S.E.X buzzing

[greyhamster]

Hi everyone,

I've just finish my S.E.X amp. All the measurements are within 10% except voltage at C5 is 3.8V (should be 3.15V).
The tubes light up; both at the same time and illuminate quite brightly.
However, there is loud buzzing sound on both channels even at no volume, and there is popping and crackling sometimes. I triple check to make sure there is no lifted ground connection as well as bad soldering joint; and no wire touches each other neither. Please help me I don't know what to do now.
 
ps: I used it with a headphone.

[mcandmar]

I'm curious about this too as mine measured 3.93v @ C5, and -2.5v @ C4. What did you get at C4?

I measured 3.06v AC @ terminal 4, and 3.05v AC @ terminal 5 on the transformer which looks perfect.

For the tubes try reseating them a few times to clean up the contacts, also if you tap them lightly while running can you hear any popping or crackling on the headphones?

[Doc B.]

It sure sounds like a bad ground connection.

[greyhamster]

               Real            Expect
A7,B7     -2.49            -3.15
A8,B8      3.74             3.15
C1          -2.51            -3.15
C2           3.77             3.15
C4          -2.51             -3.15
C5           3.94              3.15
15,25      16.9               19
A3,B3       16.5              19

other measurements are within 10%

[greyhamster]

I have check every ground connection and re-flow them (not sure if I miss any) but the buzzing hasn't gone away.
Can the plate chokes PC-3 touching the chassis cause the problem?

[Doc B.]

Substantial buzzing is signal ground-related. Popping and cracking could be a noisy tube, but most often it is due to a missed solder joint. Often we have posters who say they can't find it, then come back and say it was some connection that was hidden by other components. Is the buzz in both channels? If so it is probably a problem in the power supply. Filter capacitors might be a good place to start looking. If it is just in one channel it is probably in the amplifier circuit around the tubes.

[greyhamster]

Substantial buzzing is signal ground-related. Popping and cracking could be a noisy tube, but most often it is due to a missed solder joint. Often we have posters who say they can't find it, then come back and say it was some connection that was hidden by other components. Is the buzz in both channels? If so it is probably a problem in the power supply. Filter capacitors might be a good place to start looking. If it is just in one channel it is probably in the amplifier circuit around the tubes.

Thanks Doc,
I was well aware of that so I had checked many times before posted here.
The buzzing was in both channel and very loud regardless of the position of the volume control. At first I thought it was an easy fix because there should have been a missing or bad ground connection. I was looking, again and again all for night yesterday; re-soldered every possible ground connections.
I have been checking these connections again this morning but nothing is found.
Here are some pictures. I hope you could give me a hint on what to look for next.
http://imgur.com/vCFnvBf,lXAM2Nu,88QE0JF,RuSC1Hf,0YX1P83,hh10Gj1,8VkGTI9

Best,
Dave

Ps: The popping is gone after two minutes so I think the tubes is good.

[Doc B.]

Looks like a very clean and careful build. The only thing I can see that might possibly be an issue is that the connection of the black wire that goes from power trans terminal 10 to the center terminal of the T-strip right on front of the power trans might be a little strange at the center terminal of the t-strip. But it's not real clear in the photo and I could just be seeing things. Doesn't help that I left my reading glasses at home today!

[kgoss]

When I built my amp (version 2) I had the same issue. Turned out I did not extend the ground buss wire from pin 3 of the tube socket to terminal 18 and then 17 I one side and 38 - 37 on the other side. That meant the ground was not tied to the chassis plate. When I fixed that the amp was dead quiet.

I can't tell from the pictures if that is your problem but it's worth checking.

[greyhamster]

Looks like a very clean and careful build. The only thing I can see that might possibly be an issue is that the connection of the black wire that goes from power trans terminal 10 to the center terminal of the T-strip right on front of the power trans might be a little strange at the center terminal of the t-strip. But it's not real clear in the photo and I could just be seeing things. Doesn't help that I left my reading glasses at home today!

I have checked and re soldered that connection but it doesn't solve the problem 

When I built my amp (version 2) I had the same issue. Turned out I did not extend the ground buss wire from pin 3 of the tube socket to terminal 18 and then 17 I one side and 38 - 37 on the other side. That meant the ground was not tied to the chassis plate. When I fixed that the amp was dead quiet.

I can't tell from the pictures if that is your problem but it's worth checking.

can you please tell me more about this? I don't think there is any wire from the pin sockets that connects to the ground. And do you mean terminal 12 - 13 and 22 - 23?

[kgoss]

Well someone with the 2.1 version should give you the terminal numbers.  Don't know if the 2.1 version is wired the same. My problem was the ground buss was not connected to ground (on both channels) until I found the problem. Your solution might be different than mine but I think Doc's advise to look carefully at every ground point in the amp is wise. The amp should be dead quiet with no music playing.

[thomas27]

Hi everyone,

Wanted to chip in and share some experience on the topic. This is my first post on the forum btw (alhough I have spent quite a bit of time reading posts!).

