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Old 7th June 2010, 08:04 PM   #4601
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Jan, please send the link about positive feedback around trasformers. Is it the circuit that Walt Jung has described ?
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Old 7th June 2010, 08:21 PM   #4602
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Quote:
Originally Posted by Joachim Gerhard View Post
Anyway, i am dedicated to make a "good" phonostage with transformers. Provided i get the hum problem solved, the low noise of such circuits is a very attractive oportunity.
One question John : The inductance of the moving coil transformer will roll off the bass at a rate of 6dB / Octave. Does that help to suppress ultra low frequency distortion from warps ?
The inductance rolls off at low frequency - and so the current must increase during big disc warps. This makes the B field increase, to the point where Warp + signal approaches saturation of the trafo core; this is sure to increase distortion.
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Old 7th June 2010, 08:43 PM   #4603
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Yes, at lower frequencies the impedance aproaches the DC impedance. What kind of distortion is that ?
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Old 7th June 2010, 08:49 PM   #4604
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Quote:
Originally Posted by Joachim Gerhard View Post
Yes, at lower frequencies the impedance aproaches the DC impedance. What kind of distortion is that ?
If the warp signal is large enough, you reach the nonlinear part of the B-H curve. The (eg mumetal) core in MC trafos is sure to saturate at a low level - so it will be like having some dc current in the trafo.
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Old 7th June 2010, 09:09 PM   #4605
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Originally Posted by Joachim Gerhard View Post
Jan, please send the link about positive feedback around trasformers. Is it the circuit that Walt Jung has described ?
I don't think Walt talked about it, at least not as far as I know.

The Hofer patent is 4614914 (check at PAT2PDF - Free PDF copies of patents: Download and print! or I can send it).
Lundahl has a one-sheet app note about the same:

http://www.lundahl.se/pdfs/papers/feedbck.pdf

OK, now it's your turn to send me something

jd
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Old 7th June 2010, 09:10 PM   #4606
MRupp is offline MRupp  Germany
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Default Transformer non-linearity at low levels

Menno van der Veen has an interesting article on "Low level audio signal transfer through transformers conflicts with permeability behavior inside their cores" on his WEB page. Menno van der Veen, audio electronic research & consultancy . He is looking at output transformers but this may still be relevant for input transformers.
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Old 7th June 2010, 10:16 PM   #4607
pooge is offline pooge  United States
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At one or two microvolts of signal? Maybe. At one or two nanovolts of signal? Unlikely. At one or two picovolts of signal? Flat out don't believe it.

Regards, Allen
According to Sy's phono article:

"As an interesting exercise, we will compute the signal to noise of the cartridge itself, just to see how quiet the preamp needs to be in order that it does not add any significant noise. The cartridge resistance is 15R. Plugging that value into equation 1, we find that the cartridge has 72nV of thermal noise at room temperature over the usual 20Hz-20kHz audio bandwidth. The cartridge’s nominal output is 0.2mV. Doing a quick decibel check, signal to noise works out to -69dB."

Sounds like you've already decided that pV level signals are audible over noise.
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Old 7th June 2010, 10:18 PM   #4608
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Jan and Martin ! I know both papers, but this is for ouput and not for input transformers.
What about using two input transformers per signal in a kind of distortion cancelation arangement ? Maybe this is too crazy.
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Old 7th June 2010, 10:20 PM   #4609
jlsem is offline jlsem  United States
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Quote:
Originally Posted by Rod Coleman View Post
The inductance rolls off at low frequency - and so the current must increase during big disc warps. This makes the B field increase, to the point where Warp + signal approaches saturation of the trafo core; this is sure to increase distortion.
How does inductance roll off as permeability increases?

John
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Old 7th June 2010, 11:33 PM   #4610
jcx is offline jcx  United States
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we should also be careful about mixing/misapplying material state dependent effects when searching for explanations - hard vs soft magnetic material - analog tape um particles vs annealed NiFe alloy have very different behaviors

my understanding is that quality transformer core material has large oriented crystals (or none for amorphous) specifically to minimize the "quantization" of fully saturating a crystal - and that ferromagnetic domain wall movement within a crystal is very low energy - and as Sy alludes the motion may be dithered by thermal noise power at room temperature

we shouldn't make the mistake of thinking of "The hysteresis curve" or that it shows a "deadband" that necessarily affects low level signals - we usually see plotted a high peak excitation cycle that does show saturation effects
but at any starting (low) magnetization point there is a incremental u_r and local "hysteresis loop" for a small (far from saturation level) signal
and the initial u_r may be very high for low excitation in (su)permalloy tape cores and the local loop is vanishingly "narrow" for low signal excitation

material selection is clearly critical, from the Menno van der Veen paper: (starting with Grain Oriented Silicon Steel showing low intial permability)

“The results of this comparison are clear and totally as
expected; higher perm cores have enough perm at
threshold levels to create almost no weakening between
20Hz and 1kHz.”

He also mentions the possibility of analog magnetic tape style dithering of the core with a inaudible bias frequency

and returning to record cutting: a 80 kHz bais was added to the cutter drive with Teldec's Direct Metal Mastering – although their justification was that the bias frequency “polished” the groove


I think Hawksford's noise paper misses the Ohmic connection from the external signal source into the transistor base region - the conduction band electrons are often modeled as a "gas" with a temperature dependent "pressure" and net current is drift velocity - not exactly quantized integer number of electrons per second
this model fully allows for averaged "fractional charge" to create a continuous - i.e. not observably quantized - electric field modulation in the transistor base region by the source voltage – due to thermal “Brownian motion” in the “electron gas”
he also just "pulls out of his hat" a number for the quantization "sampling time"

Last edited by jcx; 7th June 2010 at 11:36 PM.
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