Strange ground loop between ladder voice cards [solved]

So I finally got the ladder filters working correctly on both voicecards. Now there is another issue which has me even more confused and worried… No matter which positions I put them in, if I have both ladder cards in the Ambika and have them stacked with other voicecards or each other (my voices are 2 smr4, 2 4p, 2 ladder), the ladder voicecards let through 60 hz hum, which seems stronger if they are played with other cards simultaneously. When only one ladder filter is in the Ambika, it behaves fine. WTF?

What is your power supply providing? AC or DC? How many amps? You may need more amps.
The other issue is the screen. Are you using an LCD, VDF, or OLED? An LCD won’t cause major hum issues with the TubeOhm Ladder filter, but an OLED can.

Thanks for the reply. The power supply is 9 v AC 1300 MA. I’m using this LCD:

After some more experimenting, I think it might be an issue of insufficient current. I can have 5 cards in the ambika including the 2 ladder cards, but the 6th card causes all the ladder cards to let hum through.

And those 5 cards can be stacked any way desired

Try another power supply.

Install C12 (2200µF).

I installed C12. No improvement. One other thing I should mention is that all the cards except the ladders have this mod on them:

Could that cause extra current draw somehow?

not at all.

Hmm. Well I went over the bottoms of the boards and cleaned out all the flux and checked for shorts. On one of them I think there might be a bad connection where vacuum pump desoldering exposed some copper, but I can’t seem to get rid of it. The problem is intermittent seemingly as well… I’ll order a beefier power supply and see if that helps. Otherwise I might be SOL until I can afford a replacement PCB.

1300 mA seems a bit low, and you may not even get 1300mA out of it anyway. I would suggest a good 1.5A to 2A power supply for it. Just make sure it is AC to AC. :slight_smile:

Got a 2 amp supply and seems like everything works. Thanks everybody for the support!

I had a similiar problem -

The thing that helped me was adding a resistor to the lcd.

Interesting… I did have the brighter Optrex display, but had already placed the 100 ohm current limiting resistor and put in C12 and 1300 mA still wasn’t enough. Impressive that you made do with 700.

Golden rule, if you need 1A get a PSU with more than 1A. A PSU often won’t deliver 100% of its rating well if at all.

Well, if anybody wants the 1300 mA supply, I’ll send it to you for $2 plus shipping :slight_smile: