Pearl -> Hum -> Choke Input?

[tnargs]

Hi, I am battling hum on my Pearl phono amp. I read this article in the latest SC mag, Silicon Chip Online - Fixing Transformer Buzz In The Class-A Amplifier, where they reduced tx-based hum (class A amp) by adding a light choke input. Cost them about 1V on the 20V rails.

I would like to try this on my Pearl, but what is the right size of choke to use? The article uses 470 uH chokes with 82 m-ohm DCR, but that is a 20W power amp. The Pearl has much lower bias current, is this a factor in choosing the choke value? Or is the choke value calculated from the frequency of the buzz (50Hz AC here in Oz = 100Hz rectified)?

[ra7]

The choke should have a small DC resistance to minimize heat dissippiation and voltage drop. If you are drawing 2 amps of current, and the DCR is 0.5 ohm, the voltage drop (IR) will be 1V and the heat dissipated will be (I^2R) 2W.

The pearl doesn't draw much current and so the choke can be of small gage wire. The 470uH choke will work, in fact any tube-amp supply chokes should work as long as they can pass the current drawn by the pearl without overheating. Look at hammond chokes on their website. Also, you can search for 'choke input power supplies' in google.

[tnargs]

Thanks for the advice. So the desirable choke value (to limit pulse currents) is not related to the bias current? The 470 uH choke is equally well suited to 2A current and the Pearl's 0.1A?

[eLarson]

As I understand the term "choke input" you need a certain minimum amount of inductance for a given desired voltage output and current draw.

There's a formula for it around this very forum. Search for "critical inductance" and see if that gets you there.

Here's the post I was thinking of:
The Aleph-X

[tnargs]

Ok, thanks very much. 2002, you have a good memory!

[tmblack]

I am getting hum on the phono also.
Anyone tried using capacitors across the bridge diodes, typically 0.01uf?
I have some ceramic ones, but are there special ones for this purpose?

[nar]

Hum can also come from perfectible grounding scheme Also, input ceramic discs on input RCAs (between signal and ground) can help curing noise pick up, especially in HF cases.

tnargs, is your build of the Pearl recent ? Did you use PSU proposed by Passlabs ? What is the output level of your cart ? Does the phono stage still hum when you short Pearl inputs to ground ?

Best,

nAr