Transformer options for passive DI

[beegandy]

Hi all,

I'm wanting to build a passive DI for line-level high-output sources (keyboards, SPDs, drum machines, ect.). I'm aware of the usual options for transformers for passive DI's but I'm not sure why I should choose one over another. For this project the transformer will need to be PCB mountable and ideally as small as possible. Any schematics or tips for such a build would also be appreciated as I'm fairly new to this!

Some options I've shortlisted are:
- Jensen Jt-db-epc
- Lundahl LL1530
- Lundhal LL1531 (smaller size)
- OEP A187A10C

Cheers!

 

[JohnRoberts]

small is not typical for high output with LF response (drum machines?) transformers.

JR

 

[abbey road d enfer]

- Jensen Jt-db-epc

Probably the best of the lot because of its dual shield.

- Lundahl LL1530

Not a bad second choice

- Lundhal LL1531 (smaller size)

Not suitable, it's a 1:1 line input xfmr

- OEP A187A10C

Run away from it! OEP have acquired the rights to the brand names of Carnhill, Marinair and Sowter but they haven't kept any of the guys who can design audio xfmrs.

 

[dmp]

I'm not sure why I should choose one over another.

A DI will step down the signal to mic level. Do you want to step down to mic level? The Jensen Jt-db-epc will be a 12:1 step down for instance.
Or are you trying to isolate and balance but stay at line level? For instance, if you have a unbalanced line out you could use a line level transformer to balance and use for a line level input.

 

[beegandy]

Thanks for your replies everyone.

A DI will step down the signal to mic level. Do you want to step down to mic level? The Jensen Jt-db-epc will be a 12:1 step down for instance.
Or are you trying to isolate and balance but stay at line level? For instance, if you have a unbalanced line out you could use a line level transformer to balance and use for a line level input.

It needs to step down to mic level and be balanced. It will exclusively be used in live audio environments.

Probably the best of the lot because of its dual shield.

Not a bad second choice

Not suitable, it's a 1:1 line input xfmr

Run away from it! OEP have acquired the rights to the brand names of Carnhill, Marinair and Sowter but they haven't kept any of the guys who can design audio xfmrs.

Jensen is ideal, but I’ve heard of long wait times (I’m in Australia). Would you say OEP is out of the question or are they just not what those other brands used to be regarding quality? It’d be nice to try with something cheaper than the Jensen (again quite pricey in Australia), although I’m not looking for the cheapest alternative.

Also, the primary and secondary impedances are different between the Jensen and LundhalLL1530, is that something I should be concerned about or would they both be comparable?

Thanks!

 

[abbey road d enfer]

Would you say OEP is out of the question or are they just not what those other brands used to be regarding quality?

OEP is just not the same quality. The core is M6, typical power xfmr stuff, when the others use a combo of steel and nickel, which results in much better distortion.
I must say that, driven by a typical synth/drum machine output, it does work and the difference may be subtle at best.
I must also say that I'm somewhat influenced by personal grievance with the Carnhill conglomerate that owns OEP, who bought the assets, trade marks and designs from several winders, including Sowter, fired all the persons that can design audio xfmrs, and use these prestigious names for marketing their inferior designs.

Also, the primary and secondary impedances are different between the Jensen and LundhalLL1530, is that something I should be concerned about or would they both be comparable?

Indeed, the LL has a 7:1 ratio vs. the Jensen's 12:1 ratio. That would make a significant difference with a passive guitar or bass, but not much with buffered outputs. The LL would be somewhat hotter than the others, which may be a good or a bad thing. Good thing if the preamp is noisy, bad thing if it has poor headroom...

 

[swpaskett]

I have always used the Jensen with very good results. They have a schematic on their webpage somewhere that works will with their tranny. I have had to place these things right next to AC power lines and had no problem with hum pickup, unlike many commercial DIs that cost as much or more.

 

[beegandy]

I know not everyone is a fan of OEP, but what do you think of this schematic? The OEP A187A10C has a winding ration more closely aligned with the Jensen compared to OEP A262A1E - any thoughts on the differences between these transformers?

@abbey road d enfer you mentioned winding ratio may not be too noticable with buffered outputs. Do active outputs (keyboards) generally have buffered outputs? I'm not too across this...

Also, does anyone know what the capacitator connected to A3 interwinding screen 2 (Black wire) does? Is it for RFI shielding? Document: https://docs.rs-online.com/8313/0900766b815288d8.pdf

Many thanks!

 

[abbey road d enfer]

I know not everyone is a fan of OEP, but what do you think of this schematic? The OEP A187A10C has a winding ration more closely aligned with the Jensen compared to OEP A262A1E - any thoughts on the differences between these transformers?

The main difference is the core, but it is not too significant with a medium impedance source such as a synth.

