Using "Line Input" transformer for output
« on: June 13, 2020, 10:57:11 AM »
Hi folks, any drawback to this? I need a 4:1 shielded output transformer,  to mitigate hum from power transformer  in a small enclosure. Source is 150 ohms, +10dBu max... I'm looking at:

https://www.hammfg.com/files/parts/pdf/1140-LN-D.pdf
https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-10kb-d.pdf

Thank you ---


mjrippe

Re: Using "Line Input" transformer for output
« Reply #1 on: June 13, 2020, 02:13:23 PM »
Altec/Peerless 15095 were used both ways.  Some say it is a sin.  I'm sure there are scientific explanations of why it is or isn't a good idea.  Level handling comes to mind - using a mic input reversed for an output would probably not work so well with +20dB...

Re: Using "Line Input" transformer for output
« Reply #2 on: June 13, 2020, 03:12:15 PM »
Well, these are "Line Input", not mic input transformers.. for example the Hammond is  40K: 2.4K  +21dBu
I'm wondering technically if there is a difference between 40K: 2.4K and 10K: 600... same 4:1 ratio

CJ

Re: Using "Line Input" transformer for output
« Reply #3 on: June 13, 2020, 03:17:23 PM »

probably depends on the circuit,

if you have a cap feeding the OPT then it is easier on the transformer,

current handling, flux and unbalanced DC are the tings you might want to look at,

how much voltage you are swinging on your output amp,  most inputs use a nickel alloy core  so you have to watch out or saturation,

the turns count and size of the core  will determine how much DC imbalance the transformer will tolerate,

wire size can be a problem, #46 is rated at 2.5 ma, so you would not want an SE amp with 10 ma driving that,

a lower ratio transformer will have bigger wire as there is more room on the core and they like to keep DCR down so they us as big of wire as possible.

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

ruffrecords

Re: Using "Line Input" transformer for output
« Reply #4 on: June 13, 2020, 04:09:35 PM »
Hi folks, any drawback to this? I need a 4:1 shielded output transformer,  to mitigate hum from power transformer  in a small enclosure. Source is 150 ohms, +10dBu max... I'm looking at:


Is that +10dBu at the primary which means -2dBu at the output or do you mean +10dBu at the output which means +22dBu at the input?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Using "Line Input" transformer for output
« Reply #5 on: June 13, 2020, 08:23:25 PM »
Yep cap coupled single ended output. It's an instrument preamp, a couple triodes, tone controls, etc.. .with a mosfet source follower at the end. I'm seeing about 2VRMS max output at the primary, well under the transformers 21dBu rating.
Normally I use 10K:600 but can't seem to find an off the shelf shielded version.

CJ

Re: Using "Line Input" transformer for output
« Reply #6 on: June 13, 2020, 09:31:32 PM »
a nickel core would be the way to go for a cap coupled low  level OPT.

we did a one off on a Neve input core,  which is a fairly small 80 Ni core, it was for an Alembic style preamp.
« Last Edit: June 13, 2020, 09:39:33 PM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Using "Line Input" transformer for output
« Reply #7 on: June 14, 2020, 10:16:16 AM »
Great, I'll give it a try. Thanks fellows.

CJ

Re: Using "Line Input" transformer for output
« Reply #8 on: June 14, 2020, 01:51:21 PM »
if you get a big chunk of 49 Ni for the core then you might be able to ditch the output  cap, this could improve the sound especially on the low end.

if you do keep the cap then a larger value like 10 or 20 uf might be better, that cap forms a resonant circuit with the transformer primary,  so a bigger cap might avoid a hi freq peak somewhere depending on the pri inductamce.

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

 

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