Please help with adding a transformer to EHX 12AY7 mic pre

[jdmcgee]

Greetings all,

Can anyone give me pointers or maybe point me in the right direction for some info on wiring up this Edcor 1:10 tranny in a box with XLR male and female jacks so I can put some "iron" in front of my EHX 12AY7 mic pre. I'm trying to get a little more input gain per the designers recommendation in a post on this board.

I haven't bought the Edcor tranny yet so if any one has something they would like to recommend please reply as well.

Thanks, JD

[lobstman]

I couldn't get your link to work, try this-

http://www.edcorusa.com/products/transf ... mxl10.html

On the product page, there's a tab marked "schematic"- that gives you the pinout. "CT" is center tap, wire that to ground. This tranny is made for PCB mounting, so you'll need to mount it to perfboard and solder jumpers from the pins to adjacent holes where you'll solder the wiring to the jacks.

I didn't see the post you're referring to, but 1:10 is a pretty high ratio for an input tranny- did they recommend something that high? For $23, this particular one isn't exactly going to sound that great, either- decent iron ain't cheap. Jensen, Cinemag or Sowter will be more expensive but won't degrade the sound of your preamp.

[Jeff White]

So let me get this straight.

You can add a transformer into a little box and basically go into and come out of it with XLR? The tranny needs power, though. right?

I mean, one could build a box with 2 trandformers in there and use it in front of or behind any mic preamp.

Jeff

[adadan]

What led you to that particular Edcor transformer? The Edcor transformers are actually excellent for the price (this is one of those cases where you shouldn't go by what they cost), but they are not necessarily your best choice for mic level signals since they don't have any magnetic shielding. The models that they call "mic" transformers do have a copper shield (I think in between windings), but that does not shield against magnetic fields in the environment of the transformer, so for mic level signals it is fairly likely that you will have problems.

To answer another poster's question, a transformer is passive by nature, so you don't need a power supply for a box like this, although the transformer will block phantom power if used between a mic and a board or preamp input that would normally provide phantom power, so a power supply would be necessary to provide phantom power from your transformer box.

If you really want a mic input transformer, you might look at Cinemag, where for about $40 you can get something with shielding and flying leads (which would make it somewhat easier to hook up in your box than a PC mount transformer like that Edcor).

[KennyLusk]

I couldn't get your link to work, try this-

I didn't see the post you're referring to, but 1:10 is a pretty high ratio for an input tranny- did they recommend something that high?

I could be wrong but if I recall correctly, analoghacker recommended 1:1 for the input tranny - didn't he?

[Jeff White]

So... would an output transformer box be better? This way it could be passive and not block phantom. Could you still run a balanced input into it? Would a short cable run from the 12AY7's 1/4" out into this box be the ticket to ride if you need an unbalanced input?

Jeff

[adadan]

I looked back at the thread with analoghacker, who designed the 12AY7 pre, and there were some different opinions about the best ratio transformer to make sure not to load down a mic too much, but from his description it sounds like there is a fairly obvious spot to wire in the transformer (in place of the blocking caps for the phantom) that would allow you to still use the phantom power.

The problem is that removing those caps makes unusable without the transformer, so the transformer couldn't really be in a separate box if done that way.

If your goal in adding a transformer is increased gain, using a typical output transformer is not likely to help (more likely would hurt, since output transformers are usually step down rather than step up.) The pre will drive 600 ohm stock, so you could possibly add a step-up on the output if you don't mind limiting your output drive capability, but that doesn't seem like the most elegant way to go about this. If you just want some iron sound (without adding gain) you could add a 600:600 Edcor WSM on the output for ~$12, which might get you a sound you like.

I need to buy one of these and try racking it up with transformers. I have a great 1:7 cinemag mic input lying around that might work well with this pre.

[jdmcgee]

I looked back at the thread with analoghacker, who designed the 12AY7 pre, and there were some different opinions about the best ratio transformer to make sure not to load down a mic too much, but from his description it sounds like there is a fairly obvious spot to wire in the transformer (in place of the blocking caps for the phantom) that would allow you to still use the phantom power.

The problem is that removing those caps makes unusable without the transformer, so the transformer couldn't really be in a separate box if done that way.

If your goal in adding a transformer is increased gain, using a typical output transformer is not likely to help (more likely would hurt, since output transformers are usually step down rather than step up.) The pre will drive 600 ohm stock, so you could possibly add a step-up on the output if you don't mind limiting your output drive capability, but that doesn't seem like the most elegant way to go about this. If you just want some iron sound (without adding gain) you could add a 600:600 Edcor WSM on the output for ~$12, which might get you a sound you like.

I need to buy one of these and try racking it up with transformers. I have a great 1:7 cinemag mic input lying around that might work well with this pre.

Thanks to everyone for the replies... As far as my original post about the input tranny, I would just really like to get some more usable gain on the input of the 12AY7 pre... It sounds wonderful with condensers but there's not enough gain to efficiently use it with my Heil PR-40, Shure SM-57 or Sennheiser MD-421. In this message board post (at the bottom of page 1 in the thread) a guy talks about building a box with a 1:10 input transformer in it to use with his 12AY7. He had an 8db boost in usable gain for his ribbon mic. I'm wanting to try something like that.

Also, analoghacker, the 12AY7 designer, states:

"the input tranny can be any 150-600 ohm input and the step up can be 6 to 20 dB (5K to 20K), depending on what you need. a 20dB step up would make the pre ready for any conceivable low output mic. a center tap on the input is handy for the phantom supply but not strictly necessary because you can simply add a voltage divider (2 each 6.8K and attach the center tap to a 47K and the phantom supply...)." Anyone want to take a stab at explaining all that to me (or pointing me towards somewhere to do a little reading on it)?


Any ideas or suggestions are greatly appreciated & Happy Holidays!!!

Thanks Again, jd

[jdmcgee]

just a bump