Are active outputs (keyboards) generally have buffered outputs?

All synths and electronic keyboards have buffered outputs, even a venerable Wurly200 or Vox Continental. OTOH Fender Rhodes without tremolo are fully passive, like an electric guitar.

Also, does anyone know what the capacitator connected to A3 interwinding screen 2 (Black wire) does? Is it for RFI shielding? Document: https://docs.rs-online.com/8313/0900766b815288d8.pdf

It replaces a hard connection, that could generate significant ground currents, with a capacitive path that still allows "dumping" EMI/RFI, whilst interrupting the DC path.
If the "Link" output is connected to an amp and the DI to a mixer, any differential voltage between the two "grounds" may create a current circulating in the cable shield, and thus introduce hum and buzz.
By providing a high impedance at low frequencies, the current is reduced, while maintaining a path for the high frequencies.
The value is not critical at all. Many woud recommand 0.1uF, but 47nF is just fine.

 

[beegandy]

The main difference is the core, but it is not too significant with a medium impedance source such as a synth.

All synths and electronic keyboards have buffered outputs, even a venerable Wurly200 or Vox Continental. OTOH Fender Rhodes without tremolo are fully passive, like an electric guitar.

This capacitor is there to dump RFI/EMI into earth. The value is not critical at all. Many woud recommand 0.1uF, but 47nF is just fine.

Thanks for all your help on this! I might try both the OEP A262A1E and A187A10C and see what the difference is.

Cheers

 

[JohnX]

Finished a beefy, passive DI using a FAB DI 1 transformer from Chris at FAB.
So far, I’ve only tried it on guitar in an A/B with our Cinemag passive. Can immediately say that the FAB has a thicker lower midrange and maybe a little bit more of a rolloff up top. Other than that, they were extremely similar. This one weighs about 1 pound, I have a feeling I’m really gonna like it on bass.
The enclosure design was a hydrodip and this is the first time I put a matte clearcoat on top of it as opposed to gloss - I think I may prefer the matte.

 

[PermO]

What is that orange XLR ? ..never seen that before.

 

[Newmarket]

Also, does anyone know what the capacitator connected to A3 interwinding screen 2 (Black wire) does? Is it for RFI shielding? Document: https://docs.rs-online.com/8313/0900766b815288d8.pdf

It provides a capacitive path to "ground" from the screen when the earth lift switch is in the position shown.
So the screen still screens.

 

[abbey road d enfer]

What is that orange XLR ? ..never seen that before.

It replaces a hard connection, that could generate significant ground currents, with a capacitive path that still allows "dumping" EMI/RFI, whilst interrupting the DC path.
If the "Link" output is connected to an amp and the DI to a mixer, any differential voltage between the two "grounds" may create a current circulating in the cable shield, and thus introduce hum and buzz.
By providing a high impedance at low frequencies, the current is reduced, while maintaining a path for the high frequencies.

 

[Newmarket]

What is that orange XLR ? ..never seen that before.

It appears to isolate the shell of an inserted XLR connector from the metal (?) of the chassis plug.
So isolating the "fouth" XLR connection as well as having pin 1 connectivity controlled by the earth-lift switch. I haven't seen that part before though. Manufacturer ?

 

[PermO]

Think I found it:

https://www.amazon.com/Keadic-8-Pack-Male-Female-Jack/dp/B089R19X6C

 

[Walrus]

Earth tab inside.

 

[JohnX]

What is that orange XLR ? ..never seen that before.

I had originally been planning on using the round Switchcraft panel mount XLR, but then I found these on Amazon and decided to try them out. I thought they would better match the pop art graphic that I was using for the hydrodip covering and I think it worked well.

It looks like you found them in the Amazon link. FYI, the panel itself is metal and they seem to be pretty well-made.

 

[JohnX]

Just finished another passive DI. This one is using a Sowter 1585. I just compared it to another one that uses the FAB DI1 transformer as well as our Cinemag DBX. I only had the opportunity to try it out on funk electric guitar but it was immediately one click hotter on my Neve 1073 than either of the other two (which were both almost identical in gain).
The showdown really came down between the Cinemag, and this one in terms of tone. Both very similar, but this one seems to have slightly more definition in the upper mid. It felt slightly more open and clear, TBH. I could be imagining it, but I also had a sense that it felt more “even” across the frequency range that it was playing in.
I was really shooting for a small form factor. I know that I can get it even smaller than this if I put the transformer on the inside, but I love the way it looks sticking out of the top.

 

[abbey road d enfer]

I was really shooting for a small form factor. I know that I can get it even smaller than this if I put the transformer on the inside, but I love the way it looks sticking out of the top.

Small form factor is an invitation for thievery.
I speak from experience...