Linear reg PSU ripple - too much?

[abbey road d enfer]

It's unclear to me what was the root cause, given it's common to bond the 0V reference to the safety earth to prevent them from being too far apart.

Was this a case of there being multiple connections from 0V to the chassis in this case?

No. The cause is the dirty "ground" from the filter caps was routed to the distant "ground" via the multiple safety earth connections.

 

[Ilya]

I've made a picture that illustrates two scenarios. There were no multiple connections, but it still made a difference.

 

[abbey road d enfer]

I've made a picture that illustrates two scenarios. There were no multiple connections, but it still made a difference.

Are you sure there is no connection between the circuit and the chassis in the device?

 

[Ilya]

Are you sure there is no connection between the circuit and the chassis in the device?

Absolutely! I've rechecked once again and the only connection is the one shown on the diagrams.

 

[abbey road d enfer]

Absolutely! I've rechecked once again and the only connection is the one shown on the diagrams.

What's in the "device"?

 

[Matador]

See, that picture doesn't make sense to me as to why one is high versus low ripple.

I could see if the PSU was dumping high charging currents into the chassis, and then the 0V reference 'downstream' was again bonded to the chassis, causing large charging currents to flow through the chassis and modulate the downstream 0V node.

The 'high ripple' scenario is commonly how I connect my 0V reference to earth (typically where it comes out of the high-voltage regulation section.

 

[Ilya]

What's in the "device"?

In this case it's a class A mic preamp. With the linear regulation section posted earlier in this thread.

 

[abbey road d enfer]

In this case it's a class A mic preamp. With the linear regulation section posted earlier in this thread.

Do you have phantom power on it? How is the XLR grounded?

 

[moamps]

Poor grounding scheme or too low IN-OUT voltage difference.

And it is very important where the ground of the scope is connected to the measuring object because sometimes a measuring procedure introduces itself an error in the measuring results (forming a new ground loop etc.).

 

[Winston OBoogie]

It's unclear to me where exactly the "high ripple" scheme had its 0V tied to the power supply chassis.  In your diagram, it looks like the connection is post rectifier but pre the main smoothing capacitor.  Betwixt the two yes?

 

[Ilya]

Do you have phantom power on it? How is the XLR grounded?

Phantom power has a separate global regulator in the PSU enclosure. It's ground is connected to the smoothing cap ground on the PCB. XLRs pin 1 is wired directly to the chassis (and nowhere else).

And it is very important where the ground of the scope is connected to the measuring object because sometimes a measuring procedure introduces itself an error in the measuring results (forming a new ground loop etc.).

Yes indeed. However, I've checked the output of the preamp with the analyzer (with scope probes removed), and I can clearly see the 100 Hz harmonic when I connect it as in the "high ripple" circuit.

It's unclear to me where exactly the "high ripple" scheme had its 0V tied to the power supply chassis.  In your diagram, it looks like the connection is post rectifier but pre the main smoothing capacitor.  Betwixt the two yes?

No, my apologies. This is not exactly correct. The point is AFTER the main smoothin cap.

 

[abbey road d enfer]

Phantom power has a separate global regulator in the PSU enclosure. It's ground is connected to the smoothing cap ground on the PCB. XLRs pin 1 is wired directly to the chassis (and nowhere else).

So your phantom voltage is dirty...

 

[Ilya]

So your phantom voltage is dirty...

Seems like it is. I've measured approximately 20 mV ripple. But it's common mode, I'll need to rethink the phantom power grounding scheme. But for the main rails ripple it's not that relevant now IMHO.

 

[alexc]

And on the subject, I did some tests in a typical opamp circuit ..

  ...  of my usual  'lm series regulator' bipolar supply pcb module [+/-17Vdc @ 100mA ] and an 'ebay class-a discrete bipolar regulator' pcb module for same.

I did see a couple of dB improvement in the 'hum floor' using  the discrete class A  setup.

Not so much really, but in a 'class A audio path' [which was what I was mostly interest in] it did end up between 1 and 2 dB of 'better'.

At higher current levels, say 150mA .. 500mA the difference was more pronounced

ie. circuit under test changed from  'ne5534' chips  to  'class A discrete low power'  to  'class A discrete moderate power'  etc

For a headphone amp circuit, I did find the class-A regulated and class-A audio circuit to be very good performing, but of course at a cost of heat  ...  both in the regulator  (a dc amplifier)  and the  audio  (an  ac amplifier)  ...  for class A all round

...

Anyway ... something I found interesting last I measured these things ...

 

[Ilya]

I have hard time figuring what's the class A discrete voltage regulator is. Is it a pass transistor design? Zener based?

 

[abbey road d enfer]

I have hard time figuring what's the class A discrete voltage regulator is. Is it a pass transistor design? Zener based?

As long as current passes in the same direction, i.e. the supply sourcing current to the load, it is class A. If the load was reactive, it could send current to the PSU, then the regulator would have to be capable of receiving current.
For a typical audio circuit, this is not the case, current circulates always in the same direction. As long as the regulation is within limits, there is no case of going class B (or C).
Amplification class is not related to topology, but generally class B is well supported by push-pull arrangement.