I guess I just happened to see this first. Have you tried swapping the input tubes? I diagnosed hum on a CSP2 for a couple of months and tried a dozen things, but noise/hum on the input tubes always made the biggest difference.

Thanks,
Richard

Awesome Try 3-4 others first and see how you feel. I've also heard that if it's a brand new amp, hum noise can go away over time as the amp settles in.

Yes I can confirm that even after thousands of hours, my amps still hum loudly for a couple of seconds when they first power on. Not sure whether other Decware owners will confirm the same, but it probably has to do with the amp powering up process.

After that initial phase passes, at this point I only have hum if the tubes in the amp have hum. All of the tubes can contribute or take hum away, but I think 75% of the impact is with the input tube, followed by 20% with the rectifier, and then 5% with the output tubes.

Thanks,
Richard

I'll defer to other, much more knowledgeable, members at this point.

Another thread on hum with the UFO2 here, which seems to have been resolved with burn in:
https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1615046257

All of my music is digital, so I've attacked hum by swapping tubes (input and rectifier both made a difference), adding a Hum X to the plug (didn't work), adding an iDefender (didn't work), adding a power regenerator (made a small difference), adding a USB signal regenerator/galvanic isolator (made a difference), and some other odds and ends that didn't substantially reduce hum. I also swapped plugs in the same room and that didn't help.

Good luck! I run headphones so I'm also ultra sensitive to hum.

Richard

Thanks, I will try that.  Good news is the amp is easy to move!

Time to contact Steve?  Several years ago I sent my ZP3 back to him for a hum that he fixed.  He can help you troubleshoot.

1) we need to know exactly which full-range speaker is being used and in what cabinet.

2) we need to know how loud the hum is and from how far away. (SPL meter at 1 foot away)

Based on that info we can determine if what you are hearing is normal operation or not.

The MV meter has no way to distinguish between hum and ultrasonic noise, so we can't tell much from that either.


When testing for hum, first step is have the amp warmed up for at least a couple minutes.  Then remove the input cables and turn the volume control all the way down.  What you hear is your baseline hum with the tubes that are in the amp at the time of the test.

There is nothing inside a Zen amp that can cause hum other than the rectifier tube fuses.  There are a total of 3.  Two of the fuses are protecting the amp from shorted high voltage plates in the rectifier tube, and the other larger value fuse is protecting the amp from shorted filament in the rectifier.

We know the larger fuse is OK because the rectifier tube is glowing.  We don't know if both of the remaining fuses are OK because the amplifier will still work with only one.  It will however have audible hum if one of these fuses blew.

Check the two smaller value fuses (200 or 250 mA) and see if they are both good.


Steve

Wanted to follow up.  There were some delays (lost package, someone on vacation), but have the replacement fuses in and that fixed almost all of the problem.

Just for the sake of mentioning it, the readings went from 7.5 to 2.1 on the meter - but in practical terms, I can no longer 'feel' the hum by touching the drives and it is significantly reduced.  I still hear the hum in a dead quiet room, but it is no longer what it was.  I wish there was something I could do to further reduce it (obviously none would be best), but at least at the point where it is no longer what it was.  For the sake of objectivity, it is possible the hum I am getting is from an outside source (it is still there with the RCA leads unplugged), power or other interference.  I just don't know if 'some' hum is inevitable.

Also curious...how could the amp operate at all with one of the fuses blown?  

Update for me, and I’ll update my thread as well Monday or Tuesday: spoke to canonken a bit via pm and also spoke to Steve. Pulled both small 200mA/250V fuses, and thought they were both blown. Turns out my middle age eyes are struggling to see that very thin element and once I measured them only one was blown.  Ordered a replacement, should be here Mon pm, and hopefully good to go. Canonken, appreciate you taking time to respond, and Steve, thank you for fitting me in Thurs afternoon to chat!

Ok.  Fuses came a day early (yesterday).  Replaced the blown one, hooked everything back up, and got to listen quite a bit.  Hum is definitely MUCH improved.  Not noticeable at all from my listening position which is in a smallish room (11x12).  I can still hear it slightly if I put my ear directly beside the speakers, but cannot detect cone movement while touching it.  All in all, I am good with that.  The sound is noticeably....  tighter, focused, deeper, real, more THERE.  Certainly a significant improvement, which was nice since it was kinda like delivery day all over again.

Hello,

I would like to check the fuses near the rectifier in my SE84UFO2.1. What is the best way to access those?

Thank You,
Chris

Thank You, do you know if the screws are located on the top or bottom of the unit?

Thank you for your help. I was able to access the 3 rectifier fuses and check them. All 3 are good. I am still trying to track down the cause of the speaker hum.

I learned that the hum is normal.  I am hearing the power supply ripple when I place my ear close to the speaker. I do not notice the hum from the listening position about 4 feet from the speaker. These are the Lii Audio Silver 10 which have a sensitivity rating of 98 dB. I can also hear hum when using the Klipsch R-51m speakers that have an 89 dB sensitivity rating.

I don’t know that all tube amps have hum, this is my first tube amp. It sounds like hum is more common in single ended amplifiers, because there is no common mode rejection. Also, the more capacitance that you have for the ‘smoothing capacitor’ the less power supply ripple there will be. However, the higher smoothing capacitance puts more demand on the power transformer and rectifier tube. So that would limit the types of tubes available for use.

