Just a quick power supply capacitance question

[markiemrboo]

Originally posted by BWRX
Hey guys, let's all just calm down. It's too early in the morning for me

Heh.

I most certainly am capable of error, and the "correct" schematic I posted may in fact contain error. It was correct as far as I could see, which isn't much at times.

From what mark has explained thus far I do not think he has miswired anything. Clearly the regulators for the negative rail are interacting because they share the same secondary even though they are isolated by separate bridge rectifiers.

I think I could even see why separate bridge rectifiers would work as you suggested, but it does indeed somehow still seem to be interacting in a very similar manner as before.

I didn't get a chance to think about this last night and probably won't again until the weekend. Luckily others are willing to help out as well!

Until then, using a different transformer with dual secondaries for the preamp regulators will definitely, without a doubt work and is a better solution.

Yeah. This was my backup plan, but really I wanted to have it all off the same transformer as I said. I also did really want one regulated power amp supply per channel, but it would have to be unregulated or one for both channels. This isn't so much of a problem mind, I wouldn't be able to fit it all in the case if I had a separate transformer for the preamp and three regulated supplies.

I am hesitant to finish it off by using a seperate transformer for the power amp and pre amps, as I am currently the only person able to test things (I think). I don't really want to leave it without a conclusion, you know?

Another option would be to use positive regulators for the power amp rails and then use a positive and negative regulator for the preamp rails. That will also work.

Sadly I don't have any negative regulators, and have no electronics shops near by. Postage for small orders kills me

[markiemrboo]

Ok, some measurements from Andrew's questions.

With both supplies connected, the AC to AC of bridge's seem OK and "normal" at ~27v (25v transformer).

The DC output terminals of the bridges also seem OK at ~36v. I remeasured the power amp caps, and I got the same ~36v (this is probably expected).

I measured the preamp caps, and the positive side of that supply (remembering the problem is apparently with the negative rail) was at ~36v, rose pretty quickly to ~44v and then slowed down, but was (I think) still rising.

This seems strangely similar in behaviour to how the negative rail regulated output starts at -28v and drops to ~-18v then slows down dropping. The negative rail, at the output of the caps / input to the regulator is OK / "normal" at ~36v.

With the preamp regulator unplugged both lots of the caps (on the preamp side) are normal at the expected ~36v. The output of the negative rail after the regulator, as mentioned, starts at -28v and drops to ~-18v.

[AndrewT]

Originally posted by markiemrboo
The DC output terminals of the bridges also seem OK at ~36v. I remeasured the power amp caps, and I got the same ~36v (this is probably expected).

which caps measure 36Vdc? the smoothing on the rectifier should measure 36Vdc, the power amp caps after the regs should measure +28Vdc and -28Vdc

Originally posted by markiemrboo
I measured the preamp caps, and the positive side of that supply (remembering the problem is apparently with the negative rail) was at ~36v, rose pretty quickly to ~44v and then slowed down, but was (I think) still rising.

This seems strangely similar in behaviour to how the negative rail regulated output starts at -28v and drops to ~-18v then slows down dropping. The negative rail, at the output of the caps / input to the regulator is OK / "normal" at ~36v.

With the preamp regulator unplugged both lots of the caps (on the preamp side) are normal at the expected ~36v. The output of the negative rail after the regulator, as mentioned, starts at -28v and drops to ~-18v.

can the connection between the two halves of the reg PCB be separated to create two completely independant 28Vdc supplies?
Does the swing to 44v coincide with the swing to 18v?
Is the regulator breaking down on excess voltage? 36Vdc rising to 44Vdc might be over maximum spec.
A bias building up on a capacitor?
A capacitor breaking down? you are losing 2V.

Note also that the 15V regulators are seeing a voltage drop 36Vdc -15Vdc =21Vdc with just 25mA they are getting warm and by the time they pass 50mA they are hot. I wonder if the changing voltage is a clue to an excess temperature effect?

Confirm again,
If you connect both 15V reg with the 28V regs disconnected, the +-15V supplies are OK and with a lower value dummy load (watch that dissipation).
Then with the the 15V regs disconnected, the +-28V supplies work correctly?

The 15Vdc regs can avoid some of the dissipation by adding a series R between the rectifier and the smoothing caps.

[markiemrboo]

Originally posted by AndrewT

which caps measure 36Vdc? the smoothing on the rectifier should measure 36Vdc, the power amp caps after the regs should measure +28Vdc and -28Vdc

This is correct.

can the connection between the two halves of the reg PCB be separated to create two completely independant 28Vdc supplies?

I have three entirely separate, but identical PCB's. Two for the power amp left and right channels with 2.2k / 100R, giving 28v and one (the preamp supply) with 1k / 100R, giving ~14v.

Does the swing to 44v coincide with the swing to 18v?

I'm not sure I follow here? Any chance you could you explain? I have a feeling I might be wanting to look at the scope to answer this question?

