SOLVED: Built Two Bo Hansen DIs- both distort very quickly

[PhilipMarlowe]

Hey there,

I finally got around to building two Bo Hansen DIs with Lundahl 1538s. Unfortunately I apparently did something wrong as connecting them to even a quiet line level signal makes them distort very fast before I use the mic pre gain at all. The source I'm connecting it to is not even hitting -12dbu so that can't be right, can it? I'm aware these are built for instruments in mind, but line level signals should be able to be handled by this DI too, as I read the input limit should be around 19dbu.
I'm wondering if the red LED I connected to both ( i had a few lying around for PCB stuff) is faulty, or draws too little/too much voltage.
How would I trouble shoot this issue? What is the exact milliamp/voltage requirement for the LED?
Thank you in advance.

Edit: Looks like I may have used the wrong diode on the output?

Edit 2: It was the wrong transistors that caused this issue. Fixed now. Thanks everyone!

 

[Newmarket]

Edit: Looks like I may have used the wrong diode on the output?

What Diode on the Output - Do you mean the Zeners across the Input ? Or the two diodes in the "Ground Lift" circuit (XLR Pin1) ?

 

[PhilipMarlowe]

What Diode on the Output - Do you mean the Zeners across the Input ? Or the two diodes in the "Ground Lift" circuit (XLR Pin1) ?

View attachment 100463

Thank you for the schematic.
I attached a pic to clarify what I meant. I accidentally used the 12v zener diodes for the circled part too since I apparently forgot to order different ones ( the 1n4004 to be specific). Could this be the reason why the DI distorts so quickly? Going by Bo's specifications and user comments, it should have massive headroom (up to 19db I believe before distorting).

I doubt it's an issue with the phantom power of my mixer since I tested it on both my interface as well as the analog mixer- same issue of distorting before the preamp gain.

 

[Newmarket]

Thank you for the schematic.
I attached a pic to clarify what I meant. I accidentally used the 12v zener diodes for the circled part too since I apparently forgot to order different ones ( the 1n4004 to be specific). Could this be the reason why the DI distorts so quickly? Going by Bo's specifications and user comments, it should have massive headroom (up to 19db I believe before distorting).

I doubt it's an issue with the phantom power of my mixer since I tested it on both my interface as well as the analog mixer- same issue of distorting before the preamp gain.

The circled diodes aren't the problem.
Is the Red LED lit okay ? What are its anode/cathode voltages ?

 

[PhilipMarlowe]

The circled diodes aren't the problem.
Is the Red LED lit okay ? What are its anode/cathode voltages ?

It is very bright, but I'm not sure how to measure the voltages. That I don't know, how would you check that?

 

[Newmarket]

It is very bright, but I'm not sure how to measure the voltages. That I don't know, how would you check that?

You have a voltmeter / multimeter ?

 

[PhilipMarlowe]

You have a voltmeter / multimeter ?

yes. Sorry, I'm quite new to DIY troubleshooting so excuse my newbiness.

 

[Newmarket]

yes. Sorry, I'm quite new to DIY troubleshooting so excuse my newbiness.

no problem. So measure from ground to each side of the LED. Then, double check by measuring across the LED. Use the black (-) lead on the cathode to give a positive voltage reading.

 

[Ricardus]

Why don't you post a real pic of the PCB while you're at it. Hi res.

 

[PhilipMarlowe]

 

[PhilipMarlowe]

Thanks for the replies!
So i measured the LED with 48v active. Without an LED attached the readings are 44v without an LED attached and 1.8v measured with the LED active. Does this make any sense?
I will try to get replacements for the diodes that I misused at the XLR out. Maybe it is what is causing this issue after all?

 

[ccaudle]

When I zoom in the picture the transistor installed in the spot marked BC550C has BC557C on the markings. Did you mix up your NPN and PNP transistors? I can't see the transistor installed in the BC560C spot clearly enough to see what is there.

 

[Ricardus]

Those aren't 1N4004s, but otherwise notihng looks too offensive. Make sure you electrolytic caps are oriented properly.

 

[Ricardus]

When I zoom in the picture the transistor installed in the spot marked BC550C has BC557C on the markings. Did you mix up your NPN and PNP transistors? I can't see the transistor installed in the BC560C spot clearly enough to see what is there.

Ah yeah, good call. The 557 is supposed to go where it says BC560.

If your other transister says BC547 that goes where it says BC550.

 

[Newmarket]

I was just going to say what ccaudle pointed out.
BC550 is NPN
BC557 is PNP

The "not 1N4004" diodes do no functional harm.

 

[Ricardus]

Out of curiosity, why are the caps and transistors mounted so high off the PCB?

 

[Mr. Moscode]

Switching the 2 transistors will turn the box in a poorly biased voltage amplifier. It's supposed to be a 2 stage follower and only add current to the input signal. The 2 12 V diodes in front limit the input signal to 12.6 v peak.

I'm guessing the other diodes are part of a ground loop elimination or limiting circuit. The 2 1n4004 diodes will conduct AC ground loop current if it exceeds the .6 volt diode drop for safety. Otherwise ground loop current sees the 100 ohm and 100n cap which should limit the current and keep it out of the audio. I can see its usefulness with Fender, etc gtr amps with their ground switch circuits.

Let us know how it turns out.

 

[PhilipMarlowe]

Out of curiosity, why are the caps and transistors mounted so high off the PCB?

Just in case I need to unsolder them, like now.

When I zoom in the picture the transistor installed in the spot marked BC550C has BC557C on the markings. Did you mix up your NPN and PNP transistors? I can't see the transistor installed in the BC560C spot clearly enough to see what is there.

That's probably it - seems like I ordered a different part since the one I needed was out of stock and the specs were similar. Will report back once I get my hands on the new parts! Thanks a bunch everyone!!!

Switching the 2 transistors will turn the box in a poorly biased voltage amplifier.

What do you mean? Isn't this meant to be a DI Box and not an amp?

 

[Newmarket]

What do you mean? Isn't this meant to be a DI Box and not an amp?

To confirm the whole unit is a dedicated DI box.
I think the reference is to the two transistor 'active' stage before the transformer.
Seems to think it is 'poorly biased' ?
I haven't looked at the detail of that. But the circuit is a big favourite with many on here.
Let us know how it goes.

 

[TwentyTrees]

It's a poorly biased voltage amplifier if you switch the two transistors by accident, as the OP has. If they're in the correct positions, it's all golden.