50w pp 60hz preamp hum...

[joshdfrazier]

i recently built a 50w bass amp using these schematics:

http://www.ax84.com/static/corepreamps/ ... ematic.pdf
http://www.ax84.com/static/corepoweramp ... ematic.pdf

sounds good, and is plenty loud, but i've got some really irritating hum issues that i've traced back to the preamp. with the poweramp dimed, there is almost none to be heard. with the preamp turned up its really loud.

my heater ct is referenced appropriately. moving wires around does absolutely nothing. i really don't think this is a lead dress issue. tried disconnecting the filament leads entirely, no dice. replaced the preamp b+ cap to no avail. removing b+ from preamp tube stops the hum.

im at a loss. any ideas?

[sepulchre]

When you say "with the power amp dimed" do you mean that the volume on the preamp is turned down? Also, when you said you disconnected the B+ from the preamp do you mean just the first stage, or both stages?
I ask about these because I'm wondering if swapping out the preamp tube might help.

BTW, I've built both of these with great success.

[JazzGuitarGimp]

Are you following a known-good layout? What grounding scheme did you use?

[sepulchre]

I don't know if he's using the layout supplied with the plans from AX84 but if so it does work well; no hum issues.

[joshdfrazier]

yes, i mean the volume on the preamp is turned down, and the master is up all the way. i disconnected b+ from both stages, did not think to test them individually as my 100k resistors are soldered together.. it's definitely not a tube problem. i've tried about 5 different ones.

space was at a premium for this project so i went with a hammond 13.5x5 chassis. things are pretty tight.

what do you mean by grounding scheme? there is no distinction between earth and chassis ground.

[sepulchre]

The way the grounds are run is very important. Improper grounding can look right but still cause a ground loop which will induce hum. All grounds should be tied together either in a "star" scheme or a "bus bar" style. I generally use a bus bar (ie a wire, 20ga or bigger) that runs along the board on the control side and attaches to the actual ground (the chassis) at or right next to the input jack. The AX84 layout plans show how this should be done. Look at their amp layouts; P1, Hi Octane, etc. The only ground that's not in with the others would be the one attached to the power cord or IEC connector where the power enters the chassis. That one should be firmly attached to the chassis right at or next to the entry point.

If your grounds are already done properly (no loops - all going to one point) I would go to the trouble of lifting the initial preamp stage's B+ to see if that's where the hum originates.

Also, being as it's tight in there, a gut shot might be useful. Someone might see something helpful.

[pops]

Don't rule out tubes, i have had an order of 10 preamp tubes that all made noise. Bad run of tubes i guess. Do the tubes you tried run quiet in a different amp?

[joshdfrazier]

pops, these are all working tubes pulled from equipment with no issues.

i feel i should also point out that the b+ on pins 1 and 6 is 234 and 240, respectively. seems a little high to me, maybe not. suppose i could bump up the value of those resistors.

the green circles are ground spots. the wire circled in red comes from my presence control and is supposed to connect to the tip of my output, but i get an unbelievably loud squeal/negative feedback sound. an issue i'm tackling later.

yes, it is literally a birds nest inside there.

[Tone Lover]

I would say almost definatly your grounding scheme.
Thats a lessen i have had to learn the hard way over and over again. You just cant ground anywhere and not have it make a ground loop.
Im not trying to ne negative, its a problem I chase to much in my own builds.

Just my 2 cents, from the messed up ground loop king. (LOL)

Bill

PS. IF I would quit changing how I ground things I wouldnt have to spend so much time chasing this problem, but still I do it anyway.

[joshdfrazier]

no, please be critical! all of my builds have been low powered SE designs, so i guess i wasn't really hearing my mistakes.

what could i do to correct this? do i literally just ground everything at one point? trace everything that is grounded back to the PS caps? what about pots and 1/4" jacks? not much i can do about those since they make bare metal contact with the chassis...

thanks!

[sepulchre]

Use ONE of the jack grounds, preferably the input jack. Attach a loop of bare wire to the ground pin big enough so that all the grounds can be connected to it. Yes, run Every ground to it with the exception of the power cord ground which should be grounded to the chassis as close to the place where the power comes as possible. That will get rid of the ground loop hum. All the grounds you have will guarantee a ground loop.

Btw, point to point construction is usually a good thing. It keeps noise down but only if sensitive wires are kept apart from the emitter wires - especially the ones with AC current like the heater supply lines. And generally speaking keeping the signal lines away from the power supply side of the amp helps keep noise down. Also, you might consider putting some sleeving on the bare leads on the parts. There is some nice clear sleeving available but I usually just use pieces of insulation stripped from some wire of a similar gauge.

[joshdfrazier]

attached an updated pic after i rerouted all of the grounds. circled the two ground points. there were actually more than i thought. hum has been reduced, but it's still very obvious when the amp is cranked.

i am also getting a sweeping "whooshing" sound (sort of like a phaser) when i turn some of the preamp knobs, primarily the gain. it's nothing terrible, but it isn't great either. chalking it up to lead dress due to picking a stupid small chassis.

the hum definitely has me irked, though.

[sepulchre]

That's certainly a little space. You did good getting everything in there. Well done!

So is the noise now at an acceptable level? Can you live with it?

[joshdfrazier]

I really won't be able to tell until we're able to use it live. i would really love to get all of the issues squared away without compromise considering the investment i've made in this thing...

one last pic, 95% complete i think. just need to tolex the flip top lid and add the latches.

[bal704]

Have you chop-sticked the amp while running to see if you can reduce the hum/whooshing?