1/4" unbalanced output into xlr balanced input

[zombywoof5050]

If I want to run a cable from the 1/4" unbalanced output of my Alembic F-2B into the balanced xlr input of a power amp, should the 1/4" plug be TRS or TS (or does it matter?)?

Can I even do this, or is it not adviseable?

[jastacey]

Yes, you can do that, keep you cable lengths as short as possible, to minimize any induced noise, go to this link and view page 9, this will show you how to connect the 2 conductor cable to the XLR plug/jack

http://www.crownaudio.com/pdf/amps/139548.pdf

[ulrich]

All your how do I connect this to that questions answered:

http://www.rane.com/note110.html

[4Mal]

Originally Posted by zombywoof5050

If I want to run a cable from the 1/4" unbalanced output of my Alembic F-2B into the balanced xlr input of a power amp, should the 1/4" plug be TRS or TS (or does it matter?)?

Can I even do this, or is it not adviseable?

see above and note that a TS jack is all you need on the Alembic side. It is absolutely adviseable as the F2B can be a beautiful sounding preamp.

[Bob Lee (QSC)]

Often the best way to connect an unbalanced output to a balanced input is a quasi-balanced cable like this:



It's unbalanced at the source end and balanced at the other. Avoiding a direct chassis-to-chassis connection eliminates an entry point for differences in chassis ground voltage between the two devices to contaminate the signal--IOW, there's less likelihood of hum or buzz entering in the interconnection.

Ground voltage differences between two pieces of gear are normal, but they're likely to be greater if the devices are plugged into non-adjacent electrical outlets. Balanced connections are more immune to this noise because the shield or ground connection is not one of the signal conductors as it is in an unbalanced connection.

[zombywoof5050]

Thanks for the replies!
I've had this preamp for 28 years and never tried to (or had to) hook it up this way.

Here's an idea (didn't even think of this until now)...What if I put my Radial JDI in between the preamp and power amp, would that work better than just the cable? I have a JDI Duplex, and one half of it is not even being used at live shows, so I wouldn't even have to buy anything. Actually, I presently use the second channel of my F-2B through (half of) the JDI Duplex to feed FOH, so I could easily use the other half of the JDI Duplex to feed the balanced input of the power amp. Thoughts?

[zombywoof5050]

So, I guess that no one here has tried using a passive DI between an unbalanced preamp and balanced power amp (instead of an adapter cable)? I know the preamp output is way higher than the output of a bass (it has to be in order to drive the power amp), but I believe the high quality Jensen transformer on the JDI should be able to handle the increased signal level.

[zombywoof5050]

Ahh c'mon...no one here has ever used a DI between their unbalanced pre and balanced power amp? Well, it seems appropriate to me so I guess I'll just have to try it.

[4Mal]

Nope I just wired it up as Bob described - except that I used a TS 1/4 " - adding another box into the signal chain is well, unneccessary ...

I think that with the Alembic, Bob's TRS method wouldn't work as the output of the pre is TS not TRS. I'm ging to rewire a couple of the interconnects in my PA based on Bob's diagram as my board has TRS based pre out and pwr in which I use to route to my outboard gear. It's good now but if this is potentially better - it's a worth a test.

[Bob Lee (QSC)]

I didn't recommend a TRS method. The only one I've mentioned is the one in my diagram, and it's for unbalanced outputs (such as TS). Substitute a 1/4" TS for the RCA phono plug; it'll work just as well.

[zombywoof5050]

Thanks again for the info!
True the DI in between may be uneccessary, but it's sitting right there anyway (feeding FOH from the second channel of the F-2B), so it's not like I'm going out of my way to use it, plus it's job is to convert unbalanced into balanced which seems appropriate for this application. I'll give it a try, and I'll also make a cable to try for comparison of noise and signal levels regarding the DI vs the cable.

[Bob Lee (QSC)]

There's nothing wrong with using a DI to convert unbalanced to balanced, as long as it can put out a suitable line-level signal into the power amp.

[zombywoof5050]

Well, this exercise turned out to be for naught (not really, I guess, as it has been a learning experience)..

