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jlw 21st August 2008 05:26 PM

Ripple filtering help... strange slow rise and fall of voltage
 
Greetings,

I wonder if I could solicit some assistance from fellow DIYers.

Attached please find a picture of the rectifier/filter section of a Ming Da MC-2A3 preamp before it goes to regulation.
(Oh well, I can't seem to upload the picture but its here:
http://www.audioasylum.com/cgi/t.mpl?f=amp&m=133536
)

The problem I am seeing is that while the general AC ripple level across the right-most filter caps (they are balanced with bleeder resistors) is almost unmeasurable, I see regular rise and fall of voltage (1/4 to 1/2 a second per rise/fall but not even) on my scope, in additon to spikes up to 25mV at times.

Measuring across the 1st 2 caps as expected shows AC ripple but
the absolute voltage does not seem to drift up and down.

Measuring the ripples across the 20H choke also shows only AC ripples but no up and down in absolute voltage.

I have changed out the caps to known good Nichicons but the problem remains. I have also disconnected the regulator and measured the across the right most caps with the same results.

Any suggestions and opinions welcome.
Thanks for your time.

Robert McLean 22nd August 2008 12:29 AM

I dont necessarily see a problem here. 25mv before the regulator does not sound too bad to me. The regulator may be quick enough to catch it. Have you measured the spike after the regulator ?

Also the circuit is going to have a certain amount of power supply rejection, 25 mv on the main B+ may not result in an audible effect. When the spikes occur, do you hear anything ?

I realize that it would be good to know the cause of the spikes and rise and fall of voltage even if there is no audible effect.

I may be off base here, you may have a very solid test set up, but sometimes when I get results like those you describe I find out it is my scope probe connections that are flakey, ie the probe moving around on the terminal and producing fluctuations. I end up re-routing all my leads and duct taping them to the bench, and then all is well.

jlw 22nd August 2008 04:52 PM

Thanks Robert
 
I ended up disconnecting things 1 stage at a time and found that either 1 or both 1st stage filter caps right after the choke was faulty and making noise. A view on the scope of voltage across that cap is outright horror. Disconnected the 2 caps, checked the waveform after the choke and its the expected ripples.
So I got 2 470uF 450v Nippon Chemicons of the same size from the local surplus store, swapped them in, and no more weird spikes and drops. Reconnected the downstream choke and caps and regulator and while I still have a bit ( a few mV) of rise and fall at regular intervals I have no more noisy spikes and it's a lot better now. THANKS!!!

GordonW 22nd August 2008 05:54 PM

I wonder what the resonant frequency of the combination of the first choke and first cap is, and what the resonant frequency of the combination of the second choke and second cap is. If they're too close together, you might be getting a sort of "mutual resonance" between them, that's causing that ultra-low frequency 'waveform' you're noticing. I've seen that effect, sometimes, modeling stuff in PSUDII, in fact... and usually, if you change one of the values (a choke or a cap) a bit, it can usually be made to basically go away...

That, IME, can be one drawback of using big caps... you can get high-Q resonance between them and a choke.

If that's the case, it might be "damped down" by using more aggressive bleeder resistors (lower resistance, to draw a bit more current and apply more 'damping'), or a small resistance in series with one or both of the chokes (like maybe 50 ohms)...

Regards,
Gordon.

Ty_Bower 22nd August 2008 06:21 PM

Re: Thanks Robert
 
Quote:

Originally posted by jlw
A view on the scope of voltage across that cap is outright horror.
I've sometimes wondered what is the proper method for checking high voltage with the oscilloscope. I assumed I can't just stick my 'scope probe on the 450 volt rail.

pchw 22nd August 2008 06:54 PM

Re: Thanks Robert
 
Quote:

Originally posted by jlw

So I got 2 470uF 450v Nippon Chemicons of the same size from the local surplus store, swapped them in, and no more weird spikes and drops. Reconnected the downstream choke and caps and regulator and while I still have a bit ( a few mV) of rise and fall at regular intervals I have no more noisy spikes and it's a lot better now. THANKS!!!

Hey jlw,
OT is little, which surplus store did you go to? For some reasons, I don't seem to have any luck to find them in Bay Area.

Thanks!!

Svein_B 22nd August 2008 07:16 PM

Re: Re: Thanks Robert
 
Quote:

Originally posted by Ty_Bower

I've sometimes wondered what is the proper method for checking high voltage with the oscilloscope. I assumed I can't just stick my 'scope probe on the 450 volt rail.

I do.

As I remember the Tek scopes are specified for around 250V DC mode, and 500V AC. With 10x probe maybe even more.


SveinB

phi70 30th August 2008 05:50 AM

Thanks to all who took the time
 
to give me suggestions.
Yes... 450V across 2 caps in series is dang hard for some scopes and even with the 10x probe my entry level Velleman scope fried and wont calibrate after a few measurements.
Needed to take the pre to a friend's who's got a Tektronics.
Regarding that ultra low frequency ripple...
I got rid of some spikes by replacing the 2 noisy caps after the 1st
choke and now my next step is to increase the value of that to perhaps 560uF (or should I be so bold to try 820uF) so that it's not too close to the 470uF Nichicons I am using after the 2nd choke. Not sure if too high a capacitance might cause my
2A3 rectifiers to arc or not (there's a choke separating rectifier and 1st stage filter).
BTW I got the 470 uF 40mm diameter guys from HSC Electronics in Santa Clara, CA. Cheap cheap @ $4.95 each :)


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