D.i.

[jenks]

Hi, I want to incorporate a D.I. section after a preamp circuit that I've made so that I can have one line-level output going straight from my preamp but also one female XLR output so that I can plug straight into a mixing desk.

I've been looking at transformers, impedance etc and I understand that I need to reduce the signals' impedance as well as its' level quite considerably - but how much??

And, will a transformer alone be enough to decrease both parameters by a sufficient amount - if the amount of windings are appropriate?

Cheers

 

[jenks]

Lack of standards for line and mic levels

Okay, I've done some research and I've discovered lots of articles contradict each other when it comes to altering impedance and level of a line signal to that of a microphone signal!

I've read that line-level ranges from 0.3 to 3 volts, I've also read that it is +4dBu which corresponds to a sine wave of 1.228Vrms, and I've also read that it is -10dBv!

I've read as well that the impedance of line signal is 600 ohm in some places but 50 kilo ohm in others!

I've also read that microphone level is in the micro-volts range or -30 dBv. And that a typical microphone input should 'see' an impedance of 150 ohm to 3 kilo ohm!

All this doesn't help in my quest to build a D.I.! I just want to know how many turns my transformer should have in order to alter my active bass guitar signal so that it will be suitable for the input of a microphone input on a mixing console!

Any attempt to demystify all this stuff surrounding impedance and level of different signals will be greatly appreciated!

Cheers (Angry Jenks)

 

[imix500]

Hi Jenks, you've run across one of the challenges when dealing with pro/consumer audio. All of those levels are correct, but the pro and consumer world uses two different sets of standards. +4 is a pro/broadcast standard into a balanced 600 ohm line. -10 is a consumer standard into a high impedance unbalanced line. Usually around 10k. To build a passive DI, all you need is a good quality transformer that has at least a 10k single primary winding and at minimum a 600ohm secondary. Jensen makes a fantastic transformer for this purpose. It has ruler flat response and excellent shielding. All this can be yours for about $75. There are many sources for the transformer you need. A little google search will give you lots. Good luck.

 

[jenks]

I found a great article from sound on sound that explains it all!...

http://www.soundonsound.com/sos/1994_articles/feb94/decibels.html

At last!

 

[jenks]

Hmm, I'm still rather confused atm.

As i understand it the signal level of a professional line signal is at +4dBu and the level of a professional microphone level is -50dBu (according to sound on sound) so I would have thought that working out their respective voltages and dividing them would give a factor of ten - because this is what all the audio transformers I've researched do. They reduce the signal voltage by a factor of 10. To my suprise I came out with a factor of 500!

What's that all about??

+4dBu = 1.585Vp
-50dBu = 3.16 mVp

1.585/(3.16 x 10^-3) = 502!

Crazy stuff!