S.E.X. developed hum "overnight"

[Loquah]

Hi all, my S.E.X. amp isn't getting a lot of use these days since I sold my planars, but I tried it recently with a pair of Nighthawks I was reviewing and it sounded great. I've since bought a pair of Nighthawk, but now I'm hearing hum / noise through the S.E.X. with the Hawks. Nothing has changed since I used the review pair and the amp has been untouched for over a year now (regarding the wiring I mean). What are the likely culprits for noise suddenly appearing?

For some further clarity, the noise is very much a power-type hum (50Hz or thereabouts) and is mostly consistent volume at all steps on the attenuator except that it jumps significantly in volume on the last 1-2 steps of the attenuator - particularly when the attenuator is fully "open". I think the sound is slightly more prevalent on the right channel, but I have a head cold and can't fully trust my hearing right now. My S.E.X. is wired permanently in 8 ohm configuration with a 4-pin XLR output. The noise gets slightly worse if interconnects are all detached. Everything has worked just fine until recently and nothing has been changed internally.

I'm wondering if there could be a dry joint in the input wiring based on the effect of the interconnects being connected or not - does this sound plausible?

[Paul Birkeland]

Is there any hum with the level controls all the way down? (Assuming that your attenuator has a mute position)

[Loquah]

Yes. I'm not sure if the attenuator's lowest setting is mute or just max resistance, but the hum is still there and at the same volume as for almost the entire range on the attenuator.

[Paul Birkeland]

If you tap on the chassis, does it vary?

It sounds like you have a loose connection in the amplifier, and most likely somewhere in the power supply or shared grounding based on your description.

-PB

[Loquah]

Great. Thanks for the help, Paul. I'll flip it over and take a look - maybe reflow the joints and see where that leaves me.

[Loquah]

I've just finished a reflow of all the solder joints in the S.E.X. and the hum is still present, but it switches sides with the tubes - it's stronger on one than the other, but present on both. Is it possible that the tubes have developed a hum while not being used and will extended use rectify the issue?

I'll try so other tubes to confirm the source of the issue so I can answer part 1 of the question above, but am keen to know if continued use of the tubes in question can rectify the hum issue.

[Loquah]

New tubes and the basic hum is still there although the channel variation may be gone, but I'm wondering now if it was always there and I just didn't notice it with the Nighthawks the first time around. At listening levels, the hum is too low to hear over the music and my music is all cross-faded so there's no silence.

With the HD800s, the S.E.X. sounds as good as ever so maybe I just never tried the Hawks with the S.E.X. with no music playing the first time around.

Would it be normal to hear a minor level of noise using more sensitive, low impedance 'phones with the S.E.X. wired for 8 ohm (and balanced if it makes any difference)?

Perhaps I just need to add 120ohm resistors?

[Paul Birkeland]

I would try going to the 4 Ohm tap before doing additional modifications.

With your meter on the lowest AC voltage setting, what voltage do you see at the speaker posts?

[dmhenley]

New tubes and the basic hum is still there although the channel variation may be gone, but I'm wondering now if it was always there and I just didn't notice it with the Nighthawks the first time around. At listening levels, the hum is too low to hear over the music and my music is all cross-faded so there's no silence.

With the HD800s, the S.E.X. sounds as good as ever so maybe I just never tried the Hawks with the S.E.X. with no music playing the first time around.

Would it be normal to hear a minor level of noise using more sensitive, low impedance 'phones with the S.E.X. wired for 8 ohm (and balanced if it makes any difference)?


I had the same experience, and I think it's the Nighthawks sensitivity. I rewired for 4 ohms and also completed the mod suggested by mcandmar in the ramblings thread. I'll find the bit and add it here...
This is the mod:
went ahead and added the 100ohm resistors at C1&C2 ->C3, and the hum that became more prominent with the headphone switch, is reduced to a very subtle noise. And, this is with my Nighthawks (25ohm impedence).

[Loquah]

I would try going to the 4 Ohm tap before doing additional modifications.

With your meter on the lowest AC voltage setting, what voltage do you see at the speaker posts?

Sorry for the slow response - I've been sick. I can't rewire (or at least I'm choosing not to) because I want the 8 ohm wiring for balanced operation.

I don't have speaker taps any more, but I can measure the voltage at the XLR plug. Will the resistors to ground used to wire the XLR plug as recommended cause any issues?

To dmhenley, thanks for confirming your experiences. I will take the measurements suggested by Paul, but expect that the resistors will solve the problem.