building a mic preamp... transformers?

[TimWaldvogel]

i have a edcor pc10\10k and i am wondering if there is a way to convert the input impedance to a typical 150ohms while still using this as a way to convert the balanced to unbalanced, of course by avoiding lots of noise.

or are any of those edcor pc mount transformers good for mic inoputs\outputs?

[TimWaldvogel]

i am also wondering if this would work

EDCOR Electronics Corporation. TPC150/150

[imaradiostar]

I'm not finding a pc10/10k. Do you mean the PC10k/10k?

It sounds like you'll have to get a different transformer.

jamie

[TimWaldvogel]

thats fine. but i noticed the smaller edcors are cheaper than others. what are the ideal qualities of a mic input transformer?

is it fine to have a 1:1 ratio? they also had that 150\150 1:1 transformer and then i could use my circuit to boost signal correct?

[TimWaldvogel]

this is one i found

EDCOR Electronics Corporation. WSM150/150

or this

http://www.edcorusa.com/products/141-xsm150-150.aspx

[imaradiostar]

That's kindof gigantic for a mic input transformer.

Is there a particular mic pre design you're using? If it's an op-amp based design you don't really need any voltage gain from the transformer and it's generally easier to make a decent 1:1 transformer than one with a step up. Last I check some companies recommend only using a minimal step up in spite of the fact that many vintage designs call for a very high step up because of the sonic benefits of a ratio closer to 1:1.

jamie

[TimWaldvogel]

I was thinking the 150/150 1:1 transformer to unbalance the signal and use opamps for the amplification. Also I am curious what is the usual voltage for opamps. I would like to be able to use something like an 18volt guitar power supply.

Can you refer me to any design for a simple op amp design with a input transformer this?

What kind of voltage should I use for maximum headroom

[TimWaldvogel]

And honestly how does phantom power work? I am curious to know if I can pass the phantom power from my interface through this?

And how can I simply Incorporate phantom power to my soon to be diy preamp?

this is the design i was thinkiing about

http://www.electro-tech-online.com/a...ic-preamp2.png

i can use this with dynamic microphones as well i am assuming correct? the plan was to use a trasnformer to unbalance the incoming signal, through this preamp circuit and back through a transformer to rebalance it out.

[imaradiostar]

The preamp you linked to was built with electret condenser mics in mind- it would work for a mic pre but not terribly well.

You might want to read this for some info. Good stuff there. One of his op-amps would be an excellent choice but an NE5534 would work till you were willing to spend a little more.

Here's a simple design that you can't go wrong with.

Many of the documents on the jensen page here are very helpful. They have some diagrams with pad and phantom power implementations. Hopefully they can explain better than I can. My simple explanation would be to say phantom power is between pins 2/3 and ground while audio signal is any voltage difference between pins 2 and 3.

You can use the jensen designs with parts you have around. Your existing transformer may be a usable transformer with a low enough output impedance to drive it. Remember as well- impedances are reflected from the thing you're driving. Most gear has a much higher input impedance than the old school 150 or 600 ohms.

jamie

[TimWaldvogel]

well i looked into using my transformer and i am not 100% sure if its acceptable yet. but when i popped my a input impedance of 150 ohms (typical sm57 impedance) to my input impedance of my transformer which is 10,000ohms or 10k, it said my signal loss is -0.1293.

thats less than 1db. thats acceptable right? please correct me if im wrong

[TimWaldvogel]

i like the jensen design but i just cant grasp how phantom power works i guess, i mainly just need the phantom power supplied by my interface to be able to pass through to the mic. OR i can build it with phantom. i just dont know how transformers and phantom power play together ya know?

[imaradiostar]

I don't really know if that's acceptable. How are you measuring it?

You should probably use a transformer that was designed for mic input applications.

Look at the Jensen documents- you should be able to figure out phantom from those. It's less complicated than the mic preamp designs.

jamie

[Steve Conner]

If the DC resistance of the windings is much more than the impedance of your mic, the transformer will degrade the noise floor. Also a transformer with 10k design impedance might have too much leakage inductance, spoiling the high end.

So yes, I would use a mic input transformer. Or no transformer at all: Modern mic preamp designs have made them obsolete, really, you only use them in environments with extreme interference. TI make an all-in-one mic preamp IC that gets good reviews for sound quality. I can't remember the part number.

[dai h.]

INA217?

[TimWaldvogel]

My overall goal us to use the transformer to add some color into my mic preamp. Will the below one work?

All the old preamps people love seem to color your sound and use transformers. That's my goal. I don't want to spend 120 on a single Jensen though. I want to keep it cheaper. I think the below transformer will work. And then using opamps to boost my level and another transformer or op amp stgr to re balance the signal. Would the pc10k/10k work as a balanced line output transformer

EDCOR Electronics Corporation. MXL5