Passive DI output level

[Finlaywatt]

I put together a passive DI using a carnhill VTB9072 (12:1) transformer. I prototyped it and the output seemed a bit low. Sounded great though! Is there a way I could increase the output of the circuit?

It was wired as follows -

3 - 4 (Connect primaries in series for maximum input impedence)

5 - 6 (Input cold side connected to GND)

7 - 8 and 9 - 10 (Connect secondaries in parallel for minimum output impedence)

2 - Jack Tip (Input hot)

5 - Jack Sleeve (Input screen to GND)

7 - XLR pin 2 (Output hot)

9 - XLR pin 3 (Output cold)

11 - XLR pin 1 (Output screen to GND)

Cheers, Finlay.

 

[ruffrecords]

I take it you are plugging the output into a mic pre of some kind?

12:1 step down is a loss of about 22dB. So if your source outputs 120mV then the DI output will be about 10mV. Setting the mic pre to 40dB gain would get this back to 1 volt or just over 0dBu. Some guitars can output as much a 500 to 1000mV which would need only 27dB or less gain to get it back up to nominal level.

Cheers

Ian

 

[Finlaywatt]

Well it's going into my interface. I could run it through my external pre before the interface too which would help a lot. But it seems to be pretty quiet. The circuit is very simple, the input and output are just wired to the transformer. No caps or resistors etc. Is there a better circuit to use for this?

 

[MatthisD]

You have it wired as 24:1 voltage ratio with the secondaries in parallel so your loss is a few dB more than if it was wired 12:1.

 

[Finlaywatt]

Ahh okay. That's for pointing this out. How should I wire it for the best result?

 

[MatthisD]

It would depend on the source instrument and the impedance required. 12:1 ratio with a 1.5K impedance mic preamp connected would present 12*12*1.5K=216K, the basic transformer details are here:
http://www.audiomaintenance.com/downloads/carnhill_design_guide.pdf

 

[Finlaywatt]

It's mostly going to be used for bass. My preamp (GAP Pre-73) is switchable between 300 ohms and 1200 ohms. I'm pretty new to audio electronics so I'm not really sure what to look for in the specs. Apologies if my questions are a bit stupid.

 

[abbey road d enfer]

Well it's going into my interface. I could run it through my external pre before the interface too which would help a lot. But it seems to be pretty quiet.

I take it you connect the DI to a line input. Typical impedance of such a line in is about 10k+.
So you could connect the secondaries in series for 6dB more output. The resulting impedance seen by the source would be about 1 Meh, which is correct for passive pick-ups. In fact the inductance of the windings is probably a limiting factor so the actual impedance would be lower than  Meg but still adequate. What level do you register in your DAW? It is good practice to record at about -12 dBfs peak.
Now if you want to use your GA pre, you must use the highest impedance selection (1200) with your secondaries in parallels. this will result in about 500kohms seen by the source. 300 ohms would reflect as about 120k, which is too low for most passive pick-ups.

 

[Finlaywatt]

Yeah. I wouldn't want to be using it with the preamp, but I did use it because I needed the extra boost. So for using it for a bass or guitar I'd be best to connect the secondaries in series? How would I wire this, what needs to change from how I have it wired just now?

 

[abbey road d enfer]

Yeah. I wouldn't want to be using it with the preamp, but I did use it because I needed the extra boost. So for using it for a bass or guitar I'd be best to connect the secondaries in series? How would I wire this, what needs to change from how I have it wired just now?

Connect 9 & 8. 7 is Hot (pin2), 10 is Cold (Pin 3).
As I mentioned earlier, the inductance defines a nominal impedance of 144k, but this is probably at 20 Hz. Considering the guitar lowest frequency is 80 Hz, that makes it adequate for 600kohms. 300k ohms for bass (40 Hz).

 

[Finlaywatt]

Okay, 9 & 8 as well as keeping it as is? So 7 to 8 to 9 to 10?

 

[abbey road d enfer]

Okay, 9 & 8 as well as keeping it as is? So 7 to 8 to 9 to 10?

No. Disconnect these links.

 

[Finlaywatt]

So connect 8 & 9. What do I do with 7 & 10? Again, apologies for stupid questions.

 

[abbey road d enfer]

So connect 8 & 9. What do I do with 7 & 10? Again, apologies for stupid questions.

"7 is Hot (pin2), 10 is Cold (Pin 3)."

 

[Finlaywatt]

Okay, sorry, I got confused because I was using 9 as cold before. I understand now. Thank you!