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Balanced DI out (XLR) into poweramp input (1/4")

Discussion in 'Amps and Cabs [BG]' started by el murdoque, Mar 23, 2018.


  1. el murdoque

    el murdoque

    Mar 10, 2013
    Germany
    I have a question regarding this. I want to use an output, which is a balanced signal XLR plug, to drive a poweramp which has a 1/4" input. The only cables I found which are XLR female to 1/4" are microphone cables that are 10' and longer.

    The output in question is that of an Ampeg PF20T and the input in question is that of a GSS Baby Sumo. I cannot tell if it is a balanced TRS input, but I think it is mono 1/4".

    I have a soldering iron at the ready and could make my own cable if need be.

    So which cable will do the job?
     
  2. johnpbass

    johnpbass

    Feb 18, 2008
    Glen Mills, PA
    I think you would need a low to high imp transformer to do that. Audix makes one (model number is T50 IIRC) that I think would do the trick. I've thought about doing the same thing with my 20T. Interested to hear how that works out.
     
    Zbysek likes this.
  3. el murdoque

    el murdoque

    Mar 10, 2013
    Germany
    I know a bit about electronics, but I never grasped the impedance thing on anything else but cabs.

    So the Ampeg has a low impedance output that needs to be higher to work with a poweramp ?
    Because ... the mismatch results in not enough of a signal passing through?

    The Audix is a very unfortunate length. This will be just a bit too short to run it from the back of the ampeg to the front of the GSS.
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Check your amp to see if the ¼" input is balanced or not.

    Neutrik offers a couple of adapters.

    unbalanced: NA2FP - Neutrik

    balanced: NA3FP - Neutrik

    If you decide to go with a cable, make sure that it is shielded. Some XLR to ¼" cables are not shielded. If you build one, use Neutrik connectors and a good quality cable such as Mogami or Canare. Shops that sell studio equipment will often custom build cables to any length.
     
    chupacerveza likes this.
  5. TRS 1/4" inputs can often be dual use, balanced or unbalanced.
    XLR outputs are normally balanced.
    If both output and input are balanced, get an XLR to 1/4" TRS cable.
    ^This is the best way to do this.^

    A cable is a cable. Just because it is called a mic cable does not necessarily restrict it to microphone use.

    If you are going XLR (balanced) to TS 1/4" (unbalanced) you should have a transfirmer in the line to properly interface bal to un. Without a transformer, you may not get a proper interface and get no signal or get a lot if hum. It can be done but things start getting tricky if you don't have a good grasp of the electronics.

    Just because a cable has the correct connectors on each end does not mean they are a proper balanced to unbalanced cable. Here is an example of a transformer. You want a 1:1 impedance ratio, audio isolation transformer.

    Pro Co Sound ITX Inline ISO Transformer XLRF-XLRM
     
    Swiss Frank, Mili, BooDoggie and 3 others like this.
  6. el murdoque

    el murdoque

    Mar 10, 2013
    Germany
    I know that a cable is just a cable an that before the poweramp comes into play, cables should be shielded.
    I just meant that I don't want to run a ten foot cable on a distance of about one foot on top of my cabinet.
     
  7. I *think* that both of the XLR outputs on the PF20 are electronically balanced. Even though one of them is called "transformer balanced output" the block diagram seems to indicate that it is tapped off the 8 Ohm winding on the OT and run through a buffer circuit so it's not actually transformer balanced. This matters if the power amp input is unbalanced. If the Ampeg was truly transformer balanced then you could use a standard XLR-TS adapter with pin 2 connected to the tip of the 1/4" connector and pins 1 and 3 connected to ground. This won't work if it is electronically balanced. In that case a standard XLR to TRS will work because the ring will be left unconnected in a TS input. Or get a 1:1 inline transformer so you can use the standard adapter I mentioned above. Obviously the best scenario is if the power amp has a balanced input but that information doesn't seem to be available.

    EDIT:
    @Old Garage-Bander beat me to it. One of those inline transformers and a standard XLR-TS adapter will do what you want here no matter how the balancing is done in the Ampeg. But I think a straight XLR-TRS will work too.
     
  8. If you can't find a shorter cable a 10 ft run won't be a problem at all.
     
  9. Yep! And having the transformer in the tool box never hurts. Even if you are running balanced all the way through, you can still get hum from a ground loop. Using a transformer is, IMO, the best way to solve ground loop hum.
     
