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Old 5th October 2003, 09:27 AM   #1
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Default Power supply problem???

OK I finally got around to testing the ripple on my Amps psu. I built a sound card pre-amp, and downloaded the freeware version of TrueRTA. What I see worries me somewhat!!! It looks real nasty!!!

So the question is: Is this real bad, and if so Is it likely my filter caps are cactus? There is 16,000uF capacitance on each rail, 2 X 8000uF.

Ive attached some screen dumps of what I saw in TrueRTA.

It's obvious to me the problem is at 50Hz which is what I would have expected, but the amplitude (around 100mv) seems a tad excessive, also the shape of the wave form is pretty weird, and seems to be mirrored +ve to -ve rail. BTW I have never used a scope before so wasn't sure what to expect!

The positive rail is the upper image and the negative rail the lower.

Regards,

Tony.
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Old 5th October 2003, 09:38 AM   #2
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Default Schmatic

Hi,

First of all we need to know what the load on the PSU is. Next a schmatic would be nice.

Judging from the 50 hz you must be using a single rectifier (not a bridge ??)

\Jens
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Old 5th October 2003, 10:19 AM   #3
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The measurements were taken with zero load (well 100ma quiesient current on the output transistors.

Here is the schematic. It's as basic as you can get, a metal can Bridge rectifier and some filter caps. The secondary 15V winding, and subsequent voltage regulators are fine, less than .5mv ripple, difficult to tell as the sound card has a bit of noise at that low a level. Note that the output voltage shown on the schematic is too high and is really around +- 63V.

The PSU drives two channels of my 100W Mosfet amp.

Regarding the 50Hz, is it supposed to be higher with full wave bridge rectififcation??? I just assumed that because the 240V was 50Hz, that would be the freq of the ripple.

Regards,

Tony.

edit: Forgot to mention the amp is 16 years old!
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Old 5th October 2003, 10:48 AM   #4
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Default Psu

Hi,

When building a PSU like this the two windings on the transformer are commenly out of fase, this in turn means that the caps are charged at the double freq of the mains.

The rippel you measured is only 50 hz, which leads me to think that the transformer has been (re)wired wrong??

Is there a problem with the amp?? the rippel is small

edit: try measuring the AC voltage between the two secondary windings, if you don't get something, the wirering is wrong

\Jens
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Old 5th October 2003, 11:52 AM   #5
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Hi Jens,

There is (allways has been) a hum in the amp, although it isn't just 50Hz, as you can here it in the woofers, midranges, and tweeters. It bugs me as a friend built the same amp, and had no hum at all, so I have allways suspected there was a problem somewhere.

I'm not 100% sure what you mean by wiring the secondaries out of phase. I've often wondered whether it made a difference or not which ends of the two windings were wired together for the 0V line and whichs ends went to the rectifier, is this what you mean? I recently reversed the wiring (ie the two ends that were connected together for the 0V line I put to each side of the rectifier and the other two ends I made the 0V line. This didn't seem to make any difference to the hum problem though. Of course there are four possible combinations of wiring this up (without shorting a winding), and I don't know how to tell which way is correct. If you number the secondary wires from top to bottom in the scematic from 1 to 4 you could wire up 2,3 to zero volts, 1,3 to zero volts, 2,4 to zero volts, 1,4 to zero volts!

The transformer I got with the kit had the wrong coloured sleves on two of the secondaries, and if wired up the way the instructions said, had one of the secondaries shorted (ie 1,2 to zero volts, 3,4 to rectifier) so I have never been sure which of the above combinations I should use!

I thought that 100mV seemed high (Note I don't know what is low and what is high which is the main reason I started the thread ), I tested some really cheap plug packs (admittedly only 12V) and they only had about 17mV ripple, so I was assuming that must be bad (as they are unregulated plug packs with little smoothing) , I was expecting with the amount of smoothing capacitors that I would get much less rippple than what these cheap plug packs had, so when I saw 100mV I thought it was REALY bad

On the measuring the voltage accross the secondary windings, I assume you mean the voltage accross the rectifier? If the transformer windings are in phase, then this will read what 0V? (I assume if they aren't out of phase, it will be around 90V?) I'm just trying to visualise what happens here, DC voltages I can cope with easily, I haven't really ever thought much about AC and the effects of phase.

Not going to be able to try measuring the voltage tonight, Its a major operation to get at the PS in this amp It's a public holiday tomorrow though so I might have a go at it then (and measure some more stuff with the scope while I'm at it, might shed some light .

Regards,

Tony.
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Old 6th October 2003, 01:02 AM   #6
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Default ripple with load

OK I haven't checked the wiring of the PS yet, but I decided to do a test with a load.

Input signal 1Khz sine wave at about .2V (all the soundcard will deliver) amp cranked to max volume, resulting in a 1Khz sinewave at speaker terminal at about 20V. Driving one channel only into 14 Ohm resistor (what I had lying around).

Now this looks much more nasty to me, but again, I don't know what to expect.
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Old 6th October 2003, 01:26 AM   #7
soren is offline soren  Denmark
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Since you are using a full-bridge rectifier, the fundamental of the LF supply ripple hsould be 100Hz when using 50Hz mains. Are you shure you have wired up the rectifier bridge correctly?
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Old 6th October 2003, 02:04 AM   #8
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Quote:
Originally posted by soren
Since you are using a full-bridge rectifier, the fundamental of the LF supply ripple hsould be 100Hz when using 50Hz mains. Are you shure you have wired up the rectifier bridge correctly?

Well I think so, but I'm no longer sure!!!!! I would have thought that if I didn't wire it up correctly that I wouldn't get the correct rail voltages???? I'm actually thinking about going out and buying two new rectifiers and wiring up a dual rectifier setup one each for +ve and -ve rails. This may help with my problem about not being sure which end of each secondary winding is which (if it even matters).

I guess it is possible that the rectifier bridge is faulty..........

Regards,

Tony.

edit: I am interpeting the cro output correctly aren't I. 5 peaks in 500ms (50 divisions of 10ms each) must be 50Hz correct?
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Old 6th October 2003, 02:47 AM   #9
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Well I'll be damned. I just checked the ripple on the other rectifier in the amp and guess what. 100Hz ripple! I'm now wondering whether the main rectifier has been wired up incorrectly for the last 16Years, and if that's been the cause of all my Hum problems!!!!!

I must admit that I was building this amp late at night when I was supposed to be doing uni assignments, so it is quite possible I wired up the rectifier incorrectly, but never realised, because the voltages seemed to be ok. Time to pull the amp to bits me thinks!!!!!

Regards,

Tony.
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Old 6th October 2003, 03:12 AM   #10
sam9 is offline sam9  United States
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10A is a bit of a low rating for use with a 2x100W amp. 10A may seem like enough but it is marginal for power on surge currents. This leads me to the speculation that one or two elements may be damaged. I think (just think mind you) that if only half the bridge is blown while the other is OK you end up with a half-wave rectifier. That would explain ripple at 50Hz.If ypu have a diode tester on your DMM you can check this out. But in any case, if I were in this situation I would replace it with a 25A unit regardless.

With the power OFF and the unit UNPLUGGED and the filter caps discharged you can use an ohm meter to figure out which two leads should be tied together to make the center tap. When it is connected correctly the resistance reding between the ceneter tap and each of the other two leads will be equal while the resistance between the two non-CT leads will be double that of either of the CT-to-(+/-) leads. You may have to try a couple of permutations untill you find the arrangement that behaves like this. If none of the possible combinations reads this way the transformer is kaput.
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