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Old 4th March 2009, 08:01 PM   #1
pftrvlr is offline pftrvlr  United States
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Default Tube Mic Preamp - which Edcor transformer?

I am starting to build an all-tube microphone preamp. Planning to use two stages of 12AX7 and one 6922 SRPP output, now I am looking for the input and output transformers. Edcor looks to have what I need, but their web site does not a lot of information. Any suggestions on which Edcor transformer will work better?

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Old 19th April 2009, 12:29 PM   #2
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I've never used Edcor transformers. I checked out their web page and looked at their mic input transformers, specifically their 1:8 ratio ones as that's the kind of ratio you need for a tube mic pre.

I note that:

1. Specs do not define the operating level at which measurements are made

2. There is no magnetic screening

3. They are quite inexpensive

I have no proof but I expect these are relatively poor mic transformers. You would probably be better looking at Jensen for example or Lunfdahl or Sowter.

Cheers

ian
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Old 19th April 2009, 03:09 PM   #3
SY is offline SY  United States
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Whichever transformers you choose, make sure that they will work well into the rather heavy capacitive load presented by your chosen 12AX7 input tube.

The Edcor plots I saw did specify the level. The left hand axis was marked "dBU." Distortion looked pretty good.
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Old 19th April 2009, 03:33 PM   #4
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FWIW I was not that happy with the sonics of the Edcor interstage that I tried. It could have been misapplication, I'm not a valve guy.
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Old 19th April 2009, 04:05 PM   #5
SY is offline SY  United States
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Was that the one you used in your ST-70?
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Old 19th April 2009, 05:01 PM   #6
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Yes that was my hybrid experiment, Walt Jung's AD823/AD815 line stage directly driving an ST70 output stage with no global FB via a step-up interstage. Didn't really rock my boat.
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Old 19th April 2009, 06:43 PM   #7
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Quote:
Originally posted by SY The Edcor plots I saw did specify the level. The left hand axis was marked "dBU." Distortion looked pretty good. [/B]
Yes, they did, but they fail to say whether it is input or output level; I suspect they are output levels. Also all the distortion characteristics look identical. I would very very surprised if a 23 dollar transformer can achieve better than 0.01% distortion at 10Hz at +18dBu output. If it can I'll take a truck load.

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