The implication was that below a certain B field there is no transformer action
(mu goes to 0 or muo ???). Hysteresis and residual magnetism are usually associated with large signal behavior.
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The frequency response can be as good as you want it to be, as usual cost is a factor, and a good response of an MCT can be ruined by poor selection of cable and so on. I once saw a misinformed comment about Barkhausen noise in MC transformers, well Barkhausen noise doesn’t affect materials at such low levels, this only becomes relevant near the knee of the curve as the domains reorientate.
Step-up transformers are widely used in test instruments when extremely low level signals, especially from low impedance sources such as magnetic detectors of various kinds, are to be measured, and are used in the best microphone preamps in recording studios.
Andy Grove - Audio Note design engineer
(mu goes to 0 or muo ???). Hysteresis and residual magnetism are usually associated with large signal behavior.
(Quote)
The frequency response can be as good as you want it to be, as usual cost is a factor, and a good response of an MCT can be ruined by poor selection of cable and so on. I once saw a misinformed comment about Barkhausen noise in MC transformers, well Barkhausen noise doesn’t affect materials at such low levels, this only becomes relevant near the knee of the curve as the domains reorientate.
Step-up transformers are widely used in test instruments when extremely low level signals, especially from low impedance sources such as magnetic detectors of various kinds, are to be measured, and are used in the best microphone preamps in recording studios.
Andy Grove - Audio Note design engineer
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Andy is correct to some degree. Barkhausen noise only comes into play when permeability is relatively low and it shows no effect in high quality cores, except at the knee of the magnetization curve where µ begins taper off towards unity and possibly at the intersection of the B and H axes. It's hard to say because there is little in the readily available literature that describes what is happening at diminishing signal levels.
For the record, I can't say I've experienced that a transformer of very high quality such as those made by Dave Slagle on 80% nickel cores necessitates the loss of low level detail, even with step-up ratios as high as 1:50.
Obviously there is no induction (B) where field strength (H) is zero but 4-79 Permalloy has a µ of in excess of 20K when H is zero, which is initial permeability. I would assume then "transformer action" at the slightest change in field strength.
John
For the record, I can't say I've experienced that a transformer of very high quality such as those made by Dave Slagle on 80% nickel cores necessitates the loss of low level detail, even with step-up ratios as high as 1:50.
Obviously there is no induction (B) where field strength (H) is zero but 4-79 Permalloy has a µ of in excess of 20K when H is zero, which is initial permeability. I would assume then "transformer action" at the slightest change in field strength.
John
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Sorry I'm late to this party.
The recording heads would still work as they have a very strong ultrasonic bias current applied. However, it is the playback heads that might be questionable. But obviously they also work quite well.
a modest proposal
This is such a great site! Fortunately I found this thread before posting a lengthy thing I'd composed, which will now be appropriately rewritten. And I downloaded Julian David's paper, which is quite informative.
Of course transformers are swell, and absent the concerns about low-level behavior and the usual issues with external magnetic field pickup, make all kinds of sense for transforming impedances in this application.
However, at this point mostly a thought experiment:
A direct input preamp is just a matter of spending the silicon area and the preamp power. Whether by paralleling low base spreading R bipolars and running at currents where the half-thermal noise of the emitter impedance is low enough, or paralleling JFETs, it's not conceptually difficult to design such a preamp.
But the interesting thing about JFETs, which cuts both ways as it were: the equivalent series noise spectral density only improves (is divided by) as a factor of the 1/4 power of drain current. So as Wurcer points out, this is a good thing for battery-powered condenser mic preamps, as above say a milliampere things don't improve that much, and in that application are already small compared to other noise sources.
But one can note that the e sub n of a JFET also only degrades at lower currents in a similar fashion. So paralleling devices using the same total current can improve things indefinitely! Of course this is a worthless strategy for capacitative transducers, as the increase in input capacitance entails intolerable attentuation.
But for a ribbon microhone, the extraordinarily low impedance allows a whole lot of shunt capacitance before you get into trouble.
And in fact, pursuing the massive parallelism, it becomes conceivable to have a phantom-powered transformerless ribbon microphone! Using just the available current from the net 3.4k pullup R to 48V, to roughly noise-match (~3dB noise figure, below which there's not a huge amount of improvement left) a 300 milliohm ribbon requires, for a 4 volt operation stage, a mere 5241 😱 BF862 parts (taking Wurcer's measured e sub n of 1.1nV/sq rt Hz performance at 1mA as the guideline).
But indulging this further: if the best conjugate match is made to the phantom power, and, with suitable isolation, switching regulators are used to take best advantage of the ~170mW of power, and operating a little lower (say 3V), we wind up with "only" 1110 devices. Perhaps the death of a thousand FETs, but feasible.
Initial sims on tentative designs indicate a quite useful gain as well.
Brad
This is such a great site! Fortunately I found this thread before posting a lengthy thing I'd composed, which will now be appropriately rewritten. And I downloaded Julian David's paper, which is quite informative.
Of course transformers are swell, and absent the concerns about low-level behavior and the usual issues with external magnetic field pickup, make all kinds of sense for transforming impedances in this application.
However, at this point mostly a thought experiment:
A direct input preamp is just a matter of spending the silicon area and the preamp power. Whether by paralleling low base spreading R bipolars and running at currents where the half-thermal noise of the emitter impedance is low enough, or paralleling JFETs, it's not conceptually difficult to design such a preamp.
But the interesting thing about JFETs, which cuts both ways as it were: the equivalent series noise spectral density only improves (is divided by) as a factor of the 1/4 power of drain current. So as Wurcer points out, this is a good thing for battery-powered condenser mic preamps, as above say a milliampere things don't improve that much, and in that application are already small compared to other noise sources.
But one can note that the e sub n of a JFET also only degrades at lower currents in a similar fashion. So paralleling devices using the same total current can improve things indefinitely! Of course this is a worthless strategy for capacitative transducers, as the increase in input capacitance entails intolerable attentuation.
But for a ribbon microhone, the extraordinarily low impedance allows a whole lot of shunt capacitance before you get into trouble.
And in fact, pursuing the massive parallelism, it becomes conceivable to have a phantom-powered transformerless ribbon microphone! Using just the available current from the net 3.4k pullup R to 48V, to roughly noise-match (~3dB noise figure, below which there's not a huge amount of improvement left) a 300 milliohm ribbon requires, for a 4 volt operation stage, a mere 5241 😱 BF862 parts (taking Wurcer's measured e sub n of 1.1nV/sq rt Hz performance at 1mA as the guideline).
But indulging this further: if the best conjugate match is made to the phantom power, and, with suitable isolation, switching regulators are used to take best advantage of the ~170mW of power, and operating a little lower (say 3V), we wind up with "only" 1110 devices. Perhaps the death of a thousand FETs, but feasible.
Initial sims on tentative designs indicate a quite useful gain as well.
Brad
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For audio transformer info (and other audio electronics info like grounding and safety) go to the site of some of the best, Jenson transformer.
As far as transformers working at small levels, what about the TV cable splitters that take a uV/uA antenna signal and split it into 2. (i broke one once and it had 2 wraps! per wire around a ferrite ring, think how little flux that is ) And it passes 100 ch of 5meg? bandwidth signal.
As far as transformers working at small levels, what about the TV cable splitters that take a uV/uA antenna signal and split it into 2. (i broke one once and it had 2 wraps! per wire around a ferrite ring, think how little flux that is ) And it passes 100 ch of 5meg? bandwidth signal.
For the few people in audio who have not heard of them, it will help if they search for that company with the correct spelling, namely Jensen.
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