B+ Noise Woes.

My tube preamp is ALMOST noiseless enough to satisfy me. There's one thing that is bugging me though. The B+ regulator is generating noise that I cannot get rid of. Took out the PSU toroid, moved stuff around, etc. I don't think it's related to EMI from the power transformer.

It stops if I touch the top metal of the big electrolytic capacitor.

It is the Hamptone MOSFET B+ regulator.

I have the MOSFET attached to a heatsink underneath the turret board.

Is this a layout issue? Noisy zeners? Noisy MOSFET? I'll be satisfied once I can remedy this last noise issue.

The supply looks like it has a design issue to me

The cap for the zeners is in the wrong place.
I would add series gate resistors(one should be close to the gate) with a bypass cap (move C31)  between them to ground. You should be able to figure out values if you do a search for zener diode noise reduction etc.

The series resistance can also protect the MOSFET

It is hard to bypass noise in zeners because of the low zener  resistance.

I do not see heatsink compound or a electrical (thermal pad or mica and heatsink compound) isolator in your picture.

Your gate lead is long and you could have a high frequency oscillation issue. Have you looked at the DC on a scope?

I'm not committed to this design by any means. As long as I can come up with something that fits in a tight space. This one fits on the small turret board with five turrets per side.

An International Power module gives you some UL/CE coverage for sales, if it'll fit. 

Is R31 small enough for there to be enough current though the zener string for them to actually zener?

Cheers

Ian

I've built two, and they both spit out 229vdc consistently.

You posted "It stops if I touch the top metal of the big electrolytic capacitor."
I had this in my first post "Your gate lead is long and you could have a high frequency oscillation issue. Have you looked at the DC on a scope?"

Try a series gate resistor(100 ohms) right at the body of the MOSFET to test if the noise goes away.

ruffrecords asked about the Zener current it does look to be on the low side
http://www.mouser.com/ds/2/427/1n4728a-104110.pdf
http://whites.sdsmt.edu/classes/ee320/notes/320Lecture6.pdf

Some fast calculations
(230VAC x 1.414= about 325VDC)- (3x75=225VDC) = about 100VDC
100VDC/220K = about .45mA  Zener current
I ignored R30 because it is much lower than R31

Where is the gate to source voltage protection?

One change from the schematic. I'm using a 250v secondary, not 230v.

Perhaps the Gyraf G9 PSU would work instead of this?

> It stops if I touch the top metal of the big electrolytic capacitor.

That is the 100uFd?

The "top" is the negative terminal. Which the schematic shows grounded. So either the cap is not grounded as it should be, or YOU are the problem (your body induces room hum in unshielded circuits) until your body is grounded.

Agree that the way-long Gate lead is another problem; but probably not hum.

Remedied. Put a 330 ohm resister (like a grid stopper) right up to the gate and the buzz vanished.

Thanks, gents!

This article expands on the above PSU and adds slow start as well

https://imgur.com/AY2V3LV

That document is low res and hard to read. Is there are better version somewhere?

Apologies, Imgur does not seem to do PDF to png conversion very well.....

https://imgur.com/l16PfKa

A practical example resigned for a Dynaco 70 amp

https://imgur.com/oLEaDKo

Peter

Here is the second page of the Mosfet Reg

https://imgur.com/4IVcZOT

BluegrassDan said:

The B+ regulator is generating noise that I cannot get rid of.

Click to expand...

What noise? Hum, buzz, hiss...

Just built another preamp. Used different brand zeners. Not sure what the cause of this flickering noise might be. Thoughts?

Video here: https://youtu.be/fLvUUC0wIPk

BluegrassDan said:

Just built another preamp. Used different brand zeners. Not sure what the cause of this flickering noise might be. Thoughts?

Video here: https://youtu.be/fLvUUC0wIPk

Click to expand...

Pretty green on this stuff......

Looks like some kind of wanna be feedback loop.......  motor boating almost ....... does it respond to any controls???

Tubes are good??....

Swapped out the PSU with another previous build, which has different brand zeners. Everything looks great. I think the zeners are to blame.

BluegrassDan said:

Swapped out the PSU with another previous build, which has different brand zeners. Everything looks great. I think the zeners are to blame.

Click to expand...

That's interesting.  Wonder if diodes could be an issue with an older type power supply..... ???? as far as similar symptoms???