P3A Amp hums when connected to XO

[Patrat]

In one chassis I have a P3A amp along with two ESP 09 boards set up as three-way crossovers. The amp has its own power supply, the crossovers share one. Following many threads here about star grounding have brought me to the point where the amp can be connected to any of the three outputs (or any other output for that matter) and is virtually silent. Call it "Amp A".

In another chassis I have two P3A amps, they share a toroidal transfomer, separate rectifiers, etc. Also took some time to arrange grounds and cables to arrive at near silence on all four channels. Call it "Amp B".

Any of amp Bs inputs can be connected to a source and remain silent, ---except--- when both are connected to the crossover outputs from amp A. Either one of the two can be connected with no problem, but as soon as the second one is connected, a distinct hum appears in all four channels.

I have tried a ground wire between the two chassis and a jumper from the star ground point of amp B to the star ground of the crossovers, no apparent difference. Any other suggestions? I am very new at this so please be gentle with the technical language, any suggestions are welcome.

[Jean]

Hello,

Have you seen my thread on getting rid of hum ? Did you get the p3a boards from Rod or made your own ? I was having the same problem with hum when more than one RCA jack was connected to any source. I ended up going with multiple toroids instead of sharing one and it helped alot .

[Patrat]

Jean,

Yes, I read your thread and tried some of those suggestions. I am using boards from Rod. The differences I noticed between mine and yours was that I have a 30-0-30 transformer and each amp has its own separate rectifier and capacitors. Also, I can connect either pair of inputs from a single amp without noise, the trouble starts when I try to connect the second amp. I am hoping (as I am sure you did) that I can find a simpler solution than adding another complete power supply. Thanks for checking in.

[pinkmouse]

Have you tried lifting signal ground in your interconnect cables?

[Patrat]

I disconnected the grounds on all four input cables of amp B and the noise got worse. Re-connected one pair and got silence. Actually seems quieter than the NAD 2200 I was using as a reference.

Thanks!

[pinkmouse]

No problem, glad it was so amenable to a simple solution

[eplpwr]

Hi Patrat!

Some months back I built a 6-ch amp w/ 2 x 3-way Active XO's. I also had the same problem as you descibed, and whatever I tried there was a loud or not so loud hum (50/100Hz hum in Sweden).

Although Pinkmouse has helped you solve the problem, I'd like to give you my input how I finally got rid of the hum. After trying thousand (it felt like) different approaches I finally stripped all of the Power Supply components out of the amp box. I built a separate box for the PS, which had 2 x +/- 40V outputs for the Power Amps and 2 x +/- 15V for the XO's - all outputs shared a common ground, using a 21mm2 cable to connect to the amplifier. Inside the PS as well as the Amp i used car stereo power blocks to distribute the ground connections, and all connections were made with - in my opinion - oversized cables, e g 8 mm2's. Although there was much work involved in moving the PS part out of the Amp, I finally got to hear the wonderful sound of no humming.

I'm not totally sure why the hum stopped, but I guess my PS rework got rid of some (sub-)mV differences in ground level, and thus made it work for me.

If I decide to build another amp, I'll sure go for the "monster-sized" car amp stuff for grounding purposes, it's pretty cheap too.

[Patrat]

Hi Pelle,

Thanks for the suggestions, hopefully the problem is solved and in a much simpler way. If needed, I would have followed your advice but for now all is very quiet.

Now to sort out the gain settings on the crossovers....it already sounds great although the midrange is way too high.