Opinions on the OEP Z1604

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user 37518

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I got two OEP Z1604 600:600 transformers that I bought on sale at newark for $6.5 dolars a piece, they are so cheap that I couldnt resist buying some of these tiny transformers, but I dont know how they perform, the OEP datasheet doesnt say much. Has anyone used them? opinions?
 
Martin Griffith said:
Try using them as Zero Field Transformers. http://www.lundahl.se/wp-content/uploads/datasheets/6404.pdf it would be interesting to see the LF performance
Looks interesting, I will definitely try that
 
I've been using them in "zero field" applications for line inputs, with some positive feedback to extend the LF response. It's been performing really well in this configuration
 
Henke said:
I've been using them in "zero field" applications for line inputs, with some positive feedback to extend the LF response. It's been performing really well in this configuration

Great! on what are you using it? Is there any instrument in particular that you like using it?
 
I've been simulating the Zero Field circuit, and all im getting is a good old Schmitt square wave oscillator. I guess I should spend some time on the bench and see what happens with a real circuit.
 
I measured the transformer THD+N at low frequencies, everything is relatively ok as long as the input is kept low, at -10dBu input THD+N is 0.27% @20Hz which is far from perfect, but at 0dBu input THD+N goes up to 11.3%, nasty... at +10dBu distortion is a whooping 80%@20Hz. At 40Hz and -10dBu input THD+N is at 0.08% but goes up to 0.4% as soon as the input rises to 0dBu.

Here's a pic of the scope with a 40Hz signal @+10 dBu, yikes.


20180130_173844_resized.jpg


I recently ordered an AP system one, I'll run a sweep of THD+N VS Freq as soon as I get it.
 
Martin Griffith said:
Nice old scope, I've got the 2215A. Have you had to replace the HV resistor chain yet? It just won't die :)

Thanks! no I havent replaced anything in it, these things are built to last, I also have a digital Tektronix, but takes ages to turn on and boot the software, so the good old analog scope gets used more.
 
Yeah the core is too small to be used in normal configuration for line level work, but in "current mode"/ZF into a tad negative input impedance it should be blameless. I've been successfully using it that way for generic clean and cost effective line inputs with galvanic isolation

Regarding your oscillation problem, make sure any positive feedback is AC coupled and that the net negative impedance isn't bigger than the winding DC-resistance it's working against
 
Henke said:
Regarding your oscillation problem, make sure any positive feedback is AC coupled and that the net negative impedance isn't bigger than the winding DC-resistance it's working against

I see, regarding the AC coupling, do you suggest I should add a capacitor in series with the secondary of the transformer? Do you have a schematic?
 
user 37518 said:
I see, regarding the AC coupling, do you suggest I should add a capacitor in series with the secondary of the transformer? Do you have a schematic?
The cap goes between the opamp output and the positive feedback divider resistor string, sorry i have no schematic at hand here at the moment..
 
Hey, I built the Zero Field circuit today and I'm impressed, all the low end distortion is completely gone, it can handle +10dBu signals with no problem, I'm getting nice clean sinewaves at 20Hz, the distortion is 0.05% at 20Hz at +10dBu, nothing compared to what I used to get without the Zero Field circuitry, something odd is that the distortion at +22dBu @20Hz is actually lower (0.02%) than the distortion at +10dBu  (0.05%), I just wish I could understand exactly whats going on in the circuit.

I see that there is positive feedback which causes a negative resistance to neutralize the DC secondary resistance, but what troubles me is that in the Lundahl schematic, R3 is going to ground and the capacitor connected to the top terminal of the secondary is also going to ground, I cant see exactly how this loop works. Also, whats the gain of this circuit? the 10K resistor in the NFB loop divided by the input resistors at the primary ?

Do you know if this type of arrangement can be applied to any other transformer, not just small core transformers, what about expensive line input transformers? I mean if a $6 part can perform like this, I cant imagine what a Lundahl or Jensen transformer can do.
 
The AP test set you're getting uses the negative output Z driven transformer to clean up their output transformers. Worked well enough, up to the current versions, which had to go to a conventional solid state output stage to get the distortion low enough.
 
Monte McGuire said:
The AP test set you're getting uses the negative output Z driven transformer to clean up their output transformers. Worked well enough, up to the current versions, which had to go to a conventional solid state output stage to get the distortion low enough.

Ohh I see, I never imagined that AP could get so low distortion by using transformers. I understand that one of the aspects of transformer distortion has to do with the DC resistance of the transformer and using negative resistance cancels out this DC resistance, but the real question is why does the DC resistance of the transformer causes distortion and why cancelling it out reduces distortion?
 
user 37518 said:
I measured the transformer THD+N at low frequencies, everything is relatively ok as long as the input is kept low, at -10dBu input THD+N is 0.27% @20Hz which is far from perfect, but at 0dBu input THD+N goes up to 11.3%, nasty... at +10dBu distortion is a whooping 80%@20Hz. At 40Hz and -10dBu input THD+N is at 0.08% but goes up to 0.4% as soon as the input rises to 0dBu.
You didn't specify the source impedance of your generator. For any half-decent performance, these transformers are meant to be used with almost-zero impedance drive (in fact negative-Z is even better). Anyway, the core is so small you can't even dream on passing significant level. Indeed it is adequate for zero-field (or even current transformer) inputs.
 
abbey road d enfer said:
You didn't specify the source impedance of your generator. For any half-decent performance, these transformers are meant to be used with almost-zero impedance drive (in fact negative-Z is even better). Anyway, the core is so small you can't even dream on passing significant level. Indeed it is adequate for zero-field (or even current transformer) inputs.

That is true, I was using a 50 ohm source impedance generator, but not quite zero, I could make a buffer with near zero impedance and try out the results but I dont think they will vary greatly, THD will still be very high. I tried the zero-field method and it works great, managed to keep the distortion low even at high input levels, however the opamp needs some taming, mine started to oscillate, I cured it with a simple capacitor in the feedback loop.
 
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