Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: bmkuter on May 05, 2020, 07:33:44 PM
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I just finished building the base SEX and I hear quiet noise that increases with volume. When the volume is at 0, its still barely there.
Aside from that it sounds great, but is the hum normal?
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Hum that increases with the position of the volume pot tends to be external to the amp and may change depending on where you have it.
If you have nothing plugged into the inputs when you're listening to this hum, then that is making things quite a bit worse.
Having made those two points, it's also possible to have hum problems if the layout isn't reasonably carefully followed.
-PB
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The hum is still present when I unplug the RCA inputs. It sounds like general noise.
I'll try and attach a video: https://youtu.be/z2O3VahCFoI
Video is garbage quality, but best I could do with a phone. The noise is from SEX @ 100% volume through Focal Elegia with the mic ~1.5 inches away.
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What is the noise like with the volume all the way down?
When you have nothing plugged into the SEX amp and you turn the volume all the way up, you're just amplifying whatever ambient noise is available for the input of the amp to receive. This isn't a meaningful measurement.
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With the sound all the way down, I hear a very faint hum but very faint. When I turn the volume up, static seems stack on top of the hum starting after a quarter turn.
The hum all the way down sounds like a generator through a wall.
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You could try tightening the hardware on the power transformer.
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Thanks! That helped a bit!
Anything look misplaced here?
EDIT: Weird, can't upload photos right now.
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There's a file size restriction on the photos. You can just knock them down by about 50% and they should post OK.
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Thanks!
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Nothing is jumping out at me there, everything looks to be about where it should be.
-PB
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Should the amp be silent when not playing and volume all the way down?
I re-soldered some connections and converted the transformers from 8 ohms to 4 ohms, which helped the hum a little bit.
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Something easy to try: tighten all the mounting bolts/screws of the terminal strips. Many of them are used as contacts to chassis ground and so a loose one could make for an iffy ground connection. I noticed that the terminal strip near the headphone jack has black domed nuts. I'd make sure that the nut closest to the jack that is securing a ground reference is tight. I'd also take a peek at the ground tab closest to the IEC inlet - the one with the bare ground wire running through to the ground lug on the IEC. From the pic, it looks like it could use a bit of solder (but maybe not -- might just be the angle of the pic).
cheers and good luck, Derek
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Should the amp be silent when not playing and volume all the way down?
Yes.
Having spent a lot of time capturing noise measurements of the SEX amp, sometimes it comes down to the environment too. Another issue I have seen happen is that the 8 lug strip in front of the power transformer can touch the nuts securing the power transformer, and if this happens you can get some ground loop hum. All you have to do to fix that is nudge the strip away from the transformer nuts and brush in a little nail polish or clear lacquer to keep them spaced apart.
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Any ideas on how to check if its environmental? I checked everything upstream again, including some USB extension I forgot I was running. Something up there is causing the noise, since after resoldering and converting to 4 ohms, the noise is gone when RCAs are disconnected.
The hum sounds exactly like a generator we have near my apartment building, which supplies all the power. Any connection?
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Any ideas on how to check if its environmental? I checked everything upstream again, including some USB extension I forgot I was running. Something up there is causing the noise, since after resoldering and converting to 4 ohms, the noise is gone when RCAs are disconnected.
Hey, that's great news!
I would recommend trying this mod since it's inexpensive:
https://forum.bottlehead.com/index.php?topic=11676.0 (https://forum.bottlehead.com/index.php?topic=11676.0)
The hum sounds exactly like a generator we have near my apartment building, which supplies all the power. Any connection?
If your powerline was a problem, then I would expect the noise to just be there, and for it to be independent of the level of the volume pot.
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Thanks! I'll try that and report back once the fix is implemented. Going to take a few days from Mouser.
Are there any other common grounding/shielding mistakes? Its definitely a generator hum with no noise when the RCAs are disconnected that increases with the pot.
EDIT: I actually found some 1n4007s around. Think that'll do? The data sheet I saw says peak of 30A.
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Sure, give them a shot!
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The diodes reduced a lot of the noise! Now its only noticeable at max volume.
The generator hum is still there though.
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This hum is still absent with nothing plugged into the amp? If so, what is the specific component that you plug into the Crack that causes the issue?
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This hum is still absent with nothing plugged into the amp? If so, what is the specific component that you plug into the Crack that causes the issue?
By nothing plugged in, do you mean including the power cable and headphone? I don't have any speakers connected.
When first starting the amp from off, with headphones plugged in, the hum appears after the tubes have warmed up and are working. Its still around with no RCAs connected. The hum becomes noticeable 1/4 turn in on the volume pot. If I add RCAs, the hum is still present, but I then get some upstream noise added.
