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Is it possible to change an amplifier from rca to XLR output? It does not have a balanced circuit. Anyone have a schematic. Thank you.
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I am therefore assuming, like the other on this thread, that you mean 'input' not 'output'.
If so, you don't need a transformer to have a differential input connection and the amplifier can process both phases in the differential domain, without serious modification to the amp.
The hardest part is installing the XLR connection itself. Pin 1 is the RCA circuit ground (not chassis) and pin 2 is the same as the center pin of the RCA. The real question is what to do with pin 3:
If you have a cathode bypass cap in the first stage, lift it from ground and tie it to pin 3. When running single-ended, install a jumper from pin 3 to pin 1 to restore the connection.
If there is no cathode bypass capacitor, you have to sort out what the value of one would need to be, then install it between pin 3 and the cathode of the input tube. Its OK if feedback is arriving at this point. For single ended operation the jumper (above) should not be installed.
This connection is not balanced, but it is differential. Some balanced preamps may have a problem driving pin 3 but IME its not a big deal. How differential is this connection? The numbers are pretty good- if you tie pin 2 to pin 3 of the XLR you will find the CMRR to be quite high.
Yes, Jensen has a whitepaper on doing just this. You only need a couple of resistors and a cap to do so.
See figure 2.4 in the attached PDF.
A schematic is not really necessary. A transformer does the job, provided the singled ended circuitry has a low impedance O/P.
Look into the offerings of CineMag, Jensen, and Sowter.
Eli D.
Thank you for your reply. I am a little confused by one item. How is it possible to make a cable that changes XLR to Single ended or reverse and have it still work? It seems to me that the receiving XLR amp must have a converter built in or am I missing something?
Here's a good reference:
http://www.rane.com/note110.html
Do you have a preamp that has an un-balanced output and a power amp that has a balanced input?If so then you need a small signal output transformer for your preamp.
(or just connect the center pin of the RCA to pin 2 of the XLR and the shield of the RCA to both pin 1 and pin 3 of the XLR. It's won't be a balanced connection but it will work just fine)
Or do you have a preamp with a balanced output and a power amp with only a un-balanced input?
If so then you need a small signal input transformer for your power amp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/07/14 09/07/14
I am trying to convert a single ended phono stage to balanced out. All of the amplifiers except the phono stage use balanced connectors.
Thank you all for your help! I need to clarify. I am building a tube phono amplifier. I would like to make it XLR out to connect to my pre amp. From what I have read I only need to use a female XLR connector and hook it with pin 1 chassis ground, pin 2 +, pin 3 -? I also have the chassis grounded to house and circuit. Would this be the best way? Also I have a dumb question that will help me tie this all together. Is a balanced signal just a single ended signal with the chassis grounded? I am confused as I thought that a balanced signal was different. Thanks you.
You have several choices:1) you could use some sort of transformer to do the job. This is really not a bad idea. There are a number of ways to do this, for example you could have a transformer made that can be the plate load for the final stage of gain (which means that the transformer has to be designed to have the DC current go through it).
2) Or you could have the conventional coupling cap, which then drives the transformer.
In either of these cases the secondary of the transformer is tied to pins 2 and 3 of the XLR; pin 1 is circuit ground and not connected to the transformer at all (unless there is a center tap, but if you were to use that the Common Mode Rejection Ratio would suffer, so don't do it).
3) you could create some sort of phase splitter circuit at the output of the preamp, and use the outputs of the phase splitter to drive pins 2 and 3.
To do this right you will have to do one of the scenarios above. The phase splitter is cheaper, and if done right will perform very well as long as you don't load the output too low. The transformers have the advantage of circuit simplicity and often have the ability to driver lower impedances that are common when you run balanced lines (for that you will need more gain from the phono as voltage will be lost during the step-down of the transformer).
Edits: 09/10/14
I have done more research and things are making sense. I understand the differential signal and how it works. I am always in favor of less manipulation of a signal. Would using a transformer or op amp to convert the signal degrade the sound? Wiring as I described earlier seems like a more simple solution.
The purpose of a balanced connection is to reject hum (usually in long runs).
If you don't have a hum problem (and you shouldn't between the output of a phono pre and the input of a line stage) then un-balancing one balanced input on your line stage should work just fine.
This is how you do that, connect the output of single ended phono preamp stage to pin 2, circuit ground to pin 3 and pin 1. Done!
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'd go with the transformer before an opamp, but that's me- I avoid semiconductors! Why have a solid state artifact smeared all over your vacuum-tube efforts? :)
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