I found out that the input senstivity on the new power amp I bought (a Crown XLS2500) is just too low (1.4V) for my Alembic F-2B. I obtained the greatest output level from the XLS2500 by just running a straight 1/4"-1/4" cable from the F-2b into it, but the resulting volume level was still way too low compared to my Carver PM1400 power amp (but understandibly the Carver's input sensitivity is configured at 0.775V). And then, on the XLS2500, when I connected the two RCA inputs together (neccessary for parallel operation of both channels from a single input, and achievable by jumpering any of its inputs...I used the RCAs in this case since it was the easiest to do), the output level of each channel was reduced even further, making it unfeasible.

I had originally thought that I would have to connect the XLS2500's two 1/4" inputs together (hence the need for the 1/4"-XLS cable originally inquired about) to acheive parallel mode, but I found out that you can use any of its inputs for this (and the XLS2500 has 1/4", XLR, and RCA inputs). But when you have to jumper the inputs in this way, in order to achieve parallel mode, the resulting output volume per channel is reduced. Maybe when you have to jumper the inputs for parallel mode, it's just like using a y-connector, and both inputs end up only getting about half the signal level (I don't really know, just theorizing). But this doesn't seem to happen on my Carver power amp, which uses a switch on the input board to put it in parallel mode rather than having to jumper the inputs. Anyway, I'm convinced now that any power amp to be considered for use with my Alembic F-2B will have to have an input sensitivity of 0.775V....1.4V won't suffice.

So the XLS2500 has been returned to the store.

Sorry for the rambling, but it has been a learning experience for me, and maybe this report can benefit others as well.

[Bob Lee (QSC)]

I think you mean the amp's input sensitivity is too high for that preamp.

For preamps that have an unusually low output level capability, a level matching line amp would help:

http://www.fullcompass.com/category/...hing-Amps.html

The Rolls MB15-B or some of the others would work.

[bongomania]

Also, FWIW the Radial JDI would not have been any help at all, because it is passive and it drops a huge amount of signal level going across the transformer. This is fine and not a problem when sending a sufficiently-strong instrument signal to a sufficiently-strong mic preamp in the desk, but when sending a weak preamp into a low-sensitivity power amp, that loss of signal level is the kiss of death.

[4Mal]

OK - now that I didn't expect as my old F2B drove both a PLX 2402 and a Crown CE-1000 to face melting levels... My F2B was one of the old (paleolithic, old) Blue models.

Originally Posted by Bob Lee (QSC)

I didn't recommend a TRS method. The only one I've mentioned is the one in my diagram, and it's for unbalanced outputs (such as TS). Substitute a 1/4" TS for the RCA phono plug it'll work just as well.

Sorry Bob, I was mis-reading the diagram. My bad.

[zombywoof5050]

Originally Posted by Bob Lee (QSC)

I think you mean the amp's input sensitivity is too high for that preamp.

No, I did mean to say that the input sensitivity is too low. Remember, a higher input sensitivity will have a lower number, so 0.775V is a higher sensitivity than 1.4V. But I know that you know that.

Originally Posted by Bob Lee (QSC)

For preamps that have an unusually low output level capability, a level matching line amp would help:

http://www.fullcompass.com/category/...hing-Amps.html

The Rolls MB15-B or some of the others would work.

Thanks for the info.


Yes Bongo, I did find out that the JDI dropped the level way too much to be of any use here, at least with the XLS2500's low sensitivity. It might have worked ok if the power amp had a higher sensitivity, and perhaps I'll try it with my Carver just to see how loud it will get. At most gigs, I usually only need to turn the Carver up to about 40%, and I always keep the F-2B's volume knob at 50%.

[Bob Lee (QSC)]

Actually, 0.775V is lower than 1.4V.

Input sensitivity is proportional to maximum output voltage and inversely proportional to gain. Thus, if you reduce the gain of an amp, its input sensitivity will increase.

[bongomania]

It's an easy mistake of perspective. A high rating number means a low sensitivity, and vice-versa. If you're used to using the word "sensitivity" to mean the rating number, then it's the other way around.

[Bob Lee (QSC)]

I've always disliked the term "input sensitivity" because it's so easily confused with "gain." An amp with a higher input sensitivity actually requires a higher input voltage to drive to full rated output, and that is counterintuitive.