Unbal/balanced interface consumer to pro level?

QuadraphonicQuad

Help Support QuadraphonicQuad:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

quadsearcher

1K Club - QQ Shooting Star
Joined
Nov 8, 2010
Messages
1,196
Putting together a simple 2 channel system with networked receiver that has an RCA output for subwoofer. But, want to use a JBL Pro powered subwoofer that only has XLR input.
A passive (transformer) interface will not do the -10dB to +4dB conversion.
Has anyone found a good active device to do this?
(Not high end brand, just good industry standard stuff).
Anyone try this one? TX-LC2 ‐ Unbalanced to Balanced Line Combiner Around $100.
 
Putting together a simple 2 channel system with networked receiver that has an RCA output for subwoofer. But, want to use a JBL Pro powered subwoofer that only has XLR input.
A passive (transformer) interface will not do the -10dB to +4dB conversion.
Has anyone found a good active device to do this?
(Not high end brand, just good industry standard stuff).
Anyone try this one? TX-LC2 ‐ Unbalanced to Balanced Line Combiner Around $100.
An easy project for DIY types, just need a couple of op-amps. Commercial devices will be expensive because they aren't that common.
 
Connect the + signal from the unbalanced source to pin 2 of the xlr and connect the shield from the unbalanced source to pin 3 of the xlr. Another option with the same connections but altered shielding is to use an xlr cable for the run. Use the pins 2&3 +&- wires and wire to the unbalanced end as above. Connect the shield to the xlr end but leave it unconnected on the unbalanced end. That will extend the shielding to the cable.

You shouldn't need to or want to put extra preamp stages or a transformer in the circuit. The balanced input is probably differential and good with this. Their manual probably even tells you to make the cable as described above.

That must be an older model? I thought they've had additional unbalanced inputs as well as -10/+4 switches for a long time.

Just calibrate for correct output level. As long as you have the right output level with no noise, you're good. You're not going to have any hiss. There might only be ground loop worries. One of the shield connection options should be able to avoid that.
 
Or just buy a direct-insertion box. This is what is commonly used to make such a connection.

Use an RCA to phone cable from the amp, and use an XLR cable to the sub.


di-images.jpg
 
Thanks all, I'm getting the interface just in case, but correct we shouldn't need it. The sub is a VRX918 and has only XLR in, however the level trim is calibrated +4 to -10 so a balanced cable with shield tied to pin 1 at the xlr end and pin 2 to center of rca, pin 3 to outside of rca should work fine . It helped to read the manual, ha ha.
 
Last edited:
I would agree but it sounds like he needs the extra +14dB gain .

That is not really an extra 14 dB gain.

Using unbalanced lines, the +4 dBu is only 11.8 dB higher than the -10 dBV.

That's because 0 dBu is 7.75 V while 0 dBV is 1.0 Volts. They are different standards.

Also the balanced line has two signals in opposite polarity. The input adds these in opposite phase to get the signal. That adds 6 dB.

Finally, the transformer ratio in the direct box makes the -10 dBV into the +4 dBu without any external help.
 
That is not really an extra 14 dB gain.

Using unbalanced lines, the +4 dBu is only 11.8 dB higher than the -10 dBV.

That's because 0 dBu is 7.75 V while 0 dBV is 1.0 Volts. They are different standards.

Also the balanced line has two signals in opposite polarity. The input adds these in opposite phase to get the signal. That adds 6 dB.

Finally, the transformer ratio in the direct box makes the -10 dBV into the +4 dBu without any external help.
Yes if the box has a transformer, not in the case of a simple cable adaptor, mentioned earlier. Op-amps are cheaper than transformers if gain is needed.
 
A direct box always has an isolation transformer.
They are often used with musical instruments such as guitars. I would question the quality of many of them for hifi audio use though. Those with high quality transformers are bound to be very expensive!

I had a few really good transformers that I used on the input of my tube amps, without them running long signal cables would of resulted in incurable amounts of hum. They help to reject RF interference as well. There once was an AM broadcast tower across the road from me and RF interference had been a perennial issue.

You get galvanic isolation with a transformer as well, so it breaks up ground loops!

Really they should call it something other than a direct box, direct implies that there is nothing in between!
 
Back
Top