Center-tap Choke

[SonnyOneness]

Hi everyone.

Just made some tests with different inductors in place between the power transformer center-tap and the filter caps, with no choke or inductor in place (straight wire) the voltage over the caps are about +- 36v.

When i add an inductor the positive side gos to 39v and the negative goes to 34.4v, how is this possible? the power supply i tested this with were on a gainclone typ amp.

Thanks, Sonny.

[tinitus]

are you sure its center tapped, and not just a double choke
maybe the choke is only intended for series use

or maybe simply connected wrong, out of phase

[SonnyOneness]

I made a schematic for some clarity =) if i remove the choke (just a straight wire) the voltage over the capacitors become even, +36v / -36v.

With the choke in place the voltage becomes +39v / -34.4v.

Can try to change phases on mains/choke etc. but i doubt that is going to make any difference.

Thanks, Sonny.

[tinitus]

never seen a choke in series with ground

[Frank Berry]

You're forcing the choke to pass alternating current. Chokes don't like AC. You really need two chokes ... one on the + output of the bridge and one on the - output of the bridge.

[tinitus]

Originally Posted by Frank Berry

Y ... one on the + output of the bridge and one on the - output of the bridge.

shouldnt chokes be between 1.and 2. supply cap ?

[RJM1]

It’s probably due to the difference in the amount or capacitance on each rail. Large aluminum electrolytic usually have a tolerance of +100% -10%. This could easily give you a 20% difference in the amount of capacitance per rail. (See attached)

[DF96]

Are you drawing exactly the same current from each rail? i.e. nothing in the circuit connects to the 0V rail, or if it does it draws only negligible current.

You have, in effect, two choke input supplies but with the added complication that the choke is common so it sees the difference current. Choke input supplies are very sensitive to current drawn until the current exceeds a threshold value, at which point the DC output is 90% of the secondary RMS voltage. Your supply will be very sensitive to the difference current.

What are you trying to achieve?

[SonnyOneness]

Just made some tests and measurements. There isn't much unbalance in capacitance between the positive and negative rail, about 7uF. Sure thing if you load a choke input supply lightly it will more or less become a capacitor input filter. I'm not sure if the LM3875 draws equal from + and - rail, but it should because it's not power grounded.

However it's strange that i get an increase in voltage on the positive rail from 36v to 39v

I'm trying to achieve better sound i want to isolate the signal ground from the noisy power transformer (and mains crap) and to decrease the CT ripple current.

[Frank Berry]

Reverse the connections on the choke. That may have an effect on the circuit if the imbalance is caused by the way the choke was wound. If this is the case, you'll see the same imbalance ... but in the other direction.