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-   -   Driving balanced outputs (http://www.diyaudio.com/forums/tubes-valves/207704-driving-balanced-outputs.html)

astouffer 28th February 2012 01:28 AM

Driving balanced outputs
 
A friend of mine asked me about building him a preamp for his acoustic guitar pickup. Seems simple enough except he asked for a balanced XLR output as well. Hmmm

So it looks like a phase splitter should work. My concern is the cable impedance and whatever input impedance the next amp has. I'm trying to think of an elegant solution where a phase splitter has two cathode outputs. Any ideas?

speakerfritz 28th February 2012 01:35 AM

balanced signal paths are always good, but you can drive XLR using unbalanced circuitry and a transformer, differentially arranged resistors, or a circuit that provides separate paths for the inverted and non inverted inputs.

leadbelly 28th February 2012 01:44 AM

Quote:

Originally Posted by astouffer (http://www.diyaudio.com/forums/tubes-valves/207704-driving-balanced-outputs-post2925798.html#post2925798)
I'm trying to think of an elegant solution where a phase splitter has two cathode outputs. Any ideas?

Don't think you'll find anything that meets both. If you want a CF output stage, you will need another stage such as concertina or LTP to do the splitting. If you are looking for good drive capability via fewest parts, then CCS loading a pentode LTP will do the job.

SemperFi 28th February 2012 06:57 AM

Remember you want galvanic isolation, (that is, it is preferred, tho many live without), but if you include balanced out, it will likely go to a mixer input and maybe a local amp. As soon as he wants to plug into more than one device, he should have at least one of the outputs isolated from the other.
Edcor's transformers are not bad for the price, but should be made shielded if you use them, if he wants best he'll pay for Lundahl or Jensen.
Going standard you could use E88CC into Edcor 10k:600 transformer. Has worked for me.

artosalo 28th February 2012 07:05 AM

I would try a LTP with a cathode followers for both outputs.
A douple triode 6DJ8 or 6N6P would be my favourite.

SemperFi 28th February 2012 08:10 AM

1 Attachment(s)
Here's something I've used. It is KISS, but works. The phase splitting, or symmetry at the output, is surprisingly good.
You want some kind of gain/level adjust of course, a 1megohm pot at input works. If you want more flexibility a prestage with gain and tone controls can be put in front.
Not sure about acoustics, but electrics are sensitive to input impedance, and >1megohm is usually prefered. Obviously, if the guitar has a built in preamp, the input impedance can (should?) be much lower.

astouffer 28th February 2012 10:04 PM

Thanks everyone. Looks like transformer coupling is best.

Tubelab_com 28th February 2012 10:12 PM

Quote:

I would try a LTP with a cathode followers for both outputs.
Well, maybe its just me but I would use an LTP with a mosfet follower on each output.

artosalo 29th February 2012 09:42 AM

Quote:

Thanks everyone. Looks like transformer coupling is best.
How come ?

SemperFi 29th February 2012 11:03 AM

1. In a musical instrument world you will often benefit from the galvanic isolation the transformer provides. (All high-end pro gear has in-out transformers).
2. For most instruments you usually do not want wide bandwidth as in HiFi. It will take many alpha-numeric symbols to xplain, so I'll leave it for you to google it.
3. For it's simplisity it makes for a good se-balanced conversion.
4. Musical instruments often like the 'vintage' way of doing things. Most musicians prefer the time tested tones of vintage tube and transformer gear.
5. You can chose exotic hand wound transformers and ave apiece of kit costing it's weight in gold...or just go edcor. Many fun options.
6. Ah, running out of ideas.


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