Is it possible to put a number on the impedances of the windings, that's to say, as any known xfrmr is rated, i.e. 600ohms source to 50Kohm grid, in the case of a mic input xfrmr? I've been reading the xfrmr Z thread; my mathmatical notation as written on the keyboard's less than sterling so I've gotten tripped up in a couple of spots. I'll try to distill what I've understood through quotes. Look for a line of dollar signs where I've stopped ruminating on the Z thread and ask some questions that may actually make some sense in terms of practical testing.
(and please, feel free to refer me to a book, or, even suggest it might not be too important to get a grip on the Z thread to make a close approximation of in/out Z in an fairly easy real time way)
the entire transformer z thread is here:
http://www.groupdiy.com/index.php?topic=1119&postdays=0&postorder=asc&start=0
PRR said:
So if it is wound 1:2, the output voltage is twice the input voltage, and the output current is twice the input current. So the output impedance is 2V/0.5V= 4 times the input impedance. Any other answer violates the same-power in/out fact. (Since there ARE losses, the impedance isn't exactly the square of ratio. The output impedance is a little high due to copper resistance, a little low due to iron losses. Both losses are small in a well designed transformer, and pull in opposite directions, so the output Z is V^2 for most practical purposes.
I've got it all the way to volts^2. Don't know ^.
cj responded a bit down the line:
Impedance, in this case inductive reactance, is given by 6.28 f L.
So you can see that impedance will very with frequency.
I take this to mean 6.28 times frequency time inductance. I understand how impedance increases/decreases from source across the frequency band. Is it the load impedance th
at's variable?
Okay, I'm still rereading PRR's immediate response w/fervent (but perhaps futile:?) hope that it'll sink in. I might as well quote him so there's no need to shuttle between threads:
Say you have a core and winding that is 1 Henry. At 10Hz: 6.28 * 10 * 1H = 63 ohms; at 50Hz: 6.28 * 50 * 1H = 314 ohms; at 100Hz: 6.28 * 100 * 1H = 628 ohms; at 1KHz: 6.28 * 1K * 1H = 6,280 ohms, and so forth.
That is just the inductance. With real iron and copper you also have pure resistance, which acts in series with the inductance. Without defining "Q", just assume that the Q of an iron/copper inductor at 50Hz is generally no more than about 5 or 10. The inductive reactance of 1H at 50Hz is 314 ohms, so the resistance is probably 63 to 31 ohms. This is often called "DC Resistance", though it actually affects the low-end too.
So with no load on the secondary, the input impedance of a 1H iron-core transformer is 30 to 60 ohms from DC to about 5Hz or 10Hz, then rising to 314 ohms at 50Hz, 6K ohms at 1KHz.
This impedance "shorts" the source, so we have to ask what source impedance we will use. And as far as secondary voltage is concerned, the DC Resistance adds to the source impedance. Assume 30 ohms DCR. If we used a 315 ohm source, 345 ohms total source impedance, the response would be down -3dB at 55Hz and falling below that, but flat above that. Adequate voice quality; for music we might try a 100 ohm source, total source impedance now 130 ohms, -3dB at 22Hz. So we might call this a 100 ohm winding.
For the most part I got that. Correct me if I'm wrong, Q is resonance. I understand how to check for resonant peaks at least in terms of calibrated a tape recorder. What I don't understand in PRR's post is his numerical value of Q, where he says "5 to 10." I'm also getting stuck on the relationship between DCR and load resistance.
PRR makes this practical statement:
is it OK enough to draw conclusions from the estimated Q (5 to 10) and the measured DC-resistance?
That's a starting point. 30 ohm DCR is more likely to be about-600 than about-50K.
For a next step, I would pad-out a signal generator with the estimated impedance, and check the frequency response of the secondary. If the source impedance is too high, the bass will not be good. You can try rating it at a lower impedance, but for cheap transformers you may run into the DCR before you get to 20Hz. This also limits any transformer when used with a zero-ohm source such as a feedback amp.
You should also check the high frequency response. But this is very dependent on load impedance. So you need to pick a specific application and try those values. In general you will get "most" high frequency with very low capacitance and very high resistance. But this usually (for higher-Z windings) gives a peak. The peak will be reduced if you decrease load resistance and increase source resistance, but that can reduce midband level and you usually do not want to lose that. The next step is to use an R-C network to load and damp the treble ring without loading the midrange. But the added C always lowers the ring frequency. On very hi-Z or sloppy-design transformers, there may not be a good compromise.
and I only say practical because it seems to validate my own pensive thinking in terms of just sending signal thru the mystery trafo at known resistance of the source signal.
I would love to understand this:
Peter said in reference to his Ampex SE540,
If I understand this correctly: is it that the DCR may become a problem for such low source-resistances because it starts to dominate the total effective source impedance ?
And then with the lower corner freq determined by (Rout_amp + DCR)/ 2*pi*f*L (?) there's a limit - without any further possible influence of the lower corner from lowering Rout_amp, correct ?
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
I've got HP tube stuff, oscillator, vtvm and it all outputs at 600 ohm. I've also got a matching transformer for the hp gear that will get me down to 135ohm thru the audio band. So it seems to me I can test for frequency response as you guys described in the Z thread on the signal side easy, but what about the 'scope side? My scopes got an input impedance of 3 megs on the vertical channel. Does that even make a difference?
I thought to put signal thru my own mystery trafo at 600ohm 135ohm, maybe even make a bridging xfrmer out of a UTC A-20 to get 50/125/200/333/500. Ultimately the math in the Z thread could be put aside for a moment and the mystery xfrmr could have frequency response plot thru the A-20s matching impedances and I could simple choose the best plot. Yeah?
Okay, if anybody's actually read this entire post I might owe money or extraordinary praise or something.:wink:
Brad
' There's a 200CD here and a 400H. I usually run the oscillator throught the vtvm and then to whatever's getting tested. I think I'll try to post some 'scope traces once I push some signal through some stuff here. I could push it through the distortion analyzer too, a 330B; it's not ultra sensative but quite fun to play with. Thanks for all your comments fellas, you guys always offer good stuff whenever you get your hooks into a thread.
