Interesting (or not) finding about Schoeps-style voltage converter

[pasarski]

These have been discussed before, and I'm totally out of my depth on the subject, but I still decided to post my findings. Maybe it's of interest to somebody.

So I was trying to simulate the voltage booster of Behringer B-1 using this schematic https://groupdiy.com/threads/behringer.44991/post-1163043. It was quite difficult to get running correctly and while investigating I found out that similar circuit has been used by Schoeps and it's been copied to many China mics. This is the simplified version of the circuit using only a simple ideal voltage source for Vcc. The inductors are touching each other in real life so I coupled them. (Don't know the actual coupling coefficient and used 0.7 which seems plausible.)



This is the output with 10V input:



Zoomed in to see the AC -component:



I remember reading that in some Chinese mics the inductors are not physically coupled (I think it was about U87 copy though) so tried it without the mutual inductance statement and got lower voltage, but still a hefty boost:



Zoomed in. Higher frequency and little bit less AC (is it ok to call it "ripple?)



At some point while troubleshooting the sim I started to suspect the schematic is wrong and tried to trace an actual B-1 converter board,and while (unsuccesfuly) doing it, started to suspect that the other end of the other inductor (L2 in my schematic) is floating. I know that it would have not been super hard to confirm that, but my brain was already hurting, so I went to the sim and tried this kind of schematic. While simulating transformers I've learned you can't leave an inductor floating in LTspice (at least I can't get it to work) so I connected it to ground instead and found out that this circuit indeed boosts voltage, but only with the mutual inductance statement included:



Not as much boost though, but still a nice 4X:




Marginally smaller AC component:




That's the finding. I understand that I didn't find anything useful. No sense doing it like that if you can get more boost with correct wiring. Also I understand that this is simulation and there might be misconceptions in my sims. But maybe someone with more knowledge can find some usefulness in it, though I doubt it.

 

[rogs]

I'm not sure how much the requirement of 'guessing' the mutual coupling coefficients affects the 'real world' version with this kind of sim?
Actually building Hartley oscillators has always had a element of 'suck it and see', when it comes to inductor parameters and physical placement - understandably.
Maybe that is one reason why charge pump voltage multipliers have increased in popularity ? ..... With no inductors, they are much more predictable.
OK, they use more components, but those constructed commercially using SMT devices don't really require a much greater footprint than the older, Schoeps type multipliers that used through hole components.

From a hobby perspective - and especially where stripboard is used experimentally - charge pump multipliers do take up more space, and are more complex to construct. I found that especially true for dual polarity circuits.

So I decided to have another look at trying to build a dual polarity voltage multilplier using a dual inductor oscillator, in an effort to reduce component count and footprint.
The positioning of the inductors is of course quite an important part with this type of circuit, so I decided to take the 2 inductors and lay them side by side, on a 0.1" spacing.

I'm also very much in Khron's camp when it comes to component choices... I remember the comment at the start of his 'FEELER' thread where he mentioned 'ten-mile-long BOMs'!
So where the component values are not critical, I try and use a minimum number of different components.
(In the attached circuit, there are only 2 values of both resistors and capacitors required).

The attached schematic and layout shows my experimental dual polarity Hartley oscillator voltage multiplier. I did try both orientations of the 2 inductors - and they both work! -- but one way does seem to be more 'stable' than the other.
Using a 12v zener, the output is adjustable from c. ±55v to ±90v.

It seems to work well, and is much simpler to build than a dual polarity charge pump circuit.
It does of course still involve the 'non academic' element of estimating mutual coupling, but it does remove one step between the 'sim' and 'real world' answer to that problem!

 

[kingkorg]

Maybe that is one reason why charge pump voltage multipliers have increased in popularity ?

This is exactly why it happened. In his video on EEV Doug Ford from Røde said oscillators were too unpredictable and unstable.