Whats wrong with large filter caps for Gainclone?

[janneman]

Originally posted by apassgear
well adding to the previous post the voltage will be depleated more rapidly on the smaller cap, but that does not make much difference recharging the cap, but will sure make diference on music as you listen to it.

I tried to interprete the graphs by Pedja, and I come to the following conclusions:

Extra ESR (or extra wiring resistance in the xformer, which leads to the same) makes the current pulse "flatter" and thus reduces hf content;

All other things being the same, increased load increases the rippel (of course) and from the fft's I think this increases the hf contents. In the graphs the carrier level drops but the harmonics stay about the same which means relatively more harmonic content.

Do you agree Pedja? How about those graphs with log vertical scales?

Jan Didden

[Pedja]

Here they come, logarithmic Y axis. Cap’s ESR is 0.1 Ohm.


1000uF and 100mA:



1000uF and 350mA (certain troubles begin):





And yes, finally that interesting fact can be seen below: higher capacitance can make higher noise floor at HF. Explanation goes like “it is made by those short duration pulses (which have wide bandwidth in the freq domain)”.

10000uF and 100mA:





10000uF and 350mA:





So, as seen above, the second interesting thing is, excluding the bottom end, this 10000uF likes better 350mA than 100mA load.


Again this might not be the technical answer on “why low capacitance supply works?” question. Oh yes, it works. But IMHO it is not the answer. At least, not the whole answer. YAMV*.

Pedja

* your answer may vary

[Christer]

Pedja,

did you make you simulation model detailed enough to also
get RF noise from the diodes? That is something that is very
dependent on capacitor size, among other things.

[kropf]

Pedja,

Thanks for the additional plots.

The next question is what happens when we have a 1kHz or 20kHz signal superimposed on the 120 Hz charging pulses.

Now it's time to measure; I've got a digital scope with FFT at work, so measuring is easier than simulating (for me). My IGC has 2200uF per rail and dual 24VAC transformers per channel. It does have a couping capacitor, so it won't be possible to measure a DC PS drain for a direct comparison to the previous simulations.

Jeremy

[Pedja]

Jan,

Extra ESR of the cap makes current pulses lower in amplitude and longer in time. Difference is bigger as it is bigger the drawn current. As you pointed out, added series resistance has similar (though not exactly the same) effect. But generally, noise floor is higher with higher ESR or added series resistance (I am not talking about the noise suppression, it is still only about the noise generation).

I think we all can agree that the career has lower amplitude in the case of the bigger caps and less drawn current. Generally speaking you were right claiming that the excessive large caps can cause HF problems. But as with all the problems in this world, it is the question how serious they are.

Pedja

[Pedja]

Christer,

No, that overcomes my current exercise since, if we want to go along that road, it should be modeled transformer as well and possibly should be taken into account some more parasitic phenomenon. I might be wrong, but I restrained here from fast and Schottky diodes and used conventional 1N4007 believing it will minimally mess the thing.

Pedja

[Squalish]

pedja: think you could model some parallelled film/lytic caps, based on realistic specs?

[Pedja]

Well, if you have specs... It could be modeled surely better than it is done by the capacitor as primitive passive element. Good reading here:
http://www.aeng.com/pdf/capacitor.pdf

Pedja

[Christer]

Pedja,

I didn't think you modelled diode noise, but wanted to check.
It is actually not difficult at all to get such phenomenae in a
simulation. Just adding some inductance to the AC source will
make things happen. However, I really have no idea what are
reasonable values for R and L of the secondaries, so I can't
say how close to reality my experiments are. So far I have only
bothered to see if the phenomenon can be captured in
simulations at all, which is obvoiusly can.

[apassgear]

Originally posted by janneman



Extra ESR (or extra wiring resistance in the xformer, which leads to the same) makes the current pulse "flatter" and thus reduces hf content;

All other things being the same, increased load increases the rippel (of course) and from the fft's I think this increases the hf contents. In the graphs the carrier level drops but the harmonics stay about the same which means relatively more harmonic content.

Well said Jan,

From my point of view, i agree with every word, and not taking in consideration pedja's modeling or testing myself, just as an understanding of this efects on my humble mind.