Choosing a preamp output trafo

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modulay

Well-known member
Joined
Aug 6, 2008
Messages
63
Location
Spain
Hi.
I'm looking for a cheap transformer...

This is the output stage of the preamp:

JG0.jpg


If a transformer be added, I think that R24, R25 and C12 should be skipped...

But I'm not sure about wich impedances values and turns ratio would be appropiate in this case.

Maybe 1 : 1, 600ohm : 600ohm ?

I'm thinking on buy one of these OEP's:

http://es.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106403

http://docs-europe.electrocomponents.com/webdocs/008d/0900766b8008d64a.pdf
 
It doesn't need a transformer necessarily, but it's a nice and simple way to balance an unbalanced signal.

The 1:1 OEPs might work fine despite the fact that I've heard people complain about relatively low inductance and I've measured it for the 1:2 transformer they make which was pretty low (can't find my notes though).

However, that hopefully shouldn't be an issue with a low impedance source like an opamp output.

Keeping R24 (say 47R or something) will limit current flow in the case that the transformer secondaries are shorted which might be a good thing. You shouldn't need the coupling cap at the output assuming there isn't a large voltage offset. You can check this.
 
it means that when you connect the transformer in series you have an available center tap... could be useful in certain topologies, where there is a voltage applied to the center tap. not necessary in one like this.
I've used 604s to drive a 600 ohm transformer and there is plenty of oomph for a +4 output. the 604 also has very low offset voltage, so chances are you might be able to skip the capacitor.
Don't take my word for it though, cause my memory is generally faulty. have a look at the datasheets.
Good one
Sleeper
 
I have a Triad HS 56 on the Output of a Valley People Preamp, which is a discrete opamp in a case.

It gives the otherwise clean amp a little character.

Try for 600 Ohms or above.
An API type at 150 ohms might be a drag.
Plus, a little DCR never hurts if you run it with no cap.
If the opamp rails out for some reason, like a test probe touching the wrong place, then you will not have a dead short to ground hanging off the chip.
It proboably has short circuit overload and all that, but I'm jus sayin...
 

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