U47EF Oliver Archut version

[Spencerleehorton]

Yes hum goes away when I turn off power that’s why I have cathode bypassed.
Using the passive u47 psu which Dany Bouchard designed, schematic early on in this thread.

 

[rock soderstrom]

Yes hum goes away when I turn off power that’s why I have cathode bypassed.
Using the passive u47 psu which Dany Bouchard designed, schematic early on in this thread.

Some ideas:

Does the hum level change when you increase the distance between the power supply and the microphone? Orientation of the mic may also play a role --> direct magnetic interference?

Can you measure rest ripple on B+ and F+ in AC volts with your DMM?

What happens if you temporarily add an additional RC filter stage to B+ and F+? Does the hum level change?

 

[Gus]

You need a very clean and regulated heater voltage to set the bias off the heater voltage.
The heater part of a supply is more important to get right with this type circuit.

IMO you need to design a better heater supply.

I have built microphone circuits with this type bias.

 

[rock soderstrom]

You need a very clean and regulated heater voltage to set the bias off the heater voltage.
The heater part of a supply is more important to get right with this type circuit.

I also suspect that this is the problem. I would take care of that first.

 

[Spencerleehorton]

I think it’s a gnd issue/loop, as I have iec to chassis and have B+ gnd connected to iec chassis, I also have H+ gnd connected to iec chassis and XLR pin 1 connected to chassis.
This other wiring of gnd says to only have XLR pin 1 to chassis and have IEC gnd to chassis.

 

[rock soderstrom]

I think it’s a gnd issue/loop, as I have iec to chassis and have B+ gnd connected to iec chassis, I also have H+ gnd connected to iec chassis and XLR pin 1 connected to chassis.
This other wiring of gnd says to only have XLR pin 1 to chassis and have IEC gnd to chassis.

You can simply test this by temporarily removing the corresponding connection. I do not believe it. I would connect the GNDs of the respective outputs of both power supplies to the chassis connection close to pin 1. At the moment there is only a little metal sheet between this and your solution.

I think this is the wrong spoor, your hotfix with the cathode cap speaks for a humming heater voltage.

 

[Spencerleehorton]

You are probably right but I would have thought Danys passive psu design would have allowed for ripple and made a clean H+ ?

 

[Gus]

Why is the tube so far away from the capsule?
and
Did you use an inductor in the power supply?
Do you have a picture of the PS?

 

[rock soderstrom]

but I would have thought Danys passive psu design would have allowed for ripple and made a clean H+ ?

Why is the tube so far away from the capsule?
and
Did you use an inductor in the power supply?
Do you have a picture of the PS?

I think Gus is on the right track. Do you use a choke for the heater F+? If so, does it also bring the required henries? What type and brand is it? DIY? The choke can also interact magnetically with the mains transformer. Check position and rotation!

In fact, there's also a risk that you'll catch a direct magnetic interference somewhere in the high impedance area. Hence my suggestion to change the micro position as a test...

But I still think that your F+ voltage is not clean enough.

 

[Murdock]

I’ve build fixed bias mics powered with dany’s passive PSU and it was good.
So I think the PSU circuit is clean enough if you build it “correctly”. That means the right choke, as the others pointed out, and, what I think is equal important, with the proper grounding.

 

[Spencerleehorton]

I have the exact choke required so that’s why I think it’s a gnd issue, I’ll send some pictures

 

[rock soderstrom]

Have you looked at the voltages with an oscilloscope or a good DMM? You should be able to see the ripple voltage.

 

[Sorr]

I have found when working on U47 clones that even a teeny weeny amount of noise on the heater gets into the cathode and is amplified.
As mentioned you are probably getting noise into the choke.
Which can be fixed/reduced by changing its orientation to the power Tx.
But you also might like to try one of the tricks that I use.
Which is directly from the rectifiers connect your large and small value cap.
Then using two small value resistors, 2.2 - 4.7 ohms. One to the + and one to the - of these caps to another cap. (1000 -4700mfd).
The - of that cap then goes to ground and the + to the choke as per your original cct.
Choose the value of the two resistors to suit the voltage drop.
This will help clean up the sine wave sitting on the DC before it gets to your Choke or regulator.

 

[Spencerleehorton]

I’ll get the scope out today and draw out a diagram and get some photos

 

[TLRT]

Probably you need a shielded mains transformer(s), we have a very good local company I asked them to make a single 200/20v toroid for this project, I made dozens of psu with this and never had any noise issue.

 

[Spencerleehorton]

Will draw a diagram to make sense of this picture

 

[Spencerleehorton]

 

[rock soderstrom]

Ok, you're using a back to back two power transformer solution. This offers some possibilities for problems, but can also work.

The toroid is very close to the choke, be sure to vary the position and orientation and see if anything changes.

Yes, a diagram will help solve the problem.

 

[rock soderstrom]

View attachment 111886

Are you really making the GND connections to the IEC chassis point directly at the rectifiers? I would not do that.

 

[Khron]

https://sound-au.com/earthing.htm