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Old 15th February 2013, 12:25 PM   #11
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Originally Posted by JMFahey View Post
Are you sure?
*Normal* is
Unbalanced=higher impedance
Balanced=lower impedance.
Please post specs of your amplifier.

EDIT: and what preamp are you using which can't drive a relatively low impedance?
Yes that's correct, and my power amp (Peavey IPR 1600) has a 7.5k unbalanced input impedance, which is not IMO optimal. This was why I wanted to use the balanced input (15k) instead.

My preamp currently exists only on paper, and the idea is, of course, that it should be able to drive a lo-Z input, and that, in turn, was the reason for the question about using a transformer. The last stage of the preamp was intended to be a 12AX7-triode stage, and that might not have an outout impedance that's low enough. But this is no longer a problem because:

I've since changed the design, so now it will have a small 6L6GC-power stage with an output transformer (to get a better "tube head sound"), whose output I'll connect to a 8ohm false load and attenuate to line level. This will leave the output impedance low.

This is a bit hard to explain verbally, but I'll post the schematic in the tube section soon for further discussion.
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Old 15th February 2013, 07:23 PM   #12
JMFahey is offline JMFahey  Argentina
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Don't worry, I have a reasonable picture of what you want in my mind.
Well, 7500 ohms input Z can be driven by practically *any* preamp ... except the plate of a 12AX7 because it's internal impedance ends up being around 40K to 50K so only *really* suitable (by itself) to drive the typical 500K to 1 M impedance of another tube stage.
Both 7K5 and 15K are way too low for it, so a buffer is needed anyway.
As of the uncommon Peavey input impedance, I bet they really offer a balanced impedance, 7K5 per arm (15K balanced) and for unbalanced they just drive one of them and so you meet 7K5 .
I find no other explanation for those unusual values.
But it's not worth finding a special transformer and, by the way, the 1:1 I suggested would not work, I was thinking an SS (or SS buffered) preamp before.

But of course, you are most welcome to post your project
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Old 15th February 2013, 10:13 PM   #13
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My project is here: Bass/guitar tube preamp with small power stage
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Old 19th February 2013, 04:05 PM   #14
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After your dropping resistors you still might want a switchable H-pad or two.

for driving the power amp balanced, consider also the "BM" series of tranformers from Jensen (the balanced transformer company, not the speaker company) for driving power amps. It handles lots of power and has a huge dynamic range so you don't have to constantly adjust that pad every time you change your amp volume. Expensive, and worth every penny. It's what they use on the main outputs of the very best live-performance mixing boards, where they really need the noise-cancelling balanced lines for the long runs from the board back to the crossover and power amp stacks. They run those lines with plenty of votage and enough current to drive a stack of power amp channels in parallel when necessary.
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Old 19th February 2013, 04:22 PM   #15
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If you call the guys at Jensen and tell them your application, they'll tell you exactly how to do it...using their transformers of course.
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Old 19th February 2013, 05:13 PM   #16
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Thanks, but I've pretty much abandoned the idea, the new solution is here: Bass/guitar tube preamp with small power stage

My new solution with an attenuated speaker signal will have sufficiently low output impedance even as unbalanced. I'll use the line trafo for mic level output only.

Balanced line level out would still be nice, but not worth the price of a Jensen transformer...
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