Audio Transformer Question

T

Tom Hester

Freshman
Jan 11, 2011
58
0
0
36
Southern California
It seems like there is an easy way to make this work but I am having trouble finding the best solution.

I am working for an artist who plays with a full band as well as tracks. The tracks rig is a Macbook Pro running Logic through a Avid Mbox (the new black one) Using 4 outputs (Track L, Track R, Click, Guitar). The guitar output is just sending "dry" guitars (recorded directly off the pedalboard) into an amp onstage. This has worked flawlessly for a number of shows, the issue we have run into twice now is with back lined amps we have no guarantee of quality. The major issue was an old fender twin reverb which seemed to be spitting voltage back into the Mbox and making all of the other outputs buzz, as soon as I would unplug the amp all of the outputs were clean. We eventually switched to an AC30 which solved the issues for that show, but I was hoping to find an audio transformer to mount in the rack to solve this issue before we run into it again.


Any Ideas?
 
Re: Audio Transformer Question

+1 on the Radial Engineering Re-Amp products.

If, on the other hand a passive DI box is used there are ‘gottchas’ to be aware of.

One major issue to watch out for when using a DI box backwards is too high of a load (low impedance) on your signal source; which in this case is the Mbox. Because there will be a huge increase in voltage to the amplifier (yes, you’ll need to turn down the Mbox output a lot) there will be a commensurate increase in drive current from the Mbox to keep things linear. Transformer law also dictates that the load impedance on the secondary is ‘reflected’ to the source multiplied by the square of the primary to secondary’s winding ratio. Most DI boxes have a winding ratio of about 12:1. However, when running the DI box backwards this situation is the inverse—meaning the load impedance that the Mbox will ‘see’ is the load’s input impedance divided by the square of the winding ratio.

For example; if the Mbox outputs need to see at least 2000 ohms to stay linear and the DI’s transformer ratio is 12:1 the amplifier’s input impedance would need to be:
2000 = (Amp Impedance )/Turn ratio squared.

Amp Impedance = 288K ohms. 1Meg ohms would be safer.

Obviously, only an 'instrument input' has impedance in this magnitude, not the line inputs.

David
 
Top Bottom