Re: Transformer attenuators

In article <Jtgai.124929$Ug.7295@xxxxxxxxxxxxxxxxxxxxxxxxx>,
"Ian Iveson" <IanIveson.home@xxxxxxxxxxxxxxxx> wrote:

John Byrns wrote

Ian Iveson wrote:

One thing I wonder about. It is common to use a
capacitor in series at the input of a valve power amp,
and this may be used to limit the LF response. If you
use a transformer then arguably you don't need the
series cap for its dc-blocking role. So it may seem that
you can rely on the transformer also for the role of
limiting LF. Would that be wise? I am concerned about
the possibility of ensuing distortion.

I don't see why distortion would become any more of a
problem below the lowest frequency the transformer the
transformer is rated for, assuming the LF signal level is
no higher than the signal level the transformer is
designed for in its specified range.

Distortion is more of a problem at low frequencies because of core
saturation effects:

http://www.rane.com/note159.html

"Core saturation happens when the magnetic field in the core
reaches its
maximum possible density, which is what happens when the applied
voltage
polarity remains the same for too long."

Obviously, the applied voltage polarity remains the same for a
longer time
at lower frequencies than higher frequencies.

"Saturation has nothing to do with power delivery: the onset of
saturation
depends only on the voltage waveform applied to the primary."

That quote should be corrected to state that saturation depends on
frequency as well as voltage.

*Magnetising current* is directly responsible for flux density.
Voltage and frequency relations are derived.

The reason distortion (saturation) would
not be a problem when a transformer is being used to limit the LF
response as the OP was asking about, is because what we are talking
about then is a low pass filter where the source resistance driving
the
transformer primary operates in conjunction with the transformer's
primary inductance to roll of the LF response below a specific
point.
This roll off of the primary voltage at lower frequencies prevents
the
core from going into saturation (distortion) at lower frequencies as
would happen if the primary voltage were held constant as the
frequency
decreases.

Not with my maths. Please post yours.

Think of it like this. You have current due to load, and in parallel
you have magnetising current. An increase in magnetising current at
lower frequencies is responsible for the roll-off, given a finite
source resistance. An increase in magnetising current means that the
core traverses a greater part of the BH curve, hence distortion will
increase. In the extreme, saturation will occur if Vs/Rs is great
enough at f=0, ie for DC, when *all* the current is magnetising. If
Vs/Rs is greater than that, then saturation will occur at some
frequency higher than 0.

Yes, effective primary voltage falls with frequency, but not by enough
to hold the magnetising current constant.

If your mathematical argument disagrees with this please post it. So
far you have merely asserted, not demonstrated.

My argument depends on two assumptions, please dispute one or the other,
or both if you wish, if you agree with both assumptions then my original
claim stands.

Assumption #1: I am not a transformer engineer, but those who know more
than I do about transformers have told me that for a given degree of
core saturation (distortion), if the frequency is cut in half, then the
voltage applied to the winding must also be cut in half to avoid
saturation. This means that the applied voltage must be made
proportional to the frequency (or lower) if saturation (distortion) is
to be avoided.

Assumption #2: If the transformer is being used to limit the LF
response of the system, then assuming no load on the secondary, the
source resistance feeding the primary of the transformer must be made
equal to the primary inductance at the desired cutoff frequency of the
resulting high pass filter. Below cutoff the voltage out of such a
filter, at the primary of the transformer in this case, falls at rate
that makes the voltage proportional to the frequency. This is exactly
the sort of voltage response we need on the primary to prevent
saturation of the transformer, hence saturation shouldn't be a problem.

So which one of these assumptions is wrong, or if both assumptions are
correct, where did my logic fail?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
.

Relevant Pages

  • Re: Transformer attenuators
    ... and this may be used to limit the LF response. ... "Core saturation happens when the magnetic field in the core ... the applied voltage polarity remains the same for a ... *Magnetising current* is directly responsible for flux density. ...
    (rec.audio.tubes)
  • Re: Modifying transformer
    ... If it's already close to saturation, ... The voltage only went from 24.2 to 25.7VAC so I'd have to remove a lot ... always rewind the bobbins if I got the wire. ... It seems like a whole lot of bother to make a transformer do what it was ...
    (sci.electronics.basics)
  • Re: What limits the current in an unloaded transformer?
    ... When I wound up the input voltage to the primary it got hotter. ... transformer got a lot hotter and before I knew it the primary ... At higher voltages the core will hit magnetic ... Most transformers are designed to run pretty close to core saturation ...
    (sci.electronics.basics)
  • Re: Transformer attenuators
    ... use a transformer then arguably you don't need the ... Distortion is more of a problem at low frequencies because of core ... "Core saturation happens when the magnetic field in the core reaches its ... the applied voltage polarity remains the same for a longer time ...
    (rec.audio.tubes)
  • Re: Transformer attenuators
    ... core saturation, if the frequency is cut in half, then ... This means that the applied voltage must be made ... If the transformer is being used to limit the LF ... response of the system, then assuming no load on the secondary, the ...
    (rec.audio.tubes)