makeshift di

[Al Swenson]

Hello, I've got a 10k to 150 ohm transformer here from an old project that I'd like to stick in a box and use as a DI. I'm just a beginner learning as I go here, and I'm a little foggy on my transformer theory. Please correct me if I'm wrong.

For the purpose of this project I will assume my source pickups are 50k ohms. My mic pre input is 1.3k ohms. Since the turns ratio is 10000/150 = 66.667... if I run the guitar into the 10k primary, it will see the transformer as almost 87k ohms, and the preamp will see the transformer as 750 ohms?

If that's right, can I get that 87k to look more like 500k while simultaneously lowering the 750 by adding a resister parallel to the secondary?

Thanks for your help, any input is greatly appreciated.

[geraldfryjr]

Actualy your transformation ratio is 8.1649.
Because the transformation ratio = the square root of 10000 ohms(Zp)/ 150 ohms (Zs)= 8.1649:1 .

So then 87000 ohms / 8.1649 / 8.1649=1305 ohms will be what the preamp input see's
And with 50000 ohms / 8.1649 / 8.1649=750 ohms.

I'm not sure what you are asking as adding a resistor does not change the transformation ratio.
A typo maybe?

I hope that helps. jer

[Al Swenson]

Hey Jer, yes that helps thanks. I was confused and had the wrong impression about the equations, which is why I am here. About adding a resistor, I was just wondering if there was a way to manipulate the load impedance like that, not the ratio itself, in order to change the impedance that is reflected to the primary. Anyways, clearly I'm in a little over my head so I'm gonna hit the books some more before I get excited about something else. Thanks again, I appreciate your feedback!

[cbdb]

Just found this: Heres what real pickup impedance curves look like.
http://www.google.ca/url?q=http://on...0yO-erRX82xNTg

Now I understand why you want a very high guitar amp input impedance. (1 meg ). To minimize the effect of the pickup resonance on the freq response.

Adding a resistor in series to the load (the pre input) will increase the impedance the pickup sees thru the xformer but it also drops voltage (to double the impedance you half the voltage) and burns power that should be entering the preamp. So not a good idea.

Seems like your xformer ratio isnt high enough.

[geraldfryjr]

That is a very interesting document. jer

[PeterDupont]

Hi Jer,

I use a lot of transformers in my TC-electronic G-System - mainly for hum-elimination.
I wonder why you will use a transformer. Why not use a tubed og transistor-buffer as the majority of guitarists do (mainly to impedance-match and keeping tone).
If of course you have some troubles that a galvanic isolation man cure.

Regards
Peter

[geraldfryjr]

Peter, I am not sure what you mean?
I am not the one trying to use a transformer.
I merely verified the fomula for impedence ratio vs transformation ratio in post #2.
My question is the same as yours and I am not sure as to what Al is trying to acomplish.

Cheers. jer

[chrispenycate]

Lokk, you've got the transformer; have you got phantom power coming out of the console? Because building a useable (not excellent, but generally acceptable) active DI with that transformer is child's play (building a good one is a bit more complicated).

put a battery space in the box for when there's no phantom available, or you need total ground isolation. Running magnetic pickups into anything less than the highest impedance you can managed eats sparkle and edge; it's like turning the tone control down.

[PeterDupont]

As far as I can read, Al has a quest for coupling his guitar-pu's to a low-impedance input.
And these things seldom work except for a DI-box of some kind.
Even the most tone-afficinado, gear-affordable guitarist use fet-buffered units like the Lehle Sunday-Driver (hey, you can adjust the gain to perfectly load or overload your amp-input while still keeping the impedances correct).

Just as, when it comes to hi-fi, a guitar rack or just a guitar into some pedals and further on to a amp relies on impedance-matching. The most important link is between the guitar pu's and the chosen amp's input. Forget about the signal from the guitar - forget about your chosen amp and whatever signature it's circuitry/speakers roars to the public.
Impedance-matching between the jack on the axe and the input on the amp is sooooo important.

Regards, Peter

[PeterDupont]

I left out the most important aspect:
A good buffer between your axe and the amp makes it quite easy to run long, cheap cables across the scene!
My hi-fi experience is, that expensive cables often makes up for "cheap" impedance-matching!

(0.8 mm. solid silver rules in this room - even between my guitar-amp and speakers).