Troubleshooting hum in 12b4 preamp.. finally almost done!!

[240z4u]

Alright, still haven't conquered this problem. I am not totally sure where to start. It is a 60 cycle hum and varies with volume. I am in the process of star grounding the entire preamp, hoping that will help. The umbilical is about 3' long. Is this a problem? I read in another thread that the filter caps should not be that far from the CCS.

Can I look for hum with a multimeter on the PSU without it hooked to the preamp? I read someone was able to do that as well. I assume I am just looking for AC voltage on the B+ line.

I have already converted to DC heaters.

Thoughts? I am going to continue the star ground setup tonight. I am also wondering if my ground wire from the preamp to the PSU chassis is large enough to create a quality ground. I don't recall what the gauge is, but it's not very large.

Thanks everyone.

Evan

[240z4u]

Going to start here; http://www.raleighaudio.com/Audio%20...connection.pdf

Great article!

[scott17]

Did you try to simulate your power supply using PSU Designer?

I noticed from the photos that your heater wiring appears to be giant antennas floating through the build, but you said that you have converted to DC filaments. How well filtered is the DC filament supply? The heater wiring could still pose a problem.

[240z4u]

I didn't come up with the PSU on my own so I don't know if it was modeled or not. I used the schematic that Eli Duttman provided along with a CCS that bandersnatch designed.

My DC filaments aren't all that well filtered. Eli indicated in a PM that I shouldn't even have had hum with AC heaters. I think I may go back to the AC setup honestly since my DC isn't really filtered. The hum didn't change when I switched from AC to DC if memory serves.

In this situation, what is the best way to handle running the heater wires through the chassis without turning them into a huge antenna? Ill run the damn things through tubing as a shield if I have to!

Thanks for the response, I really appreciate it.

Evan

EDIT; Embarrassingly enough, I may have installed a small switching power supply for my heaters. Stupid noob. Ill re-install the AC heater setup.

[scott17]

The best way to run the heater wiring is to have it "hug" the chassis and keep the leads as short as possible. It should be run as close as possible to the chassis. The heater wiring should also be twisted as tight as possible for best hum cancellation. All small-signal wiring should be kept as far away as possible. Any signal wiring that has to cross the heater wirings path should cross at a right angle.

Edit: Based on the photos, I would re-twist your heater wiring into a much tighter twist.

Originally Posted by 240z4u

I didn't come up with the PSU on my own so I don't know if it was modeled or not. I used the schematic that Eli Duttman provided along with a CCS that bandersnatch designed.

My DC filaments aren't all that well filtered. Eli indicated in a PM that I shouldn't even have had hum with AC heaters. I think I may go back to the AC setup honestly since my DC isn't really filtered. The hum didn't change when I switched from AC to DC if memory serves.

In this situation, what is the best way to handle running the heater wires through the chassis without turning them into a huge antenna? Ill run the damn things through tubing as a shield if I have to!

Thanks for the response, I really appreciate it.

Evan

EDIT; Embarrassingly enough, I may have installed a small switching power supply for my heaters. Stupid noob. Ill re-install the AC heater setup.

[240z4u]

Alright, I can certainly handle that. Thanks.

[scott17]

Originally Posted by 240z4u

Alright, I can certainly handle that. Thanks.

Here is a photo of an amp I built. Although it is not just a preamp, you can see the amount of twist in the heater wiring and I have absolutely no hum.

Also, if you have the component values and voltage/current measurements for your build, you can input these values into the PSU Designer II program to check for possible power supply hum.

I doubt this is the case though as the individuals you mentioned as helping you with this are highly respected in the DIY community and would have certainly noticed if there was a power supply issue.

Good luck.

[scott17]

Originally Posted by scitizen17

Here is a photo of an amp I built. Although it is not just a preamp, you can see the amount of twist in the heater wiring and I have absolutely no hum.

Also, if you have the component values and voltage/current measurements for your build, you can input these values into the PSU Designer II program to check for possible power supply hum.

I doubt this is the case though as the individuals you mentioned as helping you with this are highly respected in the DIY community and would have certainly noticed if there was a power supply issue.

Good luck.

Forgot to attach, so here it is.

[240z4u]

Oh wow, my attempt at twisting is pretty lame!

I really appreciate you taking the time to get me pointed in the right direction. As of tonight I am going to;

Star ground everything
Replace AC transformer for heaters
Twist heater wire tightly.

I read the PDF about grounding and Ill do my best to implement that and report back.

Evan

EDIT; Amp looks very nice btw!

[Doz]

Originally Posted by 240z4u

Oh wow, my attempt at twisting is pretty lame!

I really appreciate you taking the time to get me pointed in the right direction. As of tonight I am going to;

Star ground everything
Replace AC transformer for heaters
Twist heater wire tightly.

I read the PDF about grounding and Ill do my best to implement that and report back.

Evan

EDIT; Amp looks very nice btw!

I use one of those small electric drill screwdrivers with a hook in the chuck to twist the heater wiring, it's quick, tight and easy.