I've built a SEX amp a few weeks ago. I ran right from the start but I was also having a loud buzz with headphones. I measured about 5-6mV of hum in one channel and 2.5mV in the other (8 ohm tap) on the binding posts with the volume all the way down. Turns out the 5-6mV of hum/buzz was due a bad tube.

After replacing the tube, I still had about 2mV of hum/buzz on both channels. Reflowed nearly all solder joints (starting with ground connections). It seemed to have some effect, hum/buzz went down to about 1.7mV. Removed the felt pads of the PT (the ones under the chassis). Seemed to have some effect, down to probably 1.5mV. I did not intall the C4S upgrade yet.

After something like 100h of use with speakers, the hum is down to 0.6-0.8mV, which is I think is fine with most speakers and about 0.5-0.6mV when the amp has been on for a couple of hours.

I absolutely love the amp with speakers. I'm using a cheap ODAC for now and some OK PC monitors. Even in this less than ideal set up it sounds fantastic. Cannot wait to hear it with Orcas! And eventually with the C4S upgrade.

However, I still get a buzz with low impedance headphones (still on 8 ohm tap). Not super loud but annoying and very quickly tiring.
After doing some research turns out some people have had such problems with low impedance / high sensitivity headphones. I am using some Phonon headphones bought in NYC some time ago which I suppose are relatively sensitive and only 24ohm.

Anyway, my point is that I've spent a couple of nights trying to figure out what was wrong with the amp when in fact I was probably down mainly to the cans... I'm not even sure reflowing solder joints and my other initiatives had any effect at all, it may have simply been the effect of component break in. (In fact when I put new tubes in the amp I tend to have about 2mV of hum which goes down progressively)

Still have a couple of questions though:

- Is 0.5-0.6mV of hum acceptable for a SEX / an indirectly heated triode amp or is it still too much? Btw this is measured with no load not sure if that makes a difference (would have to try again when I have the Orcas). Paul Joppa wrote in a post somewhere that 0.3mV could be achieved although I don't have any details on how this was achieved.

- Is there a way to make the SEX silent enough for low impedance headphones? I understand there were some 120ohm resistors on the jack previously (but the jack was not shorting at the time) - how could I implement that? Is changing filter caps an option? Could also use the 4 ohm tap but I like the flexibility of the 8 ohm tap. And I could also invest in a pair of HD650 and not bother about it...

- Should I put back the felt pads of the PT? Does it make a difference in terms of sound quality? Safety?


In any case, thanks for this terrific piece of equipment! Intructions are super clear and it is a lot of fun building it
+ thanks in advance for any inputs

[mcandmar]

I'll had my 2c's as i also have a faint hum i am trying to cure.  With HD650's its barely noticeable, but with more sensitive Grado cans it can clearly be heard with no music playing.  And as above i have no idea what is considered normal. (maybe i'm just being too fussy)

I've tried two sets of tubes as one of the tubes supplied with the kit has a loud bzzzzt noise all the time.  And with the other tube i could hear a faint humming in the background as described above.

With the 2nd set of tubes i am still hearing the faint hum in the background so i'm pretty certain its in the amp and not the tubes that is causing it.  Unfortunately one of these two tubes is also slightly noisy and microphonic so i'm trying to source yet another set, hopefully i will be 3rd time lucky.

Volume has no effect, neither does connecting/disconnecting a source to the amp so i can only conclude its power supply related. I re soldered all the ground points, heater supply, and most of the main power supply to no avail. I also measured resistance from the chassis ground point all around the amp and dont see more than 1ohm anywhere (lower limit of DMM).  Also tried using crocodile jumper cables to add extra ground points to no avail.

Interestingly i have noticed it getting slightly better as the amp burns in, i wasn't sure if it was my imagination or if i was just getting used to it and blocking it out, but now you say it i'm almost certain that is the case.  I probably have ~16 hours play time on it so far.

I never thought to try and measure it, i will give that a go tonight. I might even dig out my scope and see if i can see it.

Another thought i had was cable routing around the output transformers.  While i was resoldering i had the amp unplugged (obviously as i'm still here) but i was still wearing the headphones and any time i moved the iron around the transformers i could hear the buzzing noise in the headphones, got me thinking maybe the issue is just the transformers picking up noise from the wiring around them, or close proximity to the heater supply diodes.

[Doc B.]

When you say you took the felt washers under the power transformer out do you mean you took the nylon or fiber shoulder washers that go around the mounting screws out? This would be most likely to increase hum, not decrease it.

The 120 ohm resistors are a good idea, You can just remove the wire connected to the "tip" and "ring" terminals of the TRS jack and install a 120 resistor between the end of each wire and its terminal.

And yes, tubes need time to burn in.

[thomas27]

Mark - good to read that you are making similar observations!
Would be very interesting to benchmark noise levels to find out what the "typical" levels are.

Doc B. - thank you so much for your input, I was indeed referring to the nylon/fiber washers around the mounting screws. Will put them back then and report in sometime this week.

Regarding the installation of the 120ohm resistors, I have a shorting TRS jack - is that an issue? I thought the 120ohm resistors were not compatible with the shorting jack (I may be wrong)

thanks