Line-input with curious LESTRONICS mic-pre topology ??

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clintrubber

Well-known member
Joined
Jun 3, 2004
Messages
5,982
Location
The Netherlands
Arriving at the input-side of an old Ampex-machine, I found transformers in the line-input circuits looking like this:

TX-in_pic11.jpg

TX-in_pic02.jpg


and wired like this:

TX-in_sch_sel01.jpg


So they're making some current through the 25k5 + TX-prim. + 25k5,
transforming that current and then feeding it to a virtual earth
(opamp TL071).
It'll obviously work, and assuming a 1:1 TX-ratio I figure the attenuation
will be (18k/51k) some 9 dB. This input is rated for -24 to +24dBm.

But I've never seen such a topology before.
What would be the idea here ?
Could they be using attenuated mic-TXs here as line-inputs ?!

And what could these TXs be for type ?
Ampex-numbering is 4290942-01 but removing the mu-cap
reveals a round TX with markings Lestronics, Inc., T-2604
/ 4580066-01 (should add a pic).

Thanks,

Peter
 
Thanks PRR ! Much appreciated :thumb:

Printing will start in a minute.

I had a look for the book you mentioned but found 'out of print' so far,
so this is really great.

Thanks,

Peter.
 
So it looks like a really neutral input arrangement -
without any of that iron-nicifying (distortion...)
that seems to have become the norm here :wink:


Thanks again, interesting to know more about that topology.


Using for instance the arrangement of fig 8.35-a it looks
like these Ampex-line-inputs could be converted to mic-inputs
if desired so:
removing the input 25k5's and controlling input-impedance
by the three resistors of the fig 8.35-a circuit.

Despite the physical TX-size I'm not so sure here though
- the TX might be more optimized to do the trick of your article
and the Ampex does use a separate TX for a mic-input (as on the
first photograph) instead of any resistor-switching.


Thanks,

Peter
 
> the book you mentioned but found 'out of print' so far

It may really be out of print. Try ABE.com
 
I first saw something like that back in the '70s and tried to do a version using a cheap small transformer and closing the feedback loop around the transformer.. My idea didn't work easily or look promising on the bench so I moved on. 

JR
 
I have used a very similar topology in a commercial product; the difference was that the opamp was connected as a diff amp. I used a 200:200 rated xfmr.
Main advantages were:
Much better THD and freq response than with a similar sized and priced voltage xfmr (10k:10k)
It provided the possibility to offer two versions, both balanced, but one with xfmr isolation
I never experienced any problem/flaw with that topology as a line input.
However, it is not suitable for high quality mic preamps because of a poor noise figure due to added resistors in source impedance.
 

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