Questions regarding 2B transformers quality and 2B emitting unipolar pulses

Hello.

I was researching /r/estim on reddit trying to find information on different power boxes, their advantages and disadvantages and I stumbled upon this post.

I'll quote some parts

(2B) emits unipolar pulses

Compare the other waveforms with the one emitted by the ET312. This is what a symmetric bipolar waveform looks like, remember it.

I guess you have been reading a lot of crap from people who don't own an oscilloscope. I've owned a 2B (briefly, out of curiosity) and I tossed it quickly. Measurements with my oscilloscope (yellow channel) clearly show a waveform that only has a positive pulse (and a tiny bit of inductor rebound that is unavoidable, no matter how cheap a circuit is designed). Anyone with the most basic knowledge of electronics would conclude this from the output stage topology (schematics courtesy of sexestim.ru) where a transformer with only one tap is used, and driven with only one FET. Compare with the three fets at the 2-tap transformer of the ET312.

The Estim 2B is also hobbled by the bizarrely chosen Eagle LT700 transformers used, probably because they are cheap as chips and plentiful, which have a whopping 1.2 kΩ resistance at the "wet" side, compared to the 6 times lower 200Ω in the ET312.

I was wondering if anyone had anything to say about this I mean.. the schematics and measurements are there...

Thank you!

Kind regards,

The person who wrote this is rather ignorant of electronics. The 2B output is through a transformer and a transformer cannot pass DC at all. There is not DC component in the 2B output, the area of the short big positive voltage spike is equal to that of the much smaller but longer negative "rebound". It may not be a symmetric waveform it is "bipolar". The 312 output stage is capable of generating symmetric waveforms which may be more versatile, but no safer. From experience I can confirm that the 2B can feel very good and never experienced a burn or other effect of a dc component.

As for the comment about the transformer, the component is designed to match a 600 + 600 ohm line signal to a 6.4 ohm loudspeaker. This has NOTHING to do with the resistance of the windings, which are likely to be only a few ohms, or in the case of the 6.4 ohm side a fraction of an ohm. The point about quoting the impedances is really to say that the turns ratio is about 10:1.

Interesting to see the schematic of the 312 - what the 3rd FET feeding the centre tap does I can't imagine, unless it's just to turn the output on and off. It shows the transformer ratio as being 1:4.7 rather than 1:10. Given the push-pull arrangement the maximum voltage at the 312 output would seem to be 9 volts x 4.7 = 42 volts peak. I can't remember what I measured on the 2B, I think it's higher but the output stage works in a different way: it relies on the inductive kick from the transformer to generate the high voltage.

Edit: realised that the purpose of the 3rd FET is to allow the output to be turned off between say a positive and a negative pulse - i.e. output ideally goes high impedance between pulses.