Help - Getting loud 60hz hum when testing my passive high-level crossover

[owdi]

I just built a voltage dividier + crossover, so I could connect the speaker outputs from my receiver to the inputs on my power amp. When I tested this circuit, I got a loud 60hz hum.

My receiver does not have external preouts for the front two speakers. I want to use my power amp with the receiver, but the power amp does not have speaker level inputs.

Reading up on the internet I found I could connect the receiver's speaker outputs to the power amp's inputs if I build a simple voltage divider. I also found a good tutorial on building passive high-level crossovers. I came up with the following circuit:



R1 and R2 make up the voltage divider. I used Dayton 2% non-inductive resistors. I chose these values based on the assumption the speaker outputs on the receiver will perform best into a load in the same ballpark as a loudspeaker.

C1 and R3 make up the passive highpass crossover, with a calculated -3db point at 83hz. C1 is a .22uF Polyester Film Capacitor from Radioshack. R3 is a 10k ohm 5% resistor from Radioshack.

The input impedance of my power amp is 56k ohms.

I built this circuit on a breadboard, and connected it to my subwoofer amplifier preouts to test. I don't know the input impedance of my subwoofer plate amp, but I figured so long as it was between 10k - 100k things should work. Also, I was already using the speaker level inputs on the subwoofer plate amp without any issues.

The problem is when I connected everything, my subwoofer started making a 60hz hum.

I'm very new to this, and am learning as I go. I suspect I have a bad ground somewhere. Any ideas? Is this happening because I'm using a breadboard, and the problem will go away once I build everything into a cable?

Any advice is appreciated, as well as links to further reading.

Dan

[DigitalJunkie]

I once built up a passive X-over for my subwoofer,and had issues with hum.

In my case I discovered that the coils I used in the X-over were *VERY* sensitive to magnetic fields (even a transformer running halfway across the room was audible in the sub.)

You don't have any coils,So i'm not entirely sure.
But a metal enclosure wouldn't hurt.
Double check all the ground connections again,just for kicks.


(edit)
Ohh,One last thought!
Your amp isn't bridged is it? (BTL/'floating' outputs?)
Measure the resistance between the Negative speaker terminals,and the amp/receiver chassis..should be very low (near 0.0x ohms.)

[AndrewT]

Hi,
I'm confused.
High pass filter set to about 80Hz into a power amp to drive a bass speaker!!!

Tell us what you want to achieve (the brief), then we can tell you how to achieve it.

[owdi]

Digital Junkie - I don't believe the amp I was testing with was bridged. It's the 250 watt plate amp from Parts Express. ink. I think I will try the voltage divider without the xover. If the hum is gone, I'll know the source of the problem.

Originally posted by AndrewT
Hi,
I'm confused.
High pass filter set to about 80Hz into a power amp to drive a bass speaker!!!

Tell us what you want to achieve (the brief), then we can tell you how to achieve it.

I was using the subwoofer for testing only, because I had already been using the speaker level inputs on the plate amp. I moved the connection from the speaker intputs to my my voltage dividier/xover, then connected that to the preouts on the plate amp. That's when the subwoofer started making the hum.

Ultimately, I want to use the speaker outputs on my Pioneer receiver with the RCA inputs on my Niles Audio SI-275 power amplifier. I added the crossover because my vented speakers were hitting their excursion limits at moderate listening levels.

I'm very new to diy electronics, so there's probably some little thing I'm missing that is common knowledge to everyone here.

Dan