best way to go from balanced XLR - RCA (unbalanced) - anyone...?

Discussion in 'Amps and Cabs [BG]' started by sixontimber, Oct 19, 2003.

  1. sixontimber


    Aug 2, 2003

    What would be the best and most trouble (noise) free way of going from a balance XLR pre-amp out to an unbalanced RCA power amp in? I know it's possible to make up a cable, but is there any way that's preferred over that - some sort of box/adapter that does it for you?

  2. sixontimber


    Aug 2, 2003
    Further to the above...

    The pre-amp output is balanced and can either be line (600 Ohm) or mic (150 Ohm) level.

    The power amp input is unbalanced RCA and has an impedance/sensitivity of 33KOhms/1V.

    Will that cause any problems? Could I just use an XLR - RCA cable, or would I need some sort of impedance converter/interface.

    I am in the dark here!

  3. Take an XLR cable and disconnect pin 3 on the end connected to the preamp. On the other end, take pin 2 as hot and pin 1 as ground and solder them to an RCA jack.
  4. sixontimber


    Aug 2, 2003
    And that will definitely do the business? No loss of signal. Basically as it would be balanced but slightly more susceptable to interference...?
  5. No, you will be cutting you signal level in half, which you MUST do to keep from overloading the RCA input and you will be completely negating the noise rejection of a balanced cable, so yes, you will be creating more opportunity for interference. That's why most pro gear uses balanced inputs.

    If you have hum problems, then buy a cheapie passive DI box and use a short XRL patch cable from the output of your preamp to the OUTPUT of the DI, and take your unbalanced line from the INPUT of the DI. The transformer in the DI will de-couple the ground loop and automatically match levels for you.
  6. Joe Smithberger

    Joe Smithberger Supporting Member

    Mar 8, 2002
    Canton, Ohio, USA
    Another option would be to buy a cheap $30 Behringer mixer and take the tape out (RCA). This would also allow you to control gain out.
  7. The quality of a $25 direct box will be greatly superior to a Behringer mixer; it will have less noise, and since the power amp will have an input attenuator, gain control from a mixer is superfluous.
  8. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    All right, here's the skinny.

    There are two types of "balanced" outputs. They require different types of wiring.

    One type is the "transformer balanced output", which means that the signal will appear between pin 2 and pin 3 of your XLR, and pin 1 will only be used as a shield. To get the signal from this type of output to your RCA plug, you'll need to do what Psycho Bass Guy said in his first post, which is to connect pin 2 of the XLR to the tip of the RCA plug, and pin 3 to the sleeve (which will then become the ground).

    The other type of "balanced output" that is most common in bass preamps is the "fake" balanced type, which uses two op amps to generate the positive and negative halves of the signal. To get the output from this type to your RCA plug, connect pin 2 of the XLR to the tip of the RCA plug, and pin 1 of the XLR to the sleeve. Do not connect pin 3 of the XLR with this type of output. And yes it is true that your signal level will be 3 db down in this mode, but that's all relative anyway and it shouldn't be a problem.

    You'll have to consult your manuals to find out what type of output you have. If it's an "ordinary" bass preamp or amp (SWR, etc) the op amp output type is most likely (most of the commercial gear doesn't have any expensive audio transformers in it).

    I wouldn't recommend using a DI box "backwards" unless you are sure of what's inside it. If there's a transformer in there you might be okay, but if it uses other type of circuitry it might not work too well.

    For impedance matching problems, use a "bridging connection" like the crusty old audio guys do. Take a resistor of the correct value to match your output impedance, and put it across the line (that is, across the RCA plug, using whatever input connection method you decided on earlier). The high impedance input of your power amp will then serve as a "bridge" and both sides will be happy.

    For more information on direct boxes, I highly recommend checking out the Jensen Transformers web site (, go to the section that has all the white papers. There's plenty of information grounding and impedance matching and etc.
  9. Actually I recommended that pin 3 be lifted and leave the ground to pin 1. I was referring to servo balancing using op amps, the most common in audio gear. If you run pin 2 hot, pin 3 ground, you'll still be running a +4dB with respect to 0vu signal and it will more than likely overload the RCA input. When you remove the -/+ balanced(pins 2+3) aspect of the signal, you'll cut the levels in half and give the RCA input a lower voltage.

    When you use the direct box, you're utilizing its transformer primary as a balanced input feeding its secondary to ground-isolated unbalanced outputs. I am referring to a passive DI which is a transformer and nothing more, should have clarified that, but I thought the rest of the post would make it obvious. You can run a DI transformer whichever way you like with respect to signal flow, since IT is the impedance matching mechanism and is far more current capable than an audio signal line could ever deliver.
  10. BottomLine


    Nov 17, 2002
    Rochester MI
    Hoping someone can help me!!!
    I have an Ampeg SVP-Pro preamp which I am going to use with my Crown K2 power amp and a DBX X-over into a couple of Mesa cabs, but when I hook up the XLR out from the SVP into the Crown's XLR in.....I get a very low volume out of my cab.
    The SVP has a "transformer" balanced out in it(per the owners manual).
    Am I doing something wrong here?
  11. Look for an output level switch or control. It sounds as though you are sending a balanced microphone level signal (-60 dBu, meant for mixer channel inputs) to your amp's XLR input which is expecting a +4 dBu (professional line) level. I couldn't find any info on Ampeg's website about the balanced level out, but I didn't really dig around.

    The transformer on the output is a passive component and either works or it doesn't. If you're using a standard, correctly wired mic cable, it should do what it claims to. Even with one signal leg lifted, you'd only notice a 3 dB signal loss and increased niose. There is also the possibility your preamp is malfunctioning.
  12. BottomLine


    Nov 17, 2002
    Rochester MI
    Thanks for your help Psycho........I am going to call my dealer and double check on the possibility of it being defective, although it looks like the XLR output is meant to drive a mixer channel. There is no level set on it. I guess I'll be stuck using the 1/4" connection even though there seems to be a bit of noise.

    Overall, the SVP-Pro does have a very nice, dirty ampeg tube sound!