diyTransformer - Frequency response changes with input level - why?

[abbey road d enfer]

yes, but you can add some paper every several layers to cut this

Adding paper increases leakage.

and also is stated that bobbin is not full , so wire size can be increased..

This is true. However, increasing the number of turns will make the core operate at lower level, which in turn results in lower permeability at very low level.

 

[CJ]

Jensen article on mic splitters by Mr. CMMR himself, >

 

[MisterCMRR]

Trying to make a low-level audio transformer from silicon-steel is comparable to making a silk purse from a sow's ear. Nothing you can do to the windings will overcome the low permeability and high magnetic hysteresis of steel. Steel's only redeeming properties are high saturation flux density and low cost - which makes it first choice for cheap power transformers.

 

[andia]

I have recored the coil with a lamination that I believe is nickel (salvaged from some transformer found on a telefunken eurocard module), it is more shiny, silvery and somewhat sparkly, when looked at at the right angle.
down below you see the result of the measurement with this new core. at around 0dBu it is -0.1dB, at -55dBu it is -0.5dB @20Hz.


I

 

[CJ]

Looks good at 20 hz,

Do an inductance plot, measure primary current and voltage
Divide voltage by current and 6.28 f

Note that test equipment might get flakey at 5 hz

 

[andia]

As I think that I wont get ahold of nickel laminations anytime soon I tried to wind a transformer on a toroidal core. TDK T38 material. I wound a bifilar 1:1 transformer, 0.125mm wire, 1080 turns. primary DCR 60ohms, primary inductance 27H. the measurements look really good. only -0.03dB at 20Hz

is there anything that speaks against such a kind of transformer for mic level applications? Besides that it took me 2 hours to wind it...

here some measurements:

frequency response (1V input, 1k load resistor)


THD @20Hz, 30Hz, 50Hz


THD @+2dBu, -10dBu, -20dBu

 

[abbey road d enfer]

I would say 27H is a tad too much. You could probably take off 1/3rd, for about 12H and less DCR.
Have you checked the very low level LF response, like you did earlier?

 

[andia]

I would say 27H is a tad too much. You could probably take off 1/3rd, for about 12H and less DCR.
Have you checked the very low level LF response, like you did earlier?

yes, it is a straight line with this transformer. lowest line is -60dBu inputlevel.

 

[Philbo King]

Looks quite decent. Nice work!

 

[abbey road d enfer]

Excellent!

 

[CJ]

Check capacitance from pri to sec

 

[gyraf]

..2 hours for 1000 turns is quite good..

 

[andia]

Check capacitance from pri to sec

Don't know if i am doing it right - capacitance between shorted primary and secondary winding is 4,5nF.

 

[CJ]

That's quite a bit , check phase shift next with dual trace scope if possible,

 

[andia]

That's quite a bit , check phase shift next with dual trace scope if possible,

like this? green is level, blue is phase. +1.2° @20Hz, -1.5° @80kHz.

 

[CJ]

that's not too bad considering the capacitance, hard to keep C down with bi fi,

capacitance should extend hi end out to 1 Mega Hertz, check out there for peaks, valleys, phase shift,

say 20 K to 1 MC,

 

[andia]

that's not too bad considering the capacitance, hard to keep C down with bi fi,

capacitance should extend hi end out to 1 Mega Hertz, check out there for peaks, valleys, phase shift,

say 20 K to 1 MC,

here is the bode plot:

20hz - 1Mhz


20hz - 5Mhz

 

[CJ]

Nice!

Last thing to ponder would be a square wave test at 1k 10 k 100 K and 1 mc

Looking at secondary waveform with scope if you can,

Bonus points for checking leakage inductance by measuring pri ind. with sec shorted

Should be very low with toroid, 10 mH max

 

[andia]

leakage inductance measures 0.55mH

here the square waves:

1k


10k:


100k:


1Mhz:

 

[gyraf]

Damn, that is some square response. I wouldn't have expected the T38 material to manage that.. You sure about the 1MHz?

/Jakob E.