lundahl "combo" transformer

Has anyone used the LL1952 transformer?

It appears to be appropriate for both mic and line signals but the data sheet states that "A transformer of this kind is for natural reasons a compromise".  What is exactly the compromise?

Also, it made me think. I have a spare Carnhill VTB9046.  Would it be possible to use it reversed as a mic TX? Are there any disadvantages or "compromises" by doing so?

Transformers do work in reverse ,Ive used a step up mic input transformer in reverse as an output transformer in a tube mic ,works great .
Ive heard of relay switching of a neve style input transformer ,so that it performs as both Mic input step up and in reverse as line input step down,of course the terminating resistances and levels have to be taken into account also.

flipping the Carnhill  Line In should work fine,  might have more headroom if they use lower perm alloy than the mic in,

there might be a drop in primary inductance,

line  input wired 2:1 has
pri 2710T  67 H          #39 wire    DCR 328 
sec 1350T  =  16.4 H     # 36 wire    DCR 112     

mic input wired 1:2 has
pri    998T          32 H     #36    Wire  DCR  47 
sec  2090 T  125 H  # 39    Wire  DCR 263 

flip the line around and use it as a mic input you have

pri  16.4 H  112 Ohms  sec  67 H    328 Ohms      6 Hz to 50 K          (1.2K : 4.8K))  Leakage: 8.2 mH

vs  original mic in:
pri  32 H      47 Ohms    sec  125 H  263 Ohms    3 Hz to  60 K Hz  (1.2K:4.8K)  Leakage: 6.0 mH

so by flipping the line in to a mic in, you go from 3 to 6 Hz due to a drop in pri ind,  no big deal, and from 60 to 50 K Hz due to a bit more leakage,  no big deal,  DCR is too close to cause any big shifts in response,

what you might get is more noise since the line in sec is the outer winding and thus lacks the natural shielding of the outer copper layers (when you flip it) that the mic input has, being wound with the pri wind first, both have copper E shields, but only the mic in will have this shield wrapped around it.

note that hi end response might drop if you use the mic or line in strapped for 1:4 operation:
spreadsheet says you drop to 33 K for the mic in and 25 K for the line in, which some people might like for vocals as your center freq drops to about 400 Hz for each.  there might be a difference in ringing between the line and mic due to the difference in coil geometry when the line in is flipped.

Fairchild 670 uses UTC A-26 output flipped for line in,

Thank you very much CJ. Great info! I was kind of hoping you to chime in  ;D

So, you're saying that all in all the mic TX would be more appropriate, right?  Well, make sense  ;D

In fact, I have both a mic and a line input transformer so I guess I'll try both and see what works best. BTW, that's not for the usual Neve circuit but for a Carlec, which originally used a Sowter input TX with 1k2 (for mic) and 10K (for line) at the primaries and 10K at the secondary. 

Thus, it occurred to me that  maybe the Carnhill line TX would be (more?) suitable than the mic TX for the mic input (when reversed).

It's not a matter of cost since I will hopefully end up with only a pair of those pres so buying one more transformer is not an issue. It's the best performance I'm after.

Tubetec said:

Transformers do work in reverse ,Ive used a step up mic input transformer in reverse as an output transformer in a tube mic ,works great .
Ive heard of relay switching of a neve style input transformer ,so that it performs as both Mic input step up and in reverse as line input step down,of course the terminating resistances and levels have to be taken into account also.

Click to expand...

Hey Tubetec, thanks for the info. Any idea what that relay switching circuit is? 

CJ said:

note that hi end response might drop if you use the mic or line in strapped for 1:4 operation:
spreadsheet says you drop to 33 K for the mic in and 25 K for the line in, which some people might like for vocals as your center freq drops to about 400 Hz for each.

Click to expand...

Very interesting post CJ.

But what is this "center freq" you refer to?

warpie said:

It appears to be appropriate for both mic and line signals but the data sheet states that "A transformer of this kind is for natural reasons a compromise".  What is exactly the compromise?

Click to expand...

Core size related compromises, distortion optimization for one or the other usage/levels.  Consider almost all quality vintage transformers were for both mic or line usage, and have the same compromises. 

IIRC somebody said ha hey actually liked he sound of he line in xfmr over he mic input, so some experiments might offer up some surprises, 

Thanks guys. I'll give it a try and whatever happens .

CJ said:

IIRC somebody said ha hey actually liked he sound of he line in xfmr over he mic input, so some experiments might offer up some surprises,

Click to expand...

I think it was Churchill  8)

CJ said:

IIRC somebody said ha hey actually liked he sound of he line in xfmr over he mic input, so some experiments might offer up some surprises,

Click to expand...

Don't mean to be off topic..But Does ANYBODY? Have a schematic or the  pin out for Hollytran input transformer:
E 5143 F ?
I can't find ANYTHING on the net.? This is on one of my RCA BA 11. My other two are the UTC G 9634. That I love!
Any help would be appreciated.....Input is all distorted even with 15db pad.......Not sure what the impedance on this transformer are......?...
Thanks!
John

CJ said:

flip the line around and use it as a mic input you have

pri  16.4 H  112 Ohms  sec  67 H    328 Ohms      6 Hz to 50 K          (1.2K : 4.8K))  Leakage: 8.2 mH

vs  original mic in:
pri  32 H      47 Ohms    sec  125 H  263 Ohms    3 Hz to  60 K Hz  (1.2K:4.8K)  Leakage: 6.0 mH

so by flipping the line in to a mic in, you go from 3 to 6 Hz due to a drop in pri ind,  no big deal, and from 60 to 50 K Hz due to a bit more leakage,  no big deal,  DCR is too close to cause any big shifts in response,

Click to expand...

Johnson noise will also be higher. The original mic 47 ohms dcr is in series with your mic (say 150 ohms) so it increases noise a little - noise factor probably 1dB or so. Increasing it to 112 ohms will make it closer to 3dB. Probably not a lot in the overall scheme of things but worth knowing if you seek the ultimate low noise mic pre.

Cheers

Ian

So, it seems that the only advantage of using a line TX (reversed) over a mic TX is a bit higher headroom.

thanks Ian, good to know that!

advantage of flipping the line input>if you are handy with switches and/or relays, you can save the price of having two transformers on your input circuit,  ie, have a switch which flips the input/output,

might not save much over Carnhill, but if using Maranair green meanies, then you are up 200 smackeroonies,