Distortion in Bo Hansen DI

[ChuckHanning]

Hi All,
I built one of Bo Hansen's DIs recently and have been comparing them to the bumblebees I built previously.

I seem to have some distortion in the signal that I can't seem to figure out. I ordered most of the parts from Mouser but did make couple substitutions based on extra supply I had:

1. NPN transistor is BC184C from SCA N72 mic preamps I made a while ago, instead of BC550C
2. PNP is 2N4403 as this was listed as substitute for PNP BC560C
3. Output capacitor is 100 uF ELNA audio capacitor (instead of 10 uF). They don't make in 10 uF size. Perhaps I need another brand in 10uF?
4. Capacitor from 1M resistor to ground is 33 uF not 10 uF. (original schematic was 1 uF then revised to 10 uF). I thought would improve bass response but perhaps this is a mistake on my part?

Would any of these changes cause the distortion?

I had read previous thread in which they had distortion problems on the forum in which Bo had stated that two Zeners should be connected together with cathode instead of anodes so I reversed how they were originally installed.

I measured the following voltages in the circuit:

1. Phantom power at where resistors connect to transformer is 36.4V (not 48V? I checked with two different mic preamps and is the same). Perhaps there is some voltage is dropped across secondaries of output transformer?
2. Other end of resistors (connected to anode of LED) is 22V.
3. After LED connected 3.9k resistor is 20.1 V
4. At collector of BC560C is 8.24 V

What am I missing here, as the bumblebees seems to sound and work fine with the same preamps?

Thanks in advance for your ideas!
Chuck

 

[ChuckHanning]

So was rereading Bo’s webpage and realized that the PNP transistor needed to be flipped 180 degrees and wasn’t.

So corrected this and tried again. Instead of distorted sound, now no sound. So thinking that the 2n4503 is toast, I replaced it with a PN2907A which I had (and is also EBC)

Unfortunately now still no sound at all.

The voltages have changed to:

1. 36.3 v at transformer secondary
2. 25.3v at LED anode
3. 23.4v at LED cathode
4. 11.6v at pn2907a collector

What should I try or test next to figure out what is wrong?

 

[ChuckHanning]

Just after I posted it, I tried the DI box with power and the thru output, which works.

So perhaps problem with the lundahl output transformer or xlr jack?

 

[ccaudle]

Would any of these changes cause the distortion?

Does not seem so.

Phantom power at where resistors connect to transformer is 36.4V (not 48V?

48V is with no load. The 48V is supplied through a 6k8 resistor to each pin, so 36.4V at each pin would imply around 1.7mA supply current from each pin, or about 3.4mA total current.

1. After LED connected 3.9k resistor is 20.1 V
2. At collector of BC560C is 8.24 V

I assume you mean at the emitter of the BC560C, since the collector is connected to the ground node. That is 11.86V across the 3k9 resistor, or 3mA through that transistor. Since there is 3.4mA total current being used, that only leaves 0.4mA for the first transistor. The emitter resistor is 100k, so lightly loaded, that could be correct.

the thru output, which works

Which way did you wire the transformer? LL1538 is an input transformer, might not be great as an output transformer, depending on the load. The schematic I saw on Hansen's web page indicated a LL1538XL, which has a larger core than the LL1538. Perhaps the transformer was being overloaded, which caused the original distortion problem.
Did you re-wire the transformer connections when changing the transistor? Any chance the dual windings were connected together in opposite polarity intead of same polarity, and the signal is cancelling?

 

[ChuckHanning]

Which way did you wire the transformer? LL1538 is an input transformer, might not be great as an output transformer, depending on the load. The schematic I saw on Hansen's web page indicated a LL1538XL, which has a larger core than the LL1538. Perhaps the transformer was being overloaded, which caused the original distortion problem.
Did you re-wire the transformer connections when changing the transistor? Any chance the dual windings were connected together in opposite polarity intead of same polarity, and the signal is cancelling?

I used the silent arts PCB for this project which, as Bo designed, uses the mic input transformer backwards as a DI transformer. The (now secondaries) are wired 1:5 in the PCB, and you just solder the transformer in the holes provided. I suppose it is possible that the transformer was overloading, but I'm thinking the distortion was because I had originally soldered the 2N4403 in the BC560 spot backward and the emitter and collector were in the wrong locations. After discarding this and replacing with a PN2907A (in the correct orientation), the guitar signal sounds fine from the amp output/thru connection and is not distorted.

So checked XLR wires last night and all had good continuity, so turned my attention to the transformer.

I then removed the transformer from the board using my solder rework gun (with vacuum) carefully, and measured the two secondaries (each about 45-50 ohms, as expected), but the now primary (pins 5&6) showed an open circuit (>1M ohm), which explains no sound from the XLR out.

I have sent an email to Lundahl to see if tis can be repaired. I had originally ordered an 1538XL, but had received the non-XL version with my order. Do people solder the Lundahl transformers at cooler temperatures? I typically use 350C for everything and just be mindful of time with soldering iron on each component, but maybe it got damaged when it was first installed?

If I have to just replace this:
1. I'm leaning to ordering a Jensen JT-13K7-A
https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-13k7-a.pdf
instead of the Lundahl which I can more easily get in the states, and is another 1:5 mic input transformer with flying leads,
2. Or the OEP A262A3 which I can buy from Newark (also here in the US).
Has anyone else used these transformers and what are your thoughts?

I've made some passive DI boxes using Jensen JT-DB-C (also flying leads) and two Bumblebee jFet Di boxes which come with their own custom transformer and didn't have any problems with them.

Thanks everyone for your thoughts and advice
Chuck