866A Power Supply

[lugnut]

Would it be ok to use a small capacitor {.01uf-.1uf} at the input side of the choke to quiet things down a bit? I have some buzzing from the 10H 500ma choke running at 400ma. I know the 866A requires a choke input so any cap at the input would have to be very low.

 

[audiowize]

Can you thoroughly describe the entire power supply? (Especially the power transformer ratings, even part numbers would be great)

The best solution would be to substitute a different choke or if this is on a stereo amp, you could add a second identical choke to ease the load on the choke.

 

[6A3sUMMER]

Is your choke a Swinging choke?
Regular chokes will not work well with an 866.

Just because the current is rated conservatively does not mean the choke can swing the large voltage required for a choke input supply.

 

[audiowize]

Here's an example of such a specific part that could solve your problem (you'd need one slightly larger; I'm sure it could be made):
CHOKES FOR VALVE TUBE AMPLIFIERS

 

[lugnut]

2 866a full wave fed by 1200vct @500ma power tranny into 10h 500ma choke followed by 1250uf filter to supply 480vdc @ 400ma.for PP 6550. I am mostly concerned about the 866a feeding a small cap.

 

[6A3sUMMER]

As long as it is a Swinging choke.
. . . and also as long as it was produced with whatever it takes to be Quiet in choke input supplies.

"All parts are created equal, but some parts are more equal than others" . . . stolen and modified from George Orwell's "Animal Farm".

 

[audiowize]

I would model the supply in PSUD to check the diode current.

The small cap will increase diode current, power transformer current, and output voltage. I believe your power transformer will not be happy having that cap there at minimum, and as the cap value increases the 866s will start to whine too.

 

[6A3sUMMER]

Versus most kinds of rectifiers, the mercury vapor ones create the most noise.

600VAC is 849V, and Peak reverse volts would be 1698 Volts.
There might be HEXFREDs with a high enough voltage rating.
No good looks, but quieter than 866a's.

 

[lugnut]

The choke is a Triad C-22A filter reactor 10H 500ma 65 ohms. Would a small cap be safe to use on the 866A's?

 

[6A3sUMMER]

A 0.1uF cap has 13,263 Ohms of capacitive reactance at 120Hz.
849V peak / 13,263 Ohms is 64 mA current in the cap (just the 120Hz product).
Any higher frequency component coming off the 866A's will draw additional current.

One of the things you will do with a 0.1uF cap there is to create a very noisy ground loop at the 0.1uf cap ground connection. And a lot of that ground loop is going to have the characteristic 866A noise.

Try it at your own risk.
It probably will not change the mechanical noise of the choke.
Your mileage may vary.

 

[baudouin0]

I just had to look up 'swinging choke'.

 

[lugnut]

Mechanical buzz is the issue. I still have a few mounting/insulating/isolating tricks to try. I have used this choke in CLC supplies without issues.

 

[audiowize]

The AC voltage across such a choke in a CLC power supply is just the ripple voltage. In a choke input supply like yours, that goes up to hundreds of volts.

For your LC power supply, crude estimates are:
Transformer AC current: 313mA
Diode Peak Current: 1.4A
Diode RMS Current: 313mA
DC output voltage: 412V

If we put a 0.1uF cap ahead of the choke you will reduce the voltage across the choke slightly and pick up maybe 10-20V DC of B+. Go ahead and try it, I suspect it will not silence your choke's cries for mercy.

 

[6A3sUMMER]

A steel chassis and a swinging choke mounted on it is a great way to create a mechanical buzz.

We might be getting somewhere now.

Try using Stainless Steel screws, lock-washers, and nuts. And use a stainless steel spacer (1/4 inch stainless steel tube), between the choke mounting tabs and the steel chassis.

 

[lugnut]

I remounted the choke on rubber grommets and shimmed the sides near the chassis. It helped a bit. Still have not tried adding a small cat in front of the choke.

 

[6A3sUMMER]

The cat would either act as an attenuator, or it would simply add purring to the sound mix.

 

[baudouin0]

Cannot imagine what a swinging choke does then.

 

[lugnut]

LOL. Maybe the purring would phase cancel the buzz!

 

[Chris Hornbeck]

Swinging chokes have no magical properties. They're named for their larger than usual variation in inductance with variation in DC current. Choke input filters need to see a minimum inductance so that their DC output voltage doesn't "soar" to higher levels than desired, and the minimum inductance increases as load decreases. Search for "critical inductance".


What to do? A swinging choke maintains a higher inductance at lower currents than a conventional design, at the expense of some slightly lower inductance at higher currents.


Circuits with constant loading don't care, but if current draw varies a lot, a swinging choke can help with voltage regulation.


All good fortune,
Chris

 

[lugnut]

I tried putting a .01uf-.08uf in front of the choke with no success. Things got worse with the .08uf. I'm considering a separate chassis for the choke. Power supply is very quiet and stable with only 8mv of ripple. I can't hear the buzz from the choke while music is playing and I'm 10 feet from the choke.