Power Supply Resevoir Size

[gootee]

Originally Posted by abraxalito

Have a look at Rubycon ZL range - that's showing 330uF/50V in 10mm with a max 100kHz impedance of 28mohm. In my limited experience of extreme paralleling of caps, I found going above 330uF to be problematic in that the self-resonant freq comes down a bit too low for comfort. I doubt when paralleling 100 caps that the high frequency ESR is going to be limited by the caps' ESR, more likely the interconnections.

Thanks for reminding me about not using 1000 uF! I had noticed that the self-resonant frequency wasn't quite where I thought I would want it, in Terry's measurements. Or maybe it was KSTR's measurements. I think that one of them even mentioned it. I'll find the data and re-think the C value. Thanks again for pointing that out!

Are you thinking that something more like 330 uF would be about right?

The inductance seen by the die inside the chipamp package is surely what matters? This to include the total loop area including the die attach wires. Hard to see how that's going to be in the sub-nH territory as the rule of thumb is that for bond wires, the inductance in nH is roughly the length in mm, to a first approximation. The distance between the power pins on your chipamps is going to be higher than 0.5mm.

Yeah, I know. I should have said I didn't want to ruin it any more than I had to. It will definitely be WAY more than 0.5 nH, by the time it gets into the output devices. But I'll still fight for every nH.

Get someone to source you directly out of Shenzhen I suggest, rather than using local distribution. I just bought 500 390uF/35V Rubycon ZL caps for my chipamps and that order was around $30.

Wow. Thanks for the tip. I have a friend or two there. I haven't bought large lots for a long time and just didn't think about it.

[fas42]

You fellas are really going to town there ... in my playtime i went with good ol' FCs, 1200/50. Availability, ripple current, costing meant these were the best bang for the buck for me, I went the rounds many times looking at options. Didn't use as many caps, regulated feed took care of a lot of the sag.

And, chopped off the legs of the chip amps to give me just enough metal to solder to - every nH is precious ...

Frank

[gootee]

Originally Posted by a.wayne

Agree Dutchie ....

@Gootee,

Whats the anticipated ripple voltage at full output and what min-z value are you using to determine values, surely not 8ohm ..?

With a sine with 5-Amp 40V peaks, the ripple should be less sthan 2.5% of 40V, i.e. 1 Volt or less, peak to peak.

I used 8 Ohms and 4 Ohms. It's just an example, basically. The point, really, is that you can set the ripple (and the load resistance) to whatever you want and then calculate what you need to do to achieve that under worst-case (or any other) conditions.

I guess I just like developing equations again, for a change. But it's much easier and more-flexible to just simulate the whole power supply in LT-Spice. Then you can do things like have it automagically sweep parameters over ranges and see the resulting series of plots, for example.

With equations, it's probably difficult for others to keep in mind what ASSUMPTIONS and conditions I have set, too. I spent some time figuring out what "worst case" should be and usually that is embodied in the equations. (I state them, usually, but it's still hard to keep in mind, maybe.) But it's probably different than what most people are accustomed to seeing, as far as C values. I usually prefer to meet the ripple spec while driving DC through the load, at the max rated peak values, for example. That way, I "KNOW" that it's truly able to work how I want it to, with the worst worst case. Most people assume a sine wave. But what if there are two? And in reality there might be dozens or hundreds simultaneously, especially if you consider all of the Fourier components of anything non-sinusoidal, which includes all music. With one piece of music someone came up with, instead of one sine peak grazing the peak value periodically, you got more and more and pretty soon there was almost always a wave touching the peak rated output value. So it might be more like a square wave, with its horizontal parts at the peak rated voltages. But that's just DC, part of the time. So "all of the time" is an even better "worse".

Then I usually also look at what the resulting configuration will do at NORMAL listening levels. Peak rated level performance is good to design to. But I never ever listen at that level. If I design for the really-worst worst-case, though, everything is usually surprisingly good at listening levels.

[gootee]

Originally Posted by fas42

You fellas are really going to town there ... in my playtime i went with good ol' FCs, 1200/50. Availability, ripple current, costing meant these were the best bang for the buck for me, I went the rounds many times looking at options. Didn't use as many caps, regulated feed took care of a lot of the sag.

And, chopped off the legs of the chip amps to give me just enough metal to solder to - every nH is precious ...

Frank

Which ones have the rail voltage (or is it ground?) on the back plate? Would it help if I bolted or soldered that to the appropriate plane on the array board? (Actually, I guess I'd have to use a large/wide L-shaped piece of copper or something, so the chip could stand up and have a real heatsink, also.) Just a crazy thought.

I will at least make the pins as short as possible, like you did.

(Gotta get to bed. Early meeting tomorrow.)

[abraxalito]

Seems to me as the inductance of the paralleled caps isn't going to be the major issue, by at least an order of magnitude that to do this properly the chipamps should be paralleled up too?

@Tom - I'm settling on whatever's easy to get but below 470uF yep. Most recently 390uF but yet to build with those.

[fas42]

Originally Posted by gootee

Which ones have the rail voltage (or is it ground?) on the back plate? Would it help if I bolted or soldered that to the appropriate plane on the array board? (Actually, I guess I'd have to use a large/wide L-shaped piece of copper or something, so the chip could stand up and have a real heatsink, also.) Just a crazy thought.

I will at least make the pins as short as possible, like you did.

(Gotta get to bed. Early meeting tomorrow.)

The National LM3875 come in 2 versions, from memory mine didn't have the -ve(??) rail connected to the heatsink metal. With regard to insulating or not, if you need to decide, I would do some tests, and think a lot about it!

I used a very long, narrow, finned black heatsink, standing upright - had the best thermal rating for the size, and money. Hence chip upright, and circuit horizontal. Obviously awkward in normal situation, but I was using these effectively to create active speakers, was on a slab of material against the back of the carcase.

Frank

[fas42]

Originally Posted by gootee

Then I usually also look at what the resulting configuration will do at NORMAL listening levels. Peak rated level performance is good to design to. But I never ever listen at that level. If I design for the really-worst worst-case, though, everything is usually surprisingly good at listening levels.

Personally, I would design for the very highest level volume as being that for normal listening. I regularly drove, and still do, chip amp systems flat out, the heatsinks are stinking hot most of the time. If you do that there are no surprises with the volume control, the tonality and dynamics remain intact right up to maximum ...

Frank

[a.wayne]

Originally Posted by fas42

You fellas are really going to town there ... in my playtime i went with good ol' FCs, 1200/50. Availability, ripple current, costing meant these were the best bang for the buck for me, I went the rounds many times looking at options. Didn't use as many caps, regulated feed took care of a lot of the sag.

And, chopped off the legs of the chip amps to give me just enough metal to solder to - every nH is precious ...

Frank

Using inventory , which means 80K/2 per channel and then decoupled at board , remote transformer , Not into the million caps to achieve the same value , never liked the sound of the ones i have heard in the past ...

Suggestions on the decouple value at the board .....?

[Telstar]

Originally Posted by fas42

You fellas are really going to town there ... in my playtime i went with good ol' FCs, 1200/50. Availability, ripple current, costing meant these were the best bang for the buck for me
Frank

The panasonic FC are definitely a best buy, and so are the FM if you can use their limited value range.

[a.wayne]

What about the sonics, FM or FC ...?