tube x-over noise redux

[jlg4104]

Hi all,

A couple weeks ago I shrank away from the forum believing that my problem was of the careless-error variety. But I'm back to square one after taking lots of care. Here's the deal:

1. I built Steve Bench's tube active x-over and associated supply. X-over point hard-wired to be 2.1 kHz.

2. It does operate properly (as far as I can tell)-- the supply delivers the proper voltages, and the crossover does its job well in two contexts: between a CD player and a television (using the TV's audio inputs), and between a CD player and my pre-amp's AUX input. By "properly" I mean it splits the frequencies (lacking dual amps, I run the highs to one input channel and the lows to the other) and produces nothing ugly.

3. It does NOT operate properly between my pre-amp (Melos 111A) and my amp (Acurus A150). Using the same hook-ups as the other two contexts (highs into one channel, lows into the other), I get the following:

- initial power-on ok
- a hum starts to build in the right speaker
- the hum gradually shifts to the left speaker and becomes VERY loud
- i get scared and shut down the main power to everything
- nothing is damaged, except my pride

I understand that such a "growing" hum is some sort of feedback, but I can't tell what the source might be. I've tried shielding and moving the whole x-over assembly 6 feet from the amp, but to no avail. i've check the grounding, and that seems fine.

My main question: why does my amp seem not to "like" coming after the x-over? Could this an impedance mismatch problem? That's just a stab in the dark; unfortunately, I do not have info on the amp's input impedence.

Thanks in advance for any ideas.

Jay Gordon

[SY]

Odd one. I'd wonder about some sort of ground loop. Have you tried floating some of the power cord grounds (i.e., the third pin)? Is there anything unusual about your power amp's grounding schemes? What sort of amp is it, anyway?

[HDTVman]

Jay, I just looked at Steve's Web site and the schematic of the crossover does not show a ground for the output RCA connectors. Is that how you built the unit?

If so that's the problem. You need to ground the outside of the output RCA connectors just like the input connectors and I think your problem will go away.

Later
Bruce

[jlg4104]

I used the pcb that he mentions, which includes +/- holes for the RCA connector inputs/outputs, as well as a pair of holes for the b+, one marked 'b+' and one marked 'ground.' that last is the only point officially hard-wired to ground in my set-up (i.e., i run a green wire from that point to the ground wire on my 3-prong cable). i've assumed that the pcb layout runs all the input and output grounds to same place. on the schematic there are two ground symbols, but as far as i can tell that's just b/c the schematic shows two halves of the circuit separately, and the inputs, outs, and B+ all seem to have a corresponding conection to ground.

that said, i will see about separately grounding the RCA's, as a test.

thanks