Pre-Amp Build

[Wntrmute2]

I MAY have tracked down the issue. Grounding the audio bus to different panels seems to reduce or increase hum so I added a few jumpers between the panels. But the biggest change came from rotating the filament transformer about 45 degrees down from where it is currently located. I've secured it with a bit of RTV in the new location and will re-evaluate in a few hours. The filament transformer is basically just below the power transformer. The SMPS and power choke can also be seen in the photo. Just below the choke is the power inlet/filter if anyone sees issues with placement.

 

[paul_b]

What you're working through is one of those processes that is incredibly time consuming and painful the first 10-20 times you do it, but at some point I assure you it will become automatic! The important thing is not to give up, there's always something to be learned when chasing down noise like this.

 

[Wntrmute2]

Better but still present at higher volumes. Will try moving that transformer completely out of the box with some wires grafted into the circuit. Next experiment after that will be to try shielding the input wires to and from the attenuators but that looks like a formidable job. Well, I guess I can't shield the attenuator to tube pin as it is just the grid resistor body.

 

[Deke609]

One thing I noticed as I was fooling around today is the hum varies not only with the setting of the volume level but sort of with the position of the knobs. If I push a little or pull on the knobs I can change the level of hum. ...

Just a shot in the dark: If all else fails, you could test whether the problem is the Kozmo and/or its power supply by temporarily taking them out of circuit (leaving them in place but disconnecting them) and tacking in a simple two resistor voltage divider in place of the Kozmo. If you set the attenuator at a level where the hum is plainly noticeable, you could use your meter to measure the two resistances comprising the voltage divider at that attenuator setting and then make your simple voltage divider using those values.

With the Meanwell unpowered and the Kozmo out of circuit, if the hum is gone you'd have narrowed the problem down to the either attenuators themselves or the Meanwells. If the hum is still there, then the problem lies elsewhere.

cheers, Derek

 

[Wntrmute2]

Great idea. I was trying to figure out how to do that but was at a loss.

 

[paul_b]

Get some cheapo mono 100K log pots and just pop the wires off, then solder the pots in (and earth the pots to the chassis).

 

[Wntrmute2]

Good suggestion.

 

[Wntrmute2]

Still need to figure out how to measure the hum on the bench. The detective work would be much easier!

 

[paul_b]

You need a cheap amp and cheap speakers at your bench. Go for some little POS class D amp with a lot of gain and some fairly sensitive small speakers so problems will avail themselves without too much prodding.

 

[Wntrmute2]

You need a cheap amp and cheap speakers at your bench. Go for some little POS class D amp with a lot of gain and some fairly sensitive small speakers so problems will avail themselves without too much prodding.

I really need to do this. I have a Bob Latino amp I built I'm not using but it is a PP design so pretty good at covering up the hum. I have some Snell E-3 speakers but their sensitivity is just 91dB.

 

[paul_b]

A push-pull amp won't cover up the hum you're listening for. It will amplify what's presented at the input, and something that's vintage like a Dynaco will have a lot of extra gain that is normally undesirable, but that is helpful in this situation. With 91dB Snell speakers, that would do the job.

 

[Wntrmute2]

Well, @paulbottlehead was correct as usual. I hooked the problem child to my Latino amp and then to a pair of altec 414 naked drivers. Nice hum machine. At about half volume the hum suddenly becomes noticable. I disconnected the attenuators from the inputs (at the attenuators) and the hum stays the same. Unplugging the power cord to the attenuators completely eliminates the hum whether I disconnect the ribbon cable from the attenuators to the LED display or the 5Vdc that powers the whole thing.
At least I have a test system now

Will do. Thanks!

I recall reading specs on a Japanese moving coil cart once that indicated either the compliance or tracking spec was at a certain temp (23C / 73 F). Can't recall the brand but I will try to find it. I remembered that because I knew that it would be next to impossible at my place without cranking the furnace up.

Fortunately, there is a solution: Cartridge Warmer

Thanks Dan and Paul. I'll prepare for carnage, even if it never happens.

The power supply is a Meanwell SMPS. 5vdc
No connection to the audio PS except for the at the mains.

 

[Wntrmute2]

I BELIEVE I have found the problem. The Khozmo attenuator bodies need to be grounded separately from the audio signal as @paulbottlehead alluded to in an earlier post. There is NO indication on the attenuators that needs to be done. What stimulated me to try that is I replaced the brass volume shafts with Garolite ones thinking I might be creating a ground loop or similar as moving the volume knobs often changed the hum. Well that made the problem much worse. I then touched a grounded probe between the flexible coupler and the chassis - and lo and behold, dramatic decrease in volume!
I then moved the jumpers to a threaded hole (unmarked) on the attenuators and tried grounding to the chassis - good reduction and then to the audio bus and next to silence was the result!!!
I'm going to add those jumpers in permanently, rehook up the inputs and give it another spin.
Wish me luck 🤞

Before grounding.


After grounding.

 

[Wntrmute2]

That did it! Just got home from one of our audio meets (first in about 18 months) and after petting the wife and kissing the dogs I slid Gadget #1 into the big rig and blissful silence! Cranked all the way to 63 and no additional hum. Nada, zilch, goose egg! I can't tell you what a relief it is. So thankful for all you guys!

Gadget #2 on the cutting table in the morning.

 

[paul_b]

I did honestly find it really odd that your Khozmo attenuators didn't come with a solder pad for that.

 

[Wntrmute2]

I know right? I sent Arek an email saying an instruction sheet would help us novice builders.

 

[paul_b]

I've sent that same e-mail! When I had to wire in that motorized stereo attenuator, I was wishing for a datasheet.

 

[Wntrmute2]

Well interestingly the attenuators in Gadget #2 had the holes in odd places if one was going to use them for mounting and there was anodizing inside the threaded hole so grounding them wasn't quite so straight forward. I finally got a good ground by running a tap through the holes a few times and I also wrapped a second ground around the mounting shaft nuts. Seems to be fine but I really wish Khozmo would improve in this area as his stuff is perfect for me otherwise. Stepped (63 steps) remote volume control with balance and somewhat affordable.
I would have thought mounting these via metal brackets would have fulfilled the grounding requirements