Is the regulator breaking down on excess voltage? 36Vdc rising to 44Vdc might be over maximum spec.

Not entirely sure. I could be being blind, but I am only seeing a 35v as a maximum "differential voltage" between "in and out". Given that "out" is currently 13.6v I think this would mean it is still in spec just about, but I am not sure.

A bias building up on a capacitor?

I don't know what you mean here? Where should I be looking for this and how would I check?

A capacitor breaking down? you are losing 2V.

Losing 2v where? You mean 27 x 1.414 = 38v and I am only seeing 36v? I thought this was a rough estimate calculation, but if it's not is this not 'diode' losses from the bridge, or does this calculation take this in to account?

Note also that the 15V regulators are seeing a voltage drop 36Vdc -15Vdc =21Vdc with just 25mA they are getting warm and by the time they pass 50mA they are hot.

Yep. I've not left it on long enough for the heatsinks to get even warm.

I wonder if the changing voltage is a clue to an excess temperature effect?

Not sure what you mean here either, sorry

Confirm again,
If you connect both 15V reg with the 28V regs disconnected, the +-15V supplies are OK and with a lower value dummy load (watch that dissipation).

Yeah, the +-15v (or 13.6v) supply seems just fine with the 28v supply disconnected. I've had it connected to the preamp (without other supplies connected) and the preamp works OK from it. It is only when there is another regulated supply connected that things start going haywire.

Then with the the 15V regs disconnected, the +-28V supplies work correctly?

I've only got one +-28v supply connected at the minute, but that seems to be outputting the correct voltages regardless of whether the preamp supply is connected or not.

The 15Vdc regs can avoid some of the dissipation by adding a series R between the rectifier and the smoothing caps.

This is handy to know, thanks.

[AndrewT]

Hi,
are you powering up through a mains light bulb?

I don't want this blowing up in your face while trying to measure a series of voltages.

The caps issue.
Measure the voltage across every electrolytic with power on to find any that have an odd voltage on them.
You are looking for one that is too low or too high or reverse polarity.

[markiemrboo]

Originally posted by AndrewT
Hi,
are you powering up through a mains light bulb?

I'm not. I know I should be.

I don't want this blowing up in your face while trying to measure a series of voltages.

I appreciate the [very real!!!!] concern.

The caps issue.
Measure the voltage across every electrolytic with power on to find any that have an odd voltage on them.

Ah, now this would be why you really want me to do the light bulb thing? In this case it is next to impossible for me to do safely. No offence, but I don't think I am going to risk doing that

Unless you meant removing all of the caps and testing them one by one...

You are looking for one that is too low or too high or reverse polarity.

All I can say here is that unless some cap casings are marked wrong, all cap polarities are correct, and also none exploded, at least, while they were running 24/7 for about a month before hand!

[BWRX]

Hi mark/everyone else.

I had some time to simulate the circuit that I proposed to be correct and it is in fact not correct either. The schematic in this post will not produce one properly regulated negative rail voltage (whichever desired regulated voltage is closer to ground potential between the two negative rail regulators). The larger voltage will regulate properly as will the two positive rail regulators. The circuit still suffers from the same issue as the previous one. The positive regulators work by sending a constant current (set by the resistor between the output and reference terminals) through a resistor that is tied to the circuit's reference voltage; which is normally ground. Unfortunately, using positive regulators to derive negative rail voltages means that the reference voltage is also the rail voltage. This poses problems as mentioned earlier (when trying to use 4 positive regulators to make a dual voltage split rail supply) and I can't see any easy way around it other than to give each regulator it's own separate secondary.

[markiemrboo]

Originally posted by BWRX
Hi mark/everyone else.

I had some time to simulate the circuit that I proposed to be correct and it is in fact not correct either. The schematic in this post will not produce one properly regulated negative rail voltage (whichever desired regulated voltage is closer to ground potential between the two negative rail regulators). The larger voltage will regulate properly as will the two positive rail regulators. The circuit still suffers from the same issue as the previous one. The positive regulators work by sending a constant current (set by the resistor between the output and reference terminals) through a resistor that is tied to the circuit's reference voltage; which is normally ground. Unfortunately, using positive regulators to derive negative rail voltages means that the reference voltage is also the rail voltage. This poses problems as mentioned earlier (when trying to use 4 positive regulators to make a dual voltage split rail supply) and I can't see any easy way around it other than to give each regulator it's own separate secondary.

Hi BWRX,

Thanks for taking the time to think this over and come to a conclusion

I can confirm that separate transformers works, and so it's definatley not a fault with my boards or wiring, as that is how I am now running it and it is giving the correct voltages.

The only problem I have now is level balance in the preamp because of a crappy pot. I reckon I am going to go the stepped attenuator route. Still sounding good though! I am convinced it's better than the Cambridge Audio I had previously. It may just be my imagination, but I am happy is what counts eh

Cheers