This is an interesting article about guitar amps that applies.
https://www.valvewizard.co.uk/smoothing.html

an issue not just in tube equipment , my NAD integrated ss amp with toroidal transformer hums the loudest including mechanical buzz right off the tranny but very slight through speakers.

Ok I've done a little more experimenting. Here's what I've found/done:

Setup: only the SE84UFO25 turned on, input shorted with shorting plugs, the only thing it's connected to is speakers (home built full range with MarkAudio Alpair 10p drivers, no crossover).

I measured the sound at the speakers with my phone. It's a 55dB hum, at 120hz. The room is 40dB without the amp on. Hooking a scope up to the outputs, I'm seeing +/-4mV. The hum isn't there when I first turn the amp on, but once the OA3 regulator tubes are fully on, it's there, and constant, regardless of how long I leave things on for.

It's the same for both channels, and is audible at my listening position about 6 ft away. The transformer makes a quiet hum at the same frequency, only audible maybe 2 ft away.

Things I've tried, none of which have made any difference at all:
- Different power cords
- Different outlets
- Moved to middle of room
- Tried with/without Emotiva DC offset eliminator
- Different speaker cables
- Switched out input tubes
- Switched out power tubes
- Switched out rectifier tube
- Switched out one of the regulator tubes for an OA3 from my ZP3 phono stage
- Removed input tube

FWIW, I have a ZP3 and CSP325, they makes the same transformer noise, but don't seem to add any noise to the output. I verified this by hooking them up to a Schitt Aegir power amp. This setup (ZP3 + CSP325 + Aegir) made no noticeable noise at the speaker when powered on but nothing playing. Everything is on the same outlet.

At this point, not sure what else I should try, as literally nothing has changed the 120hz hum (nothing makes it better, nothing makes it worse), and other setups in the same space don't hum/make noise. Any ideas? Is this just inherent to the amp? If so are there any modifications I could try internally to reduce it? Thanks!

-Sam

I haven't... the amp is pretty new, ~20 hours in, and I haven't noticed any weird flashing from rectifier or anything... the hum has been there since the first time I powered it on, and is pretty quiet, but at this point, couldn't hurt, I'll give that a shot!

Hi Sam,

What oscilloscope did you use to measure the output?

I just received a Hantek DSO2D15 and would like to figure out how to measure my amp.

Thanks,
Chris

Sam,

I can share your frustration about such issues when all I want to do is plug and play. Didn't buy a product just to immediately start troubleshooting it! Now the truth here is that I have experienced this with many high quality audio products, so I am not pointing out a certain brand of equipment. Here is what I have found out usually by accident. Lets take hum for example since it seems to be a more common problem than many others. In short there have been times that the combination/synergy of products produce unwanted results, even if each product by itself is wonderful. I have a SS power amp that had/has hum in one channel, and believe me, there was not one stone left unturned including sending it back to the engineer that first worked on it (not surprising to me, he had no such issue at His location). Sounds kinda bad but I do not have a hum issue anymore since I have switched less sensitive speakers. Yes, the hum is still there, but barely audible with my ear next to the driver. The drivers before were about 94db as compared to the new set at about 88db. I am not necessarily expecting you to go out and buy new drivers, but I am pointing out that super sensitive drivers do seem to have more of an issue with this hum problem. Kind of like using headphones compared to speakers I guess.
 Also, you mention a smaller listening area in where your listening distance may be less than other larger rooms. Yup, another chance for this to be an issue, although I won't fault you for the cards that your dealt.
All things considered there may be consequences to the choices that we have before us, intended or not.

Yeah I very much appreciate everyone's help/comments. I don't think there's anything broken with my gear, it seems pretty close to being within spec, so I'm hunting down various environmental things at this point (power, general noise in the air).

The tricky thing with only having 2.3 watts to play with is going with less sensitive speakers isn't much of an option. If I ever do find a solution, I'll be sure to post it here for future reference. Even if I can't fix things, I still totally get the Decware magic, I've heard stuff I never expected to!

Sam,

What power cables are you using? Do you have a power conditioner?

What music sources are you playing from?

Just thinking out loud, after looking at your list of things you’ve tried.

Best,

Geno

Sam,

IMO, next on your list, should be a Decware power cable. Made a big difference in my rig.

Hey Chris. The setup is dead quiet now.

I have everything run through a Decware power conditioner.

I did chase a hum problem when I first put the Darlington preamp in a short time ago. A few months before this, I had gotten a new power cord, and run it from the conditioner to the wall. But I had only played from a digital source after the new cable, with no issues.

When I got the phono setup going, I had bad hum. I tried a bunch of different things, before I switched out the power cable to a Decware cable, and the hum completely disappeared.

All of my past hum issues have been in my phono chain. Seems to be much more susceptible to such problems.

Best,

Geno

I got a Furman PST-8 delivered today, plugged everything in to that, and sadly, there was no difference. Not sure what else to try, and don't super feel like spending more money on speculative guesses at this point. Oh well, I'll see if letting everything burn in a bit helps, otherwise I don't really have any other ideas.

I think one issue is just my room is small. From the listening position, I think I'm mostly hearing quiet transformer hum and not the hum from the speakers themselves (but the speaker hum hasn't changed at all, regardless of what I've done). It would be nice if everything was dead silent, but there you have it! Thanks for all the suggestions along the way, and if I ever do figure things out, I'll be sure to report back!