Ever heard of this...
Client has issues with audio transformers... at low signal levels they exhibit roll-off in coupling at low frequencies, like -10dB at 50Hz
But high signal _levels_ do not.
Strange? Or a known phenomenon? ...Jim Thompson
Ever heard of this...
Client has issues with audio transformers... at low signal levels they exhibit roll-off in coupling at low frequencies, like -10dB at 50Hz
But high signal _levels_ do not.
Strange? Or a known phenomenon? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Sure. Lots of core materials have lower permeability at low flux levels--it's the magnetic version of traps in semiconductors.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
So there's no cure? I was pondering current drive, but it doesn't sound like that would help? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Different core material, or an out-of-band bias signal, as in tape heads.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
So, I read Jim's questions and think "ooh, I know the answer!", then Phil answers it. Twice. In a row. Hmph.
(Yes, this IS a contest).
If you're clever you can probably tailor the bias signal as needed, to reduce power drain and maintain sufficient overhead.
-- Tim Wescott Wescott Design Services
Don't feel bad--I'm stuck in bed getting over pneumonia, and am sufficientl y recovered to be excruciatingly bored, so I'm checking SED much more often than someone with other options.
If somebody asks something about systems with multiple inputs and outputs, I promise I'll be quiet as a mouse. ;)
Cheers
Phil Hobbs
Current drive sounds good as you can RSense and filter to get a control voltage representing the average flux in the core, and use that to control your out-of-band bias oscillator just summed into the signal, to keep the flux above some minimum constant
Real or approximate zero-flux mode would certainly help.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Knowing little about magnetics, could you turn on some DC bias at low signal levels?
George H.
DC could introduce distortion, maybe more easily than HF AC and will need some tweaking to get the best results It *was* used in many cheap portable tape recorders.
Maybe add another secondary winding, and use it for negative feedback in the circuit that drives the primary, a bit like old valve audio power amplifiers did.
You would want the feedback secondary and the real secondary to have similar coupling to the primary.
Why would this only affect low frequencies?
John
At higher frequencies less inductance is needed and the impedance of the transformer will still be sufficient despite the reduced permeability. Not so at low frequencies. It's the same with RF transformers such as baluns. There it is more extreme in that at really high frequencies the core wouldn't even have to be there.
-- Regards, Joerg http://www.analogconsultants.com/
Yes. The client found this...
...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Interesting. (I've never used an audio transformer in real life--just pulse and RF.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Nice oscillator circuits!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
if you put a low frequency sine wave through the transformer, does it appear distorted, resembling crossover distortion?
or is it just that the client is making a measurement error?
the xformer attenuates low frequencies for all the usual reasons, but if you drive it harder, it distorts, and the clients measurements incorrectly indicate that the amplitude of the fundamental increased?
Mark
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