Tube Mic Preamp - which Edcor transformer?

pftrvlr · 4th March 2009, 09:01 PM

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?

Thx.

ruffrecords · 19th April 2009, 01:29 PM

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

SY · 19th April 2009, 04:09 PM

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.

scott wurcer · 19th April 2009, 04:33 PM

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.

SY · 19th April 2009, 05:05 PM

Was that the one you used in your ST-70?

scott wurcer · 19th April 2009, 06:01 PM

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.

ruffrecords · 19th April 2009, 07:43 PM

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.

Cheers

Ian

4th March 2009, 09:01 PM	#1
pftrvlr diyAudio Member Join Date: Feb 2007 Location: SoCal	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? Thx.

19th April 2009, 01:29 PM	#2
ruffrecords diyAudio Member Join Date: Jun 2007 Location: Holt, Norfolk	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 __________________ Ian

19th April 2009, 04:09 PM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

19th April 2009, 04:33 PM	#4
scott wurcer diyAudio Member Join Date: Jan 2004 Location: cambridge ma	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. __________________ Pain is never permanent

19th April 2009, 05:05 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Was that the one you used in your ST-70? __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

19th April 2009, 06:01 PM	#6
scott wurcer diyAudio Member Join Date: Jan 2004 Location: cambridge ma	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. __________________ Pain is never permanent

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