Alctron Stereo Passive DI120 to Jensen Mod

[guitar4444]

I just modded this Alctron stereo passive DI to Jensen transformers. Very easy mod and quite a strong case and circuit board. Took less than a couple of hours. There is more details on my blog here Alctron Passive 2 channel DI 120 Jensen Transformer Mod | Melbourne Recording Studio - Echidna Recording Studio Blog
Or should I upload it here if anyone is interested?

 

[Ike Zimbel]

Nice! If you replace C1 and C2 with 10uf film caps, I believe you will hear the difference.
EDIT: But read the rest of this thread to find out why you won't!

 

[guitar4444]

I might give that a go, did think about it and it would be pretty easy though they would be pretty big caps. The Jensen schematic specifies 10n and they are only in the circuit when the ground lift is active so you wouldn't hear a difference otherwise (a blind test would be interesting however), though the Alctron circuit is implemented slightly differently. For the price of changing them probably worth it even just for reliability.
cheers Greg

 

[guitar4444]

After a further look I've just noticed that the caps C1and C2 are 47uf 63 volt coupling caps and are in parallel with what looks like a 33k resistor. They are in series with the earth leg of the tranny on the output side so the full signal is going through the caps all the time. The lift switch literally disconects the ground connection So by the Jensen circuit they are not actually needed at all. I have some 22uf 100v wimas I could have put there if I had had have put the jensen trannies a bit closer together I might just have squeezed them in. Anyway I figure the Alcton circuit just has the coupling caps there in case the DI is inadvertently fed phantom power and it saturates the transformer? Or perhaps it provides some DC isolation when the ground lift isn't lifted? Perhaps an expert can chime in here. I might change it back to the Jensen circuit which has no coupling caps. The Jensen circuit only has a cap and resistor in series that are only in the circuit when the ground is lifted. So no DC ground but still some AC ground. If anyone has the Radial Jensen equipped pasive DI circuit I would be interested in seeing that. Aside from that the DI sounds stella with an electric guitar or archtop though perhaps has a little distortion or weakness on the low end in a good kind of way. I'm not sure if that is the coupling caps or the little jensen DI transformers, though they have pretty good specs they are tiny.

 

[abbey road d enfer]

After a further look I've just noticed that the caps C1and C2 are 47uf 63 volt coupling caps and are in parallel with what looks like a 33k resistor. They are in series with the earth leg

Earth or cold? In both cases it just doesn't make sense. Can you trace the schemo?

 

[guitar4444]

yes the grey wire goes to the 47uf cap and then to cold pin 1 on the xlr. The cap is in parallel with the resistor orange orange black orange. But after just checking now on the original Alctron tranny it's centre tapped so the cap is actually a coupling cap between centre tap and earth/cold. So with the Jensen tranny, in the end none of the signal goes through the cap as the tranny has no centre tap, the grey wire just goes to the core by the looks, so it's not actually a coupling cap in that setup. I think I'll change the config to the same as the Jensen schematic above. I'll trace out the circuit of the original Alctron configuration tomorrow.

 

[abbey road d enfer]

yes the grey wire goes to the 47uf cap and then to cold pin 1 on the xlr.

As I said earlier, it doesn't make much sense. Grounding the xfmr core makes sense though.

 

[Ike Zimbel]

I'm pretty sure those are DC blocking caps on the input side of the transformer. That's certainly the role I was referring to when I suggested the film caps.

 

[abbey road d enfer]

I'm pretty sure those are DC blocking caps on the input side of the transformer. That's certainly the role I was referring to when I suggested the film caps.

Then why is this cap connected in the ground path?

 

[guitar4444]

I'm pretty sure those are DC blocking caps on the input side of the transformer. That's certainly the role I was referring to when I suggested the film caps.

They are definitely on the output side between the transformer center tap and pin 1 of the output XLR. The ground switch disconnects the earth plane on the circuit board. There are no input coupling caps. Here's a rough diagram shwing how the output side of the Alctron DI is set up. The Cap is 47uf 63 volt and I think the resistor is 33k. The input side has a pad network but when set at zero there is nothing connected aside from the transformer primaries. I'm going to change my circuit to match the original Jensen circuit but maintain the pad network on the input. However it works perfectly fine as is as long as the ground lift is switched on. Otherwise there is some hum/buzz when running into 1073 nic pre and a guitar plugged in.

 

[abbey road d enfer]

They are definitely on the output side between the transformer center tap and pin 1 of the output XLR.

The only effect I see is when hot-plugged, it passes DC into the xfmr's secondary and it produces the dreaded "Phantom menace"which is the cause of premature death of many a xfmr-less mic preamp.