xfmr input impedance & 48v questions

Hey the ease & convenience of this diagram begs questions
[ from my mind ]

http://www.jensen-transformers.com/as/as016.pdf

Such as i presume it would be better to switch windings instead
of adding resisters to change impedance , but is that so ?
or how would one add a pot instead of the resisters ?

If some are likely to quote that the xfmr " reflects " back impedance
you may think it doesn't matter but on some mic/pre combos it makes a big difference
changing the input impedance [ xmfr to xfmr ? ]

The jlm 1272 hotrod diagram shows phantom going to the center tap ,
but for best common mode rejection , would it be better to use 6k8 resisters matched
as tight as possible in case the windings are not as close ?

tia , answers will help my upcoming pre projects
regards Greg

[quote author="okgb"]
Such as i presume it would be better to switch windings instead
of adding resisters to change impedance , but is that so ?[/quote]

Probably, yes. Depends on what trafos you have.

or how would one add a pot instead of the resisters ?

Click to expand...

Meh, not a great idea - it'd be hard to get a dual pot to be linked closesly enough between each track, thus losing good CMRR.

The jlm 1272 hotrod diagram shows phantom going to the center tap ,
but for best common mode rejection , would it be better to use 6k8 resisters matched
as tight as possible in case the windings are not as close ?

Click to expand...

I think it's much of a muchness. If you are lucky enough to have a centre-tapped trafo then it does save a resistor (and matching if you use lousy resistors).

I'm guessing that other pres that have a pot use it between the +/-
inputs , but i've haven't seen a schematic

[quote author="okgb"]Hey the ease & convenience of this diagram begs questions
[ from my mind ]

http://www.jensen-transformers.com/as/as016.pdf

Such as i presume it would be better to switch windings instead
of adding resisters to change impedance , but is that so ?[/quote]

I've found adding resistors works fine, and if you're using Jensen's transformers, they don't have multiple primary taps anyway.

or how would one add a pot instead of the resisters ?

Click to expand...

Connect between pins 2 and 3. A 1k pot in series with a 150 ohm resistor would do the job. I'd be hesitant about putting a pot in such a low-level low-current circuit, though -- personally, I'd rather use switched resistors, with a good switch.

If some are likely to quote that the xfmr " reflects " back impedance you may think it doesn't matter but on some mic/pre combos it makes a big difference changing the input impedance [ xmfr to xfmr ? ]

Click to expand...

It does indeed -- mostly with Shure SM57s and related mics, which seem to have been designed with minimal mechanical damping, relying on the electrical damping of a lower-impedance load (500-600 ohms).

The jlm 1272 hotrod diagram shows phantom going to the center tap ,
but for best common mode rejection , would it be better to use 6k8 resisters matched
as tight as possible in case the windings are not as close ?

Click to expand...

If your transformer has a center tap, it's probably better-matched than a pair of resistors; you can get them to match to maybe 5 ohms with a good DVM, but a good center-tapped transformer will be closer than that. But again, if you're using Jensens, most of their mic input transformers aren't center-tapped. And with transformers that use separate windings in series/parallel hookups, like Lundahls, what I said about center taps doesn't necessarily apply.

Peace,
Paul

You might have missed that R4/R5 do not (significantly) change the input impedance--these are series resistors and not in parallel. They increase the source impedance, which helps to avoid transformer resonance but does little to damp microphone response.

Samuel

The center tapped xfmr i refer to is a Carnhill for the 1272 circuit

And indeed , Dan Kennedy's reply was inline wioth Samual's

This isn't to change the input impedance of the preamp, it is to
compensate the input transformers' response when driven from very low
output impedance microphones, like the Schoeps circuit.

Most transformers will have a somewhat peaked response with a very low
source Z.

Click to expand...

Thanks for the replies guys

[quote author="pstamler"]

or how would one add a pot instead of the resisters ?

Click to expand...

Connect between pins 2 and 3. A 1k pot in series with a 150 ohm resistor would do the job. I'd be hesitant about putting a pot in such a low-level low-current circuit, though -- personally, I'd rather use switched resistors, with a good switch.
[/quote]


Relatd topic about mic input pads;
JLM Joe and I got into a discussion about his method here:

http://www.groupdiy.com/index.php?topic=14888&start=15

There's definitely more to be contributed to that discussion.