Kit Stitcher" with a power LED and some filtering on the the ground going back out to pin1 of the XLR. It stomps the crap out of any other Direct Box I have.

The transformer is a JLM4:1. All Semis are Fairchild.
I have a 2 channel version on the drawing table (actually pretty close to completion now) I did a PCB design myself and had a couple of test PCBs printed from ExpressPCB.
Just waiting on the 1 part. Ill post completed pictures once its done.

 

[Mylithra]

Well,
I was waiting on the transformers and they finally came in today so I was able to complete the project.

The PCBs are my own design and printed through ExpressPCB.
The switch is an Instrument/Speaker input level switcher.

To tell the truth, I was a little disappointed when I first powered it up. It buzzed badly and the prototype didnt. After a little checking, I realized I didnt ground the chassis. After grounding the chassis, all the noise went away.
Its going to get some lasertran decals and a coat of paint but it works and sound great.

 

[RedNoise]

Nice!!! :thumb:

 

[dmp]

Does anyone have the schematic for this?
I can't find it - the links are dead
Thanks

 

[CJ]

it is a j202 current source feeding a pair of 2n4401, at least that is what i used,

bill y should have it if he is not ice fishing for mackerel,

 

[lars78]

 

[dmp]

Thanks Lars78!

 

[BYacey]

For two days I've been trying to attach the correct schematic, but it doesn't seem to work.

PM me and I'll email it to you.

 

[12afael]

hey BYacey could you send it to the forum's mail? i would like to take a look at that.

 

[BYacey]

Here:

 

[12afael]

thanks! I suppose it should have a couple of resistors and maybe a zener for the phantom supply.

 

[BYacey]

No, the actual Commercial unit has a HF switching power supply to provide 100% ground isolation.  Without that you can't achieve a true ground lift to eliminate ground loops.

Bo Hansen's DI box circuit has sort of a pseudo ground lift that you might be able to incorporate into this circuit.