Whats wrong with large filter caps for Gainclone?

[Joe Rasmussen]

Originally posted by millwood



yeah. And it goes downhill from there: 60db at 10k, and 20db at 1mhz for -psrr.

Very coy indeed!

Last time I looked at my power supply, the ripple was 100Hz. It kinda strikes me that 120dB PSRR is rather useful there and less so than at 10KHz or 1MHz ???

Just as well it's not switch mode, eh? Then you might have point. Well, maybe?

Joe R.

[IanHarvey]

So far, nobody seems to have mentioned the following:

The gainclone and most other amps have a split power supply. Every cycle of the mains the power supply capacitors are charged: current flows into the + terminal of the +ve supply rail capacitor, out of its - terminal, into the + terminal of the -ve supply cap and finally out of its - terminal back to the transformer.

This is a nasty, spiky current pulse whose magnitude is directly proportional to the capacitance involved. If there are problems in the layout or grounding, this current will find its way into the output signal - via finite lead resistances, or inductive coupling. If it does, smaller PSU capacitances are one way of solving the problem...

There is another, slightly more subtle, issue to check here. When the PSU capacitances are mismatched, there is a balance of charging current which flows from the junction of the two capacitors back to the centre tap of the transformer. This current is injected directly into the grounding system; its magnitude is proportional to the absolute difference in capacitances, which in turn increases directly as the capacitors get bigger.

Looking at some of the layouts posted here (with PSU caps coupled directly to signal grounds), I wonder if this is what's happening in some cases...

Cheers
IH

[peranders]

Originally posted by IanHarvey
This is a nasty, spiky current pulse whose magnitude is directly proportional to the capacitance involved. If there are problems in the layout or grounding, this current will find its way into the output signal - via finite lead resistances, or inductive coupling. If it does, smaller PSU capacitances are one way of solving the problem...

The currents aren't that nasty if you have reasonable large caps.

The main obviuos disadvantage ís lower output power and I suspect higher distortion due you the high ripple voltage.

Yes, I agree with Jocko here

But the best you can do is to test! If you like it, it's good....for you.

[ALW]

Last time I looked at my power supply, the ripple was 100Hz. It kinda strikes me that 120dB PSRR is rather useful there and less so than at 10KHz or 1MHz ???

You should do an FFT of your raw PSU if you think it's just 100Hz

It'll scare you...

Andy.

[DrewP]

Every cycle of the mains the power supply capacitors are charged: current flows into the + terminal of the +ve supply rail capacitor, out of its - terminal, into the + terminal of the -ve supply cap and finally out of its - terminal back to the transformer.

Umm, DC current flow through a capacitor? As far as I'm aware, caps don't pass DC, that's what they do, not pass DC.

I think the whole point here is to have a small reservoir that can be very quickly topped up by the transformer as opposed to a large reservoir that can be emptied to a greater extent. It seems that dropping headroom on HUGE signals due to a smaller reservoir is preferable to having a big slow supply of DC to the amp.

That being said, i'll wait till my own version with 1000u Nichicon Muse per rail at the outboard PSU and 100u Black Gate local at the chip is done before I nail my colours to the flagpole.

drew

[millwood]

Originally posted by Joe Rasmussen
Just as well it's not switch mode, eh? Then you might have point. Well, maybe?

Joe R.

I am sorry that you cannot see the point.

[millwood]

Originally posted by IanHarvey
The gainclone and most other amps have a split power supply. Every cycle of the mains the power supply capacitors are charged: current flows into the + terminal of the +ve supply rail capacitor, out of its - terminal, into the + terminal of the -ve supply cap and finally out of its - terminal back to the transformer.IH

the same analysis applies to full wave rectification as well.

Originally posted by IanHarvey
This is a nasty, spiky current pulse whose magnitude is directly proportional to the capacitance involved. IH

I think there are two things to be concerned: one is the spiky current going through the diode (and slightly smaller ripple current going in the cap), and another is the fluctuation in PS voltage.

Smaller caps cause larger voltage fluctuations; and larger caps cause larger spiky current.

It is probably hard to argue which one is more harmful.

[IanHarvey]

Originally posted by peranders

The currents aren't that nasty if you have reasonable large caps.

These are the charging currents (through transformer and rectifier) I'm talking about.

The time-averaged current in the charging pulses must exactly equal the time-averaged discharge current (i.e. delivered to the load).

The capacitors are charging whenever the diodes are conducting i.e. when the transformer output voltage is greater than the capacitor voltage.

A larger capacitance has a smaller ripple voltage, meaning that the voltage drops by less during the discharge phase. This means that the diodes are conducting for a smaller part of the cycle (it helps to draw a graph here).

In turn, this means that the peak current is higher (the same average current in a shorter time), and so will have greater effect. The shorter duration of the current pulses also implies higher frequencies (and therefore more effective inductive coupling to other parts of the circuit).

Cheers
IH

[IanHarvey]

Originally posted by DrewP


Umm, DC current flow through a capacitor? As far as I'm aware, caps don't pass DC, that's what they do, not pass DC.

They "don't pass DC" by virtue of increasing the voltage across their terminals at a rate proportional to the current. Think of it as recharging a battery - eventually the voltage increases to the point where the charger will give up.

Does that help?

Cheers
IH

[Pedja]

The FFTs in post #55 are done for the cases of usage of ideal caps, i.e. without the ESR. Raising the capacitance and ESR, one time things are not differ much, but at one point waveform starts to look this way and this is, I suppose, the reason to expect some HF/RF content in it.




Spectrums in certain cases look like this.

1000uF, 0.5 Ohm ESR




10000uF, 0.5 Ohm ESR




1000uF, 1 Ohm ESR




And it starts to rise with 10000uF, 1 Ohm ESR




It can’t be seen from these graphs, but what is happening all this time at higher frequencies and lower levels? Yeah, there are some differences...

Pedja