zebra50

Getting something for nothing from a transfomer?
« on: July 27, 2004, 09:19:37 AM »
Help!
I've just confused myself.

If I have a 1:1 transformer with a centre tap, I know that I can wire it as 1:2 by connecting the centre and one side of the primary, and leaving the other end unconnected.

But if I do this does everything else behave as usual, in terms of impedence ratios etc? I feel like I'm getting something for nothing, which I know we never get in this life (thermodynamics 'n'all!).

Ta!
Stewart
Ribbon microphone services
http://www.xaudia.com
Microphone blog


gyraf

Getting something for nothing from a transfomer?
« Reply #1 on: July 27, 2004, 09:30:09 AM »
First of all, you won't get anything for nothing. This is a sad fact of life.

But wired 1:2 you will get double the voltage at half the current. And as voltage noise most often is dominant, you can get 6dB "free" gain.

A drawback of using only ½ primary winding could be that your primary inductance will be only 1/4 of that of the whole primary winding, meaning that you will loose low end earlier at the same generator impedance, or that your generator impedance will have to be lower in order to drive low-end through the transformer. This may not be a problem, if you have very extended low end already.

Jakob E.
..note to self: don't let Harman run your company..

zebra50

Getting something for nothing from a transfomer?
« Reply #2 on: July 27, 2004, 09:51:58 AM »
Thanks Jakob,

I was thinking of this the other way around, actually. It's for the ouptut of a cathode follower mic. I wanted to lower the output impedence by just using half of the secondary so I have 1:0.5 (= 2:1).

Stewart
Ribbon microphone services
http://www.xaudia.com
Microphone blog

gyraf

Getting something for nothing from a transfomer?
« Reply #3 on: July 27, 2004, 09:56:54 AM »
That way, your only drawback will be the slightly less efficient power transfer due to the fact that you can't use both windings in parallel (if it's centre tapped), so your secondary copper resistance will be a bit higher than with a "real" 2:1 transformer the same size. But this is probably too little a problem to be inconvenient in any way..
..note to self: don't let Harman run your company..

zebra50

Getting something for nothing from a transfomer?
« Reply #4 on: July 27, 2004, 10:29:36 AM »
Thanks for that.

 :thumb:
Ribbon microphone services
http://www.xaudia.com
Microphone blog

CJ

Getting something for nothing from a transfomer?
« Reply #5 on: July 27, 2004, 04:02:49 PM »
You might encounter more distortion also. Maybe not. Depends on the transformer construction.
If it's a single bibbin on an EE core, you should be alright. If it is a UTC type, then you have two seperate coils, and you will be using only one for the pri or sec, so the flux in the core is imbalanced a little bit.
This is why the hookup charts are important, The transformer companies want you to hook them up a certain way for max performance, so this is why you see two pri's combined in paralell, even though the impedance will be the same.
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

zebra50

Getting something for nothing from a transfomer?
« Reply #6 on: July 27, 2004, 04:22:39 PM »
Thanks for the details, CJ.
It's a small canned sowter like this...



and I have no idea about the construction or winding. I'm going to just suck it and see as changing 1 wire is easier than slipping in a new tranny. I'm so lazeeeeeee.....
Ribbon microphone services
http://www.xaudia.com
Microphone blog

PRR

Getting something for nothing from a transfomer?
« Reply #7 on: July 27, 2004, 04:56:00 PM »
> I feel like I'm getting something for nothing

You pay in copper loss, because you are using only 3/4 of the copper (assuming primary and secondary have equal amounts of copper).

Or: primary resistance loss is double what it could be if you could use the entire available space for useful copper.

In signal transformers, lightly loaded, this is not a big deal.

In power transformers (including audio when you need every last watt possible), it means poorer regulation and more heat.

A 600CT:600CT transformer will typically have 30 ohms resistance per 600 ohm winding. Using half the winding, you get nominal 150 ohms impedance with 15 ohms resistance. Or seen end-to-end: as 600:600 you have a total 60 ohms copper loss, 10% or 1dB; but as 600:150 you have 22.5 ohms copper loss against 150 ohms impedance, a 15% or 1.4dB loss. However mike inputs are not usually power-matched. 2,000 ohms is a typical input impedance. The 22.5 ohms of copper loss (instead of the possible 15 ohms) is only 0.1dB loss, negligible.

> a cathode follower mic

In that case, the 2K mike-amp input impedance reflects back into the cathode as 8K, a nice high value for good output and low distortion. But it is still a 600 ohm winding. Assume they mean "600 ohms at 20Hz", and assume your cathode impedance is 2K. Ignoring small factors, your -3dB point shifts up to 67Hz. Actually, many 600:600 trannies do better than 20Hz, and if you run the tube rich it may be less than 2K cathode impedance, so it may not be that bad. If you care, disconnect the grid from the capsule and drive a signal generator through a cap to the grid, see what comes out of the transformer. Shoot for signal levels of 10mV and 100mV, because inductance varies with signal level.

zebra50

Getting something for nothing from a transfomer?
« Reply #8 on: July 27, 2004, 05:43:31 PM »
Many thanks - that is really helpful.

In fact the transformer I'm using is nominally spec'd at 3k9:3k9, and was chosen on the basis of being around at the right time and sound good (subjectively speaking). Is this going to be significantly worse in performance thatn a 600:600 (theoretically at least)?
Ribbon microphone services
http://www.xaudia.com
Microphone blog

CJ

Getting something for nothing from a transfomer?
« Reply #9 on: July 27, 2004, 07:28:16 PM »
thats a single coil EE tranny, so ok for non symetrical operation.
3.9k means more copper loss, but more inductance which means better low end.
Try it out now.
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


hodad

Getting something for nothing from a transfomer?
« Reply #10 on: July 28, 2004, 01:48:38 PM »
Quote from: "PRR"
>

A 600CT:600CT transformer will typically have 30 ohms resistance per 600 ohm winding.



If I may hijack this thread & ask a related question:

http://www.wee-scottie.com/tcr/15432a.pdf

shows a transformer with what looks like two pretty much equal primary windings--both 760 turns.  But the 1-2 winding measures 39R (about what the datasheet indicates, & about right for a 600 ohm winding). but 1-3 measures at 260R--more than 6 times the resistance of 1-2.

What is it that I'm not understanding here?  Why do these two not measure roughly the same resistance?

Tom


 

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