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By nothing plugged in, do you mean including the power cable and headphone? I don't have any speakers connected.
Nothing plugged into the inputs.
The hum becomes noticeable 1/4 turn in on the volume pot. If I add RCAs, the hum is still present, but I then get some upstream noise added.
How old is the wiring where you live? Do you have grounded outlets? You may want to try attaching a wire to the chassis of the SEX and the chassis of your source.
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Hum exists regardless of inputs.
I would say the wiring is maybe 15 years old at most. The apartment building is relatively new, but at least a decade old. The hum is most noticeable with low impedance headphones, especially my IEMs. The power should be grounded, and I'm plugged into a Furman Power Conditioner (got it very cheap and I needed another power extension in my house).
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And just to confirm, with the amp on and the volume control all the way down, is it detectable with IEMs? (and is the amp wired for 4 ohms?)
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And just to confirm, with the amp on and the volume control all the way down, is it detectable with IEMs? (and is the amp wired for 4 ohms?)
Its very detectable with the IEMs all the way down, and the amp is wired for 4 ohms. Not sure if it matters, but I can also hear the tubes crackling/warming up (?) with the IEMs on volume off.
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Could I see a side photo of the terminal strip with 23-28?
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I bent one of the caps out of the way for the pic. Orientation normally matches the instructions.
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I just wanted to make sure the shoulder washers were in place. If a shoulder washer is missing from one of the terminal strip terminals with a plastic acorn nut, you could end up with some hum.
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Ah okay. I checked and they're both there.
Does it sound like a grounding issue? I've gone through and looked at all the solder joints, but there could be ones for grounding that need to be reinforced.
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It could be loose hardware, it could be a power line problem (still seems unlikely in your case), or external interference. You're certain 36L isn't touching the power transformer mounting nut right?
Even for me to get a proper noise floor measurement when testing the prototype SEX 3, I had to wait for a day when everything was super duper quiet.
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Can you expand on loose hardware? I just went around tightening all the screws, but is there more than just that?
Why would the power line hum not be affected by the pot? I looked at 36L and the transformer nut, and they were separated by several millimetres of space.
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Can you expand on loose hardware? I just went around tightening all the screws, but is there more than just that?
Yup, just that.
Why would the power line hum not be affected by the pot? I looked at 36L and the transformer nut, and they were separated by several millimetres of space.
If there's something nasty riding on your powerline, it may be getting into the power supply, which would be a noise that would be there regardless of volume knob position usually.
An attenuating headphone adapter may be useful here if the noise persists.
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An attenuating headphone adapter may be useful here if the noise persists.
Cool! I've never heard of those, I'll give them an order. I've noticed in my amp testing that when I'm using my Sennheisers, the noise isn't audible at all.
Thank you for all your help.
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Is there a way I can measure my amp's performance if I have an oscilloscope and a signal generator? I was thinking of just connecting the RCA input wires to the signal generator, but where would I probe on the output end?
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Take the output from the speaker terminals, to start with. If your scope is sensitive enough, you can quantify the hum and noise. Short the input (a shorting RCA is best, but usually just turning the volume all the way down is equivalent).
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Thanks!
I just remembered that on page 40 - 41 of the instructions, when attaching the 270k resistors, the ones supplied with my kit matching the picture were closer to 370k. Typo in the instructions?
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There is no 370K resistor in our inventory, and only one carbon film 1W resistor in our inventory. What color codes do you see on yours? Did you measure the resistance with them before you installed them?
In any event, as long as they are both about the same value, they could be 200K or 500K and it wouldn't make much of a difference.
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I think it must have been a typo in the number itself. The colours matched the instructions.
Weird question, should I be able to hear the amplifier working? Like with headphones off and ear a few inches from the transformers.
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If you have a tone or music playing through the amp, then yes, you will hear some very low level noise coming out of the amp. If nothing is playing and you hear something like the power transformer buzzing, that would tend to suggest that the hardware is loose or there could some DC present on your AC mains that causing a bit of noise in the power transformer.
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Yea there's definitely a slight hum when muted. Think that could be causing all this humming?
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It's a possibility.
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I listened to my Crack last night and could hear the same static, but not on my Yamaha integrated amp (or incredibly faintly).
EDIT: Went back to reread the manual and noticed this part in the FAQ, which accurately describes the hum (Hissing or low hum noise is not there at low levels, can hear it when I turn the volume up to max -That's normal and you will probably find the listening level deafening at that loudest volume setting). I can really only hear it on more sensitive headphones.
EDIT 2: The headphone attenuation adapter arrived and it completely solved the problem! Thanks for the suggestion!