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Driving balanced outputs
Driving balanced outputs
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Old 1st March 2012, 12:56 AM   #11
riccoryder is offline riccoryder
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Quote:
Originally Posted by tubelab.com View Post
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.

Last edited by riccoryder; 1st March 2012 at 01:16 AM.
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Old 1st March 2012, 01:43 AM   #12
astouffer is offline astouffer  United States
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Does this Edcor transformer look good? EDCOR - WSM10K/600

So I just drive it with a small push pull stage like a 12AU7?
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Old 1st March 2012, 02:07 AM   #13
cyclecamper is offline cyclecamper  United States
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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.
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Old 1st March 2012, 06:32 AM   #14
SemperFi is offline SemperFi  Wake Island
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Just did a google of the mentioned Jensen transformer and this came up: http://lachapellaudio.com/downloads/Brochure_992EG.pdf
12AU7s and the transformer.

Yes, the WSM10K/600 is what I've used, tho with 6DJ8s which have little less rp. Still, for instrument work it should do fine since you actually want a little low end roll-off. (Design after HiFi specs and it'll sound muddy/muffled).
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Old 1st March 2012, 01:22 PM   #15
Michael Koster is offline Michael Koster  United States
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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).

Last edited by Michael Koster; 1st March 2012 at 01:41 PM.
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Old 1st March 2012, 05:02 PM   #16
astouffer is offline astouffer  United States
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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

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
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Old 2nd March 2012, 06:52 AM   #17
SemperFi is offline SemperFi  Wake Island
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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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