Trying to undersand the correct transformer for mic circuit.

[gary o]

Im trying to understand why each mic circuit needs correct transformer .... I been reading about a U87 single FET circuit needing  a higher ratio transformer 11:1 I believe why is this......Im just trying to learn, I dont seem to be able to get my head round impedance....

I prototyped a U87 circuit & used a Hauf T14/1 tranformer as I think its about the right ratio.... would that make it correct impedance ? ......it worked & sounded great with Tim Cambell capsule, I have switch to dissable de emphassis.....

I was then lucky to find two Marinair transformers similar I understand to Neve mic pre input transformers a much lower ratio.... I didnt know anything about them at the time made a U87 circuit & just tried wiring them randomly & came across a wiring combo that sounded good the mic has much more output and sounds better to my ears, can this be wouldnt that be too low ratio for this circuit....

Thanks

 

[Matador]

The original was most likely specified into 600 ohm input impedance consoles. 

The transformer needs to provide a bridging impedance to the JFET gain stage, otherwise the signal is loaded down and frequency response suffers.  The output impedance of the 2N3819 stage is on the order of 10K, so using the "10x load" rule, the transformer needs to look like 100K.  To make a 600ohm console look like a 100K bridging load implies a 13:1 transformer.

However many modern interfaces have much higher input impedances:  my M-Audio interface specifies a 1.7k input impedance:  so to make it look like a 100K load implies a 8:1 transformer ratio.  You can get away with a much lower ratio provided you don't try to drive low input impedance equipment.

 

[micaddict]

^

Nicely explained, as always. 

 

[gary o]

Thanks Matador as Micadict say nicely explained I get the jist of what you said tho I still lack enough knowledge to fully understand but getting there thanks.....Im trying to find out more about my Marinair transformer & the ratio of how I have it wired in U87 info is sketchy & as I say due to lack of understanding I find it hard to inturpet specs & what they mean in real world

http://groupdiy.com/index.php?topic=55168.0

from same thread http://groupdiy.com/index.php?topic=6717.msg78850#msg78850

Yes Im using modern pre R M E Babyface said to be very good pre....aslo tried U87 into RCA BA2A with Sowter 9045 input transformer..... Also tried a pre I have just made EZ1290 Im using my other marinair transformer for the input so I have Marinait transformer into another Marinair tran.... they all sound good I mean really good not complaining but it kinda worries me ...

Thanks

 

[firehazard]

Hi Gary,

I'm not sure that I'm going to help you here but I took a while to get a good grip on the concept of impedance myself. I have read a good few of your posts and admire your willingness to throw yourself at the task of trying to understand why things sound good.

Please forgive me if I over simplify this. I am just trying to help you.

A signal transformer lives in between two active electronic devices. The device that is producing the signal provides one combination of voltage and current. With things like mics and guitar pickups there is little enough of either so we can't afford to throw any away.

The transformer allows us to exchange these voltage/current values so that we drive the optimum amount of each into the next stage of our signal chain.

If we use a "step up" ratio we get more voltage but less current.

A "step down" ratio gives more current but less voltage.

We always lose a little in the transformer core but the device we make the signal with and the device we are trying to drive are both as happy as we can make them.

I hope this helps.

take care....

 

[gary o]

Hey thanks firehazard a brilliant way to explain it really helped makes great sense...... Im still trying to work out why my U87 circuit works so well with a wrong spec transformer not complaining tho...thanks again.

 

[firehazard]

Hi Gary,

I'm about to follow you on the search....

Here in Ireland singers get a lot of respect.

A good singer is a powerful thing!

Take care...

 

[Matador]

Hey thanks firehazard a brilliant way to explain it really helped makes great sense...... Im still trying to work out why my U87 circuit works so well with a wrong spec transformer not complaining tho...thanks again.

Do you have a schematic?  Are you using one of the Chinese U87 copies with a PNP output follower?  Those can use 2:1 ratio transformers without any loading problems.

 

[gary o]

Thanks Firehazard all the very best...

Mattador my U87 circuit I made from original schematic just the single FET wired it point to point as I dont have a good K67 ish capsule I have a switch to disconnect the de emphasiss circuit Im using a Tin Cambell capsule for now & WAS using a Haufe T14/1 that I believe is about the right ratio for the 87....

Will try to explain the reason for this thread up till now I have always found the capsule always shines thru the circuit  I have two fav capsule Tim C is 2 nd fav usually but due to changing the transformer its almost on a par with my best capsule !, never come across this yet and Im wondering why ......on top of this as I said it seems to be the wrong transformer for the job