Any issues to be aware of if driving SET from a balanced source?

I have my eyes on a couple 45 SETs to drive the treble and midrange in a fully active 4-way system. To do this I need to use pro audio 8-channel DACs, which are balanced designs. Of course I can make the cable connecting both with the corresponding connectors (XLR or TRS on one end, RCA on the other), but is there any potential issue I should be checking/be aware of before making the move?


Balanced means that both sides are amplified. The middle pin could be at ground or something else.  There are chips specifically designed to combine the balanced signal back into a single signal.  

In short no it won't work.  Single Ended Triode ground side must be ground.  You will need a device to convert the balanced to single ended not just a cable 
Horacio (Lewinskih01), this can certainly be made to work by means of an adapter cable, although you might obtain better results by using a Jensen transformer to convert the balanced signal pair to an unbalanced signal.

See the following link:

For an XLR to RCA cable you would construct a cable corresponding to either configuration 4 or configuration 6 shown in the lower part of the page at that link. Use configuration 6 if **and only if** the DAC’s output is either transformer coupled or "cross coupled," and configuration 4 otherwise. What is meant by a "cross coupled" output is illustrated in Figure 5 at the following link:

For a TRS to RCA cable you would construct a cable corresponding to either configuration 10 or configuration 12 shown in the lower part of the page at the first link. Use configuration 12 if **and only if** the DAC’s output is either transformer coupled or "cross coupled," and configuration 10 otherwise.

Best regards,
-- Al

This assumes that the balanced system is referenced to ground and not 2.5v.

if it is referenced to 2.5v, any D.C. offset will affect the bias of the amp making it very unhappy. 

Without knowing how the DAC and the amp is wired it is hard to say what is needed to convert the balanced signal to single ended.  That amp runs at a couple hundred volts making it capable of blowing up parts it you get it wrong.

Another factor is what speaker are you trying to drive?  A set 45 only puts out 1.5 to 2 watts
Just use these on the output of whatever is driving the SET and use RCA interconnects.

Cheers George
George, I don’t know about the adapters you suggested, but IME most XLR to RCA adapters short the signal on XLR pin 3 to ground (XLR pin 1). That will be appropriate for many XLR outputs that are transformer coupled or "cross coupled" (as well as being appropriate when adapting RCA outputs to XLR inputs), but will definitely cause problems for **some** actively driven differential outputs. See this thread, for example.  And also note the following statement in the second Rane reference I provided earlier, pertaining to actively driven differential output stages:
Mark off even more points for potential distortion (depends on op-amps, and exact configuration), oscillation, and failure, resulting from asking one side to drive a short (the result of grounding one side for unbalanced operation). 
AJCrocker1, yes, use of an adapter cable will not be a suitable approach if the balanced outputs are referenced to a DC voltage. However, I have never encountered either a consumer oriented or a pro oriented XLR output that had XLR pin 1 referenced to a DC voltage, rather than to ground. And presumably likewise for the "S" of a "TRS" connector. If you are aware of an exception to that I would be interested in knowing about it.

-- Al

I was assuming the XLR output was a standard affair, as I've never had problems using these from any xlr output sources to my single ended input product.

Cheers George
There are two ways to do it if you want the amp to really get the full benefit of the balanced source:

The first is with a high quality input transformer, which can convert from balanced to single-ended. Jensen is an excellent source for that.

The second is that it happens that any SET can properly receive a balanced signal, and while its reception won't be balanced by definition, it certainly can be differential by definition and so can process both phases of the signal. This is a very minor modification- the hardest part about it is sorting out where to install the XLR connector. The mod won't change the character of the amplifier.

The technique is simple- a coupling capacitor is used to couple the other half of the balanced signal (pin 3, if pin 2 is tied to the grid) into the cathode of the input tube. If the cathode circuit employs a cathode bypass capacitor, that cap will be the coupling cap. If the circuit does not, it has to be added. It really is that simple and works quite well- I've been doing it for years with a set of type 45-based amps.

Refer this to a technician and they will have no troubles doing the hookup.