Crossover Humms during use

soundNERD · 10th June 2004, 01:42 PM

I built a crossover with the attached schematic. It works perfectly, but there is an audible humm in the speaker. Even with music it is noticable. I have it set up with 0.1uf caps and 15K resistors to get a cutoff frequency of about 100Hz. It cuts off the frequency there, and sounds good otherwise, but I cannot figure out what is causing the humm.

Right now, i'm testing it with a +/-22VDC supply. That would blow the chips so I installed a 7805 and a 7905 regulator on the board. So, the chips get +/-5V. I have checked the voltage and it is fine.

What is causing this? I am using 1/2 of a Phillips MC1458N Op-Amp. I have some sitting around collecting dust, and with the other chip on the board, the LM837 which I originally planned to use didn't fit.

markp · 10th June 2004, 01:53 PM

Maybe it just doesn't know the words!

Actually, it sounds like a grounding problem. Does it hum with nothing connected on the input side and the input shorted to ground?

soundNERD · 10th June 2004, 01:58 PM

oh, forgot to mention. only humms loudly when the input is connected. The input is the headphone out of my sterero. When I disconnect it, the humm quiets down.

What I am wondering, is it because I do not have any filtering after the requlators? What if I put a 0.01 uf cap between +5 and gnd and -5 and gnd?

djQUAN · 10th June 2004, 02:26 PM

using the regulators without output capacitors make them unstable and sometimes oscillate.

your hum may be caused by poor grounding scheme.

I also suggest you use 15 volt regulators to get better headroom for the peaks of the signal.

markp · 10th June 2004, 04:08 PM

Quote:

Originally posted by soundNERD
oh, forgot to mention. only humms loudly when the input is connected. The input is the headphone out of my sterero. When I disconnect it, the humm quiets down.

What I am wondering, is it because I do not have any filtering after the requlators? What if I put a 0.01 uf cap between +5 and gnd and -5 and gnd?

Try from 10 to 100uF after the regs. Headphone outs are notoriously bad for hum too.

soundNERD · 11th June 2004, 01:15 AM

OK, I added a 0.1uf cap on the output of the regulators. The hum is now only heard when you put your ear right next to the speaker.

Its odd since it still shows up just as much on my scope but you can't hear it.

it is a cheap onkyo reciever, so that may also be why.

li_gangyi · 11th June 2004, 04:06 AM

hmm...sin't +/- 5V a little low?? does the signal clip?? as for the little hum present...how much uF did u put before the regulators??

soundNERD · 11th June 2004, 12:37 PM

Well, I wanted to use larger chips, but I only had both the positive and negative regulators of the 5V series. I have a +12V, but not a -12V. Well, I do somewhere from when I tore a computer SMPS apart, but can't find it.

It doesn't clip though.

oh, forgot to say, the caps before the regulators are 1000uf per rail in my power supply. I've been meaning to change them for larger values, but when I built it, 1000uf was all I had.

When the amp is all done it is going to have 2 2200uf caps per rail.

10th June 2004, 01:53 PM	#2
markp diyAudio Member Join Date: Oct 2003 Location: L.A., CA	Maybe it just doesn't know the words! Actually, it sounds like a grounding problem. Does it hum with nothing connected on the input side and the input shorted to ground? __________________ If it sounds good... it is good!

11th June 2004, 04:06 AM	#7
li_gangyi diyAudio Member Join Date: Jun 2003 Location: Woodlands Circle	hmm...sin't +/- 5V a little low?? does the signal clip?? as for the little hum present...how much uF did u put before the regulators?? __________________ Kids in the back seat cause accidents...Accidents in the back seat cause kids...

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10th June 2004, 01:58 PM	#3
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	oh, forgot to mention. only humms loudly when the input is connected. The input is the headphone out of my sterero. When I disconnect it, the humm quiets down. What I am wondering, is it because I do not have any filtering after the requlators? What if I put a 0.01 uf cap between +5 and gnd and -5 and gnd?

10th June 2004, 02:26 PM	#4
djQUAN diyAudio Member Join Date: May 2004 Location: manila	using the regulators without output capacitors make them unstable and sometimes oscillate. your hum may be caused by poor grounding scheme. I also suggest you use 15 volt regulators to get better headroom for the peaks of the signal.

11th June 2004, 01:15 AM	#6
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	OK, I added a 0.1uf cap on the output of the regulators. The hum is now only heard when you put your ear right next to the speaker. Its odd since it still shows up just as much on my scope but you can't hear it. it is a cheap onkyo reciever, so that may also be why.

11th June 2004, 12:37 PM	#8
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	Well, I wanted to use larger chips, but I only had both the positive and negative regulators of the 5V series. I have a +12V, but not a -12V. Well, I do somewhere from when I tore a computer SMPS apart, but can't find it. It doesn't clip though. oh, forgot to say, the caps before the regulators are 1000uf per rail in my power supply. I've been meaning to change them for larger values, but when I built it, 1000uf was all I had. When the amp is all done it is going to have 2 2200uf caps per rail.

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