Trying to measure very small ripple voltages, but can't.

Thread Starter

Dynaman

Joined Jan 17, 2008
91
Greetings AAC'rs: I have an audio preamplifier that turns out needing a very quiet power supply in order for it to perform as quiet as I would like. Attached is the bipolar power supply for it. When I had added a second set of filter caps to the power supply, the noise drops about 17 db improving the preamplifier performance significantly. My problem is, using my scope, I can't see the ripple on the output of the regulators either way. Any suggestions on measuring minute amounts of ripple on the output of the regulators? Any help appreciated.
 

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tautech

Joined Oct 8, 2019
58
If it's at double mains frequency like from a linear PSU a DMM in AC mV range should be able to measure it if you scope isn't sensitive enough.
Also remember to use 1:1 scope probes for this sort of measurement.
 
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Thread Starter

Dynaman

Joined Jan 17, 2008
91
Yes, I have done that with the scope probe, still not sensitive enough @ 5mV/DIV. I'll retry the DMM idea, and use a shielded probe. it is a TEK 4050 6.5 digit.

Thanks
 

Audioguru again

Joined Oct 21, 2019
857
Instead of modifying the pretty good power supply why not fix the preamp? An audio opamp has an extremely high power supply ripple rejection very low distortion and low noise.
 

Thread Starter

Dynaman

Joined Jan 17, 2008
91
Instead of modifying the pretty good power supply why not fix the preamp? An audio opamp has an extremely high power supply ripple rejection very low distortion and low noise.
Well I think the preamp if fine, and I am using a good IC --NJM2068. However there may be ground routing issue, though when I apply a bench supply the noise disappears. There is a NCP7812 which touts having a good PSRR, but not available in a negative version.
 

Janis59

Joined Aug 21, 2017
1,057
Actually, why not to switch a Cocroft-Walton ove the DC output. All the AC component will me multiplied some 7...9...11...13 or let its be even 100 fold.
 

panic mode

Joined Oct 10, 2011
1,817
"My problem is, using my scope, I can't see the ripple..."
"when I apply a bench supply the noise disappears"

did you try measuring ripple of the bench power supply? there is no point in going beyond that.

"there may be ground routing issue"

this is not something scope will be very helpful with. how are things connected/shielded, what is your construction method?
 

Thread Starter

Dynaman

Joined Jan 17, 2008
91
"My problem is, using my scope, I can't see the ripple..."
"when I apply a bench supply the noise disappears"

did you try measuring ripple of the bench power supply? there is no point in going beyond that.

"there may be ground routing issue"

this is not something scope will be very helpful with. how are things connected/shielded, what is your construction method?
Well there is more to the story.... To describe the construction method is a bit long, but it is mounted on a PCB. So my issue is modifying the power supply for better performance, and do it without taking up more PCB real estate. I'm kind of in a jam (no pun intended) if you know what I mean. I was hoping to find a drop in replacement for 78/79xx series that has better PSRR.
 

Thread Starter

Dynaman

Joined Jan 17, 2008
91
Alos, I did some quick tests and noticed the PSRR on the regs were around -38Db.. well away from the specs on the sheet which touts anywhere from 60-75db. Was surprised. My load is pretty close to the spec sheets test which I think is 100mA.
 
Is the "hum" at 60Hz or is it at 120Hz?
The full wave rectifier makes it at 120Hz and the 7812 IC reduces it -60dB (1/1000 times).
The opamps reduce it about -100dB (1/100,000 times).
Then the total reduction is 1/1 million times.

I think you are hearing the ground buss resistance causing much more hum, stray pickup or you hear the heaters on your vacuum tooobs.
 

Thread Starter

Dynaman

Joined Jan 17, 2008
91
Is the "hum" at 60Hz or is it at 120Hz?
The full wave rectifier makes it at 120Hz and the 7812 IC reduces it -60dB (1/1000 times).
The opamps reduce it about -100dB (1/100,000 times).
Then the total reduction is 1/1 million times.

I think you are hearing the ground buss resistance causing much more hum, stray pickup or you hear the heaters on your vacuum tooobs.
God I hope you're right. But then why much better performance with the bench supply or the addition of extra filtering caps? It should have been quiet as is.
 

ci139

Joined Jul 11, 2016
1,177
Your GND goes to TF tap
Your Full Period Rectifier goes immediately to "2.2mF" ** . . .
. . . and to "3-terminals"

** -- there might be some resistance - either before and/or in series - to protect the rectifier diodes and
possibly the components "down the flow" + *** . . . or combined filters

In addition to that the electrolytic cap.-s require ceramics for High frequency range improvement...
...there ought to be *** a small resistor before the 3-terminals and
another .1u ceramic just at it's input to improve the transient response

nothing serious - just the potential noise sourcses . . .
 

Thread Starter

Dynaman

Joined Jan 17, 2008
91
Your GND goes to TF tap
Your Full Period Rectifier goes immediately to "2.2mF" ** . . .
. . . and to "3-terminals"

** -- there might be some resistance - either before and/or in series - to protect the rectifier diodes and
possibly the components "down the flow" + *** . . . or combined filters

In addition to that the electrolytic cap.-s require ceramics for High frequency range improvement...
...there ought to be *** a small resistor before the 3-terminals and
another .1u ceramic just at it's input to improve the transient response

nothing serious - just the potential noise sourcses . . .
Thank you Sir... One correction... originally, the caps were 1000uf. Sorry about that.
 

MrAl

Joined Jun 17, 2014
7,114
Capacitor ESR is also an issue in filter circuits.
Try using a very low ESR capacitor(s). If that doesnt help, then the problem is with something else.

Did you mention how you are noticing the problem if you cant measure the ripple?
What made you think the ripple was a problem in the first place?
 
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