    Swiss Frank, BooDoggie and johnpbass like this.
  10. Indeed. Never hurts to have tools like that on hand. I always carry isolation transformers to gigs both mixing and playing. They can be lifesavers. I built some of my own using Lundahl ll1570s. They can be used as 1:2 splitters or just straight isolation transformers.
     
    johnpbass likes this.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    A transformer is only needed if you have a hum issue. Otherwise why clutter the signal path unnecessarily.
     
    agedhorse likes this.
  12. In this case, I think the standard XLR-TRS will work fine (assuming the PF20 is electronically balanced) so there's probably no strict *need* for a transformer. Still a good tool to have on hand, though.
     
    beans-on-toast likes this.
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    No, if the output is a preamp out that is line level, a low to high impedance matching transformer will saturate way too early. The purpose of this kind of matching transformer is to step up mic level to line level (typically to -10dBu not +4dBu)

    No, impedance is different than level. Impedance is not going to be an issue because the output impedance is plenty low to drive anything you will be looking at. The output needs to be line level.

    Generally true.

    True for either electronically balanced or transformer balanced outputs IF the receiving amp's unbalanced input shorts ring to sleeve. If not, then the transformer balanced with TRS won't work, then an XLR to 1/4" TS with pin 1 connected to pin 3 is needed.

    Without knowing which power amp, this question can't be answered accurately.
     
    chupacerveza, Zbysek, FLORINS and 4 others like this.
  14. Yeah, there are a few muddying factors involved. If the ring is shorted to the sleeve, I think that presents a problem for a lot of electronically balanced outputs because you're effectively grounding the output of an opamp or other active circuit. Is that wrong? I've always thought that leaving pin 3 open was the best way to unbalance an electronically balanced output save using a transformer. You obviously can't do that with a transformer.

    I was thinking the power amp's input jack left the ring open and now that you mention it I realize that isn't necessarily the case.
     
  15. johnpbass

    johnpbass

    Feb 18, 2008
    Glen Mills, PA
    I guess I learned something new today. I thought both preamp out and tranny out on the 20T were low impedance so if you're going to an unbalanced 1/4", you need the transformer to step up from low to high impedance. I've been wanting to try running the output of the 20T into the effects return of another amp. Different scenario than what we're talking about here I suppose.
    I'm interested to see how this works out!
     
    Zbysek likes this.
  16. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Hard to say. Every balanced line driver that I design considers that either line may be shorted to ground or to each other so I insure that this is ok indefinitely. That said, I have seen line drivers that are not so forgiving but not very often and typically in tweaky audiophile or esoteric products that are full of hyperbole in their marketing.
     
  17. el murdoque

    el murdoque

    Mar 10, 2013
    Germany
    The scenario we're looking at is as follows:

    I use a newly aquired Ampeg PF20T. This will drive my cabs as long as as it is strong enough.
    Most of my perceived sound will be through IEM and I don't need too much juice in most scenarios.
    When the 20 watts fail, a Guitar Sound Systems Baby Sumo amp comes into play.
    I want to run one of the outputs of the ampeg into the Baby Sumo and one into FOH/IEM.
    Right now I think that the output from the output transformer will be the one I use for FOH/IEM. It has the poweramp tube sound and sounds more lively to my ears. So that will be the one I want to send out for the audience and my earbuds. It will go into a behringer X32 or X-Air18, so I don't see a problem with the fact that this particular output is not very hot. The regular Line out would be the one to drive the GSS Baby Sumo.

    I did experiment at home, by using a regular XLR cable and a Neutrik XLR to 1/4" TRS adapter.
    This does not work for the output transformer outlet since the signal is too weak to drive the Baby Sumo. You can use it, but even with the volume all the way up, it is not much louder than running the Ampeg into the cab.
    It does work fine with the regular preamp out. It gets loud.

    But running a XLR cable into an XLR to 1/4" adapter creates a gigantic construct that does not look safe to my eyes when protruding from the power amp's front.
     
    Zbysek likes this.
  18. This sounds like good design practice. :D I have encountered some "pro" audio outboard gear that did not like having one side of its balanced output shorted and have had to snip pin 3 to make it work properly, but it's been a while so I don't remember what units did that. So, really an XLR to TS cable with pin 3 connected to ground *should* work in most situations though you might want to check with Ampeg.

    You don't need very much output level to drive a console's mic input so that's fine. If the XLR-TRS adapter works then a straight XLR to TRS cable should work too.
     
    Zbysek likes this.
  19. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Then all you need is a cable wired identical to the adapter and you are good to go.
     
    Zbysek likes this.
  20. el murdoque

    el murdoque

    Mar 10, 2013
    Germany
    I just realized there are plenty of these - I was just too stupid to use the thomann website right.
     
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