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Choke input supply for class AB amp

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Posted on April 15, 2012 at 23:00:58
swordsman
Audiophile

Posts: 49
Joined: October 4, 2000
How do you spec the current rating for the inductor for choke input supply?

Say a Class AB amp with a idle current load of 200mA and full power current load of 300mA. Do we spec the current rating for the choke at 200mA or 300mA?

Most of the choke in the market is spec for PI filtering and for choke input usage, we can derrate it by a factor of 1.6 or 1.8 to prevent core saturation? Of course I assuming the choke have the voltage insulation for choke input purposes.

From what I see, most Class AB amp does not use choke at all and for those which uses a choke they are for PI filtering. Very few vintage amp actually spec for choke input. For a Class AB amp which see fluctuating of load current, I believe choke input for current regulation should improve the power supply design greatly. At least we are not so panaroid with ripple voltage since it is a push pull amp. Anyone willing to share your experience in using choke input supply for Class AB amp? What kind of sonic differences did you get?

 

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General comments, posted on April 15, 2012 at 23:34:18
drlowmu
Manufacturer

Posts: 7958
Location: East of Kansas City
Joined: January 10, 2005
I think chokes are really nice on all classes of high quality audio amps, class A1 and A-B, SE and P-P.

1 1/2 volts AC of ripple is NO PROBLEM in a A-B amp that is P-P. Ripple is "overrated" by designers generally speaking, not too important as to what makes an amp sound good.

Sometimes a small (under 1 uF) "c" input filter is helpful.

There are things that well-chosen chokes do, that make them indispensable.

I would consider the use (or non-use) of chokes in vintage amplifier designs, had more due to marketing and cost and weight considerations, than absolute all-out performance considerations.

My favorite choke configuration after many years of testing amps of all types, is L1/C1/L2/C2. It is far more refined sounding to my ears than L1/C1, and it always came out on top.

I will let others advise you on finding chokes for choke input duty.

One final word of advice, my original audio mentor always advised me to use chokes that are 20 Ohms or less in DCR, and I have found that to be fabulous advice.

Jeff Medwin

 

RE: Choke input supply for class AB amp, posted on April 16, 2012 at 05:41:07
Neff


 
I recommend a 400ma rated choke as the first choke actually sees a peak to average current. Too high a current rating and the choke may lose Lcrit and stop acting as a choke resulting in charging up the first capacitor to higher voltage. 40-50% of ratings is usually minimum current requirements for LCrit. 400ma can be found used. You may meet LCrit with a more common rating of 500ma from Hammond Manufacturing. The 193P is a fine choke. The PSUII power supply designer should show calculate LCrit or ask the manufacture.

Older ham radio tube amplifier equipment subject to large current swings used a swinging choke. Some report a swinging choke may affect the audio, but I doubt that.

 

Swingers, posted on April 16, 2012 at 06:34:20
drlowmu
Manufacturer

Posts: 7958
Location: East of Kansas City
Joined: January 10, 2005
Neff,

Nice post. I only had one experience with swinging chokes, in a pair of modified Fisher 50As, and when I went from stock swinging chokes to fixed chokes, my ears were much much happier. I have no idea what others have experienced in A-Bing, but "swingers" earned an "D" grade for me with the 50As.

Jeff Medwin

 

RE: Choke input supply for class AB amp, posted on April 16, 2012 at 07:11:38
swordsman
Audiophile

Posts: 49
Joined: October 4, 2000
So you are saying I should use a pi choke of 400mA for 50% derating of 200mA idle current load. And it will handle the 300mA peak load without core saturation. Am I right?

I'm getting the choke custom wind so I have no issue sourcing for what I wanted. The problem is my supplier only does pi choke and has never done a choke for choke input supply. Thats why I'm trying to find out how do we derate a pi choke for choke input supply

 

"how do we derate a pi choke for choke input supply'", posted on April 16, 2012 at 07:35:28
Not so much the electrical de-rating, but the mechanical de-rating you have to worry about. Unless your winder is really experienced with choke load inductors, expect some nice physical hum/buzz from the unit. Make sure your working voltage rating is +800 as the choke gets beaten pretty hard.

Play with DuncanAmps.com PSUII, it will really help you visualize the supply and it will tell you your problems. You can put in stepped current demand to see how the filter works at various draws.

 

RE: Choke input supply for class AB amp, posted on April 16, 2012 at 12:48:00
op48no1
Audiophile

Posts: 504
Joined: October 29, 2010
A good rule seems to be: The choke needs to be handle the worst case DC load current plus the peak current due to the AC voltage across the choke.

The best way to visualize this is to sim it in PSUD, look for the peak choke current, then spec a choke with a DC rating greater than this value.

Using a choke with a higher current rating than necessary will not cause it to lose regulation in choke-input service.

The advice about mechanical integrity is important. Tell the winder the choke has to be able to withstand half the transformer secondary RMS voltage at 120Hz across its terminals.

-Henry

 

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