Question about transformer rectification in an EHX Echoflanger

[R.G.]

There is a tendency in humans to substitute taxonomy for real problem solving, thinking that if the problem is named, it is solved. However, often once the problem is identified (as opposed to named!) it can be solved.

Hum can be (1) conducted or (2) radiated. Further, radiated can be (a) electric field (capacitive) or (b) magnetic field (inductive).  We live in a sea of 2a and 2b from many different sources, and there are many 1s that can creep in. You're working entirely on 1, based on the observation that an external DC source removes the hum.

However: you may have a different version of 1, because the wiring to rectifiers and first filter cap can induce power-line-frequency spikes on both power and ground. The cure for this is more careful wiring, especially of the wires to the first filter cap and from the first filter cap to the rest of the audio circuit. An unfortunate choice of what ground goes where can make hum for you.

You are also comparing an external supply versus an internal one. This removes the most-intimate source of 2a and 2b from the circuits, and so it changes both the conducted and both kinds of radiated noise at the same time. It does not tell you which of these it's fixing.

That's one set of things keeping you from knowing how to fix it.

Here's a possible solution. In your schematic, look at the lower left corner. For a moment, suspend your ideas about all grounds being equal, and all wires being short circuits. If you think of black marks on paper as really representing physical wires, then the schematic is actually the correct way to wire the power supply for this thing. The CT goes directly to the (-) terminal of the first filter cap, nowhere else. There is one wire from the negative of the first filter cap to the ground terminal of the regulator. Then ground goes to other stuff. The pulse currents from the rectifier and the voltages they cause in the circuit are entirely contained inside the transformer-rectifier-filter cap loop.

Here's another possible solution. Your transformer may be radiating magnetic field leakage into something near it.

Here's another possible solution. Your transformer wiring may be coupling magnetic field from the pulsed currents going through it into a loop of wiring (or PCB traces) that happen to lie near it, making an air-core transformer.

It is also possible that some other detail about moving the transformer into the box is causing hum from some quirk of how it's pasted in.

On the LC filter: LC filters are funny. They do things that one would not expect. Very often I see people thinking about using an LC filter for dropping the output voltage of a supply, which it does - as long as you can guarantee it a minimum current load. At low loads the output voltage rises precipitously to the capacitor-input filter voltage as though the inductor was not there. LC ripple filters can help, but (1) only if ripple is what's causing the hum and (2) they introduce issues of damping the now-resonant filter properly.

So... should I try a full rectifier bridge type circuit?

The only difference a FWB does is to make your first first filter cap twice as effective at suppressing rectifier ripple by doubling the ripple frequency. You can change to a FWB, or double the size of your first filter cap (C38 in the schemo) and get the same sized ripple. But that does bring up an interesting point. Your hum may be at power line frequency or (if you did use a FWB) it may be at 2x power line. If it's power line (and you had used a FWB) then the hum tells you it's not part of the ripple filtering setup, as that can only produce 2x power line.  If it's 2x power line, it's caused by the rectifier/filter setup, not power line induction. With a half wave rectifier, you can't tell by ear between power line and an octave up, because they're both at the same frequency.

Is there any way to filter the incoming mains power to try to get rid of the hum?

No.
The incoming mains is, and has to be 100% hum. The only way to filter it to get rid of hum is to turn it off.

[Govmnt_Lacky]

Thanks once again RG for the insight 

Something that immediately stabs me in the brain after reading that is the fact that I DO NOT have my transformer ground going DIRECTLY to the negative side of the input filter cap. I actually have it connected to a ground bus bar setup as this circuit has a multitude of ground points that I needed to implement some form of bus bar method to connect them all together. I now wonder if I were to solder the transformer V- as close to the input filter cap's negative terminal if this would help.. 

As far as the power leads generating a feild. IF you look at EHX's wiring of these units.... I would be surprised if this was true with this circuit. They ran their power wires ALL OVER THE PLACE inside the enclosures. They did however, solder the CT transformer's V- directly to the PCB near the first filter cap! 

Will try this out and report back!

[Govmnt_Lacky]

Well... just tried connecting the transformer V- directly to the ground trace that is shared with the 470uF filter cap's negative.....

Still humming!!   

Double checked all grounding connections... ALL GOOD.....STILL!!!!

[PRR]

> Tried running the series transformer

And what DC voltage do you get on the cap??

If you don't measure it, you are swinging blind.

[Govmnt_Lacky]

> Tried running the series transformer

And what DC voltage do you get on the cap??

If you don't measure it, you are swinging blind.

Hello Paul... Thanks for stopping in 

I am getting 29.7VDC after the rectifier diode and into the LM7815 regulator and the 470uF/63V filter cap.

I am getting 14.8VDC out of the regulator and onto the 470uF/35V cap.