Author Topic: DC effects in a transformer  (Read 1091 times)

dspruill

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DC effects in a transformer
« on: November 02, 2011, 09:19:45 PM »
Hello all, just trying to understand the mysterious world of transformers. My question is... I am experimenting with a old Ballantine 300 AC voltmeter. It can also be used as an audio amplifier and sounds really nice running various stuff through it for color. It has a transformerless output using a blocking cap. If a transformer is just added on the output after the cap what are the effects on it performance as opposed to it being in the plate circuit with DC flowing through it? The transformer I have at the moment is a UTC A-25.

Thanks
David


alexc

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Re: DC effects in a transformer
« Reply #1 on: November 03, 2011, 02:23:53 AM »
The UTC A-25 15K:600 (and lower) plate to line transformer is rated for 8mA unbalanced DC and output level of 30mW.
So it has some tolerance to dc current flowing thru it.

I guess you could use it in a single ended or a poorly balanced push-pull topology with no dc blocking, within those limitations.

Without wishing to be sound offensive,
using a UTC A-25 on some old voltmeter amp is kind of like casting pearls before swine  :)

That traffo is quite sought after and  fetches a good price.

Sell it, buy some cheaper parts and do whatever your experimental heart desires!

Cheers
I ping therefore I am

lassoharp

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Re: DC effects in a transformer
« Reply #2 on: November 03, 2011, 09:52:46 AM »
I'm not sure if you will get any better low end performance than the speced 40Hz corner using the blocking cap.  This is assuming a very large cap value - large enough so that the point it resonates with the pri L is way down near DC.  I've wondered the same thing.  The logical choice would be to guess that it would follow the other A series and give a 20Hz corner with no DC but I don't know if that's correct.

For lower cap values you can extend the low freq response by choosing a cap value that gives a resonance bump at some freq lower than 40Hz.  Probably easiest to start subbing values and monitor with some sweep software.  This technique is said to increase distortion.  I've purposely done this before and it's not necessarily objectionable distortion.

Without cap and running 8ma or less you should get the listed response and more efficient voltage transfer than with a resistor load.   As you increase the currnet in the pri the low end response corner will start rising.

abbey road d enfer

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Re: DC effects in a transformer
« Reply #3 on: November 03, 2011, 12:45:33 PM »
Hello all, just trying to understand the mysterious world of transformers.
Transformers are far from being mysterious. You need to look at them with the necessary technical background. Almost everything about transformers is on the web. Just google transformers for the (very important basic knowledge) and then audio transformers, for the specificities.
Quote
My question is... I am experimenting with a old Ballantine 300 AC voltmeter. It can also be used as an audio amplifier and sounds really nice running various stuff through it for color. It has a transformerless output using a blocking cap. If a transformer is just added on the output after the cap what are the effects on it performance
It's a very common configuration in tubes circuitry, resistor-loaded plate driving a xfmr. As lassoharp mentioned, the output cap resonates with the primary inductance, creating a VLF hump. The amplitude of this hump depends on how the resonance is damped by the tube's output impedance and the reflected load impedance.
Quote
as opposed to it being in the plate circuit with DC flowing through it?
DC flowing in an iron-core transformer's winding decreases the inductance, which restricts the LF extension. So the trade-off is reducing LF extension by having DC but no cap, or reducing it by having a cap and no DC.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dspruill

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Re: DC effects in a transformer
« Reply #4 on: November 04, 2011, 06:57:09 PM »
Thanks everyone, I have some reading to do!

David

CJ

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Re: DC effects in a transformer
« Reply #5 on: November 05, 2011, 01:51:13 AM »
a circuit would be nice

this idea is nothing new,

hp ac voltmeter, line out,

get me?

If I can't fix it, I can fix it so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

gyraf

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Re: DC effects in a transformer
« Reply #6 on: November 05, 2011, 01:54:26 AM »
..been there too - Bruel&Kjaer 2603 preamp output...

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

dspruill

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Re: DC effects in a transformer
« Reply #7 on: November 06, 2011, 12:06:51 AM »
Ask and ye shall receive..... :) This thing sounds pretty damn good.

David



 

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