AMZ Mosfet Booster mod question - Balanced (XLR) Out?

[Gyoon]

I saw on the AMZ site that there was a slight mod suggestion where you could simultaneously get an inverted and non-inverted out.  These could be combined in a three pin xlr jack for a balanced out, correct?  What would the output impedance be for this be?  Unity gain, though, I assume.  I want to make a Christmas gift for a pretty cellist I know but don't want to shell out close to $100 for a Jensen Transformer.

Help a dude out,

Glenn

[Gus]

Have you checked cinemag
http://www.cinemag.biz/

[R.G.]

The name of the game for XLR inputs is usually common mode rejection. A mosfet setup with true/inverted signals usually takes the signal from equal loads on the source and drain. This works, but is not the same a conversion to balanced line by any means.

If the cellist is worth the trouble, use one of the transfomers mentioned, or one of the opamp circuits designed for this.

[amz-fx]

I saw on the AMZ site that there was a slight mod suggestion where you could simultaneously get an inverted and non-inverted out.  These could be combined in a three pin xlr jack for a balanced out, correct?

No, that would not be a balanced output.... it's not accurate enough for that purpose. 

If you want to do it on the cheap, and don't need truly hi-fi sound, drive the primary of a Xicon 42TU019 transformer with the booster.  The secondary will be the balanced output.  Those transformers are less than $3 at Mouser.  I don't make any guarantee about bass response or distortion but it will work!

regards, Jack

[joelap]

Off topic, but what about going from XLR to 1/4inch?  Reverse the order of that mouser transformer?

[R.G.]

If you want to do it on the cheap, and don't need truly hi-fi sound, drive the primary of a Xicon 42TU019 transformer with the booster.  The secondary will be the balanced output.  Those transformers are less than $3 at Mouser.  I don't make any guarantee about bass response or distortion but it will work!

I'll add a bit to that, as I have done some lab work on the Mouser transformers.

Those Xicons are specified for 300Hz to 3kHz. They do in fact measure out much better than that on the top end, usually getting to over 15kHz, some even better. Where they have problems is the low end. The ones I've measured get down to maybe 150-200Hz. Your cellist won't be happy with that.

You can make a transformer do better than it would otherwise by driving it from a much lower impedance than is a matching one. There's a lot of math involved that I won't bore you with, but if you drive the transformer from an low impedance source like an opamp output, it will extend the low frequencies that get through the transformer.  The output of a single-device follower like a source follower or emitter follower would be better than nothing, but not as good a bass extension as an opamp, which has a lot of gain to burn up to try and make the output follow the input.

The circuit of the hum-free A/B/Y box at GEO, http://geofex.com/FX_images/spltr2.gif simplified by leaving off the switching, the second transformer output and the LEDs, and with the sub of a 42TU019 transformer will get you down to 62 Hz - I've done it and measured it with the 42TM018, a smaller version of the 42TU019. 

I can't recall the lowest cello notes right now. But I do remember that cello doesn't couple its fundmentals to air very well, so the low 60's might be low enough.