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Driving balanced outputs

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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?
 
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.
 
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.
 
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.
 

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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.
 
Well, maybe its just me but I would use an LTP with a mosfet follower on each output.

Give that LTP a pair of CCSes for power and plate load resistors to ground to set gain and operating voltage, and it will have enhanced power supply noise immunity too. Add some r.f. filtering to keep the c.b. radios and security guys out. Nuvistors are rugged and quiet.

Also wire the off switch to turn the whole thing into just a matching transformer so it is impossible for it to ever totally fail.

Maybe include a 5534 version in the same box in case there is a need for battery power. Those can deliver +27 dbm into 600 Ohms, about what the tube stage would provide with 500 V p-p into the 10K transformer. Probably best to stay below 0 dbm output. Or less... levels always seem to be higher than expected. I suppose there should be provision to drive either a microphone or line input just to be prepared.
 
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For balanced output, IMHO the Jensen BM- series transformers are excellent. I used them as an upgade on the outputs for mixing boards. Incredible dynamic range, your preamp output won't saturate them, I think they even handle something over 100 volts... They make great input and splitter transformers too. Superb electromagnetic and RF rejection.
 
The main advantage of balanced wiring is the noise immunity. This is provided by the circuit having balanced impedance to ground and high common mode impedance to ground. A simple differential signal or even single ended (one side grounded) may drive the balanced input properly but unless the impedance is also correct there will be compromised noise immunity.

The transformer is absolutely the easiest way to provide a balanced output. Why? because it has inherently balanced impedance, super high common mode impedance AND offers step-down which is great for providing good output impedance and drive capability in a tube circuit. Most balanced output stages that fake it with amplifiers have some issues, and with tubes it's really difficult to get good drive capability.

I would look carefully at the DC current rating of that Edcor transformer wrt the DC balance of the push-pull tube stage. You probably do want decent low frequency response and don't want to saturate the transformer core with DC.

Besides, if you're using a transformer it provides a nice balanced output. You can drive it with a single end parafeed stage and avoid all DC issues. You do not need a push-pull output stage for this.

For more study of the balanced impedance issue go to the jensen transformers website and look at the technical papers by Bill Whitlock.

PS What is the pickup on the guitar? If it's a direct piezoelectric with no built in amplifier, you will want to provide a compatible input circuit. A hot piezo pickup setup can develop several volts peak signal and needs very high input impedance.

If the guitar has a built in amplifier (uses a battery), all you need is a direct box (DI).
 
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The pickup in question is an add on magnetic type for an acoustic guitar. Pretty sure its magnetic as it measured like 1.3k ohms.

That Jensen transformer costs 10x more than the Edcor :eek:

I'm thinking about using a cheap tube like a 6U8 with the pentode section for gain and the triode driving the transformer. For a nice rugged case I was toying with the idea of building it inside an old ammo can :D
 
You get what you pay for... The Edcors are just wound in a single go I think, no interleaving, so they do not measure well, high leakage inductance and capacitance. But driven from a proper impedance they end up being acceptably flat 20-20k.

Ps. I've only used these in instrument situations, never even tried listening to them in a HiFi situation. Not b/c I think they aren't up to it, more b/c I haven't had time yet.

Just double checked and the EDCORs I've been using is the XSM series. Mostly I use them as IT between driver tube and output tubes, but I've done a few DI boxes with them. I've abused them with over 100volt swings and haven't had any issues yet. I've even used them SE, and since guitar is mostly > 100Hz, it has been ok, tho I dont recommend them for SE duty.

The Edcor being open frame, can pick up noise. It hasn't been a big problem for me, but you may look into how to shield transformers.

As Michael mentioned, it is important to drive them properly, so a low impedance tube is preferred, tho for instrument usage I am surprised how far from the ideal one can go before it is 'bad'. With acoustic guitar u want farely flat response down to 80-90Hz maybe, but if u go too deep it risks sounding muddy.

I've been using slightly high driving- and load impedances to make sure they don't go too deep. Rather tune the response that way than add some RC network in the signal path to do the same thing.
 
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