Just a quick power supply capacitance question

[markiemrboo]

(I was going to edit the last post to include this info, but the time limit expired sometime between clicking edit and pressing save changes, sorry! Since this has gone on a new page I will quote the previous info too)

Just thought I would post with some info, which may or may not help.

I figured exact voltages would help. So rather than try and remember them, I wrote down the outputs.

It was actually:

Power amp: +28.5v, -12.6v
Pre amp: +13.6v, -13.6v

There's a mysterious 1v difference here.

Also, I measured the output from the caps for the power amp supply, still with the regulator (+28.5v and -12.6v) connected, and I am still actually seeing an ordinary / normal +36v from both.

Just as a reminder, when I disconnect the preamp regulator the power amp supply works as normal (+-28.5v).

The outputs are *not* connected to any kind of load at all, and I am not taking the 0v outputs to safety earth. Quite frankly, until I am getting the correct output voltages I am a bit scared to! Perhaps I should be? But I think it should be measuring correct without doing this anyway..?

Maybe something here is significant somehow, I don't really know!

I got the scope out, set to AC, and measured the rails of the power amp regulator outputs, preamp regulator outputs and bridge inputs for both, and the cap output's for the power amp regulator, but with the regulator disconnected (it's hard to get the probes in with the regulator connected so I didn't risk it).

The bridge inputs look identical as expected. A sine wave. The cap outputs seem fine, it showed as a straight line until I went down to ... somewhere.... between 10-100mv/div (I didn't note what setting I was using sorry, but can do it again if you wish). Here I started to see a very very very tiny thing like this: |\|\|\. Very small in amplitude, and as far as I am aware normal (cap charging and discharging?)

The output of the preamp regulator was normal, again flat lines with a small hint of |\|\|\. The +ve rail on the power amp regulator also showed this behaviour.

The problem rail, the -ve rail on the power amp which is giving -12.6v, I am seeing a really very large version of |\|\|\|\. I didn't note the setting again, but I believe this was visible around 1v/div. I can test again and note the setting if required, but I am off to eat for now!

[markiemrboo]

It seems like the bridges don't quite isolate the supplies. I switched secondaries around on the preamp supply (just noticed I had the preamp negative fed from secondary #2, but power amp negative from secondary #1) and I see similar behaviour to before.

The power amp supply then turns normal +-28v, the preamp positive rail is OK at +13.6V, but the preamp negative rail initially shoots up (down?) to -28v, and then very slowly decreases. It went down to -18v pretty quick, then got really really slow (to the point where it seemed like it had stopped decreasing).

[AndrewT]

Hi,
put a dummy load resistor across each of your regulated outputs, no connections to any ground or earth. Just to ensure each regulator has a real load.

Measure and post the input and output voltages.

[markiemrboo]

Originally posted by AndrewT
Hi,
put a dummy load resistor across each of your regulated outputs, no connections to any ground or earth. Just to ensure each regulator has a real load.

Measure and post the input and output voltages.

Tried this with 10k resistors just now and it didn't seem to make any difference at all. I think 10k might be too high though, that would be ... 2.8mA load? The LM338 datasheet says 3.5mA is typical minimum load. 10k was just the first lot of resistors I picked up.

I can try a lower value if necessary, of course, but individually both supplies show the correct output voltages without any load.

Looking at it I think I can fit the smaller transformer in there just about should this not be possible to do for whatever reasons.

[markiemrboo]

Any ideas?

I'm kinda tempted to try, today, and fit the separate preamp transformer and go with a single regulated supply (or even unregulated, don't which would be a better idea yet) for both power amp channels...

[Nuuk]

Any ideas?

I'm kinda tempted to try, today, and fit the separate preamp transformer and go with a single regulated supply (or even unregulated, don't which would be a better idea yet) for both power amp channels...

As most of the other contributors to this thread have recommended a separate PSU for the pre amp, I'm surprised that you need to ask!

I can try a lower value if necessary, of course, but individually both supplies show the correct output voltages without any load.

If they are showing the correct voltages what is the problem?

[markiemrboo]

Originally posted by Nuuk
As most of the other contributors to this thread have recommended a separate PSU for the pre amp, I'm surprised that you need to ask!

Uh.... I am asking because I want it ( or, would like if I am to be "polite" ) all off the same transformer.

If they are showing the correct voltages what is the problem?

They're not when all hooked up [together]. That's the point!

[Nuuk]

Uh.... I am asking because I want it ( or, would like if I am to be "polite" ) all off the same transformer.

So you have made up your mind but are asking for our ideas?

I can try a lower value if necessary, of course, but individually both supplies show the correct output voltages without any load.

They're not when all hooked up [together]. That's the point!

Andrew asked what the voltages were like with a load. Please don't think that I am having a go at you but your posts are somewhat inexpliset. You are asking questions, we are trying to help, but you don't come back with the information that we need to help you.

What we really need to see is a circuit diagram showing exactly how you have this amplifier set up, wiring and all. While Brian came up with an explanation for that 'middle' circuit not working, it's strange that it doesn't work with the extra pair of bridges.

[markiemrboo]

Originally posted by Nuuk


So you have made up your mind but are asking for our ideas?

I was just asking if anyone had any further thoughts on any possible solutions to the problem, before I dismantle it and put in a separate supply for the preamp / take out one of the power amp regulators, since I seem to be the only one who can test possible solutions to the problem at this point.

Andrew asked what the voltages were like with a load.

And I have tried this. I posted, it was exactly the same as with no load.

Please don't think that I am having a go at you but your posts are somewhat inexpliset. You are asking questions, we are trying to help, but you don't come back with the information that we need to help you.

I guess this is connected to the above comment?

I have tried with a load, and I even posted this. (i'm not sure if it was *enough* of a load, and I was hoping for feedback on that). Given it was the same as without a load, I did not post voltages. Saying that "it was the same" should be quite enough, no? There was really no need. I am guessing you just missed this post...?

What other information haven't I come up with that you would like? It is possible I have missed something too.

What we really need to see is a circuit diagram showing exactly how you have this amplifier set up, wiring and all.

It's as BWRX's "correct way" diagram. I can draw a diagram / picture / both of my own if you like, but I fear it's going to be identical.

I'm not sure if I should leave the regulated supply '0v' lines to safety ground / the disconnecting network, or leave them 'floating. In Andrew's post he said not to and BWRX's post seemed to imply it didn't really matter?

While Brian came up with an explanation for that 'middle' circuit not working, it's strange that it doesn't work with the extra pair of bridges.

There is only a small difference from before, in that the preamp negative rail now does very slowly drop down from -28v. I've only left it long enough to drop down to ~ -18v as it seems to drop very slowly indeed from here on. There still seems to be some sort of interaction between the two supplies even when they're on separate bridges.

[AndrewT]

Hi,
I'm thinking of a dummy load like 500r not 10k which draws a current of less than the manufacturer recommends as minimum.

+-28V into 500r gives +-56mA Pd~=1.6W (three 1500r 600mW in parallel).

+-15V into 500r gives +-30mA Pd~=450mW (one 510r or 470r 600mW).

Fit, measure and post.

Can you take and post those pics yet?

Common transformer into separate rectifiers into separate regulators will work. I still think there is a mis-wire somewhere.

edit: typed 50r and now changed to 500r.