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