Noisy sex

[RW]

Here are some of the photos of the build. Some of the components were changed out, but they're all equal values to the stock components that came with the kit, they're just matched closer. I'm still experiencing the same issue that the stock components had.
I did a continuity test on the grounds, I'm not sure if safety ground, and signal ground are supposed to be common, but they are. If any more photos are needed, I'll be happy to post them. Thanks!

[Zimmer64]

Hi,

The top plate looks painted or powder coated. Some components require a good connection to it to be properly grounded. Have you made sure that the paint was scratched off in the relevant places?

Kind regards

Michael

[RW]

I did grind off the powder at all ground points. I did a continuity test between all grounds and they all check out. All 3 of my BH kits have the same finish, this is the only one with the hum.

[Paul Birkeland]

If you don't build it with the stock parts first, troubleshooting stuff like this becomes incredibly difficult.

My advise would be to put the stock parts (caps, wire, resistors and all!) into the amp, then let us know what you hear. 

[RW]

It was stock originally. All my measurements were the same now as they were when I first had the issue. The parts that are in now are the same value of all of the stock parts, except the 10000uf cap, it's now 35v. My issue is still the same as it was when I first reported it and was told to resolder everything. I'll put the stock stuff back in again. If that's what it's gonna take to get this issue fixed, consider it done. I don't understand why it would be any different though.

[Doc B.]

To go back over this, just to be sure I understand what is going on -

You are experiencing hum when using very sensitive headphones but not with less sensitive ones. The lowest impedance tap sounds the least hummy. That is all to be expected. 120 ohms in series did not reduce the hum further. That is not expected.

Could it be that the 120 ohm resistors were possibly not installed the way we intended? I say this because they were first suggested by PJ when I required the S.E.X. amp to be usable with sensitive headphones, specifically to lower the noise floor even more than the lowest impedance tap alone will. So the hum should be more quiet with the 120 ohm resistors in series with the outputs than without them in there. So just to verify, is each 120 ohm resistor in the hot (positive) leg of each channel? Basically one would break the connection of each red wire that goes from the impedance board POS pad to the headphone jack and solder a 120 ohm resistor into each break.

[RW]

I took the resistors out last night to start putting all of the stock parts back in. The 120 ohm resistors were right at the headphone  jack, one was on the right lead, one on the left lead, nothing was on the neutral.
I didnt notice the hum until I put on a more sensitive headphone, now i know it's there, I can hear it on all of them.
I should have it finished and back to stock after work tonight. I Just have to finish wiring the RCAs and the pot. I'll take some measurements, take some photos, and report back.
Just for clarification, would you like for me to install the 120 ohm resistors again?
Thank you for your time and efforts, I certainly appreciate it

[mcandmar]

Also while your in there snap a pic of the wiring around the volume pot, i noticed you changed the audio path wiring, its important to have the shielding properly terminated as per the stock layout.

[Doc B.]

I am going to ask in this case that the community refrain from additional commentary for a bit so we (Bottlehead staff) can work through this methodically. Three or four different people suggesting three or four different things at once may serve to confuse things.

When you get it back together - with the 120 ohm resistors installed as you will need them for sensitive headphones - if you could check the following it would give us a good point at which to start with the redone circuit-

Is the hum constant regardless of volume control setting?
Is the hum a soft 60Hz, or is it buzzy?
Does it start the instant you turn on the power, or does it come on as the tubes warm up?

[RW]

I added the shield ground to a terminal block. It followed the schematic, the stock terminal was too packed to fit it in so I added the screw strip and soldered everything there. I pulled it out last night to go completely stock again

[Doc B.]

Yes, for us to be able to offer useful suggestions regarding stock noise performance your taking the layout back to the  stock configuration will indeed be necessary. Rerouting or adding connections in the ground potential circuits seems like it wouldn't do any harm, but grounds are often not absolutely at zero potential and they can become a conduit for interference. The reason we are such sticklers about following the assembly instructions closely is that we have wrestled with these issues in many designs and the final layout we publish is the only one that we can post performance figures for with confidence.

[RW]

No problem Doc, I totally understand. It's 90% back to stock. Once I get home from work, the other 10% will be finished. The only thing that I'm not wiring in is the speaker terminals. I just have to finish the input wiring to the pot. Once im done, I will post now photos. I'll also reinstall the 2 120 ohm resistors (2 60 ohm resistors in series)  on the headphone jack.

[RW]

I finished getting everything back to stock and took measurements again. Every thing is within the 15% tollerance. The hum is still audible.

**note**
I couldn't find the solen 1.5uf caps. The ones installed are also 1.5uf at 630v

[Doc B.]

OK, so now that we are back to stock and voltages check out, let's see if we can qualify the hum level a bit.

Is it just a soft deep hum - 60Hz - or is there a buzzy 120Hz component to it? If it is buzzy there might be a ground connection that is not quite 100%.

Is it present with less sensitive headphones with the 120 ohm resistors in place? Or is it just noticeable with sensitive headphones?

[RW]

Soft 60hz hum for the most part. My least sensitive headphones (akg q701), with the impedance switches on low, I can barely hear it, but it's there. My 470 ohm r70x (highest impedance I own), I hear it on all switch positions. Everything I own in between I hear it on all